Non-hermetic plastic packaging of high voltage electronic switches utilizing a low-stress glob coating for 95/5 Pb/Sn solder joints of flip-chip bonded multi-chip module high voltage devices

AT&T's No.5 Electronic Switching System (ESS) is the state-of-the-art electronic switch that uses the solid-state gated-diode-crosspoint (GDX) for fast switching of telephone calls. These GDX's operate at 375 volts and require exceptional protection and reliability performance of the packages. Ceramic hermetic packages have been used for these types of devices. In order to cost reduce these GDX's, a new type of flip-chip bonded structure with 95/5 Pb/Sn solder joints GDX's was developed. These flip-chip GDX's were surface-mounted on a conventional ceramic substrate. A thixotropic silicone gel was used as an underfill and overfill glob-coating to provide the ultra-high reliability performance of these high voltage devices and a silicone elastomer was used to pot the hybrid surface-mounted GDX multi-chip module structure to further enhance the robustness of its structure. In this paper, the materials such as the flip-chip solder (95/5 Pb/Sn), silicone gel, the elastomer, and their processes such as flip-chip bumping, reflow, cleaning, encapsulation and reliability testing are described for this robust, high performance and low-cost package.