Moisture diffusion and heat transfer in plastic IC packages

This paper deals with two-dimensional finite element simulations of moisture diffusion during preconditioning of plastic IC packages, as well as the simultaneous diffusion of heat and moisture during vapor phase reflow soldering. The results show good agreement with experimental data. The limitations of hitherto one-dimensional analyses are also examined. The transient build-up of water vapor pressure in the delaminated pad-resin interface during vapor phase reflow soldering is obtained. It is found that, irrespective of the manner of moisture preconditioning (moisture absorption or desorption), the same critical water vapor pressure level of about 0.9 MPa is obtained for the plastic IC package under study. It is also shown that the water vapor in the delaminated interface never reached saturation during the short period when solder is reflowed and re-solidified. These findings have important implications for the analysis of "popcorn" cracking during solder reflow. They show that any package stress analysis which assumes that the vapor pressure at the delaminated pad-resin interface is equal to the saturation vapor pressure is likely to be erroneous.