Effect of PCB Surface Finish on Sn Grain Morphology and Thermal Fatigue Performance of Lead-Free Solder Joints
Authors: Babak Arfaei, Francis Mutuku, Eric Cotts Company: Universal Instruments Corporation and Binghamton University Date Published: 12/1/2015
Abstract: The continuing decrease in the pitch of interconnects brings new reliability challenges for microsystems. The volume of a solder joint affects its undercooling, microstructure and its reliability. In this work, the effects of variation in both solder volume and surface finish on Sn grain morphology, and lifetime, were assessed. Different PCB surface finishes, such as Cu-OSP, ENIG, ENEPIG, and HASL (commonly used in the microelectronic industry) and some recently developed surface finishes, ENTEK-OM and Pallaguard, were evaluated in ATC testing of SAC305 lead-free LGAs and BGAs. Sn grain morphology was characterized by crossed polarizer light microscopy (PLM). In LGA samples, a clear correlation between lifetime and Sn grain morphology was observed; the greater the degree of interlacing the longer the average lifetime. For instance, LGA samples assembled on HASL surface finish showed the highest degree of interlaced joints and outperformed joints reflowed on all other surface finishes. Use of Pallaguard surface finish resulted in very poor performance of both BGA and LGA components in ATC tests.