Helping chips keep their cool

May 23, 2002
Two-phase flow cooling comes to high-power electronics.

By Jon Zuo,
Ron Hoover and Fred Phillips
Thermacore International Inc. A Div. of Modine Manufacturing Co.
Lancaster, Pa.

Edited by Lawrence Kren

Packing more transistors into ICs and running them faster boosts computing power but it also generates more heat. And, systems using these denser chips are being packed tighter into racks and cabinets, while the enclosures themselves are crammed into ever-smaller floor space. Refrigerated enclosures are one answer to overheated circuits, though it has reliability and environmental issues, needs power to run, and increases operating cost. This makes refrigeration a circuit engineer's last resort.

At the chip level, options focus on the chip package itself. The heat flux generated in a typical die is on the order of 10 to 100 W/cm2. To minimize contact resistance losses, the heat flux leaving the chip package should be about 1 to 10 W/cm2.

One design uses a vapor-chamber heat spreader made of low-CTE (coefficient of thermal expansion) materials. Internal twophase-flow heat transfer improves heat spreading by at least an order of magnitude compared with passive copper heat spreaders of the same size. Internal, two-phase flows are thermally driven through an optimized capillary. Here, heat from a die boils a working fluid. The vapor pipes to a cooler region and condenses then returns by capillary forces to the hot zone, repeating the cycle. The low-CTE envelope directly bonds to a chip, eliminating interface thermal resistance. Tests show the design removes heat fluxes exceeding 250 W/cm2 at a thermal resistance below 0.15°C/W/cm2.

Another design uses a MEMS (microelectromechanical systems) actuated microchannel, oscillating-flow heat spreader. A microactuator, integral to the heat spreader, generates and controls fluid oscillation. It also is made of a low-CTE material and bonds directly to a chip. High-frequency oscillating, single-phase flow in microchannels also boosts thermal diffusion by at least an order of magnitude compared with copper passive heat spreaders of equal size.

Whereas both of these designs house the entire cooling apparatus on the chip, another approach integrates chip coolers with the enclosure rack itself.

Evaporators, such as Thermacore's Loop Thermosyphons serve both as racks for circuit cards and thermal buses. A cooling liquid delivers heat from a cabinet to a remote heat exchanger where it dissipates into ambient air. This lends itself to field installation and expansion because cabinets and heat exchangers are well separated. It also works well for indoor, server-center type installations in which the thermal bus couples directly to the chiller circuit of an HVAC system. This is better than dumping heat to room air, a relatively poor heat conductor.

Another scheme for cooling server enclosures uses a two-stage Loop Ther- mosyphon thermal bus. The evaporator of the first-stage is U-shaped and relatively small and locates on a rack tray proximal to multiple ICs. The condensor routes to a cooler section of the tray. For added cooling, the first-stage condenser can couple with the condensor of a second-stage Loop Thermosyphon located in the rack itself. Second-stage condensers can be cooled either by liquid or air.

Analysis reveals the junction-temperature profiles of a running server processor with a conventional aluminum-silicon-carbide (AlSiC) spreader for a package lid, and the same chip with a miniature, vapor-chamber heat spreader. The two-phase-flow heat spreader lowers junction temperature, and temperature gradients, and related thermal stresses on the lid surface. Hot spots indicate nonuniform heat flux, especially visible in the AlSiC spreader.

Thermacore's active cooling systems may help turn a thermal budget deficit into a surplus by boosting circuit-cooling capacity more than chip thermal output levels are climbing.

To better understand circuit cooling needs, Thermacore has de-fined seven basic heat-transfer paths or levels, starting with the chip package (Level 1), moving outward through the circuit boards and racks (Levels 2 through 6), and eventually to the airspace around the equipment enclosure (Level 7).

This design integrates evaporators of several Loop Thermosyphons into the cabinet. Loop Thermosyphons act both as circuit-card racks and thermal buses. The system successfully cooled a prototype communications gear enclosure with twelve card modules dissipating over 15 kW.

A thermal-budget plot of power and temperature helps define enclosure cooling needs. One line charts decreasing chip-case temperature as demanded by increasing clock speed. The other shows rising enclosure temperatures due to increasing heat from components inside. The difference between the lines is the thermal budget available for cooling devices, and that budget shrinks as the two lines converge. When the difference between the two lines approaches zero, engineers must use refrigeration for cooling.

Linking chip thermal loads directly to HVAC chiller loops helps shrink HVAC system size as well as lower air temperatures surrounding enclosures. Technical challenges remain, however, including developing a basic understanding of micro-scale heat transfer and flow phenomena. Thermal stresses from materials with mismatched thermal-expansion coefficients and the impact of integrating electronic packaging with thermal-management hardware are also concerns.

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