Shear Fatigue Analysis of SAC-Bi Solder Joint Exposed to Varying Stress Cycling Conditions

Sn–Ag–Cu (SAC) solder alloys are frequently used in electronic assemblies, and their fatigue resistance under various stress cycles is a significant factor that influences the reliability of electronics. Three distinct SAC-based solder joints (SAC305, SAC-Q, and SAC-R) were examined under various stress amplitude cycling and ambient temperature conditions. The stress on individual solder joints alternates between mild stress (MS) and harsh stress (HS) until complete failure. The hysteresis loop was investigated and compared under various stress amplitude cycles. In addition, the morphology and microstructure of the solder joints were analyzed. The results revealed that SAC-Q, with high Bi and Ag content, demonstrated superior fatigue resistance compared with SAC305 and SAC-R under varying stress amplitude conditions. The inelastic work increased with each change between the MS and HS cycles. SAC-Q exhibited solid solution and precipitate hardening mechanisms through the presence of bismuth (Bi) and silver (Ag) in the tin (Sn) lattice. At the same time, SAC305 showed Ag3Sn precipitates hardening, whereas SAC-R, with no Ag, demonstrated solid solution hardening exclusively. Different failure mechanisms were observed depending on the alloy composition and strain rate. SAC305 and SAC-R exhibited ductile failure. However, SAC-Q experienced an interfacial fracture. Excessive Bi may lead to the embrittlement of solder joints and a change in the failure mechanisms during mechanical testing.