Investigation of the underfill delamination and cracking in flip-chip modules under temperature cyclic loading

In this paper, stress singularity in electronic packaging is described and three general cases are summarized. The characteristics of each stress singularity are briefed. In order to predict the likelihood of delamination at a bimaterial wedge, where two interfaces are involved, a criterion is proposed and the corresponding parameters are defined. The propagation of a crack inside a homogeneous material with the effects of delamination and stress singularity is predicted by the maximum hoop stress criterion. The proposed criteria are adopted in the analysis of a flip-chip with underfill under thermal cyclic loading. A finite element (FE) model for the package is built and the proper procedures in processing FE data are described. The proposed criterion can correctly predict the interface where delamination is more likely to occur. It can be seen that the opening stress intensity factor along the interface (or peeling stress) plays a very important role in causing interfacial failure. The analytical results are compared with experimental ones and good agreement is found. The effects of delamination and cracking inside the package on the solder balls are also mentioned. Further investigation into the fatigue model of the underfilled solder ball is discussed.