An investigation of reliability and solder joint microstructure evolution of select Pb-free FCBGA pad finish and solder ball alloy combinations

As Pb-free packaging technologies evolve, increased emphasis has been placed on the development of robust mechanical and metallurgical solutions. A number of different component and PCB surface finish options are now widely available, as are several different BGA solder ball alloys. Similarly, various different Pb-free C4 bumping alloys are being introduced, compounding the need for clear understanding of how the resulting interconnect systems stand up to the normal rigors of end-user applications. This paper focuses on the component-level reliability and metallurgical evolution of the C4 (flip chip) and BGA interconnect systems formed by the combination of commonly specified surface finishes and BGA solder alloys. The two surface finishes selected were electroless Ni/electroless Pd/immersion Au (ENEPIG), and immersion Sn (IT). The selected BGA alloys were Sn-3Ag-0.5Cu (SAC305) and Sn-3.5Ag. All four of these alloy/pad finish combinations were found to pass the various industry-standard (JEDEC) reliability stress tests without any failures, as measured through open/short testing and CSAM analyses of the packages. However, significant differences in microstructural evolution of the solder/substrate interfaces were observed.