Moisture Desorption and Hygro-Vapor-Thermal Induced Interfacial Delamination in Underfilled FCBGA Package During Reflow Process

As polymeric materials are widely used in integrated circuit (IC) packaging, the damage and failure of packages induced by moisture absorption of polymeric materials have become an increasing reliability concern. In this study, a two-dimensional (2D) finite element model of an underfilled flip-chip ball grid array (FCBGA) package is constructed. The moisture concentration distributions after preconditioning at 85 ℃/85% relative humidity (RH) for 168 h and reflow process are obtained, in which the internal source approach is employed to analyze the moisture diffusion during reflow process. Based on the result of moisture concentration after reflow, the vapor pressure is calculated. The effects of moisture on crack propagation are characterized using virtual crack closure technique (VCCT). Simulation results manifest that underfill, solder-mask and substrate in the package get almost saturated after preconditioning. During reflow process the desorption of moisture occurs rapidly, yet there is still a considerable amount of residual moisture in the package at the end of reflow, bringing about the saturated vapor pressure, which promotes interfacial crack propagation and delamination. For the underfilled FCBGA package subjected to moisture-dependent thermo-mechanical loads, there seems to have a higher risk of crack propagation and delamination at the underfill/solder-mask interface than the underfill/passivation interface.