Estimated reading time: 6 minutes

ECWC 2014: The Base Materials Session

Typical encapsulation materials were epoxy, polyol or silicone resins, with fillers to improve mechanical properties, thermal conductivity and flame retardancy, and additives for such purposes as reducing viscosity, degassing, improving adhesion, or humidity protection. The temperature shock resistance of these materials was a function of coefficient of thermal expansion, glass transition temperature, and elastic modulus. Frequently, the customer requested a material with a low elastic modulus, but also wanted a low coefficient of thermal expansion, which tended to be conflicting requirements. Additionally there were market requirements for materials with glass transition temperatures outside the operating temperature range, which further limited the choice of available material an could result in significant increase in cost.

The low elasticity of metal packages compared with plastic packages could provoke early failure of the potting compound, or the module itself, under thermal shock conditions. Buerger described testing procedures, programme cycles, temperature ranges, rates of change of temperature, and hold times, which could meaningfully evaluate the reliability of potting compounds under thermal cycling and thermal shock conditions. He also reviewed the causes and sources of processing defects, many of which could be traced back to the effects of poor storage conditions on the usable shelf life of the material.

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