Environmental Hysteresis in Molding Compounds and Voltage Reference Microcircuits Encapsulated in Plastics

Hygroscopic swelling characteristics of different types of molding comp ounds and electrical parameters of voltage reference microcircuits encapsulated in plastics were measured after saturation with moisture at 85 C and humidity varying from 20% to 90% RH. Sorption of moisture at high humidity conditions and desorption during baking resulted in deformation of molding compounds of approximately –0.035% and +0.028% for baking and moisturizing, respectively. The voltage output deviation at these conditions was ±(100 ppm to 300 ppm), which is large enough to cause failures in signal digitizing systems with resolution of 12 bits and higher. An increase in external mechanical compressive stresses caused linear decrease in the voltage output with a gage factor of 1.1 to 1.4. Estimations of the environmental hysteresis in the microcircuits based on the hygroscopic swelling characteristics of molding compounds and gage factors are in reasonable agreement with the experimental data. A simple model of moisture diffusion in flat plastic packages can be used to explain deviations of the voltage output with time after changes in environmental conditions.