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559 ADVANCED COOLING TECHNOLOGY FOR LEADING-EDGE COMPUTER PRODUCTS R.C. Chu System 390 Division IBM Corporation Poughkeepsie, NY USA ABSTRACT Cooling technology has been a vital prerequisite for the rapid and continued advancement of computer products, ranging 6om laptops to supercomputers. This paper provides a review of the recent development of cooling technology for computers. Both air woling and liquid cooling are included. Air cooling is discussed in terms of the advantages of impinging flow. An example of module internal conduction enhancement is given. Liquid cooling is discussed in terms of indirect liquid cooling with water coupled with enhanced conduction, and direct immersion coollng with dielectric coolants. Special cooling technology is included in terms of the application of heat pipes and Ne possibility of using liquid metal flow to cool electronic packages. INTRODUCTION During the last decade the number of circuits per chip increased while circuit power dissipation decreased for state4 thM semiconductor technology. Because of advances in microelectronic fabrication techniques, the circllit size decreased faster than the circuit power dissipation. This led to a geometric growtb in the module level power dissipation. The recent switch fmm Bipolar to CMOS 'was expected to alleviate the power dissipation problem. In reality, the switch has only resulted in a backwards shin of abut 10 years in the geometric growth of module level heat flux. The change from Bipolar has provided a short recess from the run away growul in heat flux, but the recess will soon be over. A maximum operating temperature is specified for all electronics chips in order 'to maintain a desired level of reliability. It, is therefore necessary to be able to measure and predict the junction temperature of each chip. One method often used is to express the junction temperature in terms of a total temperature budget, made up of the ambient temperature plus the temperature rises from ambient to the chip device junction. For a single or multichip module this budget can be expressed as: Tj = -+ ATh, t ATm -+ATm + Tmb where T, = chip junction temperature AT,-E =junction to chip temperature rise ATh, = module internal temperature rise = PchipRd ATm = module external temperature rise = PmdR, ATm = temperature rise of coolant = Q I pmc, Tun,, = ambient temperature As can be seen from the precedmg equation, simply minimizing one term will not necessarily result in an acceptable junctlon temperature. All terms must be considered. Usually there is no way to modify the ambient temperature other than changing the operating specification. So this is taken as fixed.

The junction to chip temperature rise is determined by the circuit technology and is also considered fixed. The other three terms may be controlled through application of cooling technology and enhancements. In virtually all cases, the dominant temperature rises will be the internal and external rises. The internal temperature rise is the product of chip power and the internal thermal resistanw from chip to case. ?he external temperature rise is the product of module power and the external thermal resistance from the case to the cooling fluid. A variety of techniques have been utilized in different cooling system designs to minimize the internal and external thermal resistances, as well as the temperature rise.of the coolant. This paper addresses some of the methods that have been used to provide reliable cooling systems for leading edge computer products. AIR COOLING not as good as for most water or immersion systems, most computers still depend wholly or in part on air woling. This includes everything from portables to mainframes. One technique now being used to extend air cooling may be called highly parallel impingement. With this technique each module on a card receives an individual, unheated air stream. The individual air streams eliminate the problem of cooling air temperature rise; the fourth term in the temperature budget. The top illustration of figure 1 shows the concept. Air is drawn from the card side of a large plenum through appropriately sized orifices in the plate separating the plenum and the modules. As shown, the air impinges against the pin fm heat sinks and flows into the return plenums. From there the air is drawn through the blower and pushed out of the system.

Although the thermal performance of an air cooled system is. Highly Parallel Impingement Flow Conventional Cross Flow Figure 1. Parallel impingement and conventional crowflow. 0-78034306-9/98/$10.00 1998 IEEE 560 R,("CIW) original Description A couple of things should be noted in this impingement scheme. First, either pin fin or parallel plate fii heat sinks could be used. Second, depending on the power dissipation of the individual modules, the vertical expanse of the supply plenum could be made quite small, and could bc incorporated into a book package.

By using many individual air streams, the sensible energy gain of an air streum cooling a given module is limited to the power dissipated by the module alone. In a conventional systan, where the air enters along one edge ofthe card and flows across several rows of modules. as shown in the bottom half of figure 2, either the pressure drop is tolerable but the air temperature rises beyond limits (low air flow rate) or the air temperature rise is not excessive but the total pressure drop becomes too great (high air flow rate). While the ducting of the highly parallel impingement cooling scheme is more complex, it can solve a number of the thermal and hydraulic problems associated with air cooling a large array of modules. MODULE INTERNAL CONDUCTION ENHANCEMENT Selection of an appropriate cooling technology must consider both internal and external resistances. As indicated in the junction temperature equation, even if any resistance is redd to zero it would still be necessary to contend with the sum of the other resistances. Ideally it would be desirable to minimize all the resistances. Much work over the past few years has lead to new heat sink designs and new cooling approaches (such as impingement) to reduce external thermal resistance. Advances in the reduction of the external resistance, while good it itself, will lead to a limit dictated by the internal resistance. Therefore, consideration must be given to techniques to enhance the internal resistance of single and multichip modules. The *que which will be discussed here is the use of a thermal space transformer.

In a typical single chip module (SCM), the thermal path betwcen chip and housing is mainly through a paste or epoxy layer between the back side of the chip and inside of the module cover. The paste or epoxy gap is relatively thick to BcMMnodate manufacturing tolerances and results in a relatively large thermal resistance. Consider a single 18.2 mm chip in a 42.5 mm ceramic column grid array (CCGA) module. Using a high thermal conductivity paste (k=3.8 W/m K) between the chip and the cover (1.0 mm aluminum) yields an internal resistance of 0.286 "CIW. As can be seen in the table, the largest contributor to the internal resistance is the chip to cover resistance. If the paste layer could be spread over a larger area, this thermal resistance could be substantially reduced. R&(oCIW) Thermal Oil Interface Heat Sink cover spreader Paste Interface Oil Interface chip Substrate Figure 2. Single chip module with thermal space transformer Table 1. Thermal resistances for conventional and thermal transformer enhanced packages. As shown in figure 2, the thermal space transformer spreads the heat out and provides a larger area for conduction through the paste. In the module with the thermal space transformer, the paste layer thickness remains the same. Although this design results in more interfaces, as can be seen in Table 1 the overall internal thermal resistance is smaller. Compared with the original design, about 40% smaller. This temperature savings on the internal si& will allow either a smaller heat sink, or a lower cooling air flow rate. LIQUID COOLING While air cooling can be thought of as the basic yardstick for amparing the efficacy of electronic cooling systems, there are many applications which require a more substantial cooling system. Air is cheap, but water provides the largest capacity for heat removal in computers. Due to Pafomce requirements, a single module may often dissipate a considerable amount of heat - much more than could be cooled by air. This is when liquids, water and others, come into the picture. Indirect Liquid Cwling with WaterKonduction Although water cannot be allowed to come into direct contact 9th the chips, cold plates may be used to bring the water vq close. A chronology of the water cooled Thermal Conduction Modules (TCMs) used in the IBM family of high-end machine:

is shown in Figure 3. These systems used completely separable cold plates, allowing for easy service in the field, since no water lines were "broken" during replacement of an electronics module. At sufficiently high power levels, even a good internal resistance can result in excessive consumption of the overall temperature budget. For instance, the internal resistance of the on@ TCM was 8.8 'C/W [I]. At this resistance, and with a 4 W chip, the internal resistance consumed 35.2 "C of the total temperature budget As the chip power dissipation level increased, the internal resistance had to derrease. If the chip power rose to 10 W and the internal resistance remained unchanged, this same internal resistance would consume 88 "C, more than the entire temperature budget. In the last generation of the TCMs, the internal resistance was redd to 1.25 'C/W through material, geometry hd manufacturing changes. 561 impingement, is typically of a much smaller magnitude and can usually be ignored.

Liquid Jet Impingement Jet impingement in both single phase and boiling systems can provide cooling capacities at least as great as that needed for today's leadingedge computer products. However, the complex- ity of such systems may limit their use to the high end only. One cooling system took a giant step forward into the high heat flux area. The SSI supercomputer SS-I [4] used fluorocarbon multi- jet impingement to handle up to 40 W on 6.5 mm chips. The module-level CPU package is shown in Figure 4. This package.

measuring roughly 150 nun on an edge, contained 8 multi-layer glass ceramic substrates. There were 120 chips on 30 interposers on each substrate. The number of jets (1 to 9) directed at each chip was determined by the chip power (up to 40 W). MLC 308X 3090 ES 9000 Figure 3. Chronology orIBM water-cooled TCMs.

The latest multi-chip module designs have replaced the piston with a thin paste layer providing the needed redud distance from the chips to the coolant. The grease layer thickness is held a1 a minimum value while accounting for manufacturing tolcrances and dikential expansion caused by thermal cycling. A 0.25 nun thick paste layer, backed by a 8 mm thick copper plate has a resistance of about 0.26 'C, considerably better than that of the last generation of TCMs. Direct Immersion with Dielectric Coolants In direct immersion cooling the coolant comes into direct contact with the electronics. Obviously only a compatible coolant can be used with this technique. The most common coolant used is one of the fluorocarbon liquids. Although these coolants have thamophysical properties which are considerably poorer than water, they are chemically compatible, and inert. A number of direct immersion cooling systems have been developed and used over the past 25 years (21. All relied on and were limited by either single phase convection or the critical heat flux of pool boiling.

The first computer application of a direct immersion cooling system was the IBM Liquid Encapsulated Module (LEM). This package enclosed a substrate and 100 chips in a container with fluorofarbon FC-72 coolant. (3M Company) The container had internal fins to provide extended surface area for transferring heat from the liquid to the walls. The container could be fitted externally with either air-cooled fins, or a water-cmled cold plate. The package was designed to handle 4 W chips and a module power up to 300 watts.

Another direct immersion package has been investigated expenmentally 131, Externally this package was either air or water cooled, but internally the fluid which came into direct contact with the electronics was also FC-72. The overall package was geometrically similar to the LEM, only smaller.

Sixteen chips, each 8.97 mm square, were mounted on a 57 nun square substrate. Opposite the substrate, across the fluid chamber was a cooling plate which utillzed pin fms to provide extended surface area. A water or air cooled cold plate, integral to the fluorocarbon immersed extended surface provided the ultimate sink. This MCM package was capable of dissipatmg 200 W making it roughly equivalent to the IBM LEM on a per chip area basis.

Temperature overshoot is a major concern with any passive boiling system and must be addressed for overall system reliability. The phenomena of temperature overshoot, while it still exists in a forced convection boiling system like jet tubes Supply tube .*'removed" to show jet plate U, Vertical backplane MLC Coolant - I flow Figure 4. SSI Supercomputer liquid impingement cooled CPU module assembly.

The search for cooling systems capable of handling heat fluxes even higher than that provided by single phase jet impingement has led to the investigation of jet boiling. Permitting the coolant to boil increases the allowable heat flux, as long as CHF is not reached. With a boiling system heat fluxes in excess of 100 W/cm2 are achievable (51. As with other direct immersion systems for high heat flux, the complexity of these cooling systems will probably limit their application to high end machmes. SPKIAL COOLING SYSTEMS Some new or less tradrtional methods for electronics cooling have surfaced over the past few years. The development of these techniques was due primarily to new applications, i.e. notebook computers, requiring a sufficiently robust cooling system with little or no power consumption, or hgh power chips in more conventional applications. As with all other cooling systems, the idea is to provide a method of moving and dinusing the heat dissipated away from the electronics. Heat pipes and liquid metal heat exchange systems for electronics have been developed to deliver the desired characteristics. Heat Pipes At fnst glance the power levels present in today's portables do not present a pure cooling challenge - that is until the package constraints are considered. Portable computers, the latest hot item in consumerelectronics, present interrelated power and 562 ~ I leal Sink ' Module :XB Figure 5. I leal pipe application to laptop computer cooling item in consumer electronics, 'present interrelated power and cooling problems. While.the rest of the computer industry is conceined with maintaining a 15 or 20 W chip at a reasonable temperature, designers of portable computers must be concerned with battery life andcooling a 6 to IO W chip. In most cases the battery life constraint precludes the use of a fan. Therefore portables arc almost always natural convection cooled. The modaate power dissipation level of the portable computer may be handled, once the energy has spread out to a SURiciently large area. The problem is one of spreading the heat out in a very conhed space. For many manufacturers heat pipes have provided the solution.

One proposed solution has the heat pipe and CPU thermally attached lhrough vias in the substrate [6]. Other solutions forego thermal vias in the substrate in favor of conduction contacts. This option is presented in figure 5. With this configuration, the evaporator section of the heat pipe is pressed against the heat producing device, and the condenser Section is in good thermal contact with the ultimate sink - typically an outside surface of the portable. In either case, the idea is to provide a path of low thermal resistance between the CPU and the portable case, and spread out the heat in the process.

The effective thermal conductivity of a heat pipe depends on the fluid, the temperature difference and also the geometry (length and dimeter). Typically effective thermal conductivities ranging Gom 100 to over 1.000 times copper are possible Uquid Metal It is well known that liquid metals provide very good thermal properties. It should be possible, for a properly collrtgured system,, to take advantage of these properties. But how would such a system gain thermal advantage? Obviously the coolant, since it is electrically conductive, could not come into direct contact with the electronics. Some sort of interface will therefore be required. This leads to the idea of a liquid metal cold plate much like the water cooled cold plates currently used in some computers.

The Qmax [7] liquid metal based cooling system provides a high thermal transport capability utilizing a liquid metal-toair heat exchanger. With this miniature heat exchanger, depicted in figure 6, a total thermal resistance, junction to air, of < 0.6 "C/W has been realized for a I cmz chip. A typical performance curve for the liquid metal heat exchange system is presenpxl in figure 7.

CONCLUSION ElectroNc cooling has evolved to a point where it is capable of handling today's CMOS chip heat fluxes. But CMOS heat flux is growing at a rate parallel to that which we experienced with Bipolars, only offset by about IO years. Enhanced air cooling is t ff !Liquid metal-to-air hat exchanger Liquid cold plate metal,/-,,,,,,,,A Electronics Figure 6. Liquid metal cooling system for electronics module. Liquid metal system g 1.0 . -.. - E 0.0 1 ' ' ' E 0.2 0.0 0.5 1 .o 1.5 2.0 Chip Edge (crn) Figure 7. Liquid metal cooling performance. needed and is-available, but serious exploration of alternative cooling techniques for the future should be initiated now. REFERENCES 1. Simons, R.E., and Chu, R.C., "Cooling Technology for High Performance Computers: Part 1 - Design Applications," Cooline of Electronic Svstems, Proceedings of the NATO Advanced Study Institute, pp. 71-95.1994.

2. Simons, R.E., "Direct Liquid Immersion Cooling for High Power Density Microe&tronics," Electronics Cooling, Vol. 2, No. 2, 1996.

3. Nelson, N.D., Sommerfeldt, S., and BarCohm, A., "Thermal performance of an Integrated Immersion Cooled Multichip Module Package," Ninth JEEE SEMI-THERM Symposium, pp. 8-18,1993.

4. Ing. P., Sperry, C., Philstrom, P., Claybaker, P., Webster, J., and Cree, R., "SS-1 Supercomputer Cooling System" F'm ceedmgs of IEEE 43rd Electronic Components and T.+nolOgy Conference, pp. 218-237, Orlando, FL, June 1993.

5. Chrysler, G.M., Chu, R.C., and Simons, R.E., "Jet Impinge- ment Bailing of a Dielectric Coolant in Narrow Gaps," IEEE Transactions on Components, Packaging, and Manufacturing Technology Part A, Vol. 18, No. 3, pp. 527-533, 1995.

6. Xi, How "The Use of Heat Pipes for Cooling Portables," IMECE, San Francisco, CA., 1995. 7. "Miniature Modular Heat Exchangers for Electrunic Circuit Cooling," technical publication from Qmax Corporation.