A study on thermomechanical reliability of flip chip package based on MUF process

Thermomechanical reliability performance of solder bumps is a critical issue in flip chip package. Traditional underfilling process in flip chip packaging applied capillary principle to make underfill material flow into the gap between die and substrate, it was called CUF (Capillary Underfill) process. In recent years, a low cost solution called MUF (Molded Underfill) process becomes a substitution of CUF process, since the process combines underfilling and transfer molding into one process by using MUF molding compound. In this study, finite element (FE) simulation was applied to analyze thermomechanical reliability property of solder bumps in flip chip package adopted MUF process, and compared to that used traditional CUF process. Reliability tests had been performed to verify FE predictions, results showed that solder bumps with the maximum stress and strain appeared at the die corners. The corner bump with maximum stress and strain was chosen as the critical bump to estimate the thermal fatigue life. Modified Coffin-Manson equation was applied, predicted thermal fatigue life of solder bumps protected by MUF was longer that used CUF process. Various design parameters have been analyzed by FE simulation, results showed increasing bump size and molding thickness can effectively improve thermal fatigue life.