Evaluation of Vapor Pressure Induced Debonding Failure in Fan-Out Package Under Reflow Condition

Polymer compounds including epoxy molding compound and underfills are widely used in high-performance multi-die package designs. In addition to the issue of thermal expansion mismatch to Si dies, the moisture absorbing nature of these compounds leads to hygroscopic swelling, interface adhesion degradation, and vapor pressure build up at high temperatures. In this study, the debonding of Si-underfill interface in a fan-out package was considered by using fracture mechanics approach. The interface fracture toughness was first measured by using four-point bending test. A numerical procedure was implemented as finite element post-process for vapor pressure calculation. Virtual crack closure procedure was applied to evaluate the crack driving force under hygro-thermo-mechanical load at reflow condition. Failure driving forces associated with vapor pressure, thermal expansion mismatch and moisture swelling were compared. The results indicate that the Si-underfill debond driving force at the Si-sidewall is significantly higher if vapor pressure is present.