A novel vapor pressure transfusion model for the popcorning analysis of Quad Flat No-lead (QFN) packages

Vapor pressure is caused by moisture vaporizing during the reflow heating process. Most existing models evaluate vapor pressure based on moisture diffusion in solids by adopting Fick's law. Differing from the traditional ones, a novel vapor pressure transfusion model is proposed in this paper with the consideration of moisture transfusion from the encapsulation molding compound into the crack opening zone. Quad Flat No-lead package is employed for demonstration. Based on certain assumptions that can be justified through physical conditions, a simple 1-D moisture transport model is established and a “moisture convection coefficient” is defined. With the “moisture convection” boundary condition, the analytical solution can be derived. In addition to simulation, an experimental study on the same type of package is also performed for model validation. Compared to the experiment results, it is found that the strain energy release rate calculated with the current novel vapor pressure model is more reasonable than those with traditional models. Several key features of the comparison between novel and traditional models are discussed in this paper.