How does electromigration induce fracture of IMC in solder joints?

Fracture is commonly observed in intermetallic compounds (IMCs) in the solder joints of flip-chip packages after serving for a certain period of time, with electrical and mechanical loadings applied. How the electromigration effect undermines the performance of IMCs in solder joints and eventually leads to structural fracture remains unclear. In this paper, a phase-field model coupling both mechanisms of electromigration and facture is proposed, which could resolve the full evolving process of defects from the early-stage mass diffusion by electromigration to the final failure by fracture. The model is carefully validated by comparing it with other celebrated electromigration and fracture models. Using this model, the evolution process of two typical defects, the void-like defect and crack-like defect, are analyzed in detail under different electrical and mechanical loadings. We find that electromigration induces fracture of solder joints by the mechanism that mass diffusion caused by electromigration helps to enlarge the defects and develop sharp corners at the defect surfaces, leading to the local stress concentration and crack initiation. Comparison among different models shows that the lifetime of solder joints is co-governed by the electromigration and fracture processes and simply dropping either mechanism leads to overestimation of the lifetime.