Henkel Pb-Free Soldering Technology - Laocsmta.org

Henkel Pb-Free Soldering TechnologyTransitioning to a Pb-free ProcessDr. Brian J. TolenoAmericas Application Engineering Team Leaderbrian.toleno@us.henkel.comHenkel TechnologiesMarketplace - Alloy Selection• Japan• still a range of materials spanning different products (resultof original broad spectrum R&D?)• focusing down on SnAg3.0Cu0.5 (SAC305)• Europe and Americas• general consensus for SnAg3.8(+0.2)Cu0.7(+0.2) (SAC387)• NEMI, IDEALS, IPC, SMART, NCMS• Industry Groups trying to establish aninternational consensus• the small differences do not matter functionally• IPC Solder Value Council – USA -> SAC305Why Pb-free?• EU Legislation – July 2006• WEEE (August 13,2005) and RoHS• Japan• Market Pressures• China• Copy EU RoHS• Testing must be done by Chinese certified lab• United States• State Legislation (WEEE) – California & Maine• CA also has a copy of the EU RoHS (Jan 2007?!?)Mechanical Properties and Reliability1

Henkel Pb-Free Soldering TechnologyMechanical Properties of Pb-free AlloysAccelerated Testing• A great deal of published data onthe bulk properties of Sn/Pb• this does not generally correlate withreliability in electronic assemblies• Limited set of data for all the Pbfreeoptions• data often contradictory need tounderstand how data was obtained.• Purpose of accelerated test• accelerate the real life failure mode• generate a “conversion factor” to predictlifetime• Danger of accelerated tests• accelerate a failure mode that does nothappen in real life• assume the same conversion factorapplies to different situations - or differentalloysFactors Affecting Joint Reliability• Assembly design - stresses & strains imposed onindividual joint• Component/PCB size & material• Assembly compliance• LCCCs, chip components give low compliance• Leaded QFPs give high compliance• Soundness of joint - interfacial defects, fillet size,standoff height, voids• Joint microstructure• Solder alloy composition, dissolution from board/componentfinishes• Local cooling rate, time since joint formationVibration Tests of Joints - IDEALSPush offstrengths as% of initialvalue0 cycles RandomVibration 6 hrs@20G @50-2000HzThermalShock–40/+125CCombinedRV+TS0805 SnAgCu 100% 82% 71% 76%1206 SnAgCu 100% 93% 92% 82%0805 SnPbAg 100% 100% 78% 70%1206 SnPbAg 100% 100% 65% 73%SnAgCu slightly worse in pure vibration, better incombined thermal shock and vibration2

Henkel Pb-Free Soldering TechnologyNCMS 2 - All Components2.502.001.501.000.500.001.401.201.000.800.600.400.20-55 / +160 o C0 / 100 o CAssembly With Pb-free Materials• Alloy Physical Properties• Impact to Process• Example of Production Study0.0063/37SnAg3.5SnAg4Cu1SnAg3.5In1.5SnAg2.5Cu0.8Sb0.5SnAg4Cu0.5SnAg3.5Cu1Bi3.3SnAg4.6Cu1.6Sb1Bi1Reliability SummaryChanging from Sn63 to SACComparison of Alloy Properties• SnPb is better than Pb-free (SAC)under severe reliability test(-55/+160°C) conditions• Pb-free alloys are superior toSnPb under low-cycle fatigueconditionsAlloy Sn63 SnAg3.8Cu0.7Melting point 183C 217CDensity (g/mm2) 8.5 7.5Surface tension 481 mNm-1* 548 mNm-1**@ 260C in airWetting angle on 12* 43Cu (deg)* Values for Sn60Pb40** Values for pure tin3

Henkel Pb-Free Soldering TechnologyPhysical Properties of Pb-free Alloys AffectingAssembly• Higher melting temperature than Sn/Pb• Effects Reflow• Change for components manufacturers• Narrower process window• Higher surface tension than Sn/Pb• Pb lowers surface tension of Sn• Observable effect in wetting behaviour• Density Difference• Changes metal loading in paste from ~90% to~88.5%• Minor impact to tack and releaseReflow Profile TypesComponent Max TempTypical Lead Free Profile217 - 221° C183° CTypical Lead ProfileAssembly Temperatures > 255° C may damage componentsTEMPERATUREPaste Chemistry During ReflowSolvent EvpnHeat SoakingViscosityViscosityHeat TransferTIMEAlloy Melts, Wetting& Wicking OccurSolidification ofSolder JointsReflow Profile Types• Linear• Small components, rapid cycles• More popular profile• High Hold• Higher mass product• Ensure component heat distribution• Double soak• Flux activity management (wettingmanagement)• Not in large useMeltingPointMeltingPointMeltingPoint4

Henkel Pb-Free Soldering TechnologyThe Pb-free Reflow Process Challenge• Achieve good wetting on the coldest part ofthe circuit board• 235°C for 10 seconds is the accepted minimum –reports we can achieve good wetting at 229 o C• Minimize the temperature of the hottest partof the circuit board• Keep the process stable• Monitor the process accuratelyAlloy Contact Angle vs. SubstrateSolder paste contact angles all in the range 20 - 25°for the same flux systemSUBSTRATE REFLOWED ALLOY PELLET (Sn +)0.5Cu 3.5Ag 3.8Ag0.7Cu 3.5Ag0.5Sb 3.8Ag0.7Cu0.5Sb 37PbCu 42 43 43 41 43 12Ag 19 26 24 30 33 13Sn37Pb 19 19 22 20 22 5Sn0.7Cu 15 11 18 11 10 17Au overNi9 6 10 14 5 4Worst Case Scenario forPb-Free Solder PasteWetting Angle on CopperBoard ∆T+20°CLonger, HotterPreheatMeltingTemperature217°CStability Error+5°CWettingAdequate+20°CHot Spot 265°CMeasurementError +5°CCold Spot235°C43 096SC14 0CopperSn/Pb5

Henkel Pb-Free Soldering TechnologyExample of WettingSummary wetting speed vs temp.1.4• Solder Melting• Wetting up theleadtime to 2/3 max wetting force, s1.31.21.110.90.80.70.660/40SnAg & SnAgCubase alloysSnCu base alloys0.50.4200 210 220 230 240 250 260 270 280Test temperature, °CPb-free Alloy Wetting BehaviourCan Wetting Rate be Enhanced?• Higher melting temperature• expect to need higher solderingtemperatures• higher soldering temperature increasesneed for flux activity• Wetting Balance Tests• quantify the wetting rate behaviour• Spread Tests• quantify the equilibrium wettingbehaviour• Flux activity is the main parameter• Alloying changes• Sb, Bi have been proposed aswetting rate enhancers• Observations show the same effectof minor and impurity elements asknown for Sn/Pb alloys6

Henkel Pb-Free Soldering TechnologyWetting Rate and Ability• Wetting rate effects:• Degree of tombstoning• Ability of component to self centerSelf-Centering AbilityBefore ReflowAfter Reflow• Both are also related to surfacetensionTombstoningComponent Self-Centering Process Window7

Henkel Pb-Free Soldering TechnologyWetting Test Conclusions• Wetting rates and spread for Pb-free alloysrender assembly processes viable• Surface tension of lead free alloys is higherthan SnPb• spread is lower• Maximum wetting force is largely independentof the alloy• Wetting speed for SnAg and SnAgCu alloys isacceptable at lower superheat temperaturesthan SnCu alloysPb-free Solder Joints• Lead-free:• Sn / Ag / Cu / Au / Ni• More variation in surface finishes• OSP, IAg, ISn, ENIG• Needs more investigation“Traditional” SnPb Solder JointsMaterials Used in This Study• Currently:• Sn / Pb / Cu• Sn / Pb / Au / Ni• Still has lamellar structure• Solder Alloy• Discrete Components• Sn 95.5 Ag 3.8 Cu 0.7 • Tin/Nickel•• Board Finishes100% Tin• OSP• µBGA• Immersion Tin• SAC 305 bumps• ENIG• QFPs• Tin / Bismuth• 100% Tin• Tin / Copper8

Henkel Pb-Free Soldering TechnologyQFP Sn/Bi + Sn / Ag / Cu + IAgBeyond EngineeringEven if you choose the best solder paste, components andprocess…it is not enough5 µm2.5 µm5 µmOthers Involved in the Process:• Quality• Need to know the differences• Logistics• Transition time period / multiple lines• Purchasing• Sourcing the correct materials• Operators• Need to be trained to understand thedifferences with Pb-freeMicroBGA (Sn/Ag/Cu) + SAC + ISnKey Inspection DifferencesµBGA + ISnLarge IM FingersPresentSAC – Rougher, dull, appearanceLess bright and shinyTHIS IS OK!9

Henkel Pb-Free Soldering TechnologyKey Inspection DifferencesWetting on cut terminationsCu OSPImmersion SnAll Pb-freeDifferent surfacefinishesImmersion AgNi/AuWeaker flux reflowed in nitrogenStronger flux reflowed in airGenerally poorer than Sn/Pb pastes but can be improvedusing flux changes and/or nitrogenSolder Wetting on PadOperator Training• Manufacturer B working with usconducted operator training• Review of characteristics of Pb-freePaste• Wetting Performance• Wetting Angle• Solder Joint Appearance• All Operators went through 2 trainingcourses• 32 hours total over 2 months10

Henkel Pb-Free Soldering TechnologyYield Performance – Full Time Manufacturing“Mixed Metal” Systems• Even with exemptions full Sn/Pbmay not be possible• Mistakes will happen• Availability of componentsAfter 2 months, yield is better with Pb-free!!Pb-free Manufacturing“Mixed Metal” SystemsPb-free “leaded” component finishes• Pb-free Cell Phones have been inproduction for over 2 years• Choosing the right materials is critical• With the right training andimplementation you can do as well aswith Sn/Pb• Pure Sn• Most common• Whiskers(?)• Sn/Bi• More popular in Japan• Reliability Questions• Ni/Pd/Au• Vendor Specific• Questions aboutwettability• Sn/Pb + Pure Sn• Wetting issues• Reflow Profile• Sn/Pb + Sn/Bi• Sn/Bi/Pb – low melttemp• Sn/Pb + Ni/Pd/Au• Least affected11

Henkel Pb-Free Soldering Technology“Mixed Metal” Systems“leaded” component finishes“Mixed Metal” SystemsPb-free bumped component finishes• Failed SOIC lead• SAC 387 Paste• Pure-Sn Lead• ENIG SurfaceFinish• Components Failafter burn-in• SAC Alloy• SAC 305• SAC 405• All SACs appear to be equal• IPC Solder Value Council• Many, Many, Many Questions“Mixed Metal” Systems“leaded” component finishes“Mixed Metal” SystemsPb-free bumped component finishes• Failed SOIC lead• Lead 80/20 Sn/Pb• Failure atcomponent lead• Quat eutectic(Sn/Pb/Cu/Ag)melts at 172• Failed in hightempburn-in• Outer Appearance – BGA bumpsNew BGA12

Henkel Pb-Free Soldering Technology“Mixed Metal” SystemsPb-free bumped component finishes• Outer Appearance – BGA bumpsLead paste Reflowed@ 210 CBall did notmelt“Mixed Metal” SystemsPb-free bumped component finishes• Pb-free BGA bumps / Sn/Pb Paste• Metals in the solder joint:• Sn (bulk)• Ag• Cu• Pb• Ni• Au• Not evenly distributed“Mixed Metal” SystemsPb-free bumped component finishes• Pb-free BGA bumps / Sn/Pb Paste &Profile• Bump doesn’t melt/collapse• Component at a higher standoff• Component tilt• Impact to reliability• Pb-free BGA bumps & Profile / Sn/PbPaste• Flux exhaustion of the Sn/Pb Paste• Increased voiding/blowholes• Poorer wetting“Mixed Metal” SystemsPb-free bumped component finishes13

Henkel Pb-Free Soldering TechnologyMixed Metal Reflowed BGAFinal Joint Compositions and Liquidus Temp. withSn3Ag0.5Cu Ball and Sn37Pb PastePitch(mm)1.27ApertureSize (mil)21 (Ro)StencilThickness(mil)65Transfer Ratio(%)100100BallDia.(mil)2828% ofPb3.42.9% ofAg2.72.8% ofCu0.50.5Liquidustemp. (C)2162161.018 (Sq)590224.92.60.42150.816 (Sq)5851411.12.20.42090.650.514 (Sq)11 (Sq)548090101017.011.91.62.10.30.3203(200*)208Hillman, et al., CMAP Pb-freeConference, May 2005.* Calculated by Dr. Ursula R. Kattner of NIST from NIST thermodynamic databaseJ. Pan, et al., IPC/JEDEC Pb-free Conference, March 2006Mixed Metal Reflowed BGAThermal Cycling-55 / +12510C/min, 11 min dwell“Normal” Sn/Pb= 1000+Mixed Metal = 267Solectron Lead-free Working Group ProjectThe fatigue life ofmixed assembly ispoorer than that ofSAC/SACHillman, et al., CMAP Pb-freeConference, May 2005.No significant differences inATC reliability betweenSAC/SAC and previousSnPb/SnPb workJ. Pan, et al., IPC/JEDEC Pb-free Conference, March 200614

Henkel Pb-Free Soldering TechnologyEffect of Pb ContentEnvironmental Health and Safety IssuesStudy showed that theamount of Pb-rich phaseincreases and becomescoarser as the Pb impurityincreases.The Pb-rich phase is theweakest region in the bulksolder, and the crack maypropagate along the Pb-richphase interface duringreliability testing.Ref. Zhu, et al., “The effect of Pbcontamination on the microstructureand mechanical properties ofSnAg/Cu and SnSb/Cu solder jointsin SMT,” Soldering and SurfaceMount Technology, Vol. 12, No. 3,2000, pp. 19-23.• Solder still contains metals• Heavy metal systems are toxic tohumans• Silver is a significant aquatic toxin• Handle as the same as Pb-containingmaterials• Good laboratory practices• Separate waste containers• Gloves, hand washing after shift/beforebreak• No need to report EPA/TRIConclusions• There are success with mixed metalsystems• Reliability window is more narrow• More work needs to be done• iNEMI committee looking into mixedmetal BGA/CSP assembly• Other companies also investigating• Avoid it if possible, understand the risksif required• Purchasing needs to understanddifferences in components• Incoming quality inspection?Lead-Free Impact on SMT – BoardFinishes15

Henkel Pb-Free Soldering TechnologyRequirements for a Pb Free surface Finish• Solderable surface for Pb freealloys.• Planar surface.• Good shelf life.• Can withstand 3 reflows.• Good contact for pin probing.ENIG Surface FinishAdvantages• Established finish• Planar surface• Good shelf life• Good solderability• Compatible with 3reflowsDisadvantages• Cost• Complex process• Black pad• Gold embritlementCandidate Surface FinishesGold Embritlement• Electroless Nickel Immersion Gold (ENIG)• Immersion Tin (IT)• Immersion Silver (IAg)• OSP Copper• Pb Free HASL (Sn/Cu/Ag)• Electroless Nickel Electroless Paladium,Immersion Gold (ENEPIG )Theoretical Average Au Content of FiletStencil ThicknessAu Thickness 4 Mil 5 Mil 6 Mil50 nm 0.19% 0.16% 0.13%100 nm 0.38% 0.32% 0.26%200 nm 0.76% 0.63% 0.52%500 nm 1.89% 1.58% 1.31%Au Flash 50 -200 nm3-6λm of NiBase Cu1 µinch = 25 nmAssumes 88% SAC 305 Paste16

Henkel Pb-Free Soldering TechnologyOSP Surface FinishImmersion Ag Surface FinishOrganic layer 10-200 nm• Poor spread of Pbfree alloys.125-250nm of Ag• New surface finish(Less than 5 years)Base Cu• Pb free OSP versionsavailable.• Thermally degradedBase Cu• Ag stabilized withorganic preservative• Chemically degradedby flux.• UL electromigrationconcerns beingaddressed.OSP Surface FinishImmersion Ag Surface FinishAdvantagesDisadvantagesAdvantagesDisadvantages• Low cost• Planar surface• Established finish• Simple intermetalics• Robustmanufacturingprocess• Low cost• Poor Pb free spread• Easily removed• Fragile• Planar Surface• Can take multiple reflows• Drop in replacement forHASL• Reasonable cost• Good wettingcharacteristics• Limited supply base• (should change)• Long term storageunknown• UL issues unresolved17

Henkel Pb-Free Soldering TechnologyImmersion Sn Surface FinishOther Possible Surface FinishesENEPIGElectroless Pd0.75-1.25µm of SnBase Cu• Relatively thickcoating of Sn.• Compatible withPb free alloys.• Most expensive complexprocess• Very stable highlysolderable surface(no black Pad)• Costly• Good solderability andshelf life150-250nm Pd coating• Gained limitedacceptance inEuropeBase Cu25-50 nm Au2.5-5 λm of Ni 200-300 nm PdBase CuImmersion Sn Surface FinishComparison of finishesAdvantagesDisadvantages• There is no one finish “fits all”• Planar Surface• Easy to rework• Drop in replacement forHASL• Reasonable cost• Good wettingcharacteristics• Potential tin whiskerproblem• limited shelf life• Still limited supply• Assemblers need to evaluate surfacefinish pros and cons• Things to consider:• Cost• Planarity• Availability• Storage• Process18

Henkel Pb-Free Soldering TechnologyAdhesion versus Temperature• Adhesion decreases with increasing temperatureManufacturing with a Pb-freeProcessMaterial Compatibility• Pb free wave requires higher pot temperatures.• It is necessary to verify adhesion at highertemperatures is sufficient to prevent lostcomponentsBeyond Solder Alloy0805 Shear Strength at 260C• Other materials in the processalso affected by the change to Pbfree• Surface Mount Adhesive• Conformal Coating• Underfill• Substrate• Board FinishAverage Shear Strength (grams)120010008006004002000Adhesive A Adhesive B Adhesive C Adhesive D19

Henkel Pb-Free Soldering TechnologySMA FailurePoor Underfill Adhesion• Example of poor underfill adhesionto flux residuePoor PerformanceFlux Compatibility Issues• Poor adhesion when first applied and cured• Delamination after stress is appliedConformal Coating Reliability• Need to retestwith new fluxsystems• SIR• Adhesion testing• Corrosion testing20

Henkel Pb-Free Soldering TechnologyOther Considerations• Wave Soldering• Rework• Logistics• Mixed Alloys (Transition Period!)• Voids in Solder Joints• One handheld device manufacturer usingunderfill to meet reliabilityThank YouQuestions?21