Modeling of Vapor Pressure during Reflow for Electronic Packages

Abstract

The plastic materials in electronic packages have high porosity and are susceptible to the moisture absorption. In order to investigate the moisture vaporization during reflow, the representative volume element (RVE) approach, by which the micro-void effect can be taken into consideration, is introduced in this paper. A theoretical model is established to calculate the whole-field vapor pressure in plastic materials. FEA models are built for both flip chip BGA (FCBGA) and wire bond PBGA packages to predict the moisture distribution, followed by the calculation of vapor pressure distribution in the package. Results show that the vapor pressure saturates much faster than the moisture diffusion, and a near uniform vapor pressure is reached in the package. The vapor pressure can’t exceed the saturated pressure at the corresponding temperature, even when more moisture is added in. The vapor pressure introduces additional mismatch to the package, which is directly related to the vapor pressure distribution, rather than the moisture distribution. Moisture desorption during reflow is also studied and it has significant effect on the moisture distribution, but not on the vapor pressure distribution. A complete solution for the vapor pressure subjected to the interface after delamination is also derived.