Effect of Reflow on Solder Joint in Low Temperature SnBi Solder Paste

Double-sided system-in-package (SiP) modules have gained popularity for advanced packaging to provide advanced solutions addressing module size, cost, performance, and time-to-market requirements for 5G. The thin package experiences notable warpage after the first reflows with SAC305 solder, followed by subsequent molding challenging the second side processing as well as overall solder joint reliability. The adoption of low temperature solder (LTS) material can help lower thermal stresses during the second reflow as the package undergoes less warpage at the lower peak reflow temperatures. Nevertheless, solder paste printability of fine pitch apertures and solder void performances are the two other major factors affecting the interconnect reliability other than the alloy itself. The adoption of LTS in electronic assembly is mainly driven by technology, reliability, and environmental requirements.The reflow profiles play a significant role in determining the intermetallic thickness formation, grain microstructure, and Bi% distribution other than solder void reduction. Extensive reflow DOEs have been conducted to include the study metrics of solder bump configuration, Bi% distribution and Bi microstructure in addition to the reflow process window for Sn42Bi57Ag1 (SnBiAg) Type6 solder paste.