Improvements in design, materials, and processes of plastic encapsulated 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 the parts of choice from cost, market-availability, performance, and reliability viewpoints. Nevertheless, PEM require appropriate vendor selection, verification by highly accelerated stress test (HAST), and manufacturing precautions. The potential failure mechanisms of package damage, internal part corrosion, and intermetallic growth, due to high temperature and humidity exposure, have been largely eliminated by improvements in the plastic and in the assembly process, and by the addition of die passivation. Models and test methodologies for accelerating (and thereby identifying) these physical phenomena have been developed, proven by testing, and generally accepted within the industry. After thoroughly defining the application temperature/humidity environments, the models are used to define a test program to qualify candidate PEM. This program consists of a high temperature life test, a humidity/temperature HAST and a completely assembled board-level HAST. To insure that subsequent manufacturing processes at Textron DS (including higher assemblies) do not damage qualified PEM, the manufacturing flow has been modified to minimize human contact with components, to eliminate any potentially corrosive chemical interaction with PEM, and to minimize exposure to moisture.
['John R. Gardner']
The appropriateness of plastic encapsulated microcircuits in a specific wooden-round application