Encapsulation of integrated circuits

Abstract

Integrated circuits mounted on ceramic substrates must be protected from moisture and from mechanical damage by an encapsulation system which must have the same long term stability as the device itself. For Bell System use, beam-lead silicon nitride protected circuits are subjected to greater than 300°C long term aging as well as to high temperature steam to accelerate failures; thus encapsulants must have the same stability. A silicone-based encapsulation system which meets these rigid requirements is described in this paper. A number of resins and rubbers have been evaluated for their ability to maintain low leakage currents under high humidity conditions by measurement of the leakage on coated metallized ceramics under both high humidity and liquid water environments. An accelerated aging program was carried out to establish the long term stability of these coating systems from the standpoint of their moisture protecting abilities. The data show that the extrapolated lifetime of the silicone coatings is about the same as that established for the integrated circuits at their operating temperature. From consideration of published permeability data and the results obtained in this study, it appears that the mechanism of protection against moisture is the inactivation of sites on the surface where water can adsorb or condense to form a continuous film of water which could lead to electrical leakage currents on either the device or substrate surfaces. The best protection is thus afforded by those materials that either chemically react with the surface or are strongly adsorbed at the sites where water would normally condense.