WhiskerFormation on Tin Plated Cu based LeadframesResultsand Conclusio - PDF document

WhiskerFormation on Tin Plated Cu based LeadframesResultsand Conclusion WhiskerFormation on Tin Plated FeNi42Resultsand Conclusion August2004 WhiskerFormation on Tin Plated Cu based Leadframes •Introduction•Experience E4•Main cause whisker growth on Cu LF•Countermeasures•Conclusions Periodof potential whiskergrowth tinplating assembly/ Cu-basedL/F FeNi42 L/F storageconditionsservicelife conditions2 years~ 15 yearsno whiskergrowthcountermeasurebakingno whiskergrowthseeFeNi42 presentation whiskerwhisker seethis presentation ElectrolytesSchlötter Slototin40Pyramid PyraTin LuxTechnic TechnistanEPOMG Reel Satin 2544 LFAtotechHSMSchlötterSlototin40Pyramid PyraTin LuxTechnic TechnistanEPOMG Reel Satin 2544 LFLucent Satin bright Tin LeadframeMaterialASTM / material numberBrand name/ short notificationCompositionOlin194, K 65 / CuFe2POlin7025, K 55 / CuNi3Si1MgAlloy420.1 Sn; 0.02 Ni/Zn0.1 Fe; 0.03 P2.4 Fe; 0.12 Zn; 0,03 P2.0 Sn; 0.2 Ni; 0,15 P; 0,15 Zn •Test data showed that whiskers gr•Explanation: irregular intermetallic growth 050100150200250300time in days 1,9 µm ambient 1,9 µm 55 °C / ambient 1,9 µm 85 °C / 85 % r.h. longest whisker broken,continued with second longest Part A time in days 1,9 µm Sn 10,8 µm Sn Cu based LF-ambient storage050100150200250300350400 continued with second longest •Observation of strong dependency on thickness may result in acceleration •Similar results available for 4 Cu-materials and 3 electrolytes •Observation of strong dependency on thickness may result in acceleration •Similar results available for 4 Cu-materials and 3 electrolytes Part A Cu Substrate SnDeposit Tin Whiskeris forced out Cu stress in the plating whichis caused by irregular growth of intermetallics Whisker Cu L/FCu Whisker Mechanism onCu based leadframes •Because of higher temperature diffusion will •Recrystallization of Sn•Diffusion barrier for further intermetallic growth•Annealing of stress•Postbake does NOT change CTE mismatch! Protection by Postbake(1h, 150 Sn depositCu based LFCu/Cu3Part A Morphology of the Intermetallics C4 h 125 •Postbake results in double layer of Cu•The average layer thickness of the resulting intermetallic is •Sngrain size 5 to 25 •No additional intermetallic after 12 months storage at Protection by underlayerChanging underlayer results in other than Cu6intermetallics, no stress build-up! Olin194 leadframeNi-underlayer after stripping Snshowing Ni underlayer:Ag underlayer: Whisker on CuPart A 050100150200250300350400time in days 4,2 µm Sn 9,4 µm Sn 0,18 µm Ni /5,2 µm SnNi-underlayer will eliminate whisker risk.Cu-based leadframe @ RT Cu-based leadframe @ RT •Small whiskers can occur due •Postbake does NOT change this mismatch 01000200030004000500060007000# cyclesMaximum Whisker Length (µm) -55/85 -55/85 precond. @ 215 C -55/85 assemb @ 215 C -40/125 -40/125 assemb. @ 215 C -40/125 assemb. @ 260 C Non postbaked Cu leadframes Postbaked Cu leadframe Test condition Preconditioning Max. Whisker Length (µm) Time (h)/ # Cycles Max. Whisker Length (Time (h)/ # Cycles 20-25 C, 30-80 % r.h. - ~ 90 � 5000 10 ~ 8000 20-25 C, 30-80 % r.h. simulated reflow @ 215 0 � 8000 0 ~ 6000 20-25 C, 30-80 % r.h. simulated reflow @ 260 - - 0 ~ 6000 20-25 C, 30-80 % r.h. assembly @ 215 C 0 � 8000 0 ~ 6000 20-25 C, 30-80 % r.h. assembly @ 260 C 0 � 8000 0 ~ 3000 20-25 C, 30-80 % r.h. with 5 V bias applied assembly @ 215 C - - 0 ~ 5000 C/85% r.h - ~ 60 � 5000 10 ~ 4500 C/93% r.h - - - 0~ 4000 C/93% r.h simulated reflow @ 215 - - 0~ 6000 C/93% r.h simulated reflow @ 260 - - 0~ 6000 C/93% r.h assembly @ 215 C - - 0 ~ 5000 C/93% r.h assembly @ 260 C - - 0 ~ 5000 C/85% r.h assembly @ 260 C 0 ~ 5000 - - C/93% r.h with 5 V bias applied assembly @ 215 C ~ 30 ~ 1000 0 ~ 4000 -40 C/125 C, TST, �7’ dwell - ~ 30 1000 ~ 30 1000 -55 C/85 C, TST, 10’ dwell - ~ 30 1000 ~ 25 1000 -55 C/85 C, TST, 10’ dwell simulated reflow @ 215 - - ~ 30 3000 -55 C/85 C, TST, 10’ dwell simulated reflow @ 260 - - ~ 30 3000 -55 C/85 C, TST, 10’ dwell assembly @ 215 C - - ~ 15 3000 -55 C/85 C, TST, 10’ dwell assembly @ 260 C - - ~ 15 3000 -40 C/125 C, TST, 20’ dwell assembly @ 215 C 10 ~ 6600 - - -40 C/125 C, TST, 20’ dwell assembly @ 260 C 10 ~ 6600 - - whiskers found after severe corrosion and exceeding 3000 h test time. Part A NEMI DoE3 Test results 60o Part A 60C/93 %RH Corrosion �Toe area of QFP after 3000 h 60 oShoulder area of QFP after � 3000 h 60o Part A 60C/93 %RH X-sections FootShoulder Compressivestress is the driving force in pure tin layers (without compressive stress no whiskers).Whisker growth on Copper leadframesis mainly caused by large, irregular growth at interface substrate / plating layer.ces longest whiskers on copper leadframe The thicker the Sn-layer the shorter is the whisker.Countermeasures are postbake, Ni-, Ag-underlayer.Temperature cycling may cause a maximum whisker length of 30 µmon Cu Maximum specified whisker length is 50 m for accelerated tests.After 2 years storage at ambient and/or soldering on board matt tin plated Cu-L/F, does not show evidence of whiskers. WhiskerFormation on Tin Plated FeNi42 Results Introduction Cu-basedL/F irregular growth of intermetallics isothermal storage at FeNi42 FeNi42= ~4 ppm/K = ~23 ppm/K mismatch of cte Major mechanismand conditionsforwhiskerformation Part B Periodof potential whiskergrowth tinplating assembly/ Cu-basedL/F FeNi42 L/F storageconditionsservicelife conditions2 years15 yearsno whiskergrowthcountermeasurebakingno whiskergrowthseethispresentation whiskerwhisker Part B 0100200300400500600700time in daysmax. whisker length in µm 3,7 µm Sn 6,6 µm Sn 9,5 µm Sn 11,7 µm Sn 14,7 µm Sn Isothermalstorageof tinplatedFeNi42Isothermal storage does not result in whisker formation for mattSnplated Furthertest w/o whiskerformation 55 °C600 days85 °C / 85 % r.h. 6 months60 °C / 93 % r.h3 months150 °C1000 h Uncontrolledambient atmosphere test package8.5 µm matt tin on FeNi42 Thermal Optical microscope 5 components per test cycles for readout (5 data per identify longest whisker per photo of longest whisker per package Part B Temperature cycling after board assembly -40 °C / +85 °C,5 K/min, 30 min dwell •SnPbplating shows whiskers after TC testing•No difference between SnAgCuand SnPbAgsoldering 0500100015002000cycleswhisker length in µm SnPb soldered SnAgCu soldered Whisker on FeNi42 -40 °C / +85 °C,5 K/min, 30 min dwell Matt pure Sn-platedcomponents 1001200500100015002000cycleswhisker length in µm SnPbAg soldered SnAgCu soldered•Sn-plated components show whisker of similar size as SnPb•Little difference between SnAgCuand SnPbAgsolderingTemperature cycling after board assembly Whisker on FeNi42 -40 °C / +85 °C,5 K/min, 30 min dwell •SnPbplating shows whiskers of similar size as Sn-plated Comparisonof Sn-and SnPb10 platedcomponentsBoard assemblywithSnAgCupasteand process 1001200500100015002000cycleswhisker length in µm SnPb10 plating Sn platingTemperature cycling after board assembly Whisker on FeNi42 -40 °C / +85 °C,5 K/min, 30 min dwell •SnPbplating shows whiskers of similar size as Sn-plated Comparisonof Sn-and SnPb10 platedcomponentsBoard assemblywithSnPbAgpasteand processTemperature cycling after board assembly 0500100015002000cycleswhisker length in µm SnPb10 plating Sn plating •Whisker length after board assembly equal for Sn-•Little influence of solder paste type and respective WhiskerriskforSn-platedcomponentsequalto actualSnPbstandardplatedcomponents Part B