Solid-state diffusion studies of lead-free solders on gold and in polymer films

Lead-free solders are commonly used in microelectronic packages in chip and board level interconnections. They primarily interact with other metals such as gold (Au), which is present in the under-bump metallization or surface finish on the chip and substrate, respectively, and with polymers, which are present surrounding the solders as underfills at the chip level and polymer collars or partial underfills at the board level. The interaction of solders with these materials at high temperatures (210–250 °C) has been widely studied and understood. However, the interaction of solders with these polymers and with Au at lower temperatures (100–120 °C), along with their diffusion behavior with these materials is not widely studied. In this research, the diffusion kinetics and mechanism of two solders, Sn57.6Bi0.4Ag (SBA), and Sn3Ag0.5Cu (SAC305), with melting points of 138 and 217 °C respectively, on bare Au, and in the presence of polymer films on Au was studied. On bare Au surface at 100 and 120 °C, diffusion rate dependence of (time) 0.2 was observed for SBA solder, while a linear dependence on time was observed for SAC305 solder. In the presence of polymer films on Au, solder diffusion was observed at the interface of Au and polymer, as opposed to diffusion through the bulk polymer. In the presence of polymer films, a parabolic dependence of solder diffusion on time was observed for SBA, while linear dependence on time was observed for SAC305 solder, indicating that the presence of polymer affects the solder diffusion rate based on the difference between the melting temperature of the solder and the operating/test temperature. Furthermore, it was also observed that solder diffusion rate is inversely proportional to the polymer Tg. This study provides a better understanding for designing polymers for their use in chip and board level interconnections in microelectronics packages.