Thermal aging of molding compounds

The mechanical properties of polymer encapsulating materials change considerably when packages are exposed to elevated temperatures for longer times. Here we report a systematic study on the change in viscoelastic properties of a commercial molding compound which is stored at 175°C for periods of time ranging up to 4 weeks. It is shown that instead of the expected degradation, the modulus and the glass transition temperature increased and that these effects are accompanied with a relatively large amount of shrinkage. All effects can be interpreted in terms of the formation of an oxidized layer where extra crosslinking occurred.