Finite Element Simulation Study of the Effects of Kirkendall Voids in IMC Layer on Interfacial Crack and Reliability of Cu–Sn Solder Joint

The past decade has seen increasing attention to Cu–Sn solid-liquid interdiffusion (SLID) bonding technology, which can be applied to achieve bonding at a low temperature and the bonded interconnect (joint) is capable of withstanding high service temperatures. However, due to different diffusion rates between Cu and Sn atoms, Kirkendall voids arise in interfacial intermetallic compound (IMC) layers, then crack may initiate in the region with large Kirkendall voids near the Cu/IMC interface, which has become a serious concern for the SLID bonding technology. In the present work, the propagation behavior of crack caused by voids at the Cu/Cu
3
Sn interfaces is studied by three-dimensional (3D) finite element analysis via virtual crack closure technique (VCCT). By calculating fracture mechanics parameters (such as energy release rate, G) at the crack tip, the influence of Kirkendall void distribution and different phase thicknesses in the solder joint on the tendency of crack propagation is investigated. The effects of crack length and temperature on fracture propagation tendency are also studied. The simulation results manifest that crack at the interface of Cu/Cu
3
Sn is more likely to propagate than inside intermetallic compound (Cu
3
Sn) layer. Moreover, a void-type crack has a higher energy release rate than an interfacial crack. The average value of G increases significantly with expansion of the void area, while G
max
tends to decrease at the crack tip. The increase in thickness of both Cu
3
Sn and Cu
6
Sn
5
layers promotes the propagation of crack. Besides, the thickness of Cu layer also influences crack propagation greatly. G
max
increases with increasing thickness of Cu (UBM) layer from 0.5 μm to 1.1 μm, while decreasing rapidly as Cu layer continues to thicken.