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Failure Analysis of SnPb Solder Joints on Cu-Fe-P Lead Frame of Power MOSFET
Keywords: Lead Frame, Failure Mode, Microstructure
The microstructure of solder joints on Cu-Fe-P lead frame of failed MOSFETs was found very uncommon. The failure has not been investigated very well and the failure mode remains unclear. In this study, a failure mode for MOSFET after long term service was analyzed, and different pins were investigated in detail. Cu-Fe-P alloy is the lead frame materials in the failed power MOSFET. The microstructure of the interface between Cu-Fe-P lead frame and SnPb solder was observed by SEM, TEM, EDS and EPMA etc. The results indicate that interface microstructure was affected by working current density and direction. The microstructure of failed pins has obvious crack, more voids and thicker IMC layer, comparing to no failed pins. Failed pins are usually the ones that worked in high current density, which can induce electro-migration and thermo-migration in the system. Both electro-migration and thermo-migration can enhance intermetallic growth and void formation in solder joints. When the direction of the electro-migration is the same as that of the thermo-migration, IMC growth and void formation occur more rapidly than the opposite direction. Fe element richness was observed close to the interface between Cu-Fe-P and SnPb in the failed pins. The different diffusion direction and speed of metal atoms induced by electro-migration and thermo-migration are attributed to such Fe richness at the interface.
Lihua Cao, Associate Professor
Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China, Huawei Technologies, Shenzhen, China
Shenyang, Liaoning

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