Reliability of lead (Pb) free SAC solder interconnects with different PWB surface finishes under mechanical loading

Tin/silver/copper (SAC) solder alloys have been introduced as primary alternatives to the eutectic tin-lead solder. SAC alloys are relatively new alloy systems in packaging applications and existing studies are limited and confined mainly to the materials, assembly process, and finite element modeling. It is essential to conduct research focusing on the material aspects of the SAC alloy coupled with its effect on reliability using both experimental methods and finite element simulations. The focus of this paper is experimental study of lead-free solder package test boards under cyclic bend load and drop impact loading. Amkor CTBGA 288 components were assembled on eight layer test boards (JESD22-B111) with Sn/Ag/Cu (95.5Sn3.8Ag0.7Cu) solder using three different PWB pad surface finishes to study the effect of the surface finish on the interconnect reliability. The pad finishes were immersion Au, immersion Ag and the organic solder preservative (OSP). The test specimens were subjected to cyclic bend fatigue and board level drop tests. Strain gage measurements were taken at numerous locations across populated and unpopulated PWBs and the data were used to create contour plots to identify variations in stress due to location and localized stiffness due to components. Results are presented with failure mode and analysis of those packages with different pad finishes and loading conditions. Finite element models were also created for the bend test boards to investigate the stress distribution and stress concentration