Closed Loop Liquid Cooling for High Performance Computer Systems

The power dissipation levels in high performance personal computers continue to increase rapidly while the silicon die temperature requirements remain unchanged or have been lowered. Advanced air cooling solutions for the major heat sources such as CPU and GPU modules use heat pipes and high flow rate fans to manage the heat load at the expense of significant increases in the sound power emitted by the computer system. Closed loop liquid cooling systems offer an excellent means to efficiently meet the combined challenges of high heat loads, low thermal resistance, and low noise while easily managing die level heat fluxes in excess of 500 W/cm. This paper describes the design and attributes of an advanced liquid cooling system that can cool single or multiple heat sources within the computer system. The cooling system described use copper cold plates with meso scale channels to pick up heat from CPU and GPU type heat sources and highly efficient liquid-to-air heat exchangers with flat copper tubes and plain fins to transfer the heat to air by forced convection. A water based coolant is used for high thermal performance and additives are used to provide burst protection to the cooling system at temperatures down to -40 C and corrosion protection to critical components. A highly reliable compact pump is used to circulate the fluid in a closed loop. The overall system is integrated using assembly methods and materials that enable very low fluid permeation for long life. INTRODUCTION The power dissipation levels in high performance electronics systems such as computers continue to increase rapidly while the silicon die temperature requirements remain unchanged or have been lowered. These electronics systems are typically cooled by forced air flow using axial fans or centrifugal blowers. When the heat loads are small and relatively diffuse, thermal conduction through aluminum plates is sufficient to spread the heat into finned heat sinks for convection from the fins into the air flow stream. In recent years, as heat loads have increased and become more concentrated, better heat conductors such as copper plates and heat pipes have been used to improve the spreading of heat from heat sources such as the central processing unit (CPU) and the graphics processing unit (GPU) modules into the heat sink fins. These heat spreaders have made it possible to extend air cooling by enabling efficient convection from finned heat sinks that can be situated within a larger volume of space around the heat sources. However, the usable space for the finned heat sinks remains limited by, among other constraints, the heat collection and transport limitations of heat pipes [1] so higher flow rate system fans or additional local fans have to be used resulting in significant increases in the sound power emitted by the electronics system. As the trend towards higher power dissipation and more concentrated heat sources continues, a more attractive solution is to use closed liquid cooling loops to efficiently spread heat to finned surfaces that can be situated almost anywhere within the electronics system. These closed loops may be two-phase capillary pumped systems such as loop heat pipes [2] or single or two-phase mechanically pumped systems such as the singlephase liquid cooling system used in the Apple G5 Power Mac computer [3]. The cooling loops may even be gravity aided thermosiphon systems using dielectric fluids [4] or vapor compression refrigeration systems such as that used in the IBM Z-series mainframe computers [5]. It is important to recognize that the closed loop cooling systems function only as efficient heat spreaders internally within the