Simulation of the Effects of Underfill Solidification on Flip Chip Fatigue Life

Underfill encapsulation provides improved reliability of controlled collapse chip connection (C4), also know as flip chip. The C4 process connects IC chips to printed circuit boards via solder bumps. During the manufacturing process, underfill flows via capillary action around the solder bumps. The underfill then solidifies to mechanically stabilize the connection between IC and PCB. The research presented in this paper assess the stress in the C4 that developing during underfill solidification on the low-cycle fatigue behavior of the solder bumps. Computational simulations assess the stress state in the C4 due to the solidification of the underfill. These simulations include the phase change of the underfill. The model is then thermally cycles the C4 to calculate the stress and strain that develop during thermal cycling. The cycles to failure are then calculated using a low-cycle fatigue damage assessment. This approach locates the solder bump at which low-cycle fatigue cracking will initiate and estimates the number of cycles required to initiate cracking.