Hermetic sealing polymer encapsulation

The concept of polymer encapsulation dates back almost to the invention of the transistor (JL). To eliminate the costly hermetic seals, it was necessary to find low cost techniques that would also yield handling capability and environmental protection. 

While attempts have been made to mitigate corrosion of electronics by encapsulating the components in plastics, it is useful to note that polymers are permeable to moisture. Hermetically sealed ceramic packaging is more successful. However, care must be taken to prevent moisture and other contaminants from being sealed in. A useful common approach for mitigating corrosion of circuits housed inside a relatively large-size chassis includes the use of volatile corrosion inhibitors. 

Polymeric encapsulation has largely displaced techniques of hermetically sealing electronic devices, due to the low cost and ease of production. However, since all polymers are permeable to some small extent to water vapour, problems of adequate protection do still occur. Interestingly, it is not the inherent permeation of water into the polymer which alone causes these problems. Rather, they arise from the presence of trace amounts of catalyst residues in the polymer, which are transported to the surface of the device by the permeating water and which there result in the formation of an aggressive electrolyte solution. This problem is of particular concern with epoxide resins and of lesser concern with silicone encapsulants. 

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