Finite Element Simulation Study of Interfacial Crack Propagation in the Underfilled FC-BGA Package

Flip chip ball grid array (FC-BGA) has been used widely since mid-late 1990s as a high-density package technology. The use of underfill can improve the stability and reliability of the package. However, due to the mismatch of coefficient of thermal expansion (CTE), cracking and subsequent propagation are more likely to occur at the interface related to underfill such as underfill/die, underfill/passivation and underfill/solder-mask interfaces, which has become a serious concern for package reliability. In this paper, the propagation behavior of crack at bimaterial interfaces of underfill/die, underfill/passivation and underfill/solder-mask are analyzed by two-dimensional (2D) Virtual Crack Closure Technique (VCCT). The propagation tendency of different interfacial cracks and possible failure forms of the package are predicted according to calculation of fracture mechanics parameters (e.g., energy release rate, G and phase angle, ψ). The effects of temperature on energy release rate are also investigated. Simulation results show that cracks located at different interfaces show distinct propagation tendencies and may lead to different failure forms. The energy release rate increases with temperature when below glass transition temperature (Tg) of underfill. But when temperature reaches Tg, the change of energy release rate depends on the degree of softening effect of underfill and the worsened mismatch of thermal expansion. The risk level of crack located at three different interfaces under thermal load seems to have the order from high to low as follows: underfill/passivation interface, underfill/die interface and underfill/solder-mask interface.