Investigating reliability of Sn-35Bi-based solder paste in SAC305 CPU applications at elevated temperature and humidity

Abstract

The Sn-35Bi alloy, has emerged as a promising alternative for low-temperature, lead-free solder applications due to its advantageous properties, including low density, a melting point range of 138–170 °C, and cost-effectiveness. However, practical applications of Sn-Bi alloys require significant improvements in their properties and joint integrity, particularly addressing the brittleness of Bi and the propensity for microstructural coarsening under thermal aging and current stress. This study employs the Arrhenius and Peck’s model to evaluate long-term reliability of the Sn-35Bi-0.5Cu-0.02Ni (melting point 141–182 °C) solder alloy on SAC305 CPU ball grid array (BGA) applications at elevated humidity and temperature. For mechanical performance of the Sn-35Bi-0.5Cu-0.02Ni solder, vibration and shock testing was assessed, while microstructural evolution was analyzed to demonstrate electrical performance. Experimental results indicated significant aggregation of Bi in the Sn-Bi-based solder joints, forming coarse, block-like structures after 1000 h at the temperature of 46 °C and 90% relative humidity, with a notable increase in the intermetallic compound (IMC) layer thickness. Continued exposure for 2607 h revealed further coarsening of the Bi phase and doubling of the IMC thickness, but no cracks were detected in the substrate. In addition, electromigration effects were noted in the solder joint microstructures compared to those without current stress. Although the findings indicate a decline in reliability after prolonged exposure, the solder joints demonstrated functionality for over 5.5 years under typical room temperature conditions. It meets the functional and mechanical–electrical performance criteria, reinforcing its potential as a viable lead-free solder for low-temperature applications.