Selection criteria for liquid encapsulants in new microelectronic packages

The relation of glass transition temperature to performance in thermal cycling is reviewed and is related to the behavior of the modulus and the coefficient of thermal expansion (CTE) with temperature. These three properties interact to dictate performance in thermal cycling, which can be the source of many failures. Each property can be adjusted to some extent with resulting tradeoffs. Fillers will dominate the measured CTE. They will also dominate the viscosity of the product and its rheology during cure. In some cases, it is possible to lower the modulus of product without appreciably affecting the CTE. The reduced modulus can yield reduced stress. From these data selection criteria for encapsulating the various package formats have been extracted. Based on the physical characteristics of the package and the constraints on curing, the user can select an appropriate encapsulant on the basis of its cured physical properties.<>