Board Level Drop Reliability of Hybrid Solder Joints with Controlled Bismuth Mixing Ratio for Carbon Neutrality

In response to global environmental concerns and requirement to overcome thermal issues in electronic packages, low-temperature solders have been increasingly used. This study focuses on hybrid ball grid array (BGA) solder joints, simultaneously soldering Sn-3.0 Ag-0.5Cu (SAC 305) BGA balls with Sn-57.6Bi-0.4Ag (SBA) paste. However, the inclusion of Bi in these joints affects their drop reliability owing to its brittle nature. To overcome this, we increased the Bi mixing ratio within the solder matrix, aiming to improve its mechanical drop shock resistance. The approach involves varying the volume ratio between SBA and SAC balls and increasing peak reflow temperature. In addition, a novel double-sided Bi diffusion structure joint for high thermal endurance is proposed and successfully fabricated. This method offers several advantages, including reduced process temperature and short Bi diffusion length, for achieving homogenized Bi dispersed joints. Drop shock tests reveal that increased Bi dispersion at the same SBA paste thickness and higher SBA paste thickness enhance drop reliability. The proposed double-sided Bi diffusion structure exhibits reliable drop reliabilities compared to those obtained using the conventional hybrid soldering method, achieving homogenized Bi dispersion without increasing the overall SBA paste volume or surpassing the melting point of SAC 305.