Evaluation of a U.K. specification for the procurement of plastic encapsulated semiconductor devices for military use

Abstract

Recommendations in a report, on plastic encapsulated semiconductor devices (PED) [1], produced by RSRE, Malvern, U.K. and subsequently adopted are described. These include the conditions of acceptance of PED for U.K. Ministry of Defence use, the setting up of a focal point to deal with PED matters, the drafting of a specification for the procurement of PED for military use and the undertaking of research into the behaviour of PED using accelerated testing and tropical trials. The contents of the specification and the programme to evaluate it are then outlined. 13,615 devices are being used in the evaluation which is not yet completed. Some hermetic devices are included for comparison. A summary of results obtained to date is then given. There were very few failures due to open circuit in the thermal cycling tests, but in many of the other tests there were variations of performance between makes of both PED and hermetics, in some cases large. The best PED performed as well as the best hermetics. Of particular note was the generally good performance of the TTL circuits. It is in the damp heat tests that some makes of PED failed relatively quickly, although others withstood such tests as the 85°C/85% RH test for over 5000hr. The application of a voltage, which reversed biased junctions and generated minimum heat, generally speeded up failure of PED in damp heat. Problems of specifying a commercially viable short term damp heat test are then touched on. The electrical performance of silicone encapsulated devices was worse than epoxy devices after the salt mist tests. Finally, some details of failure analysis are given. This shows that catastrophic failure of plastic encapsulated circuits in the damp heat tests was due to corrosion of the aluminium.