Intergranular cracking simulation of the intermetallic compound layer in solder joints

This paper presents a grain level finite element model to simulate the cracking behavior of the intermetallic compound (IMC) layer in solder joints. The grain microstructure of the IMC layer is explicitly included in the model by Voronoi tessellations. Cohesive interface elements with a coupled cohesive law are embedded along the grain boundaries to simulate microcrack initiation, propagation and coalescence in the IMC layer. A model with a Weibull distributed grain interfacial strength is adopted to account for randomly distributed grain boundary defects. The average thickness of the IMC layer and the wavelength and the roughness of the waved solder/IMC interface are used to characterize the IMC microstructure. Using the numerical approach developed, the effects of the grain shape, the randomly distributed grain boundary defects, the thickness of the IMC layer and the morphology of the solder/IMC interface on the microcrack patterns and on the overall response of solder joints are investigated. The results indicate that the overall mechanical strength is not sensitive to the grain shape, but the microcrack pattern and the crack path depend heavily on the grain shape. In the model containing randomly distributed grain boundary defects, the weak grain interface plays a critical role in the overall strength and the crack path of the model. The average thickness of the IMC layer has the greatest impact on the overall strength and the failure mode of the solder joint. The wavelength and the roughness of the solder/IMC interface have little impact on the overall strength but do have an impact on the failure mode of the solder joint. The predicted failure mode agrees well with the experimental observation in solder joints. The presented approach is feasible for simulating microcracking and the failure behavior of the IMC layer in solder joints and other quasi-brittle polycrystalline materials.