Effects of Variable Amplitude Loading on Lead-Free Solder Joint Propoerties and Damage Accumulation

Current extrapolations of accelerated test results to life in long term service may be greatly misleading when it comes to lead free solder joints. Notably, realistic service conditions are almost never well approximated by cycling with a fixed amplitude. However, recent results have demonstrated the consistent breakdown of common damage accumulation rules. In isothermal cycling tests the remaining life, after a step-down in amplitude, was invariably shorter than predicted by such a rule, while a step-up tended to have the opposite effect. The present work offers a mechanistic explanation for this and a basis for the development of a practical approach to the assessment of life under realistic service conditions. Realistic BGA joints were cycled individually in a micromechanical tester, monitoring the solder stiffness and the inelastic energy deposition. Cycling was seen to first cause rapid hardening, followed by leveling off in a "cyclic saturation" stage and eventually the initiation and growth of a crack until failure. A temporary increase in amplitude during cycling caused a lasting reduction in hardness, and thus enhanced inelastic energy deposition and damage evolution, afterwards as well. This would dominate during repeated increases and decreases, eventually shortening the remaining life dramatically.