Modeling of moisture over-saturation and vapor pressure in die-attach film for stacked-die chip scale packages

Die-attach film failure during soldering reflow is of particular concern for reliability of 3D ultra-thin stacked-die chip scale packages (CSPs), as extremely high vapor pressure can be generated from vaporized moisture to cause severe damages. Under rapid heating, pressure-driven moisture vapor flow could become as significant as concentration-driven diffusion, and should be included in moisture transport models. In this study, a convection–diffusion (CD) model is presented for analyzing vapor pressure and moisture behavior of soft die-attach films for 3D CSPs, with both vapor flow and the effect of temperature on saturated moisture content considered. By using a 1D model according to experimental observations, the over-saturation phenomenon in the thin film and vapor pressure evolution are investigated. It is found that the reflow profiles and substrate thickness could have great impact on the magnitude of vapor pressure, which agrees well with the experimental results. Sensitivity analyses show that a high vapor permeability and a large porosity could lead to low vapor pressure. Being a vapor pressure-based model, the CD model is suitable to study multiple-material systems without discontinuity issue at the material boundary. It is concluded that the CD model could serve as an effective method for predicting vapor pressure and moisture behaviors during soldering reflow for 3D stacked-die CSPs.