Damage based fatigue criterion for solders in electronic packaging

A fatigue theory with its definition of fatigue failure criterion based on physical damage mechanism is presented for solders. Mura´s micromechanical fatigue model is adopted and extended to each individual grain of the solder structure, where grain´s crystallographic orientation is taken into account. A solder structure is defined as fatigued when the portion of its failed grains reaches a critical percolation threshold, since at this point the failed grains may form a large cluster, leading to structural instability and fatigue failure. Experimental data for 96.5Pb-3.5Sn solder showed good agreement with the prediction by the theory and its failure criterion. The theory is anisotropic, and thus there is no size limitation to its application, making it applicable to anisotropic small-scale (micron scale or smaller) solder joints.