Experimental Approach to Determine Damage Curves for SnAgCu Solder Under Sequential Cyclic Loads

Miner’s rule assumes that damage in solder interconnects accumulates linearly under cyclic loading and is independent of the load sequence. Under these constraints, damage is equivalent to cycle ratio, defined as the ratio of the applied cycles to the cycles to failure for the specific cyclic loading condition. Due to these inherent assumptions, Miner’s rule can inaccurately estimate solder interconnect life under sequential loading conditions. A nonlinear damage model with load-dependent damage exponents would take into account the effect of loading sequence under sequential loading conditions. In the nonlinear damage model, damage is related to cycle ratio using a power law relationship with a load-dependent damage exponent. This paper presents an experimental approach to determine the load-dependent exponents under three load levels. Load drop in the specimens, as a result of cyclic loading induced changes, is used as the criterion for the damage state. The tests consisted of a series of constant amplitude (standalone) and blocks of sequential variable amplitude cyclic shear tests in a thermo-mechanical micro-scale analyzer. The load-dependent damage exponents were developed for SAC305 (96.5%Sn + 3.0%Ag + 0.5Cu) solder. The result of the study can be used for SAC305 damage accumulation model formulation. Further, the experimental approach can be used to generate additional fatigue data under variable amplitude loads.