Effect of protective casting materials on product level reliability under accelerated test conditions

Conformal coating and protective casting materials are widely used in electronics products for protection against environmental stresses such as humidity and contaminants and to increase the mechanical stability of the products. Furthermore, in hazardous environments protective materials can be used, for example, to lower the risk of explosion. The demands of many industrial applications have increase as regards product level reliability. Additionally, the environmental use conditions of many applications are harsh. This creates a need for detailed knowledge about the behavior of protective materials under harsh conditions. In this paper the reliability and failure mechanisms of industrial electronics products were studied by means of a high temperature high humidity test (85°C/85% RH) and a temperature shock test (-40...+125°C). The tests were conducted on products with and without protective materials. Polyurethane based rigid material was used as the protective casting material on the products and silicone based lacquer was used as conformal coating. The results from the tests showed that the protective coating changed failure modes and shortened the lifetime of the product under harsh conditions. Moreover, failure mechanisms seemed to change. For the temperature shock test failures in plated-through hole structures were the main failure mode of the samples without protective materials. On the other hand, the delamination of the protective casting and cracks caused by it seemed to be the main reason for the failures when casting was applied. In the high temperature high humidity test, the conformal coating caused component failures and was the main reason for samples to fail.