A.R.E.A. - EFFECT OF PCB SURFACE FINISH ON Sn GRAIN MORPHOLOGY AND THERMAL FATIGUE PERFORMANCE OF LEAD- FREE SOLDER JOINTS

摘要

The continuing decrease in the size of components brings new reliability challenges as the microstructure of lead-free solder alloys is significantly affected by solder volume and composition. It is well known that the microstructure of solder joints greatly affects the reliability of electronic packages. Our previous results have shown that the undercooling and thus Sn grain morphology of solder joints are dependent on solder volume. Different Sn grain morphologies affect the reliability of solder joints in thermal cycling tests. For example, some smaller LGA joints were found to outperform larger BGA joints of the same footprint. Moreover, the presence of various surface finishes can further alter the solidification behavior, microstructure and thermomechanical performance of solder joints. In this work, different PCB surface finishes such as Cu-OSP, ENIG, ENEPIG, and HASL (commonly used in the microelectronic industry) and some recently developed surface finishes such as ENTEK-OM and Pallaguard were evaluated. The effects of varying surface finish on the microstructure of SAC305 lead-free LGAs and BGAs were investigated. Sn grain morphology was characterized by crossed polarizer light microscopy (PLM). The effect of variation in solder volume and surface finish on Sn grain morphology and lifetime was assessed. A clear correlation between PCB surface finish, microstructure and reliability was observed. LGA samples assembled on HASL surface finish showed the highest degree of interlaced joints; those samples outperformed joints reflowed on other surface finishes. LGA samples showed more interlaced Sn grain morphology than larger BGA samples. Use of Pallaguard surface finish resulted in very poor performance of both BGA and LGA components in ATC tests.