Study of viscoelastic effect of EMC on FBGA block warpage by FEA simulation

Epoxy Molding Compound (EMC) is the encapsulation material that can provide mechanical support and protection to IC packages. In the meantime, it is also a key contributor to package warpage. Finite element analysis (FEA) has been widely used in packaging development to improve product performance more efficiently. EMC is usually modeled as piecewise linear elastic material in FEA model for simplification. However, EMC has viscoelastic properties like other thermosetting materials. During molding and post mold cure (PMC) processes EMC will undergo polymerization conversion and chemical aging that will change its constitutive behavior and hence affect package warpage. In this paper, block warpage of an FBGA (Fine-Pitch Ball Grid Array) package induced by molding and PMC processes was simulated using FEA method by considering viscoelastic effect of EMC. Results showed that chemical shrinkage of EMC can offset effect of thermal shrinkage on block warpage significantly; and PMC process also helped reduce warpage magnitude by a considerable proportion. To verify the simulation results, experiments had been performed to measure the FBGA block warpage after molding and PMC processes. Comparison showed the FEA prediction had good agreement with actual warpage measurement data.