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

AT&T's number five electronic switching system (ESS) is a state-of-the-art electronic switch that uses solid-state gated-diode-crosspoint (GDX) for fast switching of telephone calls. These GDXs operate at 375 V and require exceptional protection and reliability performance of these packages. Ceramic hermetic packages were used for these type of devices. In order to reduce the cost of these GDXs, a new type of flip-chip bonded structure with 95/5 Pb/Sn solder joints GDXs was developed. These flip-chip GDXs were surface-mounted on a conventional ceramic substrate. An thixotropic silicone gel was used as an underfill and overfill glob-coating to provide the ultrahigh 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, and 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 packages.