A coupled numerical and experimental study on thermo-mechanical fatigue failure in SnAgCu solder joints

In ball grid array (BGA) packages, solder balls are exposed to cyclic thermo-mechanical strains arising from the thermal mismatch between package components. Since fatigue cracks in solder balls are observed generally at the chip side junction, dedicated fatigue experiments are conducted using eutectic SnAgCu- Ni/Au specimens in order to mechanically characterize the bonding interface. Sn based solders are prone to thermal fatigue due to the intrinsic thermal anisotropy of the beta-Sn phase. Bulk SnAgCu specimens are thermally cycled and mechanical tests are conducted to quantify the thermal fatigue damage. In both damage schemes a strong size effect is observed. Experimental results are used to develop a cohesive zone based fatigue damage evolution law. Fatigue crack propagation is predicted by an irreversible linear traction-separation cohesive zone law accompanied by a non-linear damage variable. Finally, bulk damage in SnAgCu due to thermal fatigue and the interfacial fatigue failure in BGA balls are combined to simulate a BGA solder ball exposed to thermo- mechanical fatigue in 2D. This combined approach gives a more realistic outcome when determining the overall mechanical response, since the microstructural entities and the solder ball itself are on the same size scale and thus the solder ball cannot be treated as a continuum.