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1、Electronic Module Conformal Coating and Potting,MPC-T-1,2,Training Objectives,Explain what coatings and pottings are and why they are usedDescribe coating and potting application methods and controlsProvide examples of coating and potting within DelphiProvide resources for additional information,3,O
2、utline,DefinitionsCoating Reasons for coatingMaterial typesApplication methods and Delphi examplesQuality issuesPottingReasons for pottingMaterial typesApplication methods and Delphi examplesQuality issuesAdditional sources of information,4,What Is It?,Conformal CoatingA thin layer of a dielectric(e
3、lectrically insulating)chemical compound that is applied to the surface and serves to protect the electronic circuitry.The material can be transparent or opaque.Potting(also referred to as encapsulation)A process of filling a complete electronic assembly with a solid compound for resistance to shock
4、 and vibration,and for exclusion of moisture and corrosive agents.,Transparent coating on a populated printed circuit board,Opaque potting material in an electronic module cavity,Transparent“gel”potting material in an electronic module cavity,5,Additional Definitions,Ion:An atom or molecule where th
5、e total number of electrons is not equal to the total number of protons,giving it a net positive or negative electrical charge.Ionic contamination:Surface debris that contains ionsElectrochemical migration(ECM):Also known as Dendritic Growth,is the growth of conductive metal filaments on or in a pri
6、nted circuit board(PCB)under the influence of a DC voltage bias.The filament growth is facilitated by the movement of ions on the circuit board.Tin whiskers:A crystalline metallurgical phenomenon involving the spontaneous growth of tiny,filiform hairs from a metallic surface.The mechanism is encoura
7、ged by compressive mechanical stresses Silicone:Inert,man-made polymer compounds that include silicon,carbon,hydrogen and oxygen and some other chemical elements.Silicones can be used over a wide temperature range.Acrylic:Thermoplastic made by polymerizing acrylic acid or methacrylic acid.Acrylics a
8、re used for cast and molded parts or as coatings and adhesives Polyurethane:Any polymer consisting of a chain of organic units joined by urethane(carbamate)links Polyurethane formulations cover an extremely wide range of stiffness,hardness,and densities,Coatings,7,Why Do We Coat?,Encapsulate and imm
9、obilize ionic contaminationPrevent electrochemical migration(ECM),also known as dendritic growthPrevent further contamination of circuitry by environmental salts,soils,dust,smoke,and water vaporPrevent circuit leakage between conductors and possible arcing in high current situationsFor example the U
10、L94V0 standard require that coatings survive 1000V for one minute without breakdownAt minimal coating thicknessAfter prescribed thermal conditioningProvide physical protection to the circuitry and components by preventing surface scratches under normal handling and useMitigate the effect of tin whis
11、kers,8,Encapsulating Contaminants,Thin layer of water or water vapor,Various types of contamination,Circuit board laminate,Without Conformal Coating,Thin layer of water or water vapor,Conformal coating keeps moisture away and encapsulates contamination and circuitry,Contaminants dissolve in water an
12、d become mobile ions,9,What is Dendritic Growth?,It is a form of“Electro-chemical Migration(ECM).”Conductive metal filaments grow on the surface of a Printed Wiring Board(PWB)or component under the influence of a DC voltage bias ECM(or Dendritic Growth)is very similar to electro-plating.Metallic ion
13、s migrate from the anode(+)to the cathode(-)of a circuitUnder the influence of an electrical bias and in the presence of sufficient water or other electrolyte,Image from www.npl.co.uk,Direction of Growth,-+,10,What is Dendritic Growth?,Electricity+Water+Contamination=Dendrites,11,Dendritic Growth,Co
14、nditions for Electrochemical Migration An electrical potential(voltage)between the electrodes is needed to establish an ionic current in the water layer.Water or other electrolyte must be present to dissolve the ionic materials and sustain them in their mobile ionic state.Electrical carriers(such as
15、 ions)must be present.,1.Bias is present(1 to 2 volts)2.Water is present.3.Ionic contamination is present.Therefore dendritic growth!,12,Dendritic Growth,Applied correctly,Conformal Coating will prevent Electro-Chemical Migration!Provides additional dielectric material to reduce the electrical poten
16、tial(voltage)between the electrodes Creates a barrier between water or other electrolyte and ionic contaminants so that ions are not dissolved,1.Bias is present(1 to 2 volts)2.Water is present.3.Ionic contamination cannot dissolve and become mobile ions.Therefore no dendritic growth!,13,Dendritic Gr
17、owth A Cautionary Note!,Conformal coating is not a barrier to standing waterOver time water molecules will penetrate silicone and acrylic coatingsWhen the coating is thin water migrates to the surface relatively quicklySmall Chip Capacitors have been shown to be very sensitive to this condition,Veri
18、fication of coating coverage and thickness during process set-up is critical,Dendrite across capacitor terminals where coating is thin/missing,14,Another Case Where Coating Does Not Help,Circuit board laminate,Contamination bridges circuitry,Coating layer,Continuous contamination layer is in place p
19、rior to coating application,15,Is the product in a sealed case,potted,or gel encapsulated?,Coating recommended by MTT,Product remains uncoated,Is the product a mission critical or safety related device?,Try validating with no conformal coating,Product passes validation with no ECM,Product fails vali
20、dation,Try validating with no conformal coating,Does the product have pure(99%)tin finishes on the circuit board or any component surface,including connectors,surface mount components and through hole components?,Conformal Coating recommended,Failure due to cause other than ECM or moisture susceptib
21、ility,Failure due to ECM,corrosion,moisture susceptibility,Validation requires condensing moisture while under power or a“dewing test”,Product passes validation with no ECM,Product remains uncoated,Product fails validation due to ECM,corrosion,moisture susceptibility,MTT Recommended Coat/No-Coat Dec
22、ision Tree,Customer requires coating regardless of case design,MFGE RP 9.3 Appendix A,16,Key Coating Decision Factors,Customer RequirementSpecifically required regardless of product design or technologyBuild-to-printValidation test specificationsProduct Design and Application RequirementsLiquid wate
23、r condensing on circuits generally drives the decision to coatResult of product validation A function of product usage or location Sealed case design typically does not require coatingPoor component or board cleanliness or exessive water absorption drives need to coatHigh failure mode“severity”can b
24、e mitigated by coating circuit boardIn all cases MFGE RP 9.12:Liquid Water Immunity Test can be used to identify sensitive circuitry that will require coating,17,What Coating Materials Are Used?,Delphi-E&Ss current menu of conformal coatings has selections from most coating types:Commonly usedSilico
25、nes5 approved materials availableAcrylic1 approved materialApplication specific materialsPolyurethaneTypically used in EuropeParyleneMust be applied by a 3rd party(single source)Before use,materials must pass comprehensive compatibility testing(including Surface Insulation Resistance)and solder join
26、t reliability testingMFGE RP-9.21 defines the test plan and methodsMaterials COE(Hank Sanftleben)must approve usage,18,Silicone Coating Materials,Silicone:An entirely synthetic polymer containing a Si-O backbone.Organic groups are frequently attached to the backbone via a Si-C bondNote:Silicon is a
27、metal and Silicone is a polymer AdvantagesWide temperature range:Compliant to most environmental restrictions(WEEE,RoHS,REACH)Low mechanical stressConcernsCustomer sensitivity to Silicone contaminationSome relays and brush motors are sensitive to silicone volitiles Only when silicone is within 1 cm
28、or so of the arc currentOnly on relays with poorly sealed housingsMaterial SuppliersDow CorningShin Etsu,Material High Temp Flexible toSilicones+150 to 200C-45 to-115CEpoxies+150 to 180C n/aUrethanes+115 to 125C-60CAcrylics+85C n/a,19,Acrylic Coating Materials,Acrylic Resin:Thermoplastic or thermose
29、tting plastic substance derived from acrylic acid,methacrylic acid or other related compoundsAdvantagesNo risk of silicone volitilesFast ambient curingCapable of applying very thin layer(0.001”)ConcernsNot approved for high temperature applications Max temp:85CSolvent based,therefore may not satisfy
30、 REACH requirementsNot a low VOC(volitile organic compounds)materialMaterial SuppliersElantas/PD George,20,Polyurethane Coating Materials,Polyurethane:A family of polymers that offer a range of hardness values.They have good strength and environmental resistance propertiesAdvantages2-part curing sys
31、tems can be adjusted for application Vary catalyst to resin ratio to modify hardnessGood moisture resistanceConcernsLimited service temperature(120C)Not easily repairedSuppliers Primarily supplied to European manufacturing sitesLackwerke PetersHunstman,21,Coating Application Methods,Lean,Selective S
32、pray CoatingAlso have Jet Pulse precision coatingDip CoatingDispense CoatingOnly approved for use in specialized casesManual Brush CoatingNot recommended by the MTT for general use,22,Spray coating,The preferred method for applying conformal coatings in manufacturing Spray heads are mounted on 3-axi
33、s robot gantries.The system has natural tolerances and capability limitations.,Selective coating spray valves,Spray coater,23,Spray Coating Details,Example of usagePVA equipment spraying DC 3-1953(silicone coating)Kokomo plant 7-BCM for GMT900 truckDelnorte(multiple programs)Nordson-Asymtek equipmen
34、t spraying PD George 5026-21(acrylic coating)Delnorte/DDM/SZM(multiple programs)Silicone coating can be applied by both systemsAcrylic coating is best applied by Nordson-Asymtek systemWhy used/preferredControlled application of coatingReduced material and utility usageFits“Lean”principles Prevent ma
35、terial from coating“keep-out”zones without maskingReduced board to board cross-contaminationCommon IssuesNozzle cloggingThickness variation or streakingOverspray into keep-out areasShadowing around larger components,24,Precision Spray Coating,Also known as Jet Pulse Spray CoatingSpecial application
36、of Selective Spray Coating when tighter spray tolerance is requiredStandard clearance to non-coated areas is 3.2mmPrecision Coating clearance is 1.0mm,25,Dip Coating,Board is dipped in a bath of coatingExample of usageSuzhouEquipment Cheng HuaMaterial DC 4097(Silicone)Delnorte Equipment Knoedel,Pain
37、tlineMaterial DC 4097(Silicone)Equipment DifferencesKnoedel system raises a small tub of coating to the boardPaintline and Cheng Hua systems bring boards down to a larger tub of coating,26,Dip Coating,Why usedApply coating quickly across an entire boardFloods areas where small components may reside
38、under connection systemEconomical for high volume productionCommon IssuesKeep-out areas must be maskedCoating in dip bath can become contaminated with flux or other residuesDC 4097 neutralizes flux residuesSolvent borne coating is effected by fluxNeed to monitor and prevent viscosity changes in the
39、bath For DC4097,at least 20%of the material must be used or replaced per weekIf solvent borne coating,need to add solvent to maintain viscosityDrawbacksLarge equipment sizeNot scalable for lower volumeCannot dip coat with non-underfilled BGAsUnderfilled flip-chips are OK,27,Cure Processes,Depending
40、on their chemistry,coatings can be cured by HeatUV energyMoistureAmbient air,28,Coating Guidelines,DDS27 defines the minimum clearance between“must be”coated and“must not be”coated areas is 3.2mmAllows for some slump or overspray around coated areasFrom MFGE RP 9.9Coating Thickness Guidelines For Ci
41、rcuit Board Environment With Condensing Moisture,29,Quality,Quality defects are identified by their effect on adhesion,coverage,or coating homogeneity Does it compromise the coatings ability to protect the circuit?Does it compromise downstream operations?Examples:Always refer to released product wor
42、kmanship standards for specific criteria,Bubble exposes PCB,Excessive Bubbling,Insufficient Coverage,Overspray,30,Quality,IPC 610D is the primary source for coating qualityAdditional quality guidelines available through the Coatings and Adhesive Web Pagehttp:/linkThe amount of coating and coverage r
43、equired is product dependent and will be determined by validation requirements,31,Quality,The addition of UV dye to the coatings makes defects easily visible under a blacklight,32,Assembly Stacking Error Assembly Sliding On Work Surface Error,Both the coating and the components can be damaged by mis
44、handling,Assembly Handling Errors,Pottings,34,Why Use Potting?,Encapsulate and immobilize ionic contaminationProvide physical protection to the circuitry and components without need for a coverProvide mechanical stability to components with respect to vibration and shock,35,What Potting Materials Ar
45、e Used?,Silicone based gelsHard pottingEpoxyPolyurethaneSilicone adhesives,36,Selecting a potting Material,Before use,materials must pass comprehensive compatibility testing(including Surface Insulation Resistance)and solder joint reliability testingRP-9.21 defines the test plan and methodsMaterials
46、 COE approval requiredIn addition,pottings must meet thermal and mechanical requirements for the productCannot impart undo thermal stresson componentsShould not create excess voids orexpansion within a constrained space,37,Cure Processes,Pottings are applied to products by 1-part or 2-part dispense1
47、-part materialsHeatMoistureOxidation2-part materials will generally cure upon mixingCan take 24 hours or more for full cureAdditional minimum hold time may be required before validation testing Up to several daysProcess can be accelerated by heatSystem purge and frequent change of mix tubes is requi
48、red to prevent cloggingMaterial mixing can be done“statically”or“dynamically”,38,2-part mixing techniques,Static MixingMaterial is combined as it flows through a specially designed mix tubeDynamic MixingMixing equipment uses moving parts to combine the potting components prior to dispensing,39,Produ
49、ct Example,SDMFluid Research equipment dispenses PD George UH510S two part urethane,40,Quality,UV dye in a transparent gel can make it visible for automated error proofingSome level of bubbling is to be expected Air escapes from under large componentsSome air entrapped in processExcessive bubbling a
50、nd foaming can occur if material change-over not completed properlySample Workmanship Standard:,41,Bad Product Potting Design,Partial fill of an“inverted cup”will apply excessive stress on solder jointsGel shrinks during cool down after cureSolder Cracking occurred at t=0,42,Summary,Coatings or pott
