Influence of preheat and maximum temperature of the solder-reflow profile on moisture sensitive IC´s

The purpose of this work was to determine the influence of preheat and maximum solder reflow temperature on the level of moisture induced damage in plastic surface mount integrated circuits. Both an analytical moisture diffusion model and Finite Element Modeling were applied to the analysis of absorption/desorption behavior of plastic molding compounds. The models were used to analyze the moisture ingress kinetics during controlled environmental exposures and for the prediction of moisture desorption characteristics during solder reflow conditions. Moisture/reflow experiments were conducted on an 80-pin PQFP, 225-pin PBGAs, and two TSOPs (a 56-pin and a 40-pin). Devices were evaluated using C-mode Scanning Acoustic Microscopy. Effects of maximum reflow temperature on measured damage response were determined showing that as a rule the JEDEC/IPC moisture resistance drops by one level for every 20°C increase in maximum reflow temperature. The overall benefit of extended preheat is shown to be beneficial for only thin packages that are not saturated. Details of the method used for predicting the beneficial effects of preheating are described