Moisture absorption and mechanical performance of surface mountable plastic packages

The results of moisture-absorption tests on 68L plastic-leaded chip-carriers (PLCC) are presented. Environmental variables include both temperature (30, 40, 50, 60 and 85 degrees C) and relative humidity (24, 40, 60 and 85%). Control samples of pure molding compound were tested to determine the effect of leads on the PLCC samples. Comparison was made between absorbed moisture measurements and predictions based on the one-dimensional Fickian diffusion solution. Nonlinear regression analysis indicates that Fickian diffusion behavior represents the actual moisture absorption for relative humidities below 85%. At 85% RH, however, the deviation becomes significant after 500 hr. On the basis of the agreement at moderate values of relative humidity and time, analytical models were developed that show the effect of temperature and relative humidity on moisture mass gain and diffusion coefficient. A stress and deformation analysis was performed to predict the envelope in parameter space separating cracked and uncracked packages. The model used depends on the presence of saturated water between the die paddle and the molding compound. Doming of the plastic underneath the paddle is attributed to flashing of all the trapped liquid water to steam.<>