Effects of temperatures above the glass transition on properties of plastic encapsulant materials

The effects of time and temperature on four mold compounds with T _g values in the range 145-190°C were studied. The selected plastics were heated at temperatures ranging from 25°C to more than 100°C above T_g, for times ranging from 24 hours to as much as 1500 hours. The materials were then analyzed for the effects of heating on T_g, coefficients of thermal expansion, volatilized substances, and surface physical, chemical and mechanical changes. Considerable degradation in the properties of the mold compounds was observed to take place. For instance, there was clearly a change in the physical appearance of the parts and changes in T_g values and surface hardness. As expected, the extent of degradation depended strongly on exposure temperature and time. While this study did not include electrical function, changes observed in the plastics indicate that, as a general rule, PEMs should not be exposed to temperatures greater than T_g for significant lengths of time