An experimental study has been conducted into the effects of package related failures in plastic encapsulated semiconductors. As a result of an extensive literature survey particular emphasis was placed on thermal and moisture related failure mechanisms. A number of accelerated life testing techniques including Highly Accelerated Stress Testing (HAST) were used to induced parameter drift and catastrophic failures. Examples of both insertion and surface mount package types were studied. Observations were made on the parameter drift and catastrophic failure mechanisms of plastic encapsulated Bipolar Junction Transistors in dual in line packages. Gain degradation was observed to be the most sensitive parameter of those measured. It is proposed that this is a result of an increase in the extended surface depletion region area due to a degradation in the surface cleanliness. A theoretical analysis of the build up these traps is presented. Package performance test chips were used to monitor the moisture resistance properties and thermal performance of Quad Flat Pack packages. Surface leakage path effects, corrosion onset and thermally induced stresses were monitored using this technique. A computer controlled measurement system was assembled for data acquisition and logging. It is believed that these techniques will be of use in the investigation into the suitability of different package plastic styles for high reliability applications such as military and aerospace systems.