Electronics Reliability under Thermal Cycling: Microstructure and Mechanical Properties of SAC-Bi Paste Alloys with Varying Paste Volumes and Surface Finishes

Abstract

Electronic assemblies operating in harsh environments are frequently subjected to thermal cycling and mechanical stresses that can degrade solder interconnections and compromise system reliability. The transition from traditional Sn-Pb solders to lead-free alternatives has intensified reliability concerns because lead-free alloys exhibit different microstructural evolution and mechanical behavior under service conditions. In particular, thermal cycling induces cyclic stresses due to mismatches in the coefficients of thermal expansion between components and printed circuit boards, promoting the growth of intermetallic compounds, Ag3Sn coarsening, recrystallization, and the progressive degradation of solder joint integrity. This research investigates the combined effects of solder paste alloy composition, paste-to-sphere volume ratio, and printed circuit board surface finish on the microstructure, mechanical properties, and reliability of ball grid array solder joints. Assemblies were fabricated using SAC305 solder spheres and three solder paste alloys (SAC305, SAC-Bi, and SAC-Bi-Sb) with paste-to-sphere volume ratios of 0.1, 0.3, and 0.5 on organic solderability preservative (OSP) and electroless nickel immersion gold (ENIG) surface finishes. The study employed cross-sectional microscopy, SEM-EDS, nanoindentation, X-ray analysis, and accelerated thermal cycling to evaluate microstructural evolution, mechanical properties, and fatigue reliability.

Results show that increasing paste volume increases the number of Ag3Sn precipitates while reducing the interfacial intermetallic thickness. Larger paste-to-sphere volume ratios also generally improved thermal fatigue life and mechanical stability by producing larger solder joints that better accommodate thermo-mechanical strain during cycling. OSP finishes produced thicker intermetallic layers than ENIG due to the absence of a nickel diffusion barrier. Microalloyed systems containing Bi and Sb exhibited refined microstructures and improved resistance to microstructural degradation during thermal cycling. These alloys demonstrated superior fatigue life compared with SAC305, although excessive voiding in high-volume SAC-Bi-Sb joints reduced reliability. Nanoindentation results further indicate that Bi-containing alloys exhibit higher hardness and reduced modulus, and retain these properties more effectively during thermal cycling than SAC305. The effect of paste volume on hardness and reduced modulus was alloy-dependent: SAC305 showed slightly lower hardness at higher volumes, whereas microalloyed systems exhibited improved mechanical properties and stability. Overall, the findings clarify the interactions among alloy composition, solder volume, and surface finish, providing guidance for optimizing the reliability of lead-free solder joints in harsh-environment electronic systems.