Solder Fatigue Modeling of Flip-Chip Bumps in Molded Packages

Underfilling with capillary liquid encapsulants is a common process for the majority of flip-chip packages. A secondary process with plastic encapsulation using a molding compound may follow, serving to protect the die from mechanical damage. Due to the need to cost savings, recent developments of suitably engineered epoxy molding compounds have made a single step transfer underfilling/overmolding technically feasible. In this paper, solder joint fatigue, which perhaps represents a failure mode that receives the most attention in flip-chip devices, is studied for a flip-chip based molded package. The solder joint reliability for the transfer molded underfilled package as well as for a conventional underfilled design (molded and non-molded) was assessed and compared via a nonlinear viscoplastic finite element analysis (FEA) under accelerated temperature cycling conditions. The investigation found that the overall package reliability with the more rigid molding compound underfill was much improved over that with the conventional epoxy underfill.