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IPC-7525B
2011 - October
Stencil Design Guidelines
Supersedes IPC-7525A
February 2007
A standard developed by IPC

Association Connecting Electronics Industries

®
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The Principles of In May 1995 the IPC’s Technical Activities Executive Committee (TAEC) adopted Principles of
Standardization Standardization as a guiding principle of IPC’s standardization efforts.
Standards Should: Standards Should Not:
• Show relationship to Design for Manufacturability • Inhibit innovation
(DFM) and Design for the Environment (DFE) • Increase time-to-market
• Minimize time to market • Keep people out
• Contain simple (simplified) language • Increase cycle time
• Just include spec information • Tell you how to make something
• Focus on end product performance • Contain anything that cannot
• Include a feedback system on use and be defended with data
problems for future improvement

Notice IPC Standards and Publications are designed to serve the public interest through eliminating mis-
understandings between manufacturers and purchasers, facilitating interchangeability and improve-
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proper product for his particular need. Existence of such Standards and Publications shall not in
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Recommended Standards and Publications are adopted by IPC without regard to whether their adop-
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Revision Change that the use of the new revision as part of an existing relationship is not automatic unless required
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©Copyright 2011. IPC, Bannockburn, Illinois, USA. All rights reserved under both international and Pan-American copyright conventions. Any
copying, scanning or other reproduction of these materials without the prior written consent of the copyright holder is strictly prohibited and
constitutes infringement under the Copyright Law of the United States.
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IPC-7525B
®

Stencil Design Guidelines

Developed by the Stencil Design Task Group (5-21e) of the Assembly

and Joining Processes Committee (5-20) of IPC

Supersedes: Users of this publication are encouraged to participate in the

IPC-7525A - February 2007 development of future revisions.
IPC-7525 - May 2000
Contact:

IPC
3000 Lakeside Drive, Suite 309S
Bannockburn, Illinois
60015-1249
Tel 847 615.7100
Fax 847 615.7105
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October 2011 IPC-7525B

Acknowledgment
Any document involving a complex technology draws material from a vast number of sources. While the principal members
of the Stencil Design Task Group (5-21e) of the Assembly and Joining Processes Committee (5-20) are shown below, it is
not possible to include all of those who assisted in the evolution of this standard. To each of them, the members of the IPC
extend their gratitude.

Assembly and Joining Stencil Design Technical Liaisons of the

Processes Committee Task Group IPC Board of Directors
Chair Co-Chairs
Leo Lambert William E. Coleman, Ph.D Dongkai Shangguan
EPTAC Corporation Photo Stencil Inc. Flextronics International
George Oxx Shane Whiteside
Jabil Circuit, Inc. (HQ) TTM Technologies

Stencil Design Task Group

Russell Nowland, Alcatel-Lucent Joseph Brown, Hewlett-Packard Todd Woods, Photo Stencil Inc.
Christopher Sattler, AQS - All Co- ProCurve Networking Dale Kratz, Plexus Corporation
Quality & Services, Inc. Jan Kilen, HP Etch AB Timothy Pitsch, Plexus Corporation
Ricky Bennett, Assembly Process Rongxiang Yang, Huawei Robert Rowland, RadiSys
Technologies Technologies Co., Ltd. Corporation
Jay Hinerman, BAE Systems CNI Chris Anglin, Indium Corporation Guillermo Velazquez, Rain Bird
Div. of America Corporation
Ron Tripp, Cookson Electronics Tim Jensen, Indium Corporation David Nelson, Raytheon Company
Jeff Schake, DEK International of America
Jeff Shubrooks, Raytheon Company
Craig Brown, DEK USA Inc. William Kunkle, MET Associates
Mark Quealy, Schneider Automation
Inc.
Richard Lieske, DEK USA Inc. Inc.
Holly Wise, MicroScreen, LLC
Glenn Dody, Dody Consulting Steve Sangillo, Swemco
Robert Cass, Northrop Grumman
Robert Dervaes, FCT Assembly Daan Terstegge, Thales Nederland
Amherst Systems
Michael Yuen, Foxconn B.v. Huizen
William May, NSWC Crane
CMMSG-NVPD Richard Lathrop
Narinder Kumar, Pelco by Schneider
Deepak Pai, General Dynamics Ahne Oosterhof
Electronics
Info. Sys., Inc

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October 2011 IPC-7525B

Table of Contents
1 PURPOSE ................................................................. 1 3.2 Aperture Design ................................................ 4
1.1 Terms and Definitions ....................................... 1 3.2.1 Aperture Size ..................................................... 4
1.1.1 *Aperture ............................................................. 1 3.2.2 Aperture Size versus Board Land Size
1.1.2 *Area Ratio ......................................................... 1 for Tin Lead Solder Paste ................................. 8
1.1.3 *Aspect Ratio ...................................................... 1 3.2.3 Aperture Size versus Board Land Size
for Lead Free Solder Paste ............................... 9
1.1.4 Border ................................................................ 1
3.2.4 Glue Aperture Chip Component ..................... 10
1.1.5 Enclosed Print Head .......................................... 1
3.2.5 Glue Apertures for Combination of Chip
1.1.6 Etch Factor ........................................................ 1 Components and Leaded Devices ................... 10
1.1.7 Relief Etch ......................................................... 1 3.2.6 Relief Etch with Glue Stencils ....................... 11
1.1.8 Fiducials ............................................................. 1 3.3 Mixed Technology Surface-Mount/
1.1.9 Fine-Pitch BGA ................................................. 1 Through-Hole (Intrusive Soldering) ............... 11
1.1.10 Fine-Pitch Technology (FPT) ............................ 1 3.3.1 Solder Paste Volume ........................................ 11
1.1.11 Foil ..................................................................... 1 3.4 Mixed Technology Surface-Mount/
Flip Chip .......................................................... 13
1.1.12 Frame ................................................................. 1
3.4.1 Two-Print Stencil for Surface-Mount/
1.1.13 Intrusive Soldering ............................................ 1
Flip Chip .......................................................... 13
1.1.14 *Land ................................................................... 1
3.5 Step Stencil Design ......................................... 13
1.1.15 Modification ....................................................... 1
3.5.1 Step-Down Stencil ........................................... 14
1.1.16 *Overprinting ....................................................... 2
3.5.2 Step-Up Stencil ............................................... 14
1.1.17 *Pad ..................................................................... 2
3.5.3 Step Stencil for Enclosed Print Heads ........... 14
1.1.18 Squeegee ............................................................ 2
3.5.4 Relief-Etch Stencil .......................................... 14
1.1.19 Squeegee Direction ........................................... 2
3.6 Fiducials ........................................................... 14
1.1.20 Standard BGA ................................................... 2
3.6.1 Global Fiducials .............................................. 14
1.1.21 *Stencil ................................................................ 2
3.6.2 Local Fiducials ................................................ 14
1.1.22 Step Stencil ........................................................ 2 3.7 Rework and Repair Stencils ........................... 14
1.1.23 *Surface-Mounting Technology (SMT) .............. 2 3.7.1 Mini Stencils ................................................... 14
1.1.24 *Through-Hole Technology (THT) .................... 2 3.7.2 Repair Tool for Printing Paste Directly
1.1.25 Transfer Efficiency ............................................ 2 on the Component ........................................... 15
1.1.26 Ultra-Fine Pitch Technology ............................. 2
4 STENCIL FABRICATION ........................................ 15
2 APPLICABLE DOCUMENTS .................................... 2 4.1 Foils ................................................................. 15
2.1 IPC ..................................................................... 2 4.2 Frames ............................................................. 15
4.3 Stencil Border .................................................. 15
3 STENCIL DESIGN ..................................................... 3
3.1 4.4 Stencil Fabrication Technologies .................... 15
Stencil Data ....................................................... 3
3.1.1 4.4.1 Chemical Etch ................................................. 15
Data Format ....................................................... 3
3.1.2 4.4.2 Laser-Cut Stencils ........................................... 16
Gerber® Format ................................................ 3
3.1.3 4.4.3 Electroform ...................................................... 16
Aperture List ..................................................... 3
3.1.4 4.4.4 Hybrid .............................................................. 16
Solder Paste Layer ............................................ 3
3.1.5 4.4.5 Trapezoidal Apertures ..................................... 16
Data Transfer ..................................................... 3
3.1.6 4.4.6 Additional Options .......................................... 16
Panelized Stencils .............................................. 3
3.1.7 Step-and-Repeat ................................................. 3 5 STENCIL MOUNTING ............................................. 16
3.1.8 Image Orientation/Rotation ............................... 3 5.1 Location of Image on Metal ........................... 16
3.1.9 Image Location .................................................. 4 5.2 Centering ......................................................... 16
3.1.10 Identification ...................................................... 4 5.3 Additional Design Guidelines ......................... 16

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IPC-7525B October 2011

6 STENCIL ORDERING ............................................. 16 Figure 3-13 Print Only Mode 15 mil Thick Stencil .............. 10
Figure 3-14 Glue Stencil With Glue Reservoir .................... 10
7 STENCIL USER’S INSPECTION/VERIFICATION .. 17
Figure 3-15 Through-Hole Solder Paste Volume ................ 11
8 STENCIL CLEANING .............................................. 17 Figure 3-16 Overprint Without Step .................................... 12

9 END OF LIFE .......................................................... 17 Figure 3-17 Overprint With Step (Squeegee Side) ............. 12
Figure 3-18 Overprint With Step (Contact/Board Side) ...... 12
APPENDIX A: EXAMPLE ORDER FORM ................ 19 Figure 3-19 Two-Print Through-Hole Stencil ....................... 13

Figures Figure 3-20 Two-Print Stencil for Mixed Technology .......... 13

Figure 3-21 Print With Step ................................................. 13
Figure 3-1 4 mil Thick Stencil Tin Lead and Lead Free ..... 6
Figure 3-22 Step Down ....................................................... 14
Figure 3-2 5 mil Thick Stencil Tin Lead and Lead Free ..... 6
Figure 3-23 Step Up ............................................................ 14
Figure 3-3 6 mil Thick Stencil Tin Lead and Lead Free ..... 7
Figure 3-24 BTC .................................................................. 15
Figure 3-4 8 mil Thick Stencil Tin Lead and Lead Free ..... 7
Figure 3-25 BGA .................................................................. 15
Figure 3-5 Cross-Sectional View of A Stencil ..................... 8
Figure 4-1 Trapezoidal Apertures ...................................... 16
Figure 3-6 Home Plate Aperture Design ............................. 8
Figure 3-7 Bow Tie Aperture Design ................................... 9
Tables
Figure 3-8 Oblong Aperture Design .................................... 9
Table 3-1 Stencil Use Clauses ............................................ 4
Figure 3-9 Aperture Design for Cylindrical Components
and Chip Components (All Corners Rounded) .. 9 Table 3-2 General Aperture Design Guideline Examples
for Selective Surface-Mount Devices (Tin Lead
Figure 3-10 Window Pane Design for Ground Plane ........... 9
Solder Paste) ...................................................... 5
Figure 3-11 Glue Stencil Aperture Design .......................... 10
Table 3-3 Process Window for Intrusive Soldering -
Figure 3-12 Chip Component and SOIC Present Maximum Limits Desirable ................................ 11
on Board .......................................................... 10

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Stencil Design Guidelines

1 PURPOSE
This document provides a guide for the design and fabrication of stencils for solder paste and surface-mount adhesive. It is
intended as a guideline only. Much of the content is based on the experience of stencil designers, fabricators, and users.
Printing performance depends on many different variables and therefore no single set of design rules can be established.

1.1 Terms and Definitions All terms and definitions used throughout this handbook are in accordance with IPC-T-50.
Definitions noted with an asterisk (*) are quoted from IPC-T-50. Other specific terms and definitions, essential for the dis-
cussion of the subject, are provided below.

1.1.1 *Aperture An opening in the stencil foil.

1.1.2 *Area Ratio The ratio of the area of aperture opening to the area of aperture walls.

1.1.3 *Aspect Ratio The ratio of the width of the aperture to the thickness of the stencil foil.

1.1.4 Border Peripheral tensioned mesh, either polyester or stainless steel, which keeps the stencil foil flat and taut. The
border connects the foil to the frame.

1.1.5 Enclosed Print Head A stencil printer head that holds, in a single replaceable component, the squeegee blades and
a pressurized chamber filled with solder paste.

1.1.6 Etch Factor Etched Depth/Lateral; Etch in a chemical etching process.

1.1.7 Relief Etch Also known as Etch Relief and Under Etch. Adding an under etch of the foil to create a pocket for raised
features, labels, or a multi-print function.

1.1.8 Fiducials Reference marks on the stencil foil (and other board layers) for aligning the board and the stencil when
using a vision system in a printer.

1.1.9 Fine-Pitch BGA Ball grid array (BGA) with less than 1 mm [39 mil] pitch. Also known as chip scale package (CSP)
when the package size is no more than 1.2X the area of the original die size.

1.1.10 Fine-Pitch Technology (FPT) A surface-mount assembly technology with component terminations on centers less
than or equal to 0.625 mm [24.61 mil].

1.1.11 Foil The sheet used to create the stencil.

1.1.12 Frame A frame may be made of tubular or cast aluminum to which a tensioned mesh (border) is permanently
bonded using an adhesive.

1.1.13 Intrusive Soldering A process in which the solder paste for the through-hole components is applied using the sten-
cil. The through-hole components are inserted and reflow-soldered together with the surface-mount components. Also known
as Paste-In-Hole, Pin-In-Hole, or Pin-In-Paste Soldering.

1.1.14 *Land A portion of a conductive pattern usually used for the connection and/or attachment of components.

1.1.15 Modification The process of changing an aperture in size or shape.

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IPC-7525B October 2011

1.1.16 *Overprinting The use of stencils with apertures larger than the lands or annular rings on the board.

1.1.17 *Pad See ‘‘Land.’’

1.1.18 Squeegee A metal or polymer blade used to wipe a material (ink, solder paste, or adhesive) across a stencil or silk
screen to force the material through the openings in the stencil or screen onto the surface of a printed board or mounting
structure.
Note: Normally the squeegee is mounted at an angle such that the contacting edge of the squeegee trails behind the print
head and the face of the squeegee slopes forward.

1.1.19 Squeegee Direction The direction in which the squeegee travels.

Note: The typical squeegee direction is front-to-back and back-to-front. However some machines may travel from right-to-
left and left-to-right. Other machines are programmable and can travel at various angles. Squeegee Direction can be impor-
tant when designing step stencils.

1.1.20 Standard BGA Ball grid array (BGA) with 1 mm [39 mil] pitch or larger.

1.1.21 *Stencil A thin sheet of material containing openings to reflect a specific pattern, designed to transfer a paste-like
material to a substrate for the purpose of component attachment.

1.1.22 Step Stencil A stencil with more than one foil thickness level.

1.1.23 *Surface-Mounting Technology (SMT) The electrical connection of components to the surface of a conductive pat-
tern that does not utilize component holes.

1.1.24 Through-Hole Technology (THT) The electrical connection of components to a conductive pattern by the use of
component holes.

1.1.25 Transfer Efficiency Used to describe how well solder paste is released from a stencil aperture. It is a percentage
number based on the actual volume of solder paste released divided by the theoretical volume at 100% release.

1.1.26 Ultra-Fine Pitch Technology A surface-mount assembly technology with component terminations on centers less
than or equal to 0.40 mm [15.7 mil].

2 APPLICABLE DOCUMENTS
The following documents, of the issue in effect on the invitation for bid, form a part of this specification to the extent speci-
fied herein.

2.1 IPC1

IPC-T-50 Terms and Definitions for Interconnecting and Packaging Electronic Circuits

IPC-2581 Generic Requirements for Printed Board Assembly Products Manufacturing Description Data and Transfer Meth-
odology (Offspring)

IPC-7093 Design and Assembly Process Implementation for Bottom Termination Components

IPC-7095 Design and Assembly Process Implementation of BGAs

IPC-7526 Stencil Misprinted Board Cleaning Handbook

1. www.ipc.org

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October 2011 IPC-7525B

3 STENCIL DESIGN

3.1 Stencil Data

3.1.1 Data Format The file format that provides a language for communicating with the photo plotting system to produce
a tool for chemically etched stencils. It is also used to produce the laser cut or electroformed stencils. While the actual data
format may vary from file to file depending on the software package or designer, the data format commonly used by photo
plotter and laser equipment is known as Gerber®.
Regardless of the stencil fabrication method used, Gerber® is the preferred data format. Possible alternative formats are
IPC-2581, DXF, HP-GL, Barco, and ODB++, etc.; however, the data may need to be converted to Gerber® format prior to
the stencil manufacturing process.

3.1.2 Gerber® Format There are two standard Gerber® formats available:
• RS-274X – embeds the Gerber® aperture list in the data file. (Preferred)
• RS-274D – requires a data file listing the X-Y coordinates on the stencil where apertures are to be placed and formed and
a separate Gerber® aperture list that describes the size and shape of the various Gerber® apertures used to prepare the
image.

3.1.3 Aperture List An ASCII text file containing ‘‘D’’ codes. ‘‘D’’ codes define the size and shape for all apertures used
within the Gerber® file. Without the aperture list, the software and photo plotting system cannot read the Gerber® data. Only
the X-Y coordinates would be available with no size and shape data.

3.1.4 Solder Paste Layer The solder paste layer data is necessary to produce a stencil. If fiducials and/or outline infor-
mation are required on the stencil, they should be included in the solder paste layer.

3.1.5 Data Transfer Data can be transmitted to the stencil supplier via modem, FTP (file transfer protocol), e-mail attach-
ment, or disk. To ensure data integrity after transmitting and to reduce the large size of data files it is suggested that the files
be compressed prior to sending data. It is recommended that the full data file (the solder paste, solder mask, silk screen, and
copper layers) sent to the printed circuit board manufacturer be supplied to the stencil manufacturer. This allows the stencil
manufacturer to optimize or make recommendations on aperture sizes based on actual land sizes.

3.1.6 Panelized Stencils In cases where it is desired to have more than one image on the stencil, the stencil patterns will
be panelized (step-and-repeat) and included in the data file.

3.1.7 Step-and-Repeat In cases where more than one image of the same design is to be printed, the data file for stencil
fabrication should contain the stencil design in the step-and-repeat array. In instances where the data file does not contain
the step-and-repeat pattern a readme file, panel drawing, or order information should specify:
• Total number of steps for the final array.
• Number of steps in the X-direction along with dimensions from a specific feature to corresponding feature (such as fidu-
cials, component land locations, etc.).
• Number of steps in the Y-direction along with dimensions from a specific feature to corresponding feature (such as fidu-
cials, component land locations, etc.).

3.1.8 Image Orientation/Rotation In cases where image orientation is not parallel to the frame or the step and repeat is
not rectilinear (one or more images are rotated) the data for stencil fabrication should contain the oriented image. In cases
where it does not a readme file, panel drawing, or order information should specify this information (X-and Y-offsets) ref-
erencing stencil features.

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IPC-7525B October 2011

3.1.9 Image Location To accommodate specific printers the stencil image may have to be located in different positions
inside the frame:
• Center image
• Center board/panel – requires board/panel outlines
• Offset board/panel – requires board/panel outlines and reference locations
In cases where this data is not included in the Gerber® data a read me file, panel drawing, or order information should
specify this information referencing stencil features.

3.1.9.1 Multiple Assembly Images It is possible to create a stencil with multiple images that could be used for two dif-
ferent assemblies. This will be dependent on the size of the images but some printers will allow the stencil to be rotated
which will allow two or more images to be placed in one stencil.

3.1.10 Identification Each stencil should contain identification information such as part number, revision number, thick-
ness, supplier’s name and control number, date, and method of manufacture.

3.2 Aperture Design Table 3-1 indicates where information on each stencil use can be found in this standard.

Table 3-1 Stencil Use Clauses

Stencil Use Clause
Tin lead solder paste 3.2.2 - 3.2.2.7
Lead free solder paste 3.2.3 - 3.2.3.7
Either tin lead or lead-free solder paste 3.2.1.2
Glue apertures for chip components 3.2.4
Glue apertures for combination of chip components and leaded 3.2.5

3.2.1 Aperture Size The volume of solder paste applied to the board is mainly determined by the aperture size and foil
thickness. Solder paste fills the stencil aperture during the squeegee cycle of the print operation. Small aperture sizes may
require smaller solder paste particle sizes. A typical guideline is a minimum of four to five particles of paste powder across
the width of the aperture. The paste should completely release to the lands on the board during the board/stencil separation
cycle of the print operation. From the stencil viewpoint, the ability of the paste to release from the inner aperture walls to
the board land depends primarily on these major factors:
• Area ratios/aspect ratios for the aperture design.
• Aperture side wall geometry.
• Aperture wall finish.
• Stencil-board separation speed.
• Stencil-board gasketing (no air gaps).
• Accuracy and repeatability of finished stencil aperture size and shape in the Gerber file.

3.2.1.1 Aperture Position The position of the aperture in the stencil is important so that the solder paste can be printed
on the printed circuit land and not misregistered with respect to the land. As a guideline, the mismatch should be less than
0.00254 mm [0.1 mil] for every 25.4 mm [1 in] of aperture pattern or 0.0254 mm [1 mil] whichever is larger. In general,
most lead free solder pastes do not wet the land as well as tin lead solder pastes. If part of the land is left uncovered when
using lead free solder paste during the printing operation it remains uncovered after reflow. Therefore stencil aperture to
board land registration is very important.

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3.2.1.2 Area Ratio/Aspect Ratio A general design guide for acceptable paste release should be >1.5 for aspect ratio and
>0.66 for area ratio. Advances in stencil technology (finishing processes, electroform, etc.), may result in lower guideline
ratios. These ratios will affect solder paste release. Table 3-2 shows general aperture design guideline examples for a range
of Surface Mounted devices. Aperture dimensions, stencil thickness ranges and acceptable ranges for Aspect Ratio and Area
Ratio, along with recommended Solder Paste Types are shown. Please note that the land pattern footprints and aperture sizes
are typical values only and could be lead-free or tin-lead designs not necessarily recommended by this document. The graphs
shown in Figures 3-1, 3-2, 3-3, and 3-4 may be a useful guide to select the proper stencil technology per given aperture
sizes and area ratios for 4, 5, 6, and 8 mil thick stencils. These graphs apply to tin lead as well as lead free solder pastes.
Note that Figures 3-1 through 3-4 are general design guides based on area ratios. It must be recognized that there are a wide
variety of stencils available in the industry made with electroform, laser, chemical etch, and high-precision etching pro-
cesses. It is quite possible that some electroform stencils may not perform satisfactory with area ratios between 0.5 and 0.66.
On the other hand it is possible that some high-precision etched stencils (with post processing) or laser-cut stencils do per-
form satisfactory with area ratios between 0.5 and 0.66.
When the stencil separates from the board, paste release encounters a competing process. Solder paste will either transfer
to the land on the board or stick to the aperture side walls. When the area is greater than 0.66 of the inside aperture wall
area, a complete paste transfer should occur for both laser and electroform stencils.
Table 3-2 General Aperture Design Guideline Examples for
Selective Surface-Mount Devices (Tin Lead Solder Paste)
Land Land Stencil Aspect Solder
Footprint Footprint Aperture Aperture Thickness Ratio Area Ratio Paste
Part Type Pitch Width Length Width Length Range Range Range Type
1.25 mm 0.65 mm 2.00 mm 0.60 mm 1.95 mm 0.15 - 0.25 mm
PLCC 2.4 - 4.0 0.92 - 1.53 Type 3
[49.2 mil] [25.6 mil] [78.7 mil] [23.6 mil] [76.8 mil] [5.91 - 9.84 mil]
0.65 mm 0.35 mm 1.50 mm 0.30 mm 1.45 mm 0.15 - 0.175 mm
QFP 1.7 - 2.0 0.71 - 0.83 Type 3
[25.6 mil] [13.8 mil] [59.1 mil] [11.8 mil] [57.1 mil] [5.91 - 6.89 mil]
0.50 mm 0.30 mm 1.25 mm 0.25 mm [1.20 mm] 0.125 - 0.15 mm
QFP 1.7 - 2.0 0.69 - 0.83 Type 3
[19.7 mil] [11.8 mil] [49.2 mil] [9.84 mil] 47.2 mil [4.92 - 5.91 mil]
0.40 mm 0.25 mm 1.25 mm 0.20 mm [1.20 mm] 0.10 - 0.125 mm
QFP 1.6 - 2.0 0.69 - 0.86 Type 3
[15.7 mil] [9.84 mil] [49.2 mil] [7.87 mil] 47.2 mil [3.94 - 4.92 mil]
0.30 mm 0.20 mm 1.00 mm 0.15 mm 0.95 mm 0.075 - 0.125 mm
QFP 1.2 - 2.0 0.52 - 0.86 Type 3
[11.8 mil] [7.87 mil] [39.4 mil] [5.91 mil] [37.4 mil] [2.95 - 4.92 mil]
0.60 mm 0.65 mm 0.45 mm 0.60 mm 0.125 - 0.15 mm
0402 N/A N/A 0.86-1.03 Type 3
[19.7 mil] [25.6 mil] [17.7 mil] [23.6 mil] [4.92 - 5.91 mil]
0.4 mm 0.45 mm 0.23 mm 0.35 mm 0.075 - 0.125 mm
0201 N/A N/A 0.56 - 0.93 Type 3
[9.84 mil] [15.7 mil] [9.06 mil] [13.8 mil] [2.95 - 4.92 mil]
0.200 mm 0.300 mm 0.175 mm 0.250 mm 0.063 - 0.089 mm
01005 N/A N/A 0.58 - 0.81 Type 4
[7.87 mil] [11.81 mil] [6.89 mil] [9.87 mil] [2.5 - 3.5 mil]
1.25 mm CIR CIR 0.15 - 0.20 mm
BGA N/A 0.65 - 0.86 Type 3
[49.2 mil] 0.55 mm [21.6 mil] 0.52 mm [20.45 mil] [5.91 - 7.87 mil]
Fine-pitch 1.00 mm CIR SQ 0.115 - 0.135 mm
N/A 0.65 - 0.76 Type 3
BGA [39.4 mil] 0.45 mm [15.7 mil] 0.42 mm [13.8 mil] [4.53 - 5.31 mil]
SQ
Fine-pitch 0.50 mm CIR 0.075 - 0.125 mm
Overprint 0.28 mm N/A 0.56 - 0.93 Type 3
BGA [19.7 mil] 0.25 mm [9.84 mil] [2.95 - 4.92 mil]
[11.0 mil]
SQ
Fine-pitch 0.40 mm CIR 0.075 - 0.100 mm
Overprint 0.23 mm N/A 0.56 - 0.75 Type 4
BGA [15.7 mil] 0.20 mm [7.87 mil] [2.95 - 4 mil]
[9 mil]
Note 1: It is assumed that the fine-pitch BGA lands are not solder mask defined.
Note 2: N/A implies that only the area ratio should be considered.

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IPC-7525B October 2011

80

70

60
1
Aperture Length (mils)

50

40

30

20 2
10
3
4
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Aperture Width (mils) IPC-7525b-3-1

Figure 3-1 4 mil Thick Stencil Tin Lead and Lead Free
1. Electroformed, Laser, High-Precision Etch or Chem-Etch Range (AR >0.9)
2. Electroformed, Laser, High-Precision Etch (0.66< AR <0.9)
3 Electroform Range (0.5< AR <0.66)
4. Recommended Aperture Redesign or Reduce Stencil Thickness (AR <0.5)

80

70

60
1
Aperture Length (mils)

50

40

30
2
20
3
10
4
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Aperture Width (mils)

IPC-7525b-3-2

Figure 3-2 5 mil Thick Stencil Tin Lead and Lead Free
1. Electroformed, Laser, High-Precision Etch or Chem-Etch Range (AR >0.9)
2. Electroformed, Laser, High-Precision Etch (0.66< AR <0.9)
3. Electroform Range (0.5< AR <0.66)
4. Recommended Aperture Redesign or Reduce Stencil Thickness (AR <0.5)

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October 2011 IPC-7525B

80

70

60
1
Aperture Length (mils)

50

40

2
30

20
3
10
4
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Aperture Width (mils) IPC-7525b-3-3

Figure 3-3 6 mil Thick Stencil Tin Lead and Lead Free
1. Electroformed, Laser, High-Precision Etch or Chem-Etch Range (AR >0.9)
2. Electroformed, Laser, High-Precision Etch (0.66< AR <0.9)
3. Electroform Range (0.5< AR <0.66)
4. Recommended Aperture Redesign or Reduce Stencil Thickness (AR <0.5)

80

70
1
60
Aperture Length (mils)

50
2
40

30
3
20
4
10

0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Aperture Width (mils)

IPC-7525b-3-4

Figure 3-4 8 mil Thick Stencil Tin Lead and Lead Free
1. Electroformed, Laser, High-Precision Etch or Chem-Etch Range (AR >0.9)
2. Electroformed, Laser, High-Precision Etch (0.66< AR <0.9)
3. Electroform Range (0.5< AR <0.66)
4. Recommended Aperture Redesign or Reduce Stencil Thickness (AR <0.5)

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IPC-7525B October 2011

Both area ratio and aspect ratio are illustrated in Figure 3-5, using
the following formulas.
L

Width of Aperture W
Aspect Ratio = =
Thickness of Stencil T
T
W
Area of Aperture LxW IPC-7525b-3-5
Area Ratio = =
Area of Aperture Walls 2 x (L + W) x T Figure 3-5 Cross-Sectional View of A Stencil

3.2.2 Aperture Size versus Board Land Size for Tin Lead Solder Paste As a general design guide, the aperture size
should be reduced compared to the board land size. The stencil aperture is commonly modified with respect to the original
land design. Reductions in the area or changes in aperture shape are often desirable to enhance the processes of printing,
reflow, or stencil cleaning. For instance, reducing the aperture size will decrease the possibility of stencil aperture to board
land misalignment. This reduces the chance for solder paste to be printed off the land which may lead to solder balls or
solder bridging. Having a radiused corner for all apertures can reduce stencil cleaning frequency. However, there are
instances where additional solder volume is desired and adding foil thickness is not an option. In those instances, overprint-
ing solder is a viable option. When designing a stencil that utilizes overprinting, verifying clearances is very important.
Typically, when working with the stencil layer, you do not have visibility to the adjacent board features like vias, exposed
traces, etc.

3.2.2.1 Leaded SMDs For leaded SMDs (e.g., J-leaded or gull-wing components with 1.3 - 0.4 mm [51.2 - 15.7 mil]
pitch), the aperture size reduction is typically 0.03 - 0.08 mm [1.2 - 3.1 mil] in width and 0.05 - 0.13 mm [2.0 - 5.1 mil]
in length.

3.2.2.2 Plastic BGAs Reduce circular aperture diameter by 0.05 mm [2.0 mil]. However, with the issues of head-in-pillow
defects device warpage, device co-planarity, solder heights, and solder volume should be analyzed when designing a sten-
cil for BGAs.

3.2.2.3 Ceramic Grid Arrays Ceramic grid array packages require a specific solder paste volume to ensure long-term reli-
ability of the solder joint. Greater volume is required for ceramic ball grid arrays than for ceramic column grid arrays. Infor-
mation regarding proper solder paste volumes for these packages can be found in IPC-7095.

3.2.2.4 Fine-Pitch BGA and CSP Apertures should be square with the width of the square equal to or up to 0.025 mm
[0.98 mil] less than the diameter of the land circle on the board. The square should have rounded corners. A guideline is
0.06 mm [2.4 mil] radiused corners for a 0.25 mm [9.8 mil] square and 0.09 mm [3.5 mil] corners for a 0.35 mm [14 mil]
square. With the issues of head-in-pillow defects, device warpage, and device co-planarity solder heights and solder volume
should be analyzed when designing a stencil for BGAs and Bottom Termination Components (BTCs). With fine-pitch BGAs
and CSPs it is very important to monitor the area and aspect ratios.

3.2.2.5 Chip Components - Resistors and Capacitors 1

Several aperture geometries are effective in reducing the 2/3 L
occurrence of solder balls. All these designs are aimed at
reducing excess solder paste trapped under the chip compo-
1/2 W
nent. The most popular designs are shown in Figures 3-6, 3-7
and 3-8. These designs are commonly used for no-clean pro- W
cesses.

3.2.2.6 Cylindrical, Mini-MELF and Chip Components

‘‘C’’-shaped apertures are suggested for these components
L
(see Figure 3-9) but care has to be taken to prevent the parts 2
IPC-7525b-3-6
from bouncing off their locations before reflow due to mini-
mal paste contact and shaking conveyors. Dimensions of these Figure 3-6 Home Plate Aperture Design
apertures should be designed to match the geometry of com- 1. Aperture
2. Land
ponent terminals.

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October 2011 IPC-7525B

0.1 L to
1 0.2 L 1/2 W

1/2 W 1/2 W

W W

2 L 3
1 2
IPC-7525b-3-7 IPC-7525b-3-8

Figure 3-7 Bow Tie Aperture Design Figure 3-8 Oblong Aperture Design
1. Aperture 1. Aperture
2. Land 2. Land
3. Full radius
3.2.2.7 LCC/BTC Devices LCC Aperture sizes for the ter-
mination lands are the same size as recommended for the
2 1
QFPs (see Table 3-2). If skewing is observed on the package
during reflow, an increase in aperture width of 1.25 times
wider than the board land can assist in keeping the package
from rotating during reflow.
The apertures for the heat sink/ground plane should be
reduced by 20% to 50% of the area of the ground plane. This
can be accomplished by creating a window pane design in the
apertures (see Figure 3-10).
Additional design guidelines for bottom termination compo-
nents are available in IPC-7093. IPC-7525a-3-9

If vias are imbedded in the ground land design, it is recom- Figure 3-9 Aperture Design for Cylindrical Components
mended that the web of the window-pane grid is designed to and Chip Components (All Corners Rounded)
cover each via which will prevent the solder paste being 1. Aperture
2. Land
screened directly into the vias. This design may also prevent
solder wicking into the via. Solder wicking may not allow you
to achieve a >50% solder coverage of the ground plane area. 1

2 3
3.2.3 Aperture Size versus Board Land Size for Lead Free
Solder Paste As a general design guide, the aperture size
should be as close as possible to a one-to-one ratio when
compared to the board land size. This is done to assure com-
plete coverage of the land with solder after reflow. Some
slight reduction (e.g., 0.5 mils [0.0127 mm] per side of land)
is acceptable since pushing the component into the solder
paste will cause the paste to spread and cover the land. Reduc-
tion of the aperture size for the ground plane of BTC, or LCC
devices is an exception and is desirable. Radiused corners are
also acceptable as it reduces the chance of solder paste stick-
ing in sharp corners of the aperture. IPC-7525a-3-10

The use of a slight stencil aperture reduction will also assist Figure 3-10 Window Pane Design for Ground Plane
1. BTC Package
in obtaining a tight board to stencil gasket during the print 2. Aperture Design 20% to 50% Reduction
process, especially on metal defined lands that have been over 3. Ground Plane
etched.

3.2.3.1 Leaded SMDs For leaded SMDs (e.g., J-lead or gull-wing lead components with 1.3 - 0.4 mm [51.2 - 15.7 mil]
pitch) the reduction is typically 0.254 [1.0 mil] in width and no reduction in length.

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IPC-7525B October 2011

3.2.3.2 Plastic BGAs No reduction in the aperture compared to the land.

3.2.3.3 Ceramic Grid Arrays Ceramic grid array packages require a specific solder paste volume to ensure long-term reli-
ability of the solder joint. Greater volume is required for ceramic ball grid arrays than for ceramic column grid arrays. Infor-
mation regarding proper solder paste volumes for these packages can be found in IPC-7095.
3.2.3.4 Fine-Pitch BGA and CSP Square aperture with the width of the square equal to the diameter of the land circle
on the board. The square should have radiused corners. A guideline is 0.06 mm [2.4 mil] radiused corners for a 0.25 mm
[9.8 mil] square and 0.09 [3.5 mil] radiused corners for a 0.35 mm [14 mil] square.
When additional solder volume is needed the stencil apertures can be greater than the dimension of the land. This is com-
monly referred to as overprinting.
3.2.3.5 Chip Components - Resistors and Capacitors Several aperture geometries are effective in reducing the occur-
rence of solder balls. All these designs are aimed at reducing excess solder paste trapped under the chip component. The
most popular designs are shown in Figure 3-9 (same aperture design as the cylindrical device) and are the ‘‘C’’-shaped aper-
tures. This aperture design reduces the amount of solder paste
under the chip component but maintains land coverage on the 1/3 G 2
1
periphery of the lands.

3.2.3.6 MELF, Mini-MELF Components ‘‘C’’-shaped aper-

tures are suggested for these components (see Figure 3-9).
Dimensions of these apertures should be designed to match
the geometry of component terminals. This design also can be
used for chip components.

3.2.3.7 BTC/LCC Devices LCC aperture sizes for the termi- G IPC-7525b-3-11

nation lands are either no reduction or a slight reduction typi- Figure 3-11 Glue Stencil Aperture Design
cally 0.254 mm [1.0 mil] in width and no reduction in length 1. Glue Aperture
with the exception of the corner apertures. These apertures 2. Land
should be 1.25 times wider than the board land to assist in
keeping the package from rotating during reflow.
The apertures for the heat sink/ground plane should be 30 mil 15 mil
reduced by 20% to 50% of the area of the ground plane. This 4 mil
can be accomplished by creating a window pane aperture as 1 2
shown in Figure 3-10. IPC-7525b-3-12

Figure 3-12 Chip Component and SOIC Present on Board

3.2.4 Glue Aperture Chip Component The glue stencil is
1. Chip Component
typically 0.15 - 0.2 mm [5.9 - 7.9 mil] thick. The glue aper- 2. SOIC
ture is placed in the center of the component solder lands. It
is 1/3 the spacing between lands and 110% of the component
width (see Figure 3-11).
3.2.5 Glue Apertures for Combination of Chip Components 6 mil
and Leaded Devices Chip components with a typical stand- 15 mil
off of between 0.102 - 0.127 mm [4 - 5 mils] require a sten-
cil of 0.150 - 0.200 mm [5.9 - 7.9 mils], but leaded devices 1 IPC-7525b-3-13

have much higher stand-offs ranging from 0.381 mm [15 mils]

Figure 3-13 Print Only Mode 15 mil Thick Stencil
to more than 0.762 mm [30 mils]. In this case a thicker stencil 1. Board
is required. Figure 3-12 shows the off-sets for typical chip and
leaded devices. Glue prints differently than paste in that by 5
making the aperture small a defined height of glue is depos- 1
ited from the glue aperture. Figure 3-13 shows a 0.381 mm 4
[15 mil] thick stencil with a small aperture for printing 3
0.150 mm [5.9 mil] and a larger aperture for printing 2 IPC-7525b-3-14

0.381 mm [15 mil] of glue. In cases where higher glue prints Figure 3-14 Glue Stencil with Glue Reservoir
are necessary (for instance 0.762 mm [30 mil]) a special sten- 1. Glue Reservoir 3. Large Glue Aperture 5. Glue Reservoir
cil with a glue reservoir may be used (see Figure 3-14). 2. Small Glue Aperture 4. Aperture Metal Foil Metal Foil

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October 2011 IPC-7525B

3.2.6 Relief Etch with Glue Stencils Surface mount glue has a lower viscosity than solder paste, and it tends to bleed out
under the stencil. Therefore, a tight gasket seal is needed. Raised features on the surface can be alleviated by under-etching
the bottom side of the stencil to a relief pocket for the feature and allow for a tight seal. Some of the features that can pre-
vent proper gasketing are labels, some silkscreens, even a raised printed wiring board surface finish like HASL (Hot Air
Solder Leveled). In those instances adding a etch relief can eliminate the situation.

3.3 Mixed Technology Surface-Mount/Through-Hole (Intrusive Soldering) It is desirable to have a process where SMT
and THT devices can both be:
• Provided with printed solder paste
• Placed on or in the board
• Reflowed together
The objective of stencil printing of solder paste for the intrusive soldering process is to provide enough solder volume after
reflow to fill the hole and create acceptable solder fillets around the pins. Table 3-3 shows process window for a typical
intrusive soldering process.
Table 3-3 Process Window for Intrusive Soldering - Maximum Limits Desirable
Maximum Limits Desirable
Hole Diameter 0.65 - 1.60 mm [25.6 - 63.0 mil] 0.75 - 1.25 mm [29.5 - 49.2 mil]
Lead Diameter Up to hole diameter minus 0.075 mm [2.95 mil] Hole diameter minus 0.125 mm [4.92 mil]
Paste Overprinting 6.35 mm [250 mil] <4.0 mm [157 mil]
Stencil Thickness 0.125 - 0.635 mm [4.92 - 25.0 mil] 0.15 mm [7.87 mil], 0.20 mm [5.91 mil] for fine-pitch

3.3.1 Solder Paste Volume A simple equation listed below describes

the volume of solder paste required as shown in Figure 3-15. By doing 1
this less solder paste volume will be required. Following are three
stencil designs used to deliver through-hole solder paste: 2
• Non-step stencil 3
• Step stencil Wo Wp 4
• Two-print stencil
5
V = TS (LO x WO) + VH
= S1 {TB (AH - AP) + (FT + FB) + VP} - VH
Lp
Lo
Where:
V = volume of solder paste required
Vp = solder volume left on the top and/or bottom
6
board land
S = solder paste shrink factor Lo 7
Lp
AH = cross-sectional area of the through-hole
AP = cross-sectional area of the through-hole pin FT
TB = thickness of the board
FT + FB = total fillet volume required
TS = thickness of the stencil foil 8
TB
LO = length of the overprint aperture
LP = length of the land
WO = width of the overprint aperture
9
WP = width of the land VH FB
VH = solder paste filling the hole during the
printing operation 10 IPC-7525b-3-15

Note: Solder paste volume filling the hole can vary from 0% to 100% Figure 3-15 Through-Hole Solder Paste Volume
depending on the print setup. Enclosed print heads are effective in 1. Top View 6. Cross-Sectional View
2. Pin 7. Stencil
achieving close to 100% while metal squeegee blades with a high angle 3. Through-Hole 8. Board
of attack and high print speed will deliver minimum paste into the hole. 4. Annular Ring 9. Solder Paste
5. Stencil Aperture 10. Pin

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IPC-7525B October 2011

3.3.1.1 Overprint Without Step This is the stencil of choice

when it can deliver enough solder paste to satisfy the through- 1
hole requirement. A cross section of this type stencil is shown
in Figure 3-16.
An example of when this stencil could be used is a two row
connector on 2.5 mm [98.4 mil] pitch with 1.1 mm [43.3 mil] 2
diameter though-holes, 0.9 mm [35.4 mil] diameter pins, 3
1.2 mm [47.2 mil] thick board and no other components 5
within 3.8 mm [150 mil] of the through-hole openings. 4 IPC-7525b-3-16

An overprint stencil aperture of 2.2 mm [86.6 mil] wide Figure 3-16 Overprint Without Step
and 5.1 mm [200 mil] long with a stencil foil thickness of 1. Step Stencil 3. Through-Hole Land 5. SMT Land
0.15 mm [5.91 mil] can deliver sufficient solder paste. 2. Board 4. Through-Hole

3.3.1.2 Overprint With Step If the board is thicker, the hole K1

is bigger, or the pin is smaller, more solder paste volume will K2
be required. In this case, a step stencil may be needed to pro- 1
vide sufficient solder paste volume for the THT parts without
providing too much paste on the SMT lands. An example of
this type stencil is shown in Figure 3-17. K1 and K2 are
keep-out distances. K1 is the distance from the step edge to 2
the nearest aperture in the step-down area. K2 is the distance
between the through-hole aperture and the step edge. As a 3
5
general design guide K2 can be as low as 0.65 mm [25.6 mil]. 4 IPC-7525b-3-17

As a general design guide, K1 should be 0.9 mm [35.4 mil]

Figure 3-17 Overprint With Step (Squeegee Side)
for every 0.025 mm [0.98 mil] of step-down thickness. As a 1. Step Stencil 3. Through-Hole Land 5. SMT Land
simple guideline, K1 should be 36x the step-down thickness. 2. Board 4. Through-Hole
For example, a 0.2 mm [7.9 mil] stencil foil with a step down
to 0.15 mm [5.9 mil] would require a K1 keep-out distance of K1 1
1.8 mm [70.9 mil]. It is also possible to put the step on the K2
contact side of the stencil instead of the squeegee side. This is
shown in Figure 3-18. This type of step is sometimes more
convenient when using metal squeegee blades and is highly
recommended for enclosed print heads. 2

Special keep-out rules may apply. Contact side steps should 3

be evaluated on the available keep-out area as well as the need 5
4 IPC-7525b-3-18
to gasket well. When placing the step on the contact side of
the foil, verify there are no ultra-fine pitch devices in close Figure 3-18 Overprint With Step (Contact/Board Side)
1. Step Stencil 3. Through-Hole Land 5. SMT Land
proximity to the edge of the step area. This method can create
2. Board 4. Through-Hole
an issue with stencil gasketing in the area and require exces-
sive underside cleaning of the stencil.

3.3.1.3 Two-Print Stencil Some through-hole devices have small pins with large holes or dense spacing with thick boards.
In either case, insufficient solder paste volume is deliverable using the first two stencil designs. The two-print stencil
can deliver large amounts of solder paste into the through-holes. In this design, a normal surface-mount stencil, typically
0.15 mm [5.9 mil] thick, is used to print to the surface-mount lands. While the surface-mount paste is still tacky, a thick
stencil is used to print the through-hole paste. Normally this requires a second stencil printer set up in line to perform this
printing. This stencil can be as thick as required (while maintaining an acceptable area and aspect Ratio). However, 0.4 to
0.75 mm [16 to 30 mil] is typical. When stencil foil thickness requirements exceed 0.5 mm [20 mil], laser cut electro-
polished apertures provide better paste release and overall print performance due to the excellent wall geometry. The

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October 2011 IPC-7525B

contact side of this stencil foil is relief etched at least 0.25 mm

1
[9.84 mil] deep in any area where solder paste for has been
previously printed on surface-mount lands. A cross section of
the two-print through-hole stencil is shown in Figure 3-19. 6
7
3.4 Mixed Technology Surface-Mount/Flip Chip A sample
application for this technology is a PCMCIA card having flip 2
chips, TSOPs, and chip components. It is desirable to place 3
flip chip and SMT components on the card and reflow all the 5
components simultaneously. 4 IPC-7525b-3-19

Figure 3-19 Two-Print Through-Hole Stencil

3.4.1 Two-Print Stencil for Surface-Mount/Flip Chip The 1. Two Print Stencil 4. Through-Hole 7. Relief Etch Area
two-print stencil configuration can perform this task. The first 2. Board 5. SMT Land for Solder Paste
3. Through-Hole Land 6. Solder Paste Clearance
step in this process is to print flip chip solder paste or flip chip
flux on the board flip chip land sites. A stencil to do this is
1
normally 0.05 or 0.075 mm [2.0 or 3.0 mil] thick with 0.13 to
0.18 mm [5.12 to 7.09 mil] apertures. While the flip chip
paste/flux is still tacky, the surface mount stencil is used to
print solder paste to the remaining surface mount lands.
An example of this stencil would be one that is 0.18 mm
[7.09 mil] thick with a relief etch of 0.10 mm [3.93 mil] in the
area of the flip chip paste/flux. An example of a two-print
stencil for this application is shown in Figure 3-20.

3.5 Step Stencil Design There are several applications

where a stencil with multiple foil thicknesses may be desir-
able. These designs are outlined below. (See Figures 3-21 and
3-22.)
2

1
2 3

4 4

5
IPC-7525b-3-21 IPC-7525b-3-20

Figure 3-21 Print With Step Figure 3-20 Two-Print Stencil for Mixed Technology
1. Squeegee Blade 1. ‘‘Flip Chip’’ Apertures
2. Paste Residues (Side Parallel to Direction of Travel) 2. Two Print Stencil #1 for ‘‘Flip Chip’’ Paste
3. Step Area 3. Relief Etch Area for ‘‘Flip Chip’’ Paste Clearance
4. Direction of Travel 4. SMD Apertures
5. Two Print Stencil #2 for SMD Paste

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IPC-7525B October 2011

3.5.1 Step-Down Stencil This type of stencil is useful

when it is desirable to print fine-pitch devices using a thinner
1
stencil foil but print other devices using a thicker stencil foil.
For example, there may be a fine-pitch BGA of 0.5 mm 2
[20 mil] pitch that requires a 0.1 mm [3.9 mil] foil thickness
to achieve an area ratio of greater than 0.66 but at the same
3
time there are other devices on the same board that need a
thickness of 0.13 to 0.15 mm [5.1 to 5.9 mil]. The stencil 4 IPC-7525b-3-22

design would have a step area at 0.1 mm [3.9 mil] thick in the Figure 3-22 Step Down
fine-pitch BGA portion while the remainder of the stencil foil 1. Squeegee Blade
is 0.15 mm [5.9 mil] thick. The step can be on the squeegee 2. Solder Paste - Paste not Extracted by the Squeegee
3. Board
side or on the contact side. See Section 3.3.1.2 for keep-out 4. Stencil
design guidelines.

3.5.2 Step-Up Stencil This type stencil is useful when it is 1

desirable to print thicker solder paste in a small portion of the
stencil. An example would be a ceramic BGA where it is nec- 2
essary to get 0.2 mm [7.9 mil] paste height because of ball
coplanarity but 0.15 mm [5.9 mil] height on all other surface-
3
mount component lands. In this case the stencil foil is stepped
4 IPC-7525b-3-23
up from 0.15 mm [5.9 mil] to 0.2 mm [7.9 mil] in the area of
the ceramic BGA. Another example is a through-hole edge Figure 3-23 Step Up
connector that requires additional solder paste volume. In this 1. Squeegee Blade
2. Solder Paste - Paste not Extracted by the Squeegee
case the stencil foil may be 0.15 mm [5.9 mil] thick every- 3. Board
where except in the area of the edge connector where the 4. Stencil
stencil foil may be 0.3 mm [12 mil] thick. (See Figure 3-23.)
3.5.3 Step Stencil for Enclosed Print Heads As a general design guide, a step should not exceed 0.05 mm [2.0 mil].
3.5.4 Relief-Etch Stencil This type of stencil has relief step pockets on the contact/board side of the stencil foil. There
are several applications where relief-etch stencils are useful. Some examples are:
• Relief pocket for a bar code label on the board. The stencil foil might be 0.15 mm [5.9 mil] thick with a 0.08 mm
[3.1 mil] relief for the bar code label.
• Test via relief pockets. The stencil foil has relief pockets over each raised test via to allow the stencil foil to sit flat and
gasket to the board.
• Solder mask pedestals at the corners of ceramic components. A relief etch on the stencil foil provides good gasketing. The
standoff of ceramic leadless components can be increased to accommodate cleaning under the component and to increase
the length of solder joint.
• Two-print stencil. This stencil foil has deep relief pockets in the areas where surface-mount solder paste was previously
printed (see Sections 3.3.1.3 and 3.4.1). An example of this stencil would be a stencil 0.5 mm [20 mil] thick for printing
paste in and around through-holes with relief step on the contact side 0.3 mm [12 mil] thick to clear surface mount sol-
der paste previously printed.
3.6 Fiducials Depending on the vision system, fiducials are located on the squeegee or contact side and engraved or filled
with a contrasting color epoxy. Typically, they are solid, round dots 1.0 to 1.5 mm [39.4 to 59.1 mil] in diameter. They may
be half-etched, laser engraved or etched through the stencil.
3.6.1 Global Fiducials Fiducials that are placed a minimum of 5 mm [0.20 in] from the board corners in three locations.

3.6.2 Local Fiducials Fiducials that are placed near critical components (e.g., fine-pitch QFP) are useful for pick and place
machines but useless for stencil printing. For the printing process the best fiducials are as far apart as possible.
3.7 Rework and Repair Stencils

3.7.1 Mini Stencils Mini Stencils are used to print solder paste on PCB lands once a defective component is removed.
After the component is removed from the PCB the excess solder must be removed. Then the mini stencil is lined up with
the PCB lands and a mini blade prints solder paste on the lands. The new part is placed in the solder paste and locally
reflowed to form solder joints.

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October 2011 IPC-7525B

3.7.2 Repair Tool for Printing Paste Directly on the Component Sometimes it is more convenient to print solder paste
on the replacement component rather than printing paste on the PCB. In this case, a holding tool holds the part in contact
with a stencil and paste is printed directly onto the terminations of the component or on top of the balls of a BGA. This
method is particularly useful for BTC devices with terminations flat on the bottom side of the component of small devices
ranging typically from 3 mm to 10 mm. The following figures show examples of a BTC (Figure 3-24) and a BGA (Figure
3-25) which have been printed with solder paste.

Figure 3-24 BTC Figure 3-25 BGA

4 STENCIL FABRICATION

4.1 Foils Stainless steel is the preferred metal for chemical etching and laser cutting technologies. Other metals, as well
as plastics, may be specified. For electroform technology, a hard nickel alloy is preferred.

4.2 Frames Refer to the stencil printer operation manual for available frame sizes. Frames may be tubular or cast alumi-
num with the tensioned mesh border permanently mounted using an adhesive. The foil is bonded to the mesh. Some foils
can be mounted into a re-usable tensioning master frame and do not require a mesh border and negate a permanent bond-
ing of the foil to the frame.

4.3 Stencil Border Polyester is the standard material; stainless steel is optional.

4.4 Stencil Fabrication Technologies The fabrication process for stencils may involve additive or subtractive methods.
In additive processes such as electroforming, metal is added to form stencil foils. In subtractive processes, material is
removed from foils to create apertures. Laser cut and chemical etch are examples of subtractive processes.

4.4.1 Chemical Etch Chemically etched stencils are produced using photo-imageable resist laminated on both sides of
metal foils cut to specific frame sizes. A double-sided phototool, held in precise alignment usually with registration pins, is
used to expose the stencil aperture image onto the resist. Aperture images exposed on the resists are reduced in size com-
pared with the desired aperture dimensions to account for the etch factor. The etch factor describes the amount of lateral
etching that takes place as the chemical etches through the thickness of metal foil. The exposed resist is then developed,
leaving bare metal where apertures are desired. The metal foil is etched from both sides in a liquid chemical, creating aper-
tures as specified. The remaining resist is then stripped away and a stencil foil is produced.
Chemical etching also is used to provide a step down or step up area of the stencil for standard stencils (does not include
additive process stencils). This process set up is critical to provide a smooth surface for the paste to roll on and squeegee
to wipe clean in the step area.

4.4.1.1 High-Precision Etch A chemically etching process with more nozzles that are placed closer to the stencil and with
a possibility to adjust pressure to a greater extent on top and bottom side. Also with a continuous regeneration of the ferric
chloride used for etching so the concentration is kept constant to facilitate a consistent etch result.

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IPC-7525B October 2011

4.4.2 Laser-Cut Stencils Laser-cut stencils are produced from data prepared by software of the laser equipment. Unlike
chemically etched stencils, no phototool is required. A tapered aperture wall is an inherent part of laser cut stencils. Unless
otherwise specified, the stencil is cut so that the apertures are larger on the contact side than on the squeegee side (see Sec-
tion 4.4.5). Laser cutting can also be done in Kapton®.

4.4.3 Electroform Electroforming is an additive stencil fabrication method utilizing photo-imageable resist and an elec-
troplating process. Photo-imageable resist is placed on a metal mandrel. Thickness of the resist is greater than the final sten-
cil thickness desired. The apertures are imaged onto the resist and the resist is developed, leaving resist pillars where aper-
tures are desired. The mandrel with resist pillars is placed in a nickel plating tank where nickel is electroplated onto the
mandrel. When the desired stencil thickness is reached, the mandrel is removed from the plating tank. Lastly, the resist pil-
lars are stripped and the nickel stencil foil is separated from the mandrel.

4.4.4 Hybrid Where a mixture of standard and fine-pitch 1

assemblies is present on a board, the stencil fabrication pro- 2
cess may be a combination of laser cut and chemical etch. The
stencils produced are referred to as laser-chem combination or
hybrid stencils.
3

4.4.5 Trapezoidal Apertures A trapezoidal aperture may be 1/2 Z 1/2 Z

IPC-7525b-4-1
used to enhance solder paste release. In chemical etch and
high precision chemical etch processes, the trapezoidal dimen- Figure 4-1 Trapezoidal Apertures
1. Trapezoidal Aperture
sion, Z, (see Figure 4-1) can be specified. For laser cut or 2. Squeegee Side
electroform processes trapezoidal aperture is an inherent part 3. Contact Side
of process. Stencil vendors should be contacted for dimen-
sions.

4.4.6 Additional Options Further processing or material options may be desired on certain methods of stencil fabrication
to reduce friction between solder paste and side walls for improved paste release. Options are:
• Polishing A subtractive process; either chemical polishing or electro-polishing process.
• Nickel Plating An additive process applying a thin layer of nickel on the stencil.
• Fine-Grain Steel A stainless steel stencil material specifically designed to produce an extremely smooth aperture wall
when laser-cut with a fiber laser or etched using the high-precision etch process.
• Nickel/PTFE Coating/Nano-Coating Additive surface treatments that lower the surface energy and create a hydrophobic
surface which facilitates paste release from the apertures and cleaning of the bottom side of the stencil.

5 STENCIL MOUNTING

5.1 Location of Image on Metal The image is centered or offset in the stencil. Board corner marks or the board outline
can be used to indicate its location. Global fiducials or the actual board outline should be used for alignment. Where more
than one board or panel image is placed on one stencil, a minimum of 50 mm [2.0 in] is recommended between the images.

5.2 Centering It is recommended that the stencil be centered on the frame for the most uniform mechanical tensioning
and print results. The image can be offset to meet specific requirements of the stencil printer.

5.3 Additional Design Guidelines Unless otherwise specified, additional design guidelines are:
• Minimum 20 mm [0.79 in] border is recommended from the edge of frame to the edge of metal.
• Minimum 50 mm [2.0 in] from the inside edge of glue border to the edge of image is suggested for solder paste storage
and squeegee travel.

6 STENCIL ORDERING
Stencil information is typically communicated between the user and supplier through an order form (or checklist) created by
the supplier. File data, types of material, fabrication methods, and special requests are examples of what is typically included
on an order form (see Appendix A).

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October 2011 IPC-7525B

7 STENCIL USER’S INSPECTION/VERIFICATION

After receiving a stencil from the supplier, it is recommended that users generate an inspection checklist to verify that the
stencil has been fabricated correctly and that no damage has occurred during the shipping process. The following items can
be used as a guideline for inspecting an incoming stencil:
• The stencil supplier’s QC sheet should be reviewed to ensure aperture sizes, aperture presence, and aperture positions have
all passed inspection.
• The foil should be inspected for chemical corrosion.
• The foil should be inspected for handling damage (e.g., dents, creases, or metal voids).
• Tension of the stencil foil should be checked using a method agreed on between the user and manufacturer.
• Correct spacing between the image and the frame should be verified (based on printer manufacturer’s specification). A
printed wiring board or transparent image of the board (e.g., Mylar film with backlighting equipment) should be held up
against the image of stencil to check for correct spacing between the board and the edge of stencil frame.
• The image of the stencil should be observed to match the image of the board. Modifications of aperture size and/or shape
should be verified.
• Border should be checked for proper adhesion to the foil and for any handling damage.
• The size, type, flatness, etc., of the frame should be checked.
• Correct part number and revision number etched or engraved in the stencil should be verified.
• Foil thickness should be verified.
• For a step stencil, the correct step level should be verified.
• Fiducial quality and location (correct side of foil) should be inspected.
• The stencil cleanliness should be verified to be free of lint, fibers, or loose particles.

8 STENCIL CLEANING
Proper setup and cleaning of a stencil helps ensure continued repeatable printing performance. Cleaning processes need to
be compatible with materials used in the manufacture of stencils. Paste or adhesive manufacturers, stencil manufacturers,
and cleaning equipment manufacturers should be consulted as the life of stencils, integrity of the fiducials, and quality of
the glue bead may be affected. Refer to IPC-7526.

9 END OF LIFE
Stencils should be inspected periodically for damage that would contribute to decreased printing performance. Refer to Sec-
tion 7 for inspection guidelines.
Users are invited to submit recommendations based on their experience for determining the end of life for a stencil. The
committee is particularly interested in repeatable methods that measure tension on the stencil after use. These will be con-
sidered for a future amendment or revision to this document.

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IPC-7525B October 2011

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October 2011 IPC-7525B

APPENDIX A: EXAMPLE ORDER FORM

Customer / Contact Name:

Ship To: Bill To:

Shipping Method: Due Date:

Stencil Part Number and Revision Number:

(to be half-etched or engraved on squeegee / contact side of stencil)

File(s) Name: File Type:

Data Transmitted By: [ ] Modem [ ] Email [ ] FTP [ ] Disk

Stencil Fabrication Method: [ ] Chemical Etch [ ] Laser Cut [ ] Laser-Chem Hybrid

[ ] Electroform [ ]

Frame Size:
Provided By: [ ] Customer [ ] Stencil Supplier
Frame Coated: [ ] Yes [ ] No

Metal Thickness:

Step-Down Thickness: (Drawing Required for Step Locations)

Metal Type: [ ] Stainless Steel [ ]

Location of Pattern on Stencil: [ ] Center Image [ ] Center Board [ ] Offset (Drawings Required)

Fiducials: [ ] None
[ ] Half Etch / Engraved Contact Side
[ ] Half Etch / Engraved Squeegee Side
[ ] Full Etch / Cut Through, Not Filled
[ ] Full Etch / Cut Through, Filled with Contrasting Epoxy

[ ] Panelization Provided By: [ ] Customer [ ] Stencil Supplier

Dimensions Required:

[ ] Step-and-Repeat Provided By: [ ] Customer [ ] Stencil Supplier

Dimensions Required:
Border: [ ] Polyester [ ] Stainless Steel

Polish: [ ] Yes [ ] No

Nickel Plate: [ ] Yes [ ] No

Nano-coat: [ ] Yes [ ] No

Special Modifications, Editing, or Instructions:

(This page is authorized for copying/local reproduction.)

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IPC-7525B October 2011

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ANSI/IPC-T-50 Terms and Definitions for

Interconnecting and Packaging Electronic Circuits
Definition Submission/Approval Sheet
The purpose of this form is to keep SUBMITTOR INFORMATION:
current with terms routinely used in Name:
the industry and their definitions.
Individuals or companies are Company:
invited to comment. Please
City:
complete this form and return to:
IPC State/Zip:
3000 Lakeside Drive, Suite 309S Telephone:
Bannockburn, IL 60015-1249
Fax: 847 615.7105 Date:
❑ This is a NEW term and definition being submitted.
❑ This is an ADDITION to an existing term and definition(s).
❑ This is a CHANGE to an existing definition.

Term Definition

If space not adequate, use reverse side or attach additional sheet(s).

Artwork: ❑ Not Applicable ❑ Required ❑ To be supplied

❑ Included: Electronic File Name:
Document(s) to which this term applies:

Committees affected by this term:

Office Use
IPC Office Committee 2-30
Date Received: Date of Initial Review:
Comments Collated: Comment Resolution:
Returned for Action: Committee Action: ❑ Accepted ❑ Rejected
Revision Inclusion: ❑ Accept Modify

IEC Classification
Classification Code • Serial Number
Terms and Definition Committee Final Approval Authorization:
Committee 2-30 has approved the above term for release in the next revision.
Name: Committee: IPC 2-30 Date:
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Put IPC MEMBERSHIP to WORK for your Company

A trusted leader for more
than 50 years, IPC is the
Keep on top of industry developments …
premier source for industry and how they will affect your company
standards, training, market
research and public policy • Enjoy 24/7 privileges to FREE members-only online resources, including a
searchable archive of original articles and presentations on the latest technical
advocacy — supporting the
issues and industry/market trends.
needs of the estimated $1.7
• Receive FREE exclusive statistical reports available for the EMS, PCB, laminate,
trillion global electronics
process consumables, solder and assembly equipment industries.
industry.
• IPC events, including IPC APEX EXPOTM, technical conferences, workshops,
For less than $3.00 a training and certification programs and executive management summits provide
day, IPC members enjoy unparalleled educational and networking opportunities.
unlimited access to the • Stay abreast of global environmental directives, legislation and regulations, and
tools, information and how these specifically impact each segment of our industry’s supply chain.
forums needed to thrive in
an ever-changing electronic Save enough money to easily pay for your
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“I have a responsibility to • Get discounts of up to 50 percent on IPC standards, publications and training
my customers and my materials.
shareholders. Between the • Save money on online subscription licenses of IPC standards through the world’s
savings on standards, training largest standards reseller — IHS.
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data, IPC membership • Enjoy dramatic discounts on registration fees for meetings, technical
provides immediate 100% conferences, workshops and tutorials.
return on investment for us. It • Benefit from preferred pricing on exhibit space at IPC trade shows and events.
would be irresponsible not to
be a member.”
Joseph F. O’Neil
President
Hunter Technology Corp

“Graphic PLC has enjoyed the privilege of being

an IPC member for more than 30 years and the
technical benefits derived to focus us as a world
player in the manufacture of PCBs have superceded
the cost of membership many times.”
Rex Rozario, OBE
Chairman
Graphic Plc.,UK

“Being a part of the fast-changing global electronics

marketplace requires constant intelligence about
market trends, standards and solutions to the
challenges throughout the supply chain. IPC is an
invaluable partner in providing that intelligence
through conferences, white papers and technical
standards.”
Andy Hyatt
Executive Vice President
Business Development
Creation Technologies
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Increase your knowledge and train your “IPC’s role in defining

industry technical standards,
people addressing industry concerns,
and promoting knowledging
• IPC workshops and international conferences provide an exchange of technical sharing through conferences
information that is unequalled. and training, significantly
benefits member companies
• IPC’s training and certification programs offer a cost-effective, industry- and the industry as a whole,
recognized way to demonstrate your commitment to quality. especially in today’s global
• Facilitate your staff’s continuous learning through IPC’s award-winning CD- outsourcing environment.”
and DVD-based training materials. Dongkai Shangguan, Ph.D.
Vice President
Flextronics International
Expand your network and build your
visibility
• Network with your peers through IPC committees, PCB/EMS management
councils and IPC events.
“Juki gets tremendous value
• Participate in problem-solving exchanges through IPC’s technical e-forums. from our ipc membership
• Get answers to your technical questions from IPC’s … we get quarterly market
technical staff. data which would cost us
thousands of dollars if we
commissioned it on our
Help shape the industry own. The industry standards
generated by IPC committees
• Participate in developing or updating the global industry standards that your allow us to design our
company, customers, competitors and suppliers use. equipment with certainty
that it will meet industry
• Take an active role in IPC-organized environmental and public policy activities
requirements. The returns
to advocate for regulations and legislation favorable to your company and the
for our company are so great,
global electronics community. they are beyond calculable.”
Bob Black
Market your business President and CEO
Juki Automation Systems Inc.

• Use the IPC member logo to highlight your company’s leadership in the
industry.
• Build your brand visibility through IPC’s Products and Services Index (PCB and
EMS companies only), and IPC’s annual trade shows and conferences, including
IPC APEX EXPO.
• Gain valuable exposure by sponsoring market research conferences and
executive management meetings.

Put the resources of the entire industry behind

your company by joining IPC today!
To learn more about IPC membership or to apply online, visit www.ipc.org.

IPC — Association Connecting Electronics Industries® Headquarters

3000 Lakeside Drive, Suite 309 S, Bannockburn, IL 60015
www.ipc.org
+1 847-615-7100 tel
+1 847-615-7105 fax
Visit www.IPC.org/offices for the locations of IPC offices worldwide.
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Association Connecting Electronics Industries

®
Application for Site Membership
Thank you for your decision to join IPC. Membership is site specific, which means that IPC member benefits are
available to all individuals employed at the site designated on this application.

To best serve your specific needs, please indicate the most appropriate member category for your facility.
(Check one box only.)

o Printed Circuit Board Manufacturer

Facility manufactures and sells printed circuit boards (PCBs) or other electronic interconnection products to other companies.
What products do you make for sale? (check all that apply)
o One and two-sided rigid, multilayer printed boards o Flexible printed boards o Other interconnections
o Printed electronics

__________________________________________________________________________________________________

o Electronics Manufacturing Services (EMS) Company

Facility manufactures printed circuit assemblies, on a contract basis, and may offer other electronic interconnection products
for sale.

___________________________________________________________________________________________________

o OEM — Original Equipment Manufacturer

Facility purchases, uses and/or manufactures printed circuit boards or other interconnection products for use in a final
product, which we manufacture and sell.

What is your company's primary product line?______________________________________________________________

___________________________________________________________________________________________________

o Industry Supplier
Facility supplies raw materials, equipment or services used in the manufacture or assembly of electronic products.

Which industry segment(s) do you supply? o PCB o EMS o Both o Printed electronics

What products do you supply?__________________________________________________________________________

o Government, Academia, Nonprofit

Organization is a government agency, university, college or technical or nonprofit institution which is directly concerned with
design, research and utilization of electronic interconnection devices.

o Consulting Firm
What services does the firm provide?_____________________________________________________________________

___________________________________________________________________________________________________
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Association Connecting Electronics Industries

®
Application for Site Membership
Site Information

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Main Switchboard Phone No. Main Fax

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Membership Dues
Membership will begin the day the application and dues payment are received, and will continue for one or two
years based on the choice indicated below. All fees are quoted in U.S. dollars.
Please check one:
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o One year $1,050.00 o One year $275.00
o Two years $1,890.00 (SAVE 10%) o Two years $495.00 (SAVE 10%)
Additional facility: Membership for a facility of an organization that Consulting firm (employing less than 6 individuals)
already has a different location with a primary facility membership o One year $625.00
o One year $850.00 o Two years $1,125.00 (SAVE 10%)
o Two years $1,530.00 (SAVE 10%)
Company with an annual revenue of less than $5,000,000
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*Overnight deliveries to this address only.

Contact membership@ipc.org for wire transfer details 10/10
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Standard Improvement Form IPC-7525B

The purpose of this form is to provide the Individuals or companies are invited to If you can provide input, please complete
Technical Committee of IPC with input submit comments to IPC. All comments this form and return to:
from the industry regarding usage of will be collected and dispersed to the IPC
the subject standard. appropriate committee(s). 3000 Lakeside Drive, Suite 309S
Bannockburn, IL 60015-1249
Fax: 847 615.7105
E-mail: answers@ipc.org
www.ipc.org/standards-comment

1. I recommend changes to the following:

Requirement, paragraph number
Test Method number , paragraph number

The referenced paragraph number has proven to be:

Unclear Too Rigid In Error
Other

2. Recommendations for correction:

3. Other suggestions for document improvement:

Submitted by:

Name Telephone

Company E-mail

Address

City/State/Zip Date
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Association Connecting Electronics Industries

3000 Lakeside Drive, Suite 309 S

Bannockburn, IL 60015
847-615-7100 tel
847-615-7105 fax
www.ipc.org

ISBN #978-1-61193-020-7