Backside cooling solution for high power flip chip multi-chip modules

Future high performance workstations and server applications call for chips with high I/O count, high power dissipation and large number of high frequency interconnects. The packaging envisioned for delivering this performance is flip chip multichip modules (MCM). The high I/O requirements coupled with short net lengths are driving the substrate and interconnect technology. A typical MCM configuration is 1 or 2 VLSI and multiple cache RAMs, with MCM power dissipation of 150 to 200 W for a 2 VLSI configuration. The systems are forced air cooled, and the thermal solution has to be cost effective in the workstation marketplace. This paper describes an innovative thermal solution for cooling such a high power, high performance, advanced processor MCM. A physical configuration of an MCM that provides all the ingredients for building a high performance system is outlined. The area array dice are solder bumped and flip chip bonded on a ceramic substrate. The interchip connections are made with thin film copper/polyimide structures on the ceramic substrate. The substrate is assumed to be mullite. There are two 17 by 17 mm VLSIs´ which dissipate 45 W each, and 16 SRAMs that dissipate 4 W each. The paper initially defines the cooling requirements and sets the targeted thermal resistances. The issues related to interfacing multiple dice are examined. A viable forced air cooling solution is proposed which uses a stock heatsink and an innovative die to heatsink interfacing scheme. The viability of the proposed solution is checked by a combination of experimentation to determine heatsink characteristics and computer simulation to map the temperature distribution