IPC-7S27

2012 - May
Requirements for Solder
Paste Printing
A standard developed by IPC

Association Connecting Electronics Industries

 IPC-7S27

Requirements for Solder

Paste Printing

Developed by the Solder Paste Printing Task Group Nordic (5-21 JND)
of the Component Mounting Subcommittee (5-21) of the Assembly and
. Joining Committee (5-20) of IPC

Users of this publication are encouraged to participate in the

development of future revisions.

Contact:

IPC
3000 Lakeside Drive, Suite 309S
Bannockburn, Illinois
60015-1249
Tel 847 615.7100
Fax 847 615.7105
May 2012 IPC-7527

Acknowledgment
Any document involving a complex technology draws material from a vast number of sources across many continents.
Shown below are the principal members of the IPC-7S27 development team including the Solder Paste Printing Task Group
Nordic (5-21JND) of the Component Mounting Subcommittee (5-21) of the Assembly and Joining Committee (5-20). 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 Component Mounting Solder Paste Printing

Committee Subcommittee Task Group Nordic

Chair Chair Co-Chairs

Leo P. Lambert Peggi J. Blakley Scott Pinney
NSWC Crane Grundfos Management NS

Steven Hansen
Vestas Control Systems

Technical Liaisons of the

IPC Board of Directors
Peter Bigelow
IMI Inc.

Sammy Yi
FIextronics International

Component Mounting Subcommittee

Sonic Lu, Autoliv (China) Electronics Renie Zhao, Flextronics Mfg. Poul Juul, Hytek
Co., Ltd. (Zhuhai) Co. Ltd. Seren Treeholt, Kai Toft Elektronik
Claus Nielsen, BB Electronics NS Henning Haubo, Flextronics Special ApS
Jens Chr, Jensen, Bent Hede Business Solutions Kim Poulsen, Kamstrup NS
Elektronik NS Jesper Konge, Gaasdal Erik Andresen, Linak A/S
Steven Perng, Cisco Systems Inc. Bygningsindustri NS
Allan Sigfredsen, Necas A/S
Jergen Stenstrup, Danfoss Drives NS Scott Pinney, Grundfos Management
Gregers Dybdal, Linak NS
NS
Torgrim Nordhus, EMG Norautron Kathrin Morgener Jensen, Protec
AS Svein Olav Kolbu, Hapro AS
Electronics ApS
David Qi, Flextronics Mfg. (Zhuhai) Jens Andersen, Hytek
Steven Hansen, Vestas Control
Co. Ltd. Alex Christensen, Hytek Systems
Jiyang Zhang, Flextronics Mfg.
(Zhuhai) Co. Ltd.

111
IPC-7527 May 2012

Table of Contents
1 GENERAL 1 Figure 5-6 Centered Deposit 5
1.1 Scope 1 Figure 5-7 Centered Deposit 5

1.2 Purpose 1 Figure 5-8 Offset - Acceptable 5

1.3 Background 1 Figure 5-9 Offset - Acceptable 5

Figure 5-10 Offset - Acceptable 5
1.4 Terms and Definitions 1
Figure 5-11 Offset - Acceptable 5
1.4.1 Classification 1
Figure 5-12 Offset - Acceptable 6
1.4.2 Disposition* 1
Figure 5-13 Offset - Acceptable 6
1.4.3 Slumping 1
Figu re 5-14 Offset - Defect 6
1.4.4 Combined Conditions 1
Figu re 5-15 Shorting - Defect 6
] .4.5 Designed Volume 1
Figu re 5-16 Offset - Defect 6
1.5 Specialized Designs 1 Figu re 5-17 Area - Target 7
1.6 Verification of Dimensions 2 Figu re 5-18 Area - Target 7
1.7 Magnification Aids 2 Figu re 5-19 Area - Acceptable ~ 7
1.8 Lighting 2 Figure 5-20 Area - Acceptable < 7
Figure 5-21 Area - Defect 7
2 APPLICABLE DOCUMENTS 2
Figure 5-22 Height - Target 8
2.1 IPC 2
Figure 5-23 Height - Target 8
2.2 ASTM 2
Figure 5-24 Height - Process Indicator 8
3 CHOICE OF TECHNOLOGY 2 Figure 5-25 Height - Defect 8
Figure A-1 Solder Paste Printing with Squeegee
4 MEASUREMENT OF SOLDER PASTE
Blades 9
DEPOSITS 3
Figure A-2 Solder Paste Printing with Enclosed Head
5 SHAPE OF SOLDER PASTE DEPOSIT 4 System 9
5.1 Solder Paste Deposit - Misalignment 4 Figure A-3 Plastic from Packaging 9

5.1.1 Slump 4 Figure A-4 Glass Fiber from PCB 10

Figure A-5 Effect of Solder Mask 10
5.1.2 Misregistration of Solder Paste Deposits 5
Figure A-6 Effect of Silk Screen 10
5.1.3 Solder Paste Deposit - Area 7
Figure A-7 Effect of Solder Paste 10
5.1.4 Solder Paste Deposit - Height 8
Figure A-8 Result of Silk Screen Interference 10
Appendix A Guideline for Operator Trouble- Figure A-9 Saddle Shape Deposit 10
shooting in the Solder Paste
Figure A-10 Roof Top Shape Paste Deposit 10
Screen Printing Process 9
Figure A-11 Spikes on Paste Deposit 11
Figures Figure A-12 Increased Deposit Quantity 11
Figure 3-1 Squeegee Blade 2 Figure A-13 Decreased Quantity 11
Figure 3-2 Enclosed Paste Print Head 3 Figure A-14 Slumping/Smearing 11
Figure 3-3 Needle Dispense 3 Figure A-15 Paste Residues 12
Figure 3-4 Paste Jet Dispense 3 Figure A-16 Long Smear 12
Figure 4-1 Sample of an Automated Machine Readout 3 Figure A-17 Offset 12
Figure 4-2 Sample of an Automated Inspection Result 3 Figure A-18 Excess Residue Along Direction of Travel 12
Figure 4-3 Weighing Solder Paste 3 Figure A-19 Residue Across Direction of Travel 13
Figure 5-1 Full Shape 4 Figure A-20 Solder Balls 13
Figure 5-2 Saddle Shape 4 Figure A-21 Close Up of Solder Balis 13

Figure 5-3 Roof Top Shape 4

Figure 5-4 Pyramid Shape 4 Tables

Figure 5-5 Fuli Shape 4 Table 1-1 Inspection Magnification (Land Width) 2

iv
May 2012 IPC-7527

Requirements for Solder Paste Printing

1 GENERAL cannot be tolerated, or the end-use environment may be

uncommonly harsh.
1.1 Scope This standard is a collection of visual quality
acceptability criteria for solder paste printing. Target Condition A condition that is close to perfect!
preferred, however, it is a desirable condition and not
1.2 Purpose The purpose of this guideline document is always achievable and may not be necessary to ensure reli-
to support the user in the visual evaluation of the solder ability of the assembly in its service environment.
paste printing process, which makes subsequent process
Acceptable Condition Indicates a condition that, while not
optimizing possible.
necessarily perfect, will maintain the integrity and reliabil-
The purpose of this guideline is not to inspect and evaluate ity of the assembly in its service environment.
the quality of the solder paste. For information on the
Defect Condition A condition that may be insufficient to
evaluation of solder paste, see J-SID-OOS, Requirements
for Soldering Pastes and IPC-HDBK-OOS. ensure the form, fit, or function of the assembly in its end
use environment. Defect conditions …all be dispositioned
The purpose is not to define requirements to stencil design. by the manufacturer based on design, service, and customer
For information on stencil design, see IPC-7S2S, Stencil requirements. Disposition may be to rework, repair, scrap,
Design Guideline. or use as is. Repair or use as is may require customer con-
Appendix A provides different error types and suggested currence.
solutions are listed. The guideline is intended to help/ A defect for Class I automatically implies a defect for
improve the optimizing process for paste printing. Class 2 and 3. A defect for Class 2 implies a defect for
In the case of a discrepancy, the description or written Class 3.
criteria always takes precedence over the illustrations.
1.4.2 Disposition* The determination of how defects
should be treated. Dispositions include, but are not limited
1.3 Background Standards such as J-SID-OOI and IPC-
to, rework, use as is, scrap, or repair.
A-61O provide tools that help to promote quality within the
electronics industry. However, the standards do not include
1.4.3 Slumping Solder paste flows out after it is applied.
requirements of visual appearance of the paste deposit. The See more in IPC-HDBK-OOS.
industry has, for some time, expressed a wish for a stan-
dard to support the users in the solder paste printing pro- 1.4.4 Combined Conditions There may be situations
cess. ·IPC-7S27 will help improve the quality in this very where a combination of maximum deviations in form,
sensitive process. placement, coverage, and height can result in lack of sol-
dering or wrong amount of solder paste. The manufacturer
1.4 Terms and Definitions Other than those terms listed is responsible for identification of such conditions.
below, the definitions of terms used in this standard are in
accordance with IPC- T-SO. Terms quoted from IPC- T-SO 1.4.5 Designed Volume The designed volume is equal to
are marked with *. the area of the stencil aperture times the stencil thickness
(volume = length x width x height). Volume is not a visu-
1.4.1 Classification ally inspectable condition.
CLASS 1 General Electronic Products 1.5 Specialized Designs This standard, as an industry
Includes products suitable for applications where the major consensus document, cannot address all of the possible
requirement is function of the completed assembly. components and product design combinations. Where
uncommon or specialized technologies are used, it may be
CLASS 2 Dedicated Service Electronic Products
Includes products where continued performance and necessary to develop unique acceptance criteria. However,
extended life is required, and for which uninterrupted ser- where similar characteristics exist, this document may pro-
vice is desired, but not critical. Typically the end-use envi- vide guidance for product acceptance criteria. Often,
ronment would not cause failures. unique definition is necessary to consider the specialized
characteristics while considering product performance cri-
CLASS 3 High Performance Electronic Products teria. The development should include customer involve-
Includes products where continued high performance or ment or consent. For Class 3, the criteria shall include
performance-on-demand is critical, equipment downtime agreed definition of product acceptance.
 IPC-7527 May 2012

Whenever possible these criteria should be submitted to the 2 APPLICABLE DOCUMENTS

IPC Technical Committee to be considered for inclusion in The following documents currently in effect form a part of
upcoming revisions of this standard. this specification to the extent specified herein.

2.1 IPC1
1.6 Verification of Dimensions The actual measure-
ments provided in this document (i.e., specific part mount- IPC-J-STD-005 Requirements for Soldering Pastes
ing and solder fillet dimensions and determination of per-
centages) are not required except for referee purposes. All IPC-HDBK-005 Guide to Solder Paste Assessment
dimensions in this standard are expressed in SI (System
IPC-T-50 Terms and Definitions for Interconnecting and
International) units (with Imperial English equivalent
Packaging Electronic Circuits
dimensions provided in brackets). All specified limits in
this standard are absolute limits as defined in ASTM E29. IPC-A-610 Acceptability of Electronic Assemblies

IPC-7525 Stencil Design Guidelines

1.7 Magnification Aids For visual inspection, some indi-
vidual specifications may call for magnification aids for IPC-7526 Stencil and Misprinted Board Cleaning Hand-
examining solder paste print. book

The tolerance for magnification aids is ± 15% of the 2.2 ASTM2

selected magnification power. Magnification aids, if used
for inspection, shall be appropriate for the item being ASTM E29 Standard Practice for Using Significant Digits
inspected. Unless magnification requirements are otherwise in Test Data to Determine Conformance with Specifica-
specified by contractual documentation, the magnifications tions.
in Table 1-1 are determined by the item being inspected. 3 CHOICE OF TECHNOLOGY

Referee conditions are used to verify product rejected at The requirements in this standard have been developed for
the inspection magnification power. For assemblies with solder paste printing. Since the requirements are referred to
mixed land widths, the greater magnification may be used as general, it is also possible to evaluate other application
for the entire assembly. methods according to this standard. However, it may be
necessary to develop specific acceptance requirements for
Table 1-1 Inspection Magnification (Land Width)
the chosen application method.
Magnification Power
Various methods can be used when solder paste is applied
Land Widths or Inspection Maximum
Land Diameters Range Referee to the PCB (printed circuit board). Four methods are illus-
>1.0 mm [0.0394 in] 1.5X to 3X 4X trated here:
>0.5 to ~1.0 mm • Squeegee Blade (Figure 3-1)
3X to 7.5X 10X
[0.0197 to 0.0394 in]
• Enclosed Print Head (Figure 3-2)
;:::0.25to ~0.5 mm
7.5X to 10X 20X • Needle Dispense (Figure 3-3)
[0.00984 to 0.0197 in]
<0.25 mm [0.00984 in] 20X 40X • Paste Jet Dispense (Figure 3-4)

1.8 Lighting Lighting shall be adequate for the item

being inspected.

Illumination at the surface of workstations should be at

least 1000 lm/nr' [approximately 93 foot candles). Light
sources should be selected to prevent shadows.
~c! 4~ ~~
IPC- 7527 -3- 1
Note: In selecting a light source, the color temperature of
Figure 3-1 Squeegee Blade
the light is an important consideration. Light ranges from
1. Squeegee
3000-5000° K enable users to distinguish various solder 2. Paste
paste printing features and contaminates with increased 3. Stencil
clarity. 4. PCB

1. www.ipc.org
2. www.aslm.org

2
May 2012 IPC-7527

equipment manufacturer may use different equations to

determine height, area, and volume of the solder deposit. It
is up to the user of this standard to translate from this
guideline to the individual automated inspection equip-
ment.

IPC-7527 -3-2

Figure 3-2 Enclosed Paste Print Head

1. Enclosed Paste Head
2. Stencil
3. PCB

3
Figure 4-1 Sample of an Automated Machine Readout

f 1
IPC-7527 -3-3
(Screen Capture Courtesy of CyberOptics Corporation)

Figure 3-3 Needle Dispense

1. PCB
2. Needle Dispenser
3. Paste Deposit

f 4
IPC-7527 -3-4 Figure 4-2 Sample of an Automated Inspection Result
(Image Courtesy of HYTEK)
Figure 3-4 Paste Jet Dispense
1. Motor 3. Paste Deposit Controlling the amount by weighing the PCB shows the
2. Paste 4. PCB total amount of solder paste on the board (Figure 4-3). By
4 MEASUREMENT OF SOLDER PASTE DEPOSITS comparing the result with a PCB, with a correct amount of
solder paste, you will get an exact indication of potential
Various types of equipment can be used to measure the
deviance.
solder paste print. The measurement can be done manually
or with an automatic inspection system. Figures 4-1 and
4-2 illustrate the results from two automated methods of 8 , 9

.:
paste measurement. It is often necessary to perform mea-

••
surements in order to control the amount of solder paste.
This paragraph is not a step by step process requirement, I 4.021 9 I
but an overview of technologies that can be implemented ~
when measuring the amount of solder paste. U 0
IPC-7527 -4-3

Follow the manufacturer's instructions when using auto- Figure 4-3 Weighing Solder Paste
mated measuring equipment. Each automated inspection

3
IPC-7527 May 2012

Control of the deposited amount ensures that the design

amount is obtained with reference to the thickness and
aperture of stencil. However, total weight of the assembly
cannot take the place of measurement of individual paste
deposits. Weight is only used for total volume assessment.

To weigh the boards, weigh clean, bare boards. Deposit the

paste using the normal print process. Then weigh the same Figure 5-3 Roof Top Shape
board again to determine the difference between the clean
board and the printed board. The difference will be the total
volume of paste applied to the board.

It is reconunended to use a calibrated and accurate scale.

The accuracy of the scale will be determined according to
the size and density of the board assembly.

5 SHAPE OF SOLDER PASTE DEPOSIT Figure 5-4 Pyramid Shape

The volume and placement precision of the solder paste
5.1 Solder Paste Deposit - Misalignment A process
deposits are the important factors for a correct solder paste control system is recommended to ensure the lowest pos-
deposit. Ideally the paste deposit should have the same sible error rate, e.g., Statistical Process Control (SPC).
shape as the stencil aperture (area and height). However, to
5.1.1 Slump
some extent it will be acceptable with varying paste deposit
shapes. See the appendix for possible shapes. Target - Class 1, 2, 3
• Full shape solder paste deposit (Figure 5-5).
The paste deposit shape can be defined as follows:
• Full Shape (Figure 5-1)
• Saddle Shape (Figure 5-2)
• Roof Top Shape (Figure 5-3)
• Pyramid Shape (Figure 5-4)

Figure 5-5 Full Shape

Acceptable - Class 1
Figure 5-1 Full Shape
• The paste deposit deviates from the nominal shape but
has the designed shape and volume.
Acceptable - Class 2, 3
• The paste deposit deviates from the designed sbape but
slumps less than 25% in the X and Y directions.
Defect - Class 1
• Paste has connected with adjacent deposits.
Figure 5-2 Saddle Shape Defect - Class 2, 3
• The paste has slumped more than 25% in the X and/or Y
directions.

4
May 2012 IPC-7527

5.1.2 Misregistration of Solder Paste Deposits

Target - Class 1, 2, 3
• Paste deposit is centered on solder pad (Figures 5-6 and
5-7).
• The paste deposit appears as one single block (unless
segmented by design).
• All solder balls have contact to the solder paste deposit.

Figure 5·9 Offset - Acceptable

Figure 5·6 Centered Deposit

Figure 5·10 Offset - Acceptable

Figure 5·7 Centered Deposit

Acceptable - Class 1, 2, 3
• Paste deposit (D) is off pad <25% (A) in X and/or Y
direction (Figures 5-8 through 5-13).

Figure 5·11 Offset - Acceptable

Figure 5·8 Offset - Acceptable

5
IPC-7527 May 2012

Figure 5-14 Offset - Defect

Figure 5-12 Offset - Acceptable

Figure 5-13 Offset - Acceptable Figure 5-15 Shorting - Defect

Defect - Class 1, 2, 3
• Solder paste deposit has connected with adjacent pad
(Figure 5-15),
• Paste deposit (D) is off pad >25% (A) in X and/or Y
direction (Figures 5-14 through 5-16),
• Solder deposit not in single block (except by design),
• Solder balls that do not contact the solder paste deposit.

Figure 5-16 Offset - Defect

6
May 2012 IPC-7527

5.1.3 Solder Paste Deposit - AreaIt may be necessary Process Indicator - Class 1, 2, 3
to use a measurement system to evaluate this factor. See • Coverage 75% of stencil aperture (Figures 5-19 and
Section 4, MEASUREMENT OF SOLDER PASTE 5-20).
DEPOSITS.

Target - Class 1, 2, 3
• Coverage = 100% of stencil aperture (Figures 5-17 and
5-18).

Figure 5-19 Area - Acceptable

Figure 5-17 Area - Target

Figure 5-20 Area - Acceptable

Figure 5-18 Area - Target Defect - Class 1, 2, 3

• Coverage <50% of stencil aperture (Figure 5-21).

Figure 5-21 Area - Defect

7
IPC-7527 May 2012

5.1.4 Solder Paste Deposit - Height It may be neces- Process Indicator - Class 1, 2, 3
sary to use a measurement system to evaluate this factor. • Paste deposit height 75% of stencil thickness (Figure
See Section 4, MEASUREMENT OF SOLDER PASTE 5-24).
DEPOSITS.
Example of Tolerance:
• Component pin co-planarity: 100 urn [0.0039 in]
• Stencil thickness: 127 urn [0.005 in]
• Reduction of 20%
• Result of theoretical paste deposit height: 102 urn
[0.004 in]
See Appendix A for further examples. Figure 5-24 Height - Process Indicator

Target - Class 1, 2, 3 Defect - Class 1, 2, 3

• Nominal height is achieved, (deposit height = stencil • Paste deposit height 50% of stencil thickness (Figure
thickness) (Figures 5-22 and 5-23). 5-25).

Figure 5-22 Height - Target Figure 5-25 Height - Defect

Ipc·7527-5-23

Figure 5-23 Height - Target

8
May 2012 IPC-7527

Appendix A
Guideline for Operator Troubleshooting in the
Solder Paste Screen Printing Process

This section can be used as a guideline, so that the

operator/user can identify error types and problem areas 3
with relevant solutions that can be used in connection with
correcting/optimizing of the solder paste deposits.

The following requirements should be met before the

screen printer process optimization can be started:
• The solder paste should meet the requirements of J-STD-
005 or equivalent.
• The stencil should be in good condition and have the cor-
rect tension.
• The squeegee blades/printing head should be in good con-
dition and mounted correctly .
• There should be sufficient board support for the PCB dur-
ing the printing process. IPC-7527 -A-2

• The stencil should be clean on both sides and in the aper- Figure A-2 Solder Paste Printing with Enclosed Head
tures (inspect with microscope). System
1. Printing Speed
A.1 Visual Check for Solder Paste Printing Optimization 2. Enclosed Head
• Does the solder paste roll when pushed by the squeegee 3. Paste Pressure

blades? A.2 Tolerances The nominal solder paste deposit height

• Are all apertures filled 100% with solder paste? equals the stencil thickness. However, this will not always
• Is the top side of the stencil free of solder paste residues? .be true because of other factors that influence the paste
deposit height such as solder mask, silk screen, labels,
• Is the solder paste released from the stencil apertures?
masking, and contaminates on the PCB surface (Figures
• Is there solder paste residues on the bottom side of the A-3 and AA).
stencil?
Note: There may be differences in the solder deposit
depending on the type of squeegee blade used; e.g., metal,
1 rubber or plastic. The customer and manufacturer should
agree on the acceptable conditions when other than metal
blades are used.

IPC-7527 -A-1

Figure A-1 Solder Paste Printing with Squeegee Blades

1. Squeegee Pressure
2. Blade Height
3. Squeegee Speed
4. Paste
5. Squeegee

Figure A-3 Plastic from Packaging

9
 -
IPC-7527 May 2012

be reduced by cleaning the bottom side of the stencil.

There are automatic cleaning systems that can be fixed into
the screen printer; these can be programmed so that the
stencil is cleaned with fixed intervals or as required. See
Section A.8, Issues with Stencil Cleaning.

Stencil

PCB
IPC-7527 -A-8

Figure A-8 Result of Silk Screen Interference

A.3 Varying Paste Deposit Shape This section is a

Figure A-4 Glass Fiber from PCB guideline to suggest possible causes of the listed problems.
It is not intended to be a corrective action checklist.
Figure A-5 illustrates the effect of solder mask. Due to sol-
der mask coverage on the tracks, the stencil will be lifted.
Problem A.3.1 Saddle Shape Paste D~posit
Result: Increased height of solder paste deposit and solder
(Figure A-9)
paste smearing.

Stencil

f PCB
IPC-7527 -A-5

Figure A-5 Effect of Solder Mask

Figure A-6 illustrates the effect when solder mask is com- Figure A-9 Saddle Shape Deposit
bined with silk screen. This gives a further increase in the
Possible causes:
paste deposit height and smearing.
• Printing pressure too high.
• Squeegee blades too soft.
Silk Screen
• Aperture area too large compared to squeegee type and
~tencil pressure.

f PCB
IPC-7527 -A-6
Problem A.3.2 Roof Top Shape Paste Deposit
(Figure A-10)
Figure A-6 Effect of Silk Screen

Figure A-7 illustrates the effect of solder paste smearing

resulting in solder balls after the reflow process.

Figure A-10 Roof Top Shape Paste Deposit

IPC-7527 -A-7

Figure A-7 Effect of Solder Paste Possible causes:

• Solder paste is not rolling during printing.
When the solder paste smears, a small amount of paste will
be left on the bottom side of the stencil (Figure A-8), which • Print speed too high.
may then be deposited on the next PCB. This problem can • Insufficient solder paste on stencil.

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May 2012 IPC-7527

Problem A.3.3 Spikes on Paste Deposit Problem A.4.2 Decreased Quantity Compared to Stencil
(Figure A-11) Thickness (Figure A-13)

Figure A-11 Spikes on Paste Deposit Figure A-13 Decreased Quantity

Possible causes: Possible causes:

o Incorrect solder paste viscosity. o Dried solder paste.

o Poor stencil quality. o Print speed too high.
o Dried out solder paste residues in stencil aperture. o Insufficient solder paste on stencil.
o Incorrect separation speed. o Dried solder paste in stencil aperture.
o Squeegee pressure too low.
A.S Solder Paste Residues Outsi,de Designated Print
A.4 Varying solder paste deposits quantities Area

Problem A.4.1 Increased Quantity Compared to Stencil Problem A.S.1 Slumping/Smearing

Thickness (Figure A-12) (Figure A-14)

Figure A-12 Increased Deposit Quantity Figure A-14 Slumping/Smearing

Possible causes: Possible causes:
o Incorrect down stop. o Temperature too high in printing area.
o Print pressure too low. o Incorrect solder paste viscosity.
o Incorrect parallelism. o Printing pressure too high.
o Print speed too high. o Excessive solder mask thickness.
o Incorrect snap off. o Excessive silk screen thickness.
o Excessive solder mask thickness. o Uneven surface on footprint, e.g., Hot Air Solder Level
o Excessive silk screen thickness. (HASL).
o Incorrect adjusted PCB thickness. o Incorrect snap off.
o Incorrect/insufficient board support. o Misalignment of solder paste printing.
o Insufficient stencil tension.
o Excessive paste pressure in closed head printing system.

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 IPC-7527 May 2012

Problem A.S.2 Solder Paste Residues Around Deposit

(Figure A-1S)

Figure A-15 Paste Residues

Possible causes:
• Incorrect stencil cleaning.
• Printing pressure too high .
• Excessive solder mask thickness.
• Excessive silk screen thickness.
• Double printing.

A.6 Printing Problems with Solder Past Residues on Top

Side of Stencil Figures A-16 through A-19 show how the
solder paste residues are distributed on the stencil surface.

Figure A-17 Offset

Possible causes:

• Parallelism .
• Board support.

Figure A-16 Long Smear

Figure A-18 Excess Residue Along Direction of Travel
Possible causes:
• Print pressure.
Possible causes:
• Print speed.
• Defect squeegee.
• Downstop.
• Defect stencil .
• Board support.

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May 2012 IPC-7527

Figure A-19 Residue Across Direction of Travel Figure A-21 Close Up of Solder Balls

Possible causes: Board Cleaning Handbook for more information on clean-

• Board support. ing misprinted boards.
• PCB.
It may not be possible to clean a PCB so it will be in new
• Stencil/print pressure. condition!

A.7 Issues with Cleaning Misprinted PCBs Misprinted A.8 Issues with Stencil Cleaning
PCBs that are rejected by a visual inspection system
(manual inspection, AOI, SPI) require cleaning. Manual A.8.1 Under-Screen Cleaning Reliable solder paste
cleaning will often result in solder paste being wiped over printing for fine and ultra-fine pitch components places
the PCB leaving solder balls (Figures A-20 and A-21) on high demands on the materials and equipment in the print-
the PCB surface and in via holes which can result in a short ing process. Stencils for fine pitch printing require a higher
circuit. frequency of cleaning to minimize smearing and ensure a
precise placement of the deposited solder paste. Automatic
stencil cleaning systems are normally integrated in the
screen printer that use solvents and lint free paper can be
programmed to remove solder paste residues from the bot-
tom side of the stencil.

A.8.2 Manual Cleaning of Stencil The disadvantages

and problems connected with manual cleaning are normally
greater than the advantages. Cleaning stencils is considered
as the most critical process. Manual cleaning of stencils is
normally conducted at the expense of the cleanliness of the
stencil aperture. There will often be solder balls in the
stencil apertures. Effective manual cleaning of the stencil
apertures using tools can be difficult and lead to damaging
the stencil.

Figure A-20 Solder Balls

Compressed air that is used to blow the apertures clean will
cause damage to fragile areas between fine pitch apertures.
It is often more difficult to remove solder paste and par- High pressure water cleaning may cause damage to sensi-
ticles than cleanable flux residues. The most effective tive areas. Compressed air also can spread solder paste to
method for removing solder paste is a combination of other surfaces or operators. Solder paste that has dried in
mechanical and chemical cleaning. the stencil aperture can harden and can be very difficult to
remove.
Because solder balls are held in place by the flux, an inte-
grated cleaning process will be required to first loosen the Dried solder paste is the most common cause for blocking
solder balls, followed by a rinsing process to remove them. of the stencil aperture affecting an insufficient paste
The reader is directed to IPC-7526, Stencil and Misprinted deposit. An automatic stencil washing system will normally

13
IPC-7527 May 2012

reduce these dangers and ensure a more consistent and pre- • Dissolve solder paste.
dictable result. However, this is under the condition that the • Complies with the solder paste manufacturer recommen-
cleaning process is monitored and maintained. dations.
When using certain types of stencil cleaning systems, it
may be necessary to pre-clean the stencil. This step can be The recommendations in the safety data sheet should
conducted while the stencil is in the screen printer or always be adhered to when cleaning!
immediately before the stencil is placed in the cleaning
system. Normally it is possible to remove excessive solder
A.9 Recommendations
paste by using a squeegee or spatula.

When the material for wiping the stencil is to be chosen, it

A.9.1 Test Prints A test print is as a visual check of the
is important that the material is designed for cleaning sten-
print. A non-production board can be used to avoid
cils and will not release fibers, lint, or glue residues onto
repeated cleaning of a production board.
the stencil. These pollutants can have a negative influence
on the subsequent screen printing process. According to the complexity and volume of the product,
If the cleaning process requires a pre cleaning with chemi- the determination will need to be made as to the value of
cals, a solvent should be chosen with the following proper- test printing.
ties:
After printing of the test board, the operator can visually
• Nontoxic. verify alignment and deposit appearance. If all parameters
• Dries without leaving residues or odors. are met, production can begin.

14
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 eli.minating mis-
understandings between manufacturers and purchasers, facilitating interchangeability and improve-
ment of products, and assisting the purchaser in selecting and obtaining with minimum delay the
proper product for his particular need. Existence of such Standards and Publications shall not in
any respect preclude any member or nonmember of IPC from manufacturing or selling products
not conforming to such Standards and Publication, nor shall the existence of such Standards and
Publications preclude their voluntary use by those other than IPC members, whether the standard
is to be used either domestically or internationally.

Recommended Standards and Publications are adopted by IPC without regard to whether their adop-
tion may involve patents on articles, materials, or processes. By such action, IPC does not assume
any liability to any patent owner, nor do they assume any obligation whatever to parties adopting
the Recommended Standard or Publication. Users are also wholly responsible for protecting them-
selves against all claims of liabilities for patent infringement.

IPC Position It is the position of lPC's Technical Activities Executive Committee that the use and implementation
Statement on of IPC publications is voluntary and is part of a relationship entered into by customer and supplier.
Specification When an IPC publication is updated and a new revision is published, it is the opinion of the TAEC
Revision Change that the use of the new revision as part of an existing relationship is not automatic unless required
by the contract. The TAEC recommends the use of the latest revision. Adopted October 6, 1998

Why is there Your purchase of this document contributes to the ongoing development of new and updated industry
a charge for standards and publications. Standards allow manufacturers, customers, and suppliers to understand
this document? one another better. Standards allow manufacturers greater efficiencies when they can set up their
processes to meet industry standards, allowing them to offer their customers lower costs.

IPC spends hundreds of thousands of dollars annually to support IPC's volunteers in the standards
and publications development process. There are many rounds of drafts sent out for review and
the committees spend hundreds of hours in review and development. IPC's staff attends and par-
ticipates in committee activities, typesets and circulates document drafts, and follows all necessary
procedures to qualify for ANSI approval.

IPC's membership dues have been kept low to allow as many companies as possible to participate.
Therefore, the standards and publications revenue is necessary to complement dues revenue. The
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publications, why not take advantage of this and the many other benefits of IPC membership as
well? For more information on membership in IPC, please visit www.ipc.org or call 847/597-2872.

Thank you for your continued support.

©Copyright 2012. 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
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Association Connecting Electronics Industries

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