MICROCHANNEL COOLING OF FLIP CHIPS

Abstract

The requirements of faster, smaller and cheaper packages in the semiconductor industry are spurring the IC packaging technology to its limits. Flip chip which provides the highest packaging density, performance and the lowest packaging profile over all the other face-up packaging technologies is being placed as the ultimate packaging solution. Along with these marvelous advances, come relentless challenges with extreme increases in dissipation of heat fluxes for state-of-the-art IC chips, which most conventional cooling methods are inadequate to meet. The advanced and innovative microchannel cooling technique is among the most promising to beat the challenges. This research program aims at theoretical studies of flow and thermal performance of a silicon microchannel and a proposed MEMS micro-cooling system comprising microchannel, pin-fin fan-sink and micropump technologies for cooling of flip chips. The fundamental flow and heat transfer theory of a silicon microchannel etched and bonded on the backside of an IC chip is presented in detail. The simulation analyses of flow and thermal characteristics of the microchannel are carried out on critical elements including pressure drop, hydraulic diameter, aspect ratio, ratio of wall thickness to channel width and coolant inlet temperature with the aid of a self-developed computer code which is finally validated on the adequate and effective estimation of the microchannel thermal performance by typical empirical results. Based on the preceding study, an active MEMS micro-cooling system incorporating microchannel, pin-fin fan-sink and micropump technologies is proposed for cooling of flip chips. The steady-state analytical study of flow and heat transfer with a self-developed Fortran computer program on this closed-loop single-phase water forced convection MEMS micro-cooling system demonstrates that pumping pressure head of the micropump, geometry of the silicon microchannel, ambient temperature and different types of aluminum pin-fin fan-sink coolers have vital impact on the system. Simulation results show that this MEMS micro-cooling system can successfully extract a heat flux of magnitude up to 100 W/cm2 from the flip chip into the ambient whilst maintaining the peak substrate temperature below 100 °C. The present study clearly confirms the proposed compact and considerably effective MEMS micro-cooling system as a prospective advanced cooling solution for the future high power IC devices. Furthermore, in order to deeply understand the flow and heat transfer relations of microchannel single-phase liquid forced convection, analyses and comparisons of flow and heat transfer equations for the simulation with diverse theoretical and experimental correlations as well as recent empirical data are performed by using a self-developed computer code. Results generally show good agreement between the simulation and reported experimental results in laminar flow. However, the underestimation of friction factors and the overestimation of Nusselt numbers in turbulent flow from the simulation indicate that further investigations are needed.