Mechanistic study of failed SnPb/Cu-Fe-P solder joints in power MOSFET

The failure of solder joints on Cu-Fe-P lead frame of power MOSFETs was found after long term services. The microstructures of failed and accelerated experimental solder joints have been investigated in detail in order to find out the failure mechanism. SEM, TEM, EDS and EPMA techniques were used in this study. Observed results showed that all failed MOSFETs had worked in the higher current while the unfailing ones worked in lower current during the same service conditions. Accelerated electro-migration experiment in laboratory on Cu-Fe-P solder joints showed similar IMC morphology and Fe richness to the failed pins. The results indicate that the failed pins are usually the ones that worked in high current density and especially with the same directions of electro-migration and thermo-migration in the system. 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. The main reasons of failure are the higher working current and the higher working temperature produced by the high current under long time service.