Intergranular thermal fatigue damage evolution in SnAgCu lead-free solder

At present, SnAgCu appears to be the leading lead-free solder in the electronics industry. Driven by miniaturization, decreasing the component size leads to a stronger influence of microstructure on the observed lifetime properties. The present study concentrates on the thermal fatigue response of a near-eutectic SnAgCu solder alloy with the objective of correlating damage mechanisms with the underlying microstructure, on the basis of which a thermo-mechanical fatigue damage evolution model is characterized. Bulk Sn4Ag0.5Cu specimens are thermally cycled between −40 and 125 °C up to 4000 cycles. As a result of the intrinsic thermal anisotropy of the β-Sn phase, thermal fatigue loading causes localized deformations, especially along Sn grain boundaries. Mechanical degradation of test specimens after temperature cycling is identified from a reduction of the global elasticity modulus measured at very low strains. Using OIM scans, the test specimens are modeled including the local grain orientations and the detailed microstructure. A traction-separation based cohesive zone formulation with a damage variable that traces the fatigue history is used to model interfacial interactions between grains. Damage evolution parameters are identified on the basis of the experimentally obtained global elastic moduli after a certain number of cycles. The resulting damage evolution law is applied to a number of numerical examples and the mismatch factor is discussed in detail. Finally, the damage evolution law characterized in this study is exploited towards the fatigue life prediction of a 2D microstructure-incorporated BGA solder ball.