TABLE OF CONTENTS The main reason for coating electronic devices and assemblies is to safeguard the reliability of the system; if not properly chosen, applied, and tested, a coating can also adversely affect reliability. The key elements to assure reliability include:
Initial selection and qualification of the coating for the intended application.
Adequate receiving inspection to assure the quality and reproducibility of the coating.
Clearly defined application dimensions such as thickness, coverage, and meniscus.
In-process testing to assure proper application and curing.
Final inspection and testing.
A Physics of Failure (PoF) analysis and corrective active program.
Reliability is assured by considering it in the initial design, for example, by assuring the compatibility of all the materials used with the electrical functioning of the part and by knowledge of and avoiding potential failure modes and mechanisms that can occur. Also, when failures occur, an effective reliability and corrective action program should be in place. This program consists of a plan for determining the failure modes and mechanisms and taking corrective actions.
For high-reliability military applications, the qualification tests of MIL-I-46058 have been the mainstay for decades. Although this specification has recently been inactivated for new designs, the test procedures and the specified limits are still valid and useful. Accelerated temperature/humidity tests are used to measure electrical properties on test patterns representative of the circuit. A Qualified Product List (QPL) is available for five major polymer coating types. Although military environments are quite strict, the requirements for other products, even some...
