ape IGIATION CONNECTING. ECTRONICS INDUSTRIES ® IPC-HDBK-830 Guidelines for Design, Selection and Application of Conformal Coatings Developed by the Conformal Coating Handbook Task Group (5-33c) of the Cleaning and Coating Committee (5-30) of IPC Users of this standard are encouraged to participate in the development of future revisions. Contact: Pc ‘3000 Lakeside Drive, Sute 9088 Bannockbum, Ilinois (60015-1219 “Toi 847 615.7100 Fax 847 615.7105‘This Page Intentionally Left Blank:oteber 2002 Acknowledgment 1PC-H08K-990 Any Standard involving a complex tochnology draws raatrial from a vast number of sources. While the principal members ‘of the Conformal Coating Handbook Task Group (5-33c) of the Cleaning and Coating Committee ($-30) are shown below, 11 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, Cleaning and Coating Committee Chair rank Cala, PAD. Chureh and Dwight Co, Ine. ‘Conformal Costing Handbook Task Group Co-Chairs Fonda Wa Raytheon Systems Co. and Sharon Goudie Dow Coming Corporation Technical Liatson of the IWC Board of Directors Nilesh 8. Naik Eagle Circuits inc. Conformal Coating Handbook Task Group David C. Adams, Rockwell Collins Paul Alexander, Shin-Fisu Silicone Norazmi Alias, UCB Asia Pacific Sdn Bhd Lloyd Anmogan, Rocks Collins Kim M. Atkins, Specalized Coating Services ‘Simin Bagheri CCelestca International James P. Barlett, PhD., PE., North Dakota State University Heather Benedict, Plexus Corp. Paul A. Bey, PE, Dow Coming Corporation Geruld Leslie Bogert, Bechtel Plant Machinery, Ine ‘Alan M. H. Brewin, National Physical Laboratory Ronald J. Brock. NSWC - Crane Frank Cala, Ph.D., Church & Dwight Co, Ine M, Lee Collier, Eleewa Polymers & ‘Chemicals America Graham Collins, Litton Systems ‘Canada Li. David J, Corbet, Defense Supply Center Columbos Amelia A. DeBaggis, Honeywell Inc. William E. Donges, Nordson ‘Corporation Davie A. Douthit, LoCan, LLC Jari Drlik, Lockitced Martin Space Systems ‘Thomas G. Farrell, Underwriters Labs Ine. Hugh Fay, Loctite Corporation Larry Fisher, Dexter Electronic Materials Jess V. Fosd, PhD., InSusf Mahendra S. Gandhi, Scott D. Glasspoule, Honeywell Inc Sharon K. Gousie, Dow Corning Corporation Michael R. Green, Lockheed Martin Space Systems Hue T: Green, Lockheed Martin Space Systems David Greenman, Concout Limited James A. Gryga, Jr, Rockwell Automation/Allen-Bradley Dennis Hardy, Benchmark Electronics Inc Frank R. Hart, Precision Valve & ‘Automation, Inc David Hill, 3M Company Bemard Icore, Nonthrop Grumman Corporation David H. Johnson, US. Ait Force ‘Buck Johnsov, 3M Company ‘Ted Jones, InSurt Joseph E. Kane, BAE Systems Controls Dave Kelly, Schenectady Europe Dr. William G. Kenyor, Global Centre for Process Change, Inc Phil Kinner, Concoat Limited. Gregg Klawson, General Dynamics - C4 Systems, Michael J. Knoellinger. LGA Michael J. Kroeger, Honeywell Inc. Viiay Kumar, Lockheed Martin Missile & Fire Control Xavier Lambert, Schneider Electric SA Roget H. Landolt, Cookson Blectronies James Lawrence, Homiseal Divisior/ Chase Corporation James Lieari, Avamteco ‘Curtis A. Lustig, Shipley Company, LLC. James F. Maguire, Intel Corporation Michaet Mallory, L-3 Communications Rene R. Martinez, TRW Electronics & Technology Division William Dean May, NSWC - Crane ‘Thomas MeConihay, General Electric Co, Randy McNutt, Northrop Grumman Renee J. Michalkiewiez, Trace Laboratories - Eastpo HOB-830) James H. Mofit, Mofitt Consulting Services, ‘Terry L. Munson, CSL Ine. Graham Naisbitt, Coneoat Limited Robert Newel, Northrop Grumman Corporation Debora L. Obitz, Trace Laboratories - Bast Gary Okafuji, Toro Co, Deepak K. Pai, CLD.+, General Dynamies-Advanced Information Douglas O. Pauls, Rockwell Collins ‘Marcio A. Ponce, Dow Coming, CCorporation-South America Jeffrey Quarberg, Sauer Danfoss Rick B. Ramitez, Specialized Coating Services William A, Rasmus, Jr, Northrop Grumman Space Systems Mia $. Riley, Trace Laboratories - Fast Barry Ritchic, Dow Corning Corporation Joh H. Robifing, Delphi Delco lectronies Systems Joseph C, Salvini, Underviiters Laboratories Inc Darrell C. Schneider, Viasystems Inc. ‘Mark Schumacher, Crown Equipment Corporation Stanley §. Seetig, Dynaloy, ne Chi ‘Awiom Semkow, Rockwell (onsAlen-Bradley Paul Shafer, Sealant Equipment & “Engineering Joseph L. Sherfick. NSWC - Crane Lowell Sherman, Defense Supply Center Columbus Ray Simon, L-3 Communications John C. Sines, BAE Systems Contots Garth Speny, L-3 Communications vik Tegehall, IVF Ronald E. Thompson, NSWC - Crane James R Thompson, DaimlerChrysler Huntsville Electronics Dung Q. Tiet, Lockheed Martin Space Systems Brian J. Toleno, Pb.D., Henkel Loctite Corp. Octebee 2002 James D. Tower, Precision Valve & Automation, Ine Laura J. Turbini, Ph.D, University of Toronto Paula VanDenberg, Plasma (Crystal B. Vanderpan, Undenvriters Laboratories Ine David A. Vaughan, Donald Waddell, Rockwell Avtomation Lamy Waksman, Shin-Eis Silicone John Waryold, Humiseal Division/ Chase Corporation Philip W. Witmer, Delphi Deleo Electonies Systems Henry Wong, TRW Electronics & ‘Technology Division Greg Wood, EMPF/ACI Fonda B. Wa, Raytheon Electronic Systems Lamar Young. Specialty Coating Systems Ine, Don Youngblood, Honeywell Inc.October 2002 IPO HOBK-890 Table of Contents 12 pum crear “ 2 QUALIFICATION AND SPECIFICATION 53. Component Material TYPE swssnninnnn 10 ST ae eee o2 Shunt compe m COATINGS 353.3. Leaded SMT Components i 2.1 ASTM Intemational Standards 3534 Lends SMT Component on u 23° IPC Standards 4541 igh Voltage GHVyHigh Coment (HC) sooo 12 24 Joint Industry Standard 4 $4.2 RF and Microwave 2 28 Military Standards. wo 54.3 High Speed Digital ......... se seen 13 26 Underwriters Laboratories 5 54.4 — Controlled Impedance ..... creversenrnns 13, on temas Sena ot 25 Saal erm Mews iOEN |S Rannnepang + cwommya wenn manner RAV AATERALS CHARACTERS and Other Combinations) ” wane B TAA PlAStICIZEE sor nseneen soenensersesn seve AT 47 Ovber Types of Conform! Coatings 8 1.6 ‘Mold Releaee Agent nv 43.2 Pesfiuoroether . 8 7.1.8 Temporary Masking n\poHoK 830 73.4 Components 73.2 Surface Finishes 733 Cleanliness 74 Imerlayer Adhesion 7.5. Methods of Assessing Companbility 8 PROCESSING 81 Cleanliness 811 Cleaning 8.1.2 Cleanliness Assessment Techniques. 8.13 Processing Havironment 2 Substrate Preparation 82.1 Priming 82.2 Plasme Treatment 8.2.3 Mechanical Etching 83 Masking 8.3.1 Types of Masks 8.3.2 Manual vs, Automated Masking 833 Desmasking 84 Recommended Coverage 8.4.1 Recommended Thickness 842 Uneven Coating Thickness 8.43 Eidge and Paint Coverage 844 Application Method 85 Shadowing/Bndging . 85.1 Accessibility Consideration... 85.2 Curing Consideration 853° Shadowing Techniques 854 Bridging Techniques 86 Viscosity Adjustment 86.1 Methods of Viscosity Adjustment 8.62 Objectives of Viscosity Adjustment. 87 Application Methods 87.1 Manval Spraying 87:2 Automated Spraying 873 Dipping 874 Brshing 815 Selective Coating 8.76 Vacuum Deposition (XY) 88 Mult-Layering 89 Cure Mechanisms 89.1 Room Temperature Cure 892 Heat Cure .. 8.9.3 Heat Accelerable 394 UV Cwe 895 Moisture Cure 19 19 2» 20 » a4 a a 2 2 B 24 a 25 2 25 26 n 2 2 n 28 28 28 sos 28 228 28 By 8 30 3 3 32 33 3 3B 8 3 33 33 896 8.10 ator 8.10.2 8.103 B04, 8.105 8.106 Bal 812 821 8122, 8.123 83 83 8132 8133, e134 135 etc 2008 Catalytic Core . Cure Process Considerations Cure By-Products Exotherm Shrinkage Premature Surface Cu Exceeding Cure Recommendations Layering sen Application Process Monitoring Inspection Guidelines Magnification .. UV/Light Source ‘Workmanship Environmental, Health and Safety Processing Considerations Viscosity Adjustment Spraying Dipping and Brushing Vacuum Deposition Curing Solvent Eateupaent 9 FILM PROPERTIES ou 92 920 92. 923 924 93 93.1 932 933 934 935 94 98 96 97 98 99 9.10 9.0 9.102 9.11 9.12 9.13 Appearance/Color Dielectric Properties Dielecwie Withstanding Voltage (DWV) Insulation Resistance Q-Resonance Dielectric Constant and Dissipation Factor ‘Thermal Froperties ‘Thermal Stability ‘Thermal Shock Glass Transition Temperature (T,) Coefficient of Thermal Expansion (CTE) .. ‘Temperature Gradient Flammability Flexibility ‘Abrasion Resistance Coating Creep Hydrolytie Stability Permesbility Chemical Compatibitiy and Chemical Resistance Fuel Resistance Biological Compatibility Gas Resistance Corrosion Resistance Fungus Resistance 3 3 a4 4 aM 34 3 35 as 35 35 35 36 36 36 36 36 36 wo 37 2 7 37 a7 2 sone BT on 38 38 38 38 38 8 38 39 9 30 39 40 40, 40 4010 REWORK AND REPAIR oA 9 Degradation av 10.1.2 Mechanical Abrasion 4a ‘Test Parameters 48 10.13 Media Blasting 2 Examples of Tests id ris eats ge cotemeion 10.1.7 Plasma 42 APPENDIX Flow Cup Viscosity Measurement... 52 en S Mreen® nae ty ‘ o donrnann Somes semeceamsar 8 i en “eee, Sieve acesca 11.12 Humidity “ Breach in A Conformal Coating: 70 M12 Ausemarive ~. “5 Figure 5-1 Assembly Drawing with Masking 11.3.1 Aircraft on the Ground .... 45, Figure 8-1 Options of Cleaning Systems According 3.2 Equipment Outside The Pressure: oF ype % #133 Eup ie The reste ove whores eapouls onlay 28 M14 Space Environment 45 Figure 6-6 Masking Boots 26 11S Medical Environment ... 46 Figure 8-7. Spray Bootn with a Manuat Spray Gun 28 11.6 Geothermal Environment 6 Figure 8-8 Nonatomized Curtain Coater %0 11.7 Nuclear Biological Chemical Warfare Figure 8-9 Svar Appcator * 12 LONG TERM RELIABILITY AND TESTING 46 {125 ml] for workaround Sensitive Keep-out 0 124 Failure Mechanism 46 Figure 8:11 Conveyerized Appicator 3 12.1.1 Wear/Abrasion ~ 45 gure 812 Stand-Alone Batch Applicator atIPOHDBK-250 steber 2002 Figuce 6-17 Loss af Conformal Coating Achasion Figure 8-18 Voids in Conformal Coating Table 114 Table Bt Table 0-1 Table D+ Table D2 Tables Temperature Classifications of Automotive Irusty CConwersion Chat for Flow Cup Viscosity MeRSUEMERS nnn ‘Troubleshooting Guide ‘Moleauiariderpretation Measurement Methods 88 Table £4 Table F Table G1 Table H41 Table H2 Table H Table Hs Table HS Table tical Relative Humidity (CRH) For Sovera Trorgane Compouras Time of Wotnoss Doposton Rates for Particles ‘Amblant Air Poltion Levols Diy Deposition Measurements Highest Lavals of Wer Doposition Concentration of Selected Gasoous Air CConsttuents in US {Indoor Pelton Levels in the Far East ‘Aci Fain Formula for Testing Purposes Rae eeeeeeeteber 2000 IPo-H8K-290 Guidelines for Design, Selection and Application of Conformal Coatings 4 SCoPE + To inhibit arcing, corona and St. Elmo's Fie. 44 Introduction Conformal costings are used in com Junction with printed circuit assemblies (PCAs). The designer and the users of conformal coatings for elecron- ies applications should be aware of the properties of vari= ‘us types of conformal coatings and theit interactions wit CAS to protect the PCAS in the end-use enviroament for the desigo-life of the PCA (or beyond). This document has ‘been written to assist the designers and users of conformal coatings in understanding the characteristics of various coating types. as well a5 the factors that can modify those properties when the coatings are applied. Understanding avd accounting for these materials can ensure the reliabil- lity and function of electronics 4.2 Purpose The purpose of this handbook is to assist the individuals who either make choices regarding confor ral coating or who work in coating operations. This hand: book represents the compiled knowledge and experience of the IPC Conformal Costing Handbook Task Group. It is not enough to understand the properties ofthe various con- formal coatings. the user needs to understand what i t0 be achieved by applying the conformal coating and bow to verify thatthe desired resus have been realize. 1.3 Scope Conformal coating, for the purpose of this ‘document, is defined as a thin, transparent, polymeric coat- ing tha is applied to the surfaces of PCAS to provide pro tection from the end-use environment. Typical coating thickness ranges from 12.5 yan [0.49 mil] to 200 pm (7.9 mil Processing characteristics and curing mechanisms are dependent on the coating chemistries used. The desired performance characteristics of « conformal costing depend ‘on the applicmtion and should be considered when selecting coating materials and coating processes. Users are urged 10 ‘congult the suppliers for detailed technical data This guide enables a user to select 2 conformal coating bsed on industry experience and pertinent considerations. It is the responsibility of the user to determine the suit ability, via appropriate testing, of the selected coating and application method for a particular end use application Acontormal coating may have several functions depending (on the type of application. The most common are: To inhibit current leakage and shor circuit due to humid- ity and contamination from service environment. + To inhibit corrosion, * To improve fatigue life of solder joins to leadless pack- + To provide mechanical support for small pur that cannot be secured by mechanical means, © prevent damages due to mechanical shock and vibration. 1.4 Terms and Definitions Acetone A volatile fragrant flammable liguid ketone ‘C4EI,O used chiefly as a solvent and in organie synthesis. Adhesion promotion - The chemicsl provess of preparing ‘8 surface to enhance is ability to be bonded to another sur- face, ie.. 8 layer of conformal coating Adhesion feilure - The rupture of an adhesive bond such that the reparation appears to be at the adhesive-adherent interface Anisotropic ~ Having properties that vary depending on the irection of measurement Anthropogenic — Relating to or resulting frum the influ- fence of human beings on nature, ARUR-~ Abbreviation standing for acrylic resin and ure- thane resin combination chemistries Bridging ~ Fillet or meniscus formation of coating around the leads of a component caused by capillary action, Creep ~ Strain, deformation, or movement of coatings caused by time andlor temperature Cross-linking ~ The formation of chemical bonds between molecules ina thermosetting resin during 1 polymerization reaction CTE ~ (Coeficient of Thermal Expansion) Linear dimen- sional change with respect to an original dimension dae to a change in temperature (Cure ~ A change i the physical properties of a polymer by ‘a chemical reaction Degradation — Decrease in quality or integrity. Loss of desired physical, chemical or electrical properties. Delamination - A separation between a conformal coating layer and che surface itis adhering to De-masking ~'The process of removing or disengaging a ‘maskant film, tape, boot or plug, De-wetting ~The propensity of the coating material to refuse to wet the surface evenly Dielectric constant The ratio of the capacitance of « con figuration of electrodes with a specific material as the dielectric between them fo the capacitance of the samePoHOBK.- 859 electrode configuration with a vacuum or air as the diel te. Dielectric strength Tae maximum voltage that « diclee- tric can withstand under specified conditions without resulting in a voltage breakdown, usually expressed as volts per unit din Dilatometrs ~ The process of measuring expansion, Dilution ~ Reduction in viscosity. Can be achieved by mix- ing & nonzeacting, soluble agent into the material. Dissipation factor ~ A value that represents the tendency of insolating or dielectric materials to absorb some of the ‘energy i0 an alternating-current signal Diurnal ~ Occurring every day or having a daily cycle Durometer & measure of the degree of hardness or the resistance to be deformed or fractured, EMC ~ Abbreviation for Electromagnetic Compatibility -EMI-~ (Electromagnetic Interference) Unwanted radiated electromagnetic energy ¢hat couples into electrical conduc tors Emulsion sable mixture of to oF more immiscible liquids held in suspension by small percentages of emulsi- fiers. EOS ~ Electrical Overstress) Internal result of an ‘unwanted application of electrical energy that results in damaged components ESD ~ (Electrostatic Discharge) Rapid discharge of electi- ‘al energy that was created from electrostatic sources. iller~ A substance that is added to a material to modity its solidity, bulk, or other properties, Fish eyes~ A surface defect (0 the conformal coating that resembles the eyes of a fish Gel time ~Timne taken for a liquid polymer to begin to exhibit pseudo-elastic properties or to be “inmobilized.” Glass transition temperature T,—The teinperature at ‘which an amorphous polymer, or the amorphous regions in 4 partally-crystalline polymer, changes from being in & hard and rlatively-britle condition to being ina viscous or rubbery condition. Hardness ~ A property that indicates the ability of a mate rial 16 resist penetration of a specific type of indentor when forced into the material under specified conditions. Inden- tation hercness is inversely related to the penetration and is ‘dependent on the elastic modulus and viscoelastic behavior of the material Cotober 2002 Heptane ~ Any of several isomettie alksnes C5Hyo: espe- cially the iguid normal isomer occurring in petroleum and used especially ss a solvent and in determining octane umes. Hybrid A costing system with more than one principle resin chemistry. Hydrolyie stabiity ~ The degree of resistance of a polymer {o permanent property changes from hydrolytic effets Hydrophobic-oleophobic coatings \ coating having an aversion to water and oils. Impedance ~The resistance tothe fow of caren, repre- sented by sn elecuical network of combined resistance, Capacitance and inductance reason na conductor as seen ty an AC source or varying time voltage Inhibition — Toe inability for the coating materials to ‘obtain the desired properties atthe manufctures” specified time and temperature. Insulation resistance ~ measure of the capability af a material to electrically insulate adjacent conductors from cach other. ‘Masking — The process of applying & temporary film, tape, boot or plug thet prevents the area covered from being coated. -Mealing ~ A condition in form of discrete spots or patches that reveals a separation at the interface between a confor- ‘mal costing and the surface to be costed. Monomer~ A chemical compound that can pndergo poly merization. [MSDS ~ (Material Safety Data Sheet) Provided by the manufacturer contains relevant properties of the material with regards to safety concerns ‘Multi-layering - The process of applying more than one layer of coating to make up the desired thickness NBC contamination ~ Abbreviation for Nuclear, Biologi- cal, and Chemical agents of contamination. Oligomer~ A polymer or polymer intermediate containing latvely few structural units. Orange peeling ~ A surface defect tothe conformal costing ‘that resembles the surface or skin of an orange. ‘Outgassing - The gaseous emission fram & processed coat ing layer when it is exposed (o heat or reduced air pressure, ‘or both. Permeability ~ The ability of molecules of one material 10 flow through the matrix of another material, The degree of | ‘permeability is dependant on the molecular structure of both materials.October 2002 IPO-HDBK:-890 Photoresist~ & material that is sensitive t portions of the Tight spectrum and that. when properly exposed. can mask onions of a base metal from exposure with a high degree of integrity Polymer ~ A compound of high molecular weight that is Aenved from cither the joining together or many small Similar or dissimilar molecules or by the condensation of :many sal molecules by the elimination of water, aleobol. fr some ober solvent, Polymerization ~The formation of a matrix of cross-linked Jong chain molecular structure from short chain monomer molecules. Polysiloxane ~ & polymer whose msin chemical linkage is repeating units of SiO atoms bonded together Pot life ~ The length of time a material, substance, or prod- tact can be left in an open package or éispenser, while i meets all applicable specification requirements aad renusins stable for its imtended use Priming — A surface treatment utilizing a sutactant t pro- smote adhesion of conformal coating Repair ~ The act of restoring the functional capability of a defective article in a manner thet precludes compliance of the asticle with applicable drawings or specifications. Rework The act of reprocessing soncomplying articles, through the use of original or aliemate equivalent process: ‘ng, in a manner that assures compliance of the article with applicable drawings or specifications. RTV ~ (Room Temperature Vuleanizing) The development ‘of desired dry film properties at room temperature Shedowing, coating — 1. A situation that can occur during spray coating of a PCA when components may bide or “shadow” the area undemesth them, relative to the spray direction, pre venting the surfaces beneath the companent from being contd. Also used in reference to curing of coatings by UV rays ‘when components may hide or “shadow” the area underneath, not allowing it to be exposed and cured by the UV rays. Shelf life-"The length of time a material, substance, or product can be stozed, under specific environmental condi- tions, while it meets all applicable specification require- ‘ments and remains suitable for its intended use, ‘Shrinkage ~ Reduction in volume as a wet, freshly applied layer dries/cures into a coating film with desires properties, Solids content ~ The propestion of ‘resin’ or polymer mate- Fial tothe solvent carier. Spectroscopy ~The production and investigation of spee= tua, or the process of using a spectroscope or spectrometer. Stripping ~ The process of eroding a material by chemical reaction, Stripping agents con be used to remove certain types of conformal coating for the purpose of rework or sepals ‘Surface tension ~The natural, inwerd, molecular attraction {otce that inhibits the sprend of a liquid atts interface with solid materia ‘Thermoplastic — A plastic that can be repeatedly softened and reshaped, without any significant change in inherent properties, by exposure to heat and hardened by cooling Thermoset~ A plastic that undergoes a chemical reaction when exposed to elevated temperatures that leads to it hav ing a relatively infusible or crosslinked state that cannot be softened or reshaped by subsequent heating Toluene = A liquid aromatic hydrocarbon C;Hy that resembles benzene but is less volatile, flammable, and toxic snd js used as a solvent in organic symhesis. Also known, us Methy! Benzene. Transfer efficiency ~ The ratio of volume dispensed to the desired volume of coating materials on the PCA. Vessication = The formation of blisters at the interface berween a semipermeable polymer film costing and ‘another material caused by an osmotic effect from the inter- action of water soluble matter with moisture Viscosity ~ The property of a polymer to frictionally resist internal flow that is direely proportional to the applied force ‘VOC - (Volatile organic compound) Regulated compounds containing carbon that have measurable vapor pressures. Wetting - The formation of a relatively uniform, and adler: ent film of materials on a surface. Withstand voltage ~ See dielectric strength Young’s modulus ~ Modulus of elasticity. A measure of the Aexibilty of a material. The lower the modulus, the more flexible the material 2 QUALIFICATION AND SPECIFICATION REFERENCES OF CONFORMAL COATINGS. ‘The following documents are listed for reference only: This handbook does not attempt to explain these documents 2x4 ASTM International Standards ASTIA 570. Standard Test Method for Water Absorption of Plastics {Rt vovnon ois ofthe ASTM tmatonal tnd ae not moked her iting her, Pease ste o the ren oven,1Po-HOBK 880) ASTM D635__Rate of Buming and/or Extent and Time of Burning of Self-Supporting Plastis in A Horizontal Posi [ASTM 03959. Standard Test Methods for Measuring Adhe- sion by Tape Test ASTM 03933. Standard Test Method for Water Vapor ‘Transiaission of Pressure Sensitive ASTM E96. Standard Test Methods for Water Vapor Trans mission of Materials ASTM E595. Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgas sing in a Vacoum Environment [ASTM F1249 Standard Test Method for Water Vspor ‘Transmission Rate Through Plastic Film and Sheeting Using Modulated Infrared Sensor 2.2. Federal Aviation Regulations (FAR) FAR §25.853 and Appendix F FAA material flammak tests 2.3 IPC Standards IPCHOBK-001 Handbook & Guide to Supplement J-STD- oot WC-S0-60 Post Solder Solvent Cleaning Handbook WC.$A461 Post Solder Semiaqueous Cleaning Handbook IPC-AG-62 Post Solder Aqueous Cleaning Handbook WC-CHES Guidelines for Cleaning of Printed Boards and Ascomblies| WC-0-278 Design guidelines for Reliable Surface Mount ‘Technology Printed Board Assemblies IPC-A-SIO Accepicbility of Electronics Assemblies W-TW-650 Test Method Manual’ 23.25 Detection and Measurement of lonizable Surface Contaminants by Resistivity of Solvent Extract 2.3.25.1 Ionic Cleanliness Testing of Bare PWBS 23.27 Cleanliness Test » Residual Rosin 23.27. Rosin Flux Residue Analysis-HPLC Metiod Demeeag Geteber 2 23.28 onic Analysis of Cireuit Boards, lon Chroma: tography Method 23.38 Surface Organic Contaminant Detection Test 2339 Surface Organic Contaminant Identification Test (Inirared Analytical Method) 257.1 Dielectric Withstending Voltage - Polymeric Conformal Costing 26.1.1 Fungus Resistance - Conformat Coating 263 Moisture and Insulation Resi Boards nce, Printed 263.1 Moisture and Insulation Resistance-Polymeric ‘Sokier Masks and Conformal Coatings 2.634 Moisture and Insulation Resistance - Conformal Coating 261.1 Thermal Shock - Conformal Coating 26.111 Hydrolytie Stability - Conformal Coating 1Pc-CC-690 Qualification and Performance of Electrical Insulating Compound for Printed Board Assemblies 1PC-SM-840 Qualification and Performance of Permanent Solder Mask Wc-2221 Generic Standard on Printed Board Design 1Pc-6012 Qualification end Performance Specification for Rigid Printed Boards IPO-TTIt Rework of Flectronic Assemblies Wo-7721 Repair and Molliication of Printed Boards and Electronic Assemblies 24 Joint industry Standard’ 4STD-001 Requirements for Soldered Electrical and Elec: onic Assemblies 4-STD.004 Requirements for Soldering Fluxes 2.5 mlitary Standards? MIL-STD-202 Method 106 Test Methods For Electronic ‘And Electrical Component Parts MiL-c-28803. Circuit Ca ‘And Rigid Flex" Assemblies, Rigid, Flexible MIL-+46058 Insulating Compound, Electrical (For Coating Printed Circuit Assemblies)” Curt and ris YPC Tet Moho svi trou PC-TA ESD autem enon th IPC Mab se ump orgtmiereethods en) 5 Stzuarazaton Dacureris Or sk Bulg 4D, 700 Robbins Avenue, Pasi, PA SH S098 1 boevners cance win repacement 885 1 Deeaman xen rach! new ess. MD: Sap ape md,(Cetober 2002 Bo HDAK E30 2.6 Underwrttors Laboratories” ULO6 Tests for Flammability of Plastic Materials for Parts in Devices and Appliaaces ULT46C Polymeric Materials - Use in Electrical Equip- sent Evaluations UL746E Polymeric Materials - Industrial Laminates, Fila ment Wound Tubing, Vulcanized Fiber, and Materials Used in Printed Circuit Boaeds 2.7 Intemational Standards 2.74 British Standards (DSTAN, UK Defence Standard- lization) DEF.STD-89/47 Issue 4 Conformal Coatings For Panels, Printed Circuits And Panels, Electronic Circuit (QPL main- tained, alternative to MIL-L-46058C, Independent testing and cerification) EN 61096 (BS) Specification For Coatings For Loaded Printed Wire Boards [Conformal Coatings] 2.72 IEC Standards EC 60664 Insulation coordination for equipment within low-voltage systems IEC 61006 Specification for coatings for loaded printed ‘wire boards (conformal coatings) 2.8 Original Equipment Manufacturing (OEM) Specifics ion OEM specifications are engineering documents in the form of either an engineering drawing. showing areas of electronic hardware to be conformally costed, oF a text document which contains requirements and processes that ‘conformal coatings shall meet in the “applied” condition Specifically directed to the assembly and fabrication of hardware. these requirements are usually based on electri- cal output, physical performance. life cycle reliability of = PCA in an end-use environment. Since each PCA is unique in its functional and electrical responses, conformal coat- ings have to be selected for usage based on their ability to perform as a protective coating without deleterious effects upon electronic circuit elements over the PCAs perfor- smance and life cyele requirements while also meeting the ‘material property and compatibility of the conformal coat~ ing to the PCA substrate surfaces. ‘9 ENVIRONMENTAL, HEALTH AND SAFETY CONSIDER. ATIONS Recent environmental regulations such as the Montreal Protocol and Clean Air Act have had « significant impact ‘on both coating materials and application methods, particu- larly with regard to contol of volatile organic compounds (VOCS) and ozone depleting chlorofluorocarbon (CFC) ‘compounds. VOCs can be defined as any solvent that has 2 vapor pressure greater than 0,001 mPa {1.43 x 10°" PSI] land are the primary concem, as they react in the atmo- sphere t form ground level ozone (or smog). CFCS have ‘been found to deplete earth's protective ozone layer in the ‘upper stratosphere. Both VOCs and CFCs have been exten- sively used as solvent cariers. Manufacturers and suppliers ‘of conformal coating materials have responded by develop ing nonsolvent based coatings and environmentally accept able methods of application, curing and removal. Specifié environmental, health and safety concerns are addressed in various subsections. Refer to 9.13 for process Ing considerations and 11.4 for rework and repair issues 8.4 Emissions. Emission is defined as any substance dis- ‘charged into the atmosphere such as solvents in conformal ‘coating systems, Huxes, cleaners ete. Certain types of sol- vent such as isopropyl alcohol, xylene. ete. are volatile ‘organic compound (VOC) liquids containing carbon and hydrogen that emit VOC vapors. 3.2 Disposal of Hazardous Waste Hazardous waste is defied as any material that is classified as such and stated fon the manufacturers material safety data sheet (MSDS), Disposal of hazardous waste should be in total compliance the local, state and federal regulations 3.8 Governmontal Regulations The user should be aware of any limitation with the use of any product or ‘material used in their process. Frequently asked questions about hazardous waste are answered on the Environmental Protection Agency's website at hip/www.EPA.gov 4: TYPES OF CONFORMAL COATINGS Conformal coatings are polymeric materials used to provect electronic assemblies from « wide variety of life cycle eon taminaots. Conformal costings provide @ high degree of insulative protection and are usually resistant to many types of solvents and harsh environments encountered in the product life eycle. The coating materials also act to immobilize various types of particulates on the surface oF the PCA and function as protective barriers to the various devices on the board. They are resistant to moisture end humidity, which may reduce the potential of leakage curtents, “cross talk.” elec trochemical migration, dendrite growth and arcing. These issues are becoming more critical with the reduction in component size. pitch. circuitry spacing, laminate thick- 1nd voltage plus the rise in speed (frequency) of sig- 1 Underwtrs Laborio Te, 205 Wak Whtran Rad, Med Long land, AY 1746eG HOB 320 2ctooer 2002 Conformal coatings can be broken down into two Tigaid families, organic and silicone. The primary twaditiona cas- sifcaions are acrylic (AR), epoxy (ER), silicone (SR), re- thane (UR) and poly-parasylslene (XY), Figure 41 ills- trates thete classifications based on basic resin chemistry {ype, which is futher differentiated into subtypes based on cure mechanism. Appendix A provides a comparison of various product types as e quick guide for the selection of conformal coating ‘These systems are primarily made up of monomers, oligo- ters, de-foaming agents fillers, and wetting agents. There are also water-based emulsions. Various combinations of each are added to the formulations to adjust the cured and ‘uncured properties. Solvents may be added co adjust appli- cation viscosity. Prior tn the star of curing process, sol- ‘ents evaporate ("lash off”), leaving a resin matrix 10 initiate the cure. The VOC emissions associated with this ~ type of cure created a need for more environmentally “One- and two-part formations available, saat chamseat ‘Chemistry ‘Type PERE — soos mpeg — Pos tyre —eeniene ate Se i pein ara aaa Figure 41 Conformal Coating Family Treeseteber 2002 acceptable production chemistries and cure mechanisms, Presently, solvent and water-based conformal coating chemistries exist which are exempt from environmental regulations. See Sections 3, 8.13 and 10.4 on envisonmens tal, health and safety considerations. All resins except for erylic esins are cured by an imevers- ible polymerization reaction with varying degree of eross- linking (Uhermoset polymers). The cross-linking of the epoxy, urethane, and silicone polymers provide very good chemical resistance but also make it dificult to remove the 5s When performing repair work. No polymerization reaction is taking place when applying acrylic coatings ‘Therefore, itis misleading to say that acrylic coatings are cured. Tuey are formed by drying a solution of already ‘formed acrylic polymer chains dissolved in # solvent (ther- moplastic polymer). Hence, acrylic coatings are easily dis: solved in many organic solvents providing selective chemi cal resistance but fecilitating repair work. Consequently, acrylic materials are either solvent or water-based. All, except the poly-para-rylelenes (parylenes), have tradi- tionally been solvent-based until a decade ago when envi ronmental issues dictated @ need to change. Now, many imuterials are solvent-free chemistries (100% solids). The ability to ultra violet (UV) cute is commonplace. This new generation of conformal costing materials has also given birth to hybrid coatings that contain two or more systems to achieve superior properties, 1e., ARUR. See 4.6 for examples of two-part systems Raw materials as well as film properties need to be consid cred while selecting conformal coating materials. Due to variations in conformal coating chemistry. it is recom- mended (© consult the coating manufucturers or (echnical Gata shoet for more information, 4-4 AR Acrylic Acrylics are easy to apply and the dried film can be removed using the solvent method in Section 11 Rework and Repair. Spot removal of the coating to repair solder joint or replace a component ean be easily complished by localized solvent spplication, Though not encouraged, they may be soldered through if preferred, Acrylics dry rapidly, reaching optimum physical properties Jn minutes, are fungus resistant and provide long. pot lie. Furthermore, acrylics give off litle or no heat during cure, eliminating damage to beat-sensitive components, do not shrink during cure and have good humidity resistance. Acrylics tend to soften more readily at elevated tempera- ‘tures than other polyiners 4.2 ER - Epoxy Epory systems are usually available as ‘wo part compounds. They provide reasonable humidity resistance and good abrasive and chemical resistance. They are virtually impossible to remove chemically for rework since any stripper that will remove the coating may vigor- PC-HOBK 890 ‘ously attack epoxy-potted components as well as the epoxy-glass board itself. The only effective way to repair a board or replace a component is to burn through the epoxy coating with a knife oF soldering iron Single part epoxy resin coatings with temperature-sctivated hardeners are also available. These coatings require curing at temperatures higher than 66°C [150°F}, Single part UV curable coatings are available which eliminates curing at these elevated temperatures ‘When most epoxies are applied, a “butler” material should be used around fragile components to prevent their damage from film shrinkage during polymerization, Curing at low temperature, if possible, is encouraged to reduce shrinkage Curing of epony systems takes place up to three hours at an elevated temperature or up to seven days at room tempera ture. Short pot life creates 2 limitation on their effective 4.3 SR. Silicone Silicone coatings are extremely useful materials when components will endure extreme tempers ture eyeling environments such as automotive applications ‘The useful operating range of these materials is -55°C to 4200°C [-67°F w #392°F). They provide high humidity resistance along with good thermal endurance, raking them desirable for PCAs with heat dissipating components such as power resistors, For high impedance circuitry, sili- cones offer a very low dissipation factor. They are very forgiving materials in production because they coat over and adhere to most surfaces found on & PCB and offer good resistance to polar solvents, Cross contamination factors stemming from the use of silicones and the effects on other production processes is no longer a major concern with the solvent-free, nonvolatile chemistries that are easily handled ‘with proper housekeeping practices. Secondary cure for the UY curable versions is accomplished with a very effective ambient moisture mechanism. It should also be considered that high temperature protection may generally demand that the silicone coating be cured at or near to the maxi- mam temperature itis designed to withstand. Silicone coat- ings can be applied at large thickness 30.3 mm [50.01 in}, thus immobilizing wiring Gumpers etc.) on PCAs. 44 UR ~ Polyurethane Polyurethane coatings ate avail- able as either single or two-component formulations. Both provide good humidity and chemical resistance, plus higher sustained dielectric properties, ‘Their chemical resistance, however, can be & major draw- back since rework can become difficult and costly. To repair or replace component. refer to Section It on Rework and Repai Barly polyurethane compounds exhibited instability or reversion of the cured film to a liquid under high humidity and temperature conditions. Newer formulations, however, clitainate this phenomenon.IPOMDBK 890 While polyurethane’s ean be soldered through, this usually results in a brownish residue, which allects the aesthetics of the coating Single component polyurethane's, while easy £0 apply. sometimes require 3-30 days at room temperature for opt mum cure, Two component formulations, on the other hand, reach optimum cure properties a elevated tempera tuses within 1-3 hours, but with pot fives of 30 minutes to 3 hours. 4.5 XY ~ Poly-para-xylolene (parylene) These are the vacuum deposited pol)-pare-xylelene (parylene} coatings. Both material characteristics and application technique yield XY conformal costings that are uniquely different from the liquid applied coating types. The obtained coating film yields consistent thickness with true conformance 10 PCA contour and is pintole and bubble free. The XY film is also characterized by properties such as good dieleetic, Tow thermal expansion, good abrasion resistance and out- standing chemical resistance, among others. This makes XY coating a good choice for protecting circuits against effects From harsh environments, notably high humidity with condensation, intermittent immersion, salt fog, atmo~ spheric pollutants and exposure w aggressive solvents, ‘This type of coating is frequently used in FDA approved devices for medical and biomedical upplications. Poly. para-xyletene (parylene) coating can be removed by abra- sion, conventional surface mount techniques, excimer laser, beat softening, plasma etching and several other methods. Poly-pare-aylelene (parylene) coatings are very effective in high voluge applications. owing to the capsbility of coat ing sharp edges. A fluorinated version is also available that can maintain its properties at temperatures in excess of 400°C [72°F], has Increased UV stability, and has @ lower dielectric constant. Poly-para-rylelenes will not readily adhere to ionic res ues and they exhibit poor zepairability in comparison to the other coating types. The repaired coating often does not return the protection to a monolithic film. Poly-para- xylelenes require masking of components that are not to be coated, The masking musi be 100% effective for the ‘vacuum coating process. 4.6 Two-Part Systems (Acrylic/Polyurethane and Other Combinations) 4.6.4 UV and Solvent Cure One method of curing uses ultra-violet light. This permits curing of the material in seconds rather than in minutes ot hours, They have been specifically developed for use on fat bare substrates and are of particular benefit for fiber optic filament costing as coring can be effected at speeds of up vo 122 mii [400 fumin} ‘Their use on PCAs, however, is somewhat limited because of the shadowing effect produced by components. AS 2 8 (cto 2002 ‘result of this, a catalyst is often required to ensure a chemi ‘cal reaction to cure the compound in shaded areas. How: ever, this produces the associated drawbacks of two-part systems such as short pot life and material blending for correct application. One-part materials have been devel ‘oped, although they tend to be of an epoay or polyurethane ase. ‘Two-part products are dificult to repair as subsequently reapplied coatings do not etch into the existing material surface. but produce disezete lamination, The reliability of ‘ype of coating is suspected at temperatures below °C [-67°F] or above +130°C [4266°F] because the coat- ing may become britle and less flexible, 47 Other Types of Conformal Coatings 4.7.4 Fluorocarbon (FC) Fluorechemical surfice modifi- cers are hydraphobic-oleophobie coatings that reduce the surface energy of a substrate to 11-12 mN/m, This property allows water, solvents such a8 heptane, toluene and acetone, and lubricating oils to bead up and deain off a substrate. These coatings are applied as ¢ duit fast drying. layer that can be soldered through for rework, and fre- ‘quently do aot require masking. The coatings are typically supplied in fluorocarbon solvents which are nonflammable, low in toxidty and are not VOCS. The low viscosity and low surface tension of the coating allows it to flow into, and coat in-between, small gaps and layers. Fluovochemi= ‘cal surface modifier coatings ae thin and not very resistant to abrasion, but insoluble to most organie solvents and as a fully reacted polymer in solution, has an indefinite pot life. ‘These coatings are generally unsuitable for applications where the operating temperature exceeds 40°C [104°F) because the material begins to volatize beyond this point, 4.7.2 Perfluoroother For certain applications involving ‘extended exposure to fuels, hydrocarbon oils, hydraulic fluid, nonpolar solvents, acids or bases, the use of perfiuo- roether type coatings could be considered. This polymer is based on a perfluorvether (fuorinated-carbon to oxygen) backbone combined with an addition-coring silicone crosslinker. One-part perfluoroether materials are processed (Gispensedi/sprayed and cured) in the same manner as heat- cured silicone type coatings. They can be sprayed or dis- ppensed using typical existing equipment and heat cured st 150°C [302°] for one hour. Perfluoroether elastomer provides improved low tempers ture propertis, long-term heat resistance, self-priming adhesion, excellent electrical properties. and ionie purity. ‘As a 100% solids (no-solvent) materiel it offers processing ‘advantages unmatched by other standard fluorpelastomers. "The thermal resistance and low modulus are similar to sili- cone properties while proviing protection from the harsh: cst environmentsctoner 2002 Pc: HOBK-820 5 DESIGN FOR COATING APPLICATION This section establishes design concepts, guidelines, and Procedures intended to promote appropriate “Design for Reliability (DfR)" procedures and to ensure reliable char acteristics of a conformal coated printed circuit assembly PCA). ‘The definition of reliability inthis section is: Reliability is the ability o£ product to Funetion under given canditions and for a specified period of time without failure. ‘This sect addresses veliability-telated aspects of product design, process design, as well as materil/component Selection and gualfication when using confortmal coatings in accordance with the design guidelines given in 1PC-D- 279. This section also identifies other apgropriate exiting IPC documents as reference for basic detailed information. ‘The effort of this section is directed at making the designer and the users of conformal coatings for electronic applica- ms aware of the various factors that affect the protection. atforded coated PCAS in the end-use environment for the design-life of the PCA. 5.1 Design Philosophy Before the praduct design effort can begin, the designers of the product and assembly pro- ‘cess need 10 know the customer’s reliability requirements for the product. These requirements should be defined and ranked by @ concurrent engineering or cross-functional team through a process such as Quality Function Deploy- ment (QFD) used to capture the voice of the customer ‘The design team can include, but is not limited t0, the members who participate in at least the design activities identified in the IPC-D-279, In this table, DFA/M stands for Design for Assembly/Manufactorabilty, DIT for Design for Testability, DAR for Design for Reliability ‘The design team can consider the general design guidelines presented in the body of this section as & methadology for achieving its reliability goals. The IPC-D-279 contains information that illustrates the general design steps and process flow using concurrent engineering. The IPC-D.279 also includes information that illustrates the interactive nature of the design for reiability process. 5.1.4 Defining Reliability Requirements The basic requirements to be defined include but are not limited to: + Years of service ‘+ Failure rte(s}probabilityies) as a function of time. + Repair/replacement/upgrade/service/maintenance! warranty strategy. * Life cycle environment). + Definition of acceptable performance. + Critcaity of function(s). + Available test equipment ‘+ Mean tie between maintenance + Mean time between failures 5.1.2 Understanding the Product Life Cycle The prod uct life eyele begins at the component level (including the printed board) and continues through and beyond the assembly level. Exposures thronghout the life cycle include the assembly/process conditions; the testing. storing, tans portation and operating environments Test, storage and transportation need to be considered at both component and assembly levels as well as before, during, and after the coating process. 45.1.8 Defining the Product Environment For cach ev ronment in 5.1.2, its enical to identify, characterize and ‘quantify all the parameters that may affect the performance ofthe product. These inelude the humidity and pressure of the eavironment the range, rate of change and exposare period to temperature, i. temperature eycles encountered during the operation of the product. Some products may also encounter physical vibration and shock during oper tions. Electrical parameters such as ESD, EOS, EMC, EME and high voluge exposure should be taken ino consider ation. Most environments impose a certain level of eben cal exposure from eis some common factors such as salt spray and fuel, o specie chemicals such as ux, solvents, NBC decontamination, ete. Not to be neglected are radite tion from sun ight, UV of other specific sources: reactive ses (S0,, NO,, HS, Cl, Os, ete), sirboressurface acids, submicron particles, and contaminacon fiom dust oil or human contact. When the life environments have been Identified and defined, the engineering team is prepared to analyze and select the conformal coating, coating provess, and test stra egies required, 5.2 POBs\Printed Circuit Boards (PCBs) are one of the _major subcomponents of « conformally-coated printed cir cuit assembly (PCA), Other sub-elements of the PCA that are considered in the conformal coating design and selee- tion include, but are aot limited to, surface finish, spacing between component leads or printed circuit features, dis: crete and integrated circuit components, solder joint con- figuration, exc. However, it is the PCB that is the singular largest Feature of the PCA that is conformally coated and therefore has 2 significant impact in determining which ‘conformal coating and process of application is most sppropsiate to use. The characteristics ofthe PCB that need tw be considered should include the properties of the diclectrie laminate, prepreg adhesive, metallized signal lay- ‘ers, pad pattem for surface mount attach designs, aspect ‘ratio of plated-through holes (PTH) and vias (blind or bur- ied), Therefore, the materials of construction of the PCB ‘are important in determining the appropriate conformal coating and method of application to useIPOHOBK 30 5.2.4 Plating Surfaces Plating surfaces are the metallic ‘areas that remain on the exterior surface of the PCB as welll 2a through-holes and solder pads ater etching and removal ‘of photoresists. These areas are typically bare copper that are then plated with tvlead (Sn/Pb). In some eases. large tallic areas are often plated with other organometallic metals such as nickel ar even solder mask. la most ofthese instances. the plated surfaces will have some dissimilar ‘metal interfaces thot will be coated with a conformal cos ing. In addition, there may be desidns in which a confor- rmal coating will either panially or completely cover the plated surface areas. Ip situations with paral coating cov- ‘erage, the edge of the coating may delaminate or lose adhe st the surface of the plated metal 5.2.2 Alternate Surface Finishes One of the areas on a PCA that a conformal coating has 10 adhere to is the sur- face finish of the PCB. Surface finish is the material on metallized areas of the exterior PCB layer that usually results during bare board manufacture. Previous geners- tions of these areas on a PCE where the remaining copper after etching and other PCB processing prior to reflowing tinfead (Sn/Pb). Newer technologies have resulted in alter nate materials to SniPb being used as the finish on the PCB prior to assembly processing. These materials include, but are not limited to, bare copper, immersion Un, immersion silver, immersion gold, elecwoiytic or electroless nickel combined with goit) and/or palladium, organic soldering preservative (OSP), and other similar types of alloys. Jn many situations, some surface finish areas are not being ‘changed when there is no solder joint Formation and there fore are part of the overall board substrate. Depending on the type of board surface finish, there may be dissimilar metal interfaces that could lead t0 corrosion if not pro- tected from humid environments. Conformal coating mate- Fials would then need to be considered as u corrosion pre- vention aid in addition to an environmental protection barrier. The dissimilar metal combination of an alternate surface finish over bare copper is also a surface in which conformal costing adhesion may also need to he evaluated. In many cases, these conditions may necessitate conducting 4 qualification test using representative PCR sample betore approving the intended conformal coating as agceptable for use.” 5.23 Spacing Reduced heat extraction from the PCA ‘and inereased junction temperstures) may result if ewnfor- ‘mal coating covers heat conduction surfaces on the PCA, edge or margin which mate with heat sinks such as card- ‘edge slamps and cold plates. A resolution is to widen con- ‘ductors that function as heat dssipaters. If possible, itis recommended to limit AT conductor to Jess than SC (ave) ‘The other issue concerning the aspect of spacing on PCAs involves the areas between adjacent printed circuit traces or 10 Octeoer 2002 between solder pads atthe PCB level. Other spacing issues fare the physical volume between adjuceat leads of soldered electronic components, In all these instances, a confarmal coating is usually requited to cover these areas. For sol ‘Geted components with leads, they usually require com> plete costing coverage without solder bridging in between adjacent leads to maintain the necessary dielectric insula- tion, The type of conforraal coating that would be used usually depends on spacing between the leads (lead pitch) and whether the leads are cylindrical oF rectangular. For fine pirched leaded devices the finer the pitch, i, smaller spacing and clearance between adjacent leads, a coating with lower viscosity should be considered to achieve uni- form thickness while providing edge and point coverage. 5.2.4 Solder Mask Compatibility issues beween solder ‘mask and conformal coating should be considered when ‘designing a PCB for conformal coating application. Refer {0 8.1.1 for solder mask material compatibility. 5.3 Component Various diffrent types of component packages ure widely esed in electronic circuitry. The mate rials used in packages vary greatly and consist of mold release agents. waxes, plastics, ceramics, marking inks, metals. glass and various other materials. The degree of cconformal coating adhesion, CTE mismatch, shear modu- lus and general wetting characteristics of these materials need to be considered when application of conformal cost- ing is anticipated ‘Over-application of conformal coating materials (excess thickness) could be detrimental to long-term reliability for most leaded and leadtess components. except dam and fill and glob top types of components. 15.3.1 Component Matorlal Typo 5.3.1.4 Plastic Various mold release agents are used to assist in the release of these packages from the mold after the injection molding process. Component suppliers com- sider these release agents proprietary and are reluctant to reveal the exact formula used. commonly these are silicone based products. These agents may impact the degree of wetting and the adhesion of conformal coating (or any adhesive) to the package. The plastic packages may absorb various process materials such as fluxes and cleaning agents, and the release agents may exit the porous plastic ‘matrix in subsequent heat excursions in processes or end 8.3.1.2 Coramle Ceramic packages typically do not pose ‘much of a threat (© most conformal coatings. They are sometimes color-coded and the pigments should be consid- ered, but they rarely pose problems with the conformal ‘coating. Marking inks, however, may contain polysiloxane agents, which may cause de-wetting and adhesion loss ofeteber 2002 PoH0BK 820 the conformal coating. but usual only oceurs inthe prox imity area of legend markings. 5.3.1.3 Metal Metal packages are of the least concer of all ypes of package materials curently used. Most confor ‘mal coating formulations are designed to wet and adhere well to these surfaces, The matking inks andior decals used should be considered as localized de-wetting and/or elamination may oceur 8.3.1.4 Glass Glass components, such as diodes, typi- cally pose no threat to conformal coating application and achesion, The primary concer should be the coating fillet, surrounding the component. Too thick & cured Tiguid coat ing filet can cause stress to the component body and crack andor weaken the solder fillet as a result of extreme tem- perature cycling. Low modulus “buffering” compounds ‘can be used as an adjunct in end use applications consist- Ing of bigh vibration and thermal cyclin. 5.8.2. Through-Hole Components Through-hole compo- nents are devices that penetrate substrates via holes in the ‘ards. They can have multiple leads and are composed of various metals and placed through a “tinning” process prior to insertion that enhances the solderability of leads, Once parts are inserted, the leads may be clinched and clipped. After soldering operation, the clipped leads typi cally create sharp edges on the bottom side of the PCA. These could be dificult areas to cover with conformal coat ing. Liguid conformal coatings typically do not build up on the sharp edges because gravity moves them to the base of the lead and solder fillet. When itis cured, very little coat: ing material would be left on the edges of clipped leads. Abrasion of the coating at lead edges due to subsequent handling should be considered. The ‘ype of coating chem: ‘sty, application method and cure mechanism dictates the success of creating film buildup in these areas. Multiple coats and higher viscosity costing is typically recom: ‘mended for pin cover, Fluxes used during a wave soldering process often “wick’ up the legs of such components and onto the “top-side” of the assembly surface. This may lead t0 coating reliabiltiy Droblems caused by the flux residues if « no-clean process is being used. 5.3.2.1 Axlal Leaded Components Axial components are leaded through-hole devices. They are typically bent at 4 S0-degree radius and inserted into holes in the board ‘They may be clinched, cut and soldered. Since most are cylindrical in shape, conformal coating concems include enormous filet formation under and around these types of devices, This may create shadows during the curing pro- cess of single component UV conformal coatings. It may so cause stress on the component body due to CTE mis ‘match, Axial leads typically have thinner coatings than pls: nar sites because gravity forces wet coating to flow off and down to the solder fillets, The time elapsed from coating application to cure or coating immobilization may also affect the finished coating chickness in the actual lead areas, ‘This is a particularly important issue if solvent-less coating materials are being used 5.8.3 Leaded SMT Components Devices with termins- tions formed independently of. and projecting away from, the component bodies are specified as leaded. The shape (gull wing, “J.” ete), and length of the terminations may ‘etermine the actual conformal coating thickness achieved fon these types of leads. The time elapsed from coating application to cure or coating immobilization may also affect the finished coating thickness in these leaded areas ‘The “pitch” or distance between Ieads is also a design concern for costing. Some higher viscosity coating materi- als may not penetate past and under fine pitch leads and not migrate under the device. This causes bubbles to form in the coating during cure as air voids under the device heat up and the air attempts to escape from under the compo nent. If the bubble does not pop before polymerization, it can bridge conductors and limit the degree of insulation provided by conformal coating within the cured bubble. 6.4 Leadless SMT Components Components with ter ‘minations formed as an integral part of the body are speci- fied as leads. This would inclde BGA, PGA, and Flip Chip, chip scale package (CSP), chip on board (COB), leadless chip carrier (LCC), and chip type compoaents 5.3.4.1 Ball Grid Array (@GA) Ball Grid Asray devices are leadiess. They contain solder bumps on the underside Which ean be peripheral or area dispersed, The fnished gap under the device after solder collapse can be 0.5 mm [0.020 in] to 1.3 mm (0.0512 in]. This is generally enough to allow good capillary flow of conformal coating under the device depending on how large the BGA is. BOAS in excess of 650 mm* [1,007 in") may limit the amount of coating penetrating under the device via capillary flow. Post soldering residnes are of prime concern as cleaning and inspection under these types of devices can be dificult fr even impossible. Many of these devices are now being “under-filed” with epoxy prior to the conformal coating operation. 5.9.4.2 Pin Grid Array (PGA) Pin Grid Arrays are similar te the BGA except for the fact that pins have ceplaced the solder bumps. The characteristics affecting the conformal ‘coating are identical 0 those of the BGA. 5.3.4.3 Flip Chip (COB, CSP) Flip Chip devices are package-tess die, waich have been bumped with solder on ‘the contact sites. The solder bumps are fluxed and the sili- ‘con chip is inverted, placed and passed through solder inIPO-HDK-639 reflow. The device is then “underfilled” with an epoxy er ating a complete seal and illet round the device. The con- formal coating can be applied over the die. Adhesion and wetting (othe epoxy underfll fillet is usually not an issue Underfill can also be applied at the wafer level with Bestaged anisotropic adhesives. Some versions also contain ‘ux within the epoxy matrix, hence "“Auxing underils” ‘which eliminate the fuxing step. 6.3.4.4 Dam and Fil’ This type of device is sila to lip chip except the die is turned face up and stil contains wire die bonds, Because of this, it ean te considered & leaded device before encapsulation. ‘The die is connected on the bottom side with die attach adhesive which is usually a conductive epoxy. The wire die bonds are made from she die face to the substrate or lead frame. Using a high viscosity epoxy, a dam is dispensed around the outside parameter of the device. The height profile of the dam shomld excecd the tallest die wires. A second epoxy material, which is low in viscosity. is then dispensed within the dam parameter and both materials are ‘cured simultaneously. This completely encapsulates the device, Rework is not pract cal ‘The conformal coating should primarily be used to protect other conductors and leaded devices on the board, If coated, the dam and fill epoxy package does not generally pose Wetting or adherence problems with the coating 5.3.45 Glob Top Glob top is similar to dam and All with the only diference being a high viscosity epoxy encapsu lant is dispensed in the center of the die. The epoxy should ‘encapsulate the wire die bonds without breaking the fine leads and flow over the entire device. It is then cured, The height profile of the epoxy glob needs to exceed the high- ‘est bend in the die wires by 304%. Glob-tops are aot to be confused with conformal coatings as they perform funda mentally different functions. 54 Electrical The effect of conformal coating om the cloctical funetioning of a PCB can be either beneticial or etrimental. This depends primarily oa the design param- eters of the circuit and the materia! of the conformal coat- ing. How beneficial or how detnmental depends on good engineering, as outlined below, There are several distinct categories of electronic circuits! assemblies that will be briefly discussed with regards tthe possible effects of conformal coatings. These include high voltage high current, RF and microwave, high speed digi- tal, along wit the effect on ESD and EMI. The effects dis- cussed only apply to circuit traces on the outer layers ofthe PCB. In other words, those traces in dicect contact withthe conformal coating. October 2002 ‘5.4.4 High Voltage (HVY/High Current (HC) High Voltese (HV) circuits are the one case sehere confcrmal coatings may be necessary for the PCA to function in some eaviton- iments, Conformal coatings are used to provide greater Insulation between HY leads than is provided by the Such additional insulation is needed to prevent HV arcing, ‘corona and St. Elmo's Fire. The best method to produce HY circuits with a high degree of robustness is by using ‘conformal coatings or other encapsulates after assembly. Conformal coating is not to be used in Hiew of electrical insulation on high voltage wire ‘Therefore, an important charocteristic of a conformal cout- ing used in HV circuitry is its dielectric strength. Given the dielectric stengtt of the conformal coating materi ant the maximum voltage of the circuit, the necessary thick- ress of the coating can be calculated from: TV x 644 x 10° Where T is thickness in inches of the coutiag and V is the voltage in volts. With HV circuits i is che ability 10 con- tain the HV that is critical. For High Current (HC) circuits, onthe other hand, confor- mal coatings are generally detimmental. High current means high heat and conformal cowtings interfere with the disper: sion of that heat. Therefore, good thermal conductivity is the key property for coatings used in high eusrent circuits. Other important fsctors include high melting point and glass transition temperature (T,). If the melting point and/or the T, are too Tow the coating may melt or deform from the &ssipated heat CCireuis which are both high voltage and high current need couformal coatings with both good electrical insulation to contain the HV and good thermal conductivity to dissipate the heat generated by the high current. Unfortunately, materials that are good electrical insulators are typically not good thermal conductors. Therefore, ttadeofis are required based upon the design of the circuit, how it will be used, environmental conditions, ete. Good engineering in board design, compovent selection, as well as materials, Js necessary to produce the best PCA. When seeking 2 conformal coating to aid in protection against a lightning strike. for components such as connec tors, then a film thickness as litle as 0.5 pm (12 mill may bbe adequate. Refer 10 withstand voltage of the coating ‘material. 5.42 RF and Microwave The materials used in a PCA ‘can effect the propagation of RF and microwave signals in a variety of ways. For conformal coatings, the preferred effect would be none at all, oF failing that, a well under- ‘stood, reproducible change in performance. Ideally, the dielectric constant of the coating material should be as close as possible 0 1, that of a vacuum, andcteber 2002 it should absorb as litle as possible of the RF energy. In practice, RF design engineers use materials whose ellect on their circuits they are familiar with and which they can predict and, if necessary, conect, This makes RF designers, Who usually are not very knowledgeable about materials. very cautious about trying new materials, Adding to the probletn is the fact that mo material behaves in exactly the same manner across the entire RF and microwave fe- quency bands. This means that circuits in the Very High Frequency (VHF) and Ulta High Frequency (CHF) bands employ different materials an circuits i and millimeter bands. Any matetial whose dielectric con- stant changes dramatically over frequency should be avoided in very broadband applications For conformal coatings then, a few general rules may be given. It should have a dielectric constant close to 1. It should have low RF absorption. Polyurethane based “Yoams’ have traditionally yielded good results in these cir- cumstances. If a conventional conformal coating is being used, itis important that i be applied evenly and at con trolled thickness, Finally, it should do all these things across a usable portion, at least an octave, of the RF spec- lwum. Acrylics and polyurethanes are We more commonly used coatings mainly because of ease of use, repeatability ‘of deposition and the variety of formulas allowing opi mum performance across a number of frequency bands. To achieve an even application, coatings applied with vacuum deposition are often the better choice. 5.4.3 High Speed Digital In many ways, the require ments of a conformal coating for high speed digital appli- cations are the same as those of an RF circuit of the same ‘frequency. Microprocessor and High Speed Digital signals reaching 10 Giga Bites Per Second may benefit from acrylics or poly- arethane coatings. It is important to note that AC currents induce electrons at the periphery of the conductor rather than in the center. Consequently, the edge of an AC con- ductor is eu area of higher potential for electo-chemical reactions, as the consequence of unwanted residue, than ‘with DC cureent. 5.4.4 Controlied Impedance In 6.4.2 RF and Micro- Wave, it was stated that coatings would ideally have a dielectric constant of 1, so as to alter the parameters of the circuit as little as possible. Sometimes however, itis desic- able to alter the purameters, especially the cireuit imped: ance, One method of changing the circuit impedance is to intentionally use # coating witha dielectric constant greater than 1 In the case of a microstrip the formula” a b % ar 1 atoran Gaia for Fado Engine, IT Sith Ea Howard W Same & Co, POHOBK-200 {ives the characteristic impedance of the microstrip. Here 1s the height of the stip above it’s ground plane, w the strip width and &, the relative dielectric constant of the PCB base material, When a conformal coating is applied, the tenm (r must be changed to €,, and must be calculated taking into account the dielectric constant of both the base ‘material and the coating. The added coating will raiye the “Effective Relative Permitivity” (2,5) of the environment as seen by the transmission line (trace and its signal return path). Theoretically, the impedance of a system can be alieted at will by choosing the correct material, with the correct dielectric constam, In practice, few materials allow such fine-tuning and tight control of their dielectric con- stanis, This is especially tue of 2-Part conformal coating mixtures, Even with the best automatic dispenser, some pants of the board may get more of one part than another ‘causing the dielectric constant to vary widely. Homogene- ity and reproducibility sre tie most important characteris tues for any coating material, In general silicone materials produce good results but the range of dielectric constants (6-10 usually) is rather narrow. Acrylies and epoxies need careful manufacturing to assure homogeneity but otherwise are useful and can be obtained in a wider range of values ‘Vacuum deposited coatings have good results with homo: szencity and reproducibility as long as the process param ters remain constant ‘The presence of the coating will raise the dielectric con- stant of the environment. The end result is Uaat the trans- ‘mission line impedance will be lower than it would be ‘without the presence of the coating, The closer the coat ing’s dielectric constant is (0 1, the less the effect on impedance, To prevent ‘Return Loss’ or Voltage Standing ‘Wave Ratio (VSWR), this effect should be understood and considered during the design process.'° ‘The coating witl have the samme effect on the impedance of 4 high speed digital cirvit, The difference being that the end result is usually less dramatic in high speed digital applications, because digital circuits have a much greater tolerance to signal reflections and noise than do analog cir- cults. As 2 rule of thumb, costings will lower trace imped- ance by 1 to 3 ohms, depending on thickness and dielectric constant of the coating material relative to the thickness and dielectric constant of the PCB material between the trace and its rerum path (power or ground plane). Even ‘hough the coating effect is less dramatic on signal integ- sity in digital applications, it should be understood and considered. 5.4.5 EMVESD Electronic assemblies that are going «0 be handled, repaired. modified or etc. on 2 regular basis reed some protection from the discharges produced by handling. The standard model used in ESD control is the 10 Fes erat castors, trio “Trnsmsion Line Deogh Hendoot by Bian C. adel (ISN 06000-4360),1PC-HOBK 20 lbuman body mode! consisting of « 1SpF capacitor through «4 1500 ohm resistor at a maximum of 3SKV". This means that the coating should withstand high voltage but with ‘nly a small amount of charge behind it. Therefore the ree- ‘ommendations given in 6.41 are just as applicable here. outings are not commonly used for ESD protection, (nthe other hand there is litle a conformal coating ear do to alleviste EMI, Many items need to be understood (0 properly control EMI, One of those items is how © prop erly shield a PCA to prevent radiation of EMI oF suscepti- bility to EML. Shielding an PCA from EMI requires encios- ing the PCA in a conductor that is then connected to ‘ground, In theory a conductive coating could do an excel: leat job, bur in practice the likelihood of short-cicuiting Teads of the various components makes this an impractical ‘process I is important to select a conformal coating mate Fial that does not exacerbate this problem. 5.5 Coating Coverage Responsibility to specify the areas of ‘where to’ and “where not 10" coat should lie with the design team with consideration of the end use environ: ment. The following are some factors to be considered + All solder joints and exposed leads should be costed. + Bare board aress with no traces or joints are not necessary ‘0 coat, but may be considered optional, if for cosmetic purposes only + Coating for cosmetic reasons may adversely affect assem- bly eyole time and usage cost. + Component packaging ‘Typical examples of areas free from costing would be: connectors, test points, variable value components, ground- ing poinrs, switches, display screens, socket contacts, mechanical interface areas, ete Masking prevents conformal coating from being applied in 1 specific area, However, a masking process can be labor Intensive, especially around intricate assemblies. Therefore, minimizing masking should be 2 consideration while designing & PCA. Choosing # mask that is easy to apply and remove could be helpful in the event that masking is needed Users are cautioned that certain types of masking such as liquid masking materials that subsequently harden t0 rubber-like consistency can cause problems in the event that they ate not completely removed from electrical con- tacts im connectors, sockets, and other similar electrical interface areas. There is good evidence tbat most synthetic- based latex masking compounds can aggravate the devel ‘opment of electrochemical actions on the assembly sur: face. 1 BELLZORE TReNWTpGa7O etober 2002 15.6 Masking The purpose for masking is to prevent cer fain areas on a PCA from twing costed, The conventional ‘way of specifying information on masking areas is to use & ‘combination ofa pictorial diagram and a written specifica: tion. The pictorial diagram is a very common method of demarcating the “coating-free™ zones and thus the aceas that need to be masked. Masking and de-masking js a labor intensive process, PCAS should be designed with confor smal coating operstion in mind such that the number of areas that need (© be masked is minimized. The written specification contains information on how 0 use various ‘masks and mare importantly the sequence steps especially when different types of masks are used. Usually the wnt ten specification also contains information on which mask: 8 techniques are acceptable to use. All this information (pictorial and written) is essential to prepare the PCA for conformal coating application Example of an assembly drawing with masking require. ‘ments is shown in Figure $-1 Annotations describing the components in more detait often help. For example if only the pins in a connector housing need to be free of coating then state this, otherwise the whole connector may be masked. Ic is preferable that a view of both sides of the PCA is siveo. 5.7 Drawings & Design Guidelines In order for a con- Formal coating to be used on 2 PCA, there should be a ref= ference fo 8 material and process specification as well as a pictorial drawing. The material speciicstion usually gov- ems all the requirements for the conformal coating mate- rial, The process specification usually identifies all the requirements associates with applying the coating material, ‘This specification should also include details for processes preceding and subsequent to the costing step. Examples of these would include but are not limited to cleaning after soldering. masking. handling (as ESD safe). de-masking and inspection. A pictorial diagram, as a guideline. should indicate either by chainline, hatched regio, or something similar the areas to be conformal coated. Design guidelines for coaformal coating usage should always include a checklist of requirements that need 0 be ‘examined. In most cases trade studies are performed 10 assess whether an PCA requires conformal costing cover- age and if nacessary which conformal coating type and processes should be used. This checklist should take into account all conventionally used design crteria/guidetines considered t0 be typical engineering practice 6.8 Reworkabitity/Repalrabitity If PCB and/or the components are to be replaced during the life of the prod- uct, ten the conformal coating removal and application