| Three accelerated thermal cycling (ATC) profiles (ramp rate °C/min - temperature range °C: 14-100, 95-100, 95-165) were applied to resistor 2512 and PBGA 256 constructed with either SAC or SnPb solders. The effect of reflow cooling rate on the SAC resistors was also investigated using reflow rates of 1.6, 3.8, and 6.8°C/s. The SAC joints had longer thermal fatigue lives than did the SnPb joints when subjected to the smaller temperature range, but were inferior at the larger temperature range. The overall effect of the ramp rate was relatively small. In contrast, the SAC PBGAs lasted longer than the SnPb ones regardless of the temperature range. The higher ramp rate caused more cyclic damage with both SAC and SnPb PBGAs. The reflow rate of 3.8°C/s was the best for the SAC resistor joints.; Simple yet accurate analytical models were developed for the interfacial elastic and elasto-creep stresses and strains in trilayer assemblies. The models are generic and applicable to assemblies with any aspect ratio under both plane stress and plane strain conditions, and satisfy equilibrium and compatibility requirements. They were then extended to the leadless chip resistors (LCR) with global effects incorporated as boundary conditions.; The LCR model predicted that, for the same geometry and ATC profile, the SAC joints undergo a larger stress range than do the SnPb ones, while the SnPb solders experience a larger creep strain range. The hysteresis of SnPb solders are bigger than those of the SAC solders, both showing shakedown behavior under viscous creep and ratcheting under a nonlinear creep constitutive law.; The LCR model predicted that increasing the ramp rate produces a relatively small decrease in the creep damage in both the SnPb and SAC solders. Cycling over a larger temperature range, however, greatly increases the creep damage in both solders. The strain-based Coffin-Manson model best described the SnPb solder fatigue life, while the energy-based model provided the best correlation with the SAC solder. The fatigue life of SAC solder is more sensitive than the SnPb solder to the ranges of stress, creep strain, and creep energy density per cycle. |
