THE APPROPRIATENESS OF PLASTIC ENCAPSULATED MICROCIRCUITS IN A SPECIFIC WOODEN-ROUND APPLICATION

Improvements in design, materials, and processes of plastic encapsulat ed microcircuits (PEM) have increased their reliability dramatically, to the point where PEM can achieve the '20-year lifetime in unpowered storage' required by certain wooden-round applications. PEM are now th e parts of choice from cost, market-availability, performance, and rel iability viewpoints. Nevertheless, PEM require appropriate vendor sele ction, verification by highly accelerated stress test (HAST), and manu facturing precautions. The potential failure mechanisms of package dam age, internal part corrosion, and intermetallic growth, due to high te mperature & humidity exposure, have been largely eliminated by improve ments in the plastic and in the assembly process, and by the addition of die passivation. Models and test methodologies for accelerating (an d thereby identifying) these physical phenomena have been developed, p roven by testing, and generally accepted within the industry. After th oroughly defining the application temperature/humidity environments, t he models are used to define a test program to qualify candidate PEM. This program consists of a high temperature life test, a humidity/temp erature HAST and a completely assembled board-level HAST. To insure th at subsequent manufacturing processes at Textron DS (including higher assemblies) do not damage qualified PEM, the manufacturing flow has be en modified to minimize human contact with components, to eliminate an y potentially corrosive chemical interaction with PEM, and to minimize exposure to moisture.