Numerical study of thermomechanical fatigue influence of intermetallic compounds in a lead free solder joint

The study aims at investigating the influence of IMC growth on the thermomechanical fatigue behaviour of lead free solder joints. As a potential driving parameter of the failure of such assemblies, a numerical study is carried out to conclude on the implementation in future predictive FE models. A first FE model of a Single Lap Shear test is used to highlight in which conditions neglecting the IMC thickness can be an issue for material identification. It has been shown that common IMC layers present after fusion do not represent an important source of error except for the thinnest and stiffest soldered joints. The most important impact of IMC on this kind of test is the error of estimation on the Young's modulus and the stress increasing at the ends of the joint. The creep properties determination is not impacted by IMC. A second FE model is used to simulate the consequences of IMC growth in a BGA. The simulation of a classic BGA under a thermal load shows that warpage is not really impacted by a stiffer joint contrary to the stress levels in the corner ball. Differential displacement due to CTE mismatch produces higher stress levels in the solder balls near the bi-material interfaces when IMC layers are considered. Submodelling techniques are used to analyse with more details the interfacial gradients of stress and determine if IMC implementation is necessary to reproduce all the potential conditions of failure.