since the development of the first electronic computers in the 1940s, the development of faster and denser circuit technologies and packages has been accompanied by increasing heat fluxes at the chip and package levels. over the years, significant advances have been made in the application of air cooling techniques to manage increased heat fluxes.
although air cooling continues to be the most widely used method for cooling electronic packages, it has long been recognized that significantly higher heat fluxes can be accommodated through the use of liquid cooling. application of liquid cooling for microelectronics may be categorized as either indirect or direct.
indirect liquid cooling is one in which the liquid does not contact the microelectronic chips, nor the substrate upon which the chips are mounted. in such cases a good thermal conduction path is provided from the microelectronic heat sources to a liquid cooled cold-plate attached to the module surface, as shown in figure 1. since there is no contact with the electronics, water can be used as the liquid coolant, taking advantage of its superior thermophysical properties.
figure 1: example of indirect and direct liquid immersion cooling for a
multi-chip module package.
direct liquid cooling, the focus of this article, may also be termed direct liquid immersion cooling, since there are no physical walls separating the microelectronic chips and the surface of the substrate from the liquid coolant. this form of cooling offers the opportunity to remove heat directly from the chip(s) with no intervening thermal conduction resistance, other than that between the device heat sources and the chip surfaces in contact with the liquid. interest in direct liquid immersion as a method for cooling integrated circuit chips may be traced back as early as the 1960s.
direct liquid immersion cooling offers a high heat transfer coefficient which reduces the temperature rise of the chip surface above the liquid coolant temperature. as shown in figure 2, the relative magnitude of a heat transfer coefficient is affected by both the coolant and the mode of convective heat transfer (i.e. natural convection, forced convection, or boiling). water is the most effective coolant and the boiling mode offers the highest heat transfer coefficient. direct liquid immersion cooling also offers greater uniformity of chip temperatures than is provided by air cooling.
figure 2: relative magnitude of heat transfer coefficients for various
coolants and modes of convection.
the selection of a liquid for direct immersion cooling cannot be made on the basis of heat transfer characteristics alone. chemical compatibility of the coolant with the chips and other packaging materials exposed to the liquid must be a primary consideration.
there may be several coolants which can provide adequate cooling, but only a few will be chemically compatible. water is an example of a liquid which has very desirable heat transfer characteristics, but which is generally unsuitable for direct immersion cooling on account of its chemical characteristics. fluorocarbon liquids (e.g. fc-72, fc-86, fc-77, etc.) are generally considered to be the most suitable liquids for direct immersion cooling, in spite of their poorer thermo-physical properties.
as shown in table 1, the thermal conductivity, specific heat, and heat of vaporization of fluorocarbon coolants are lower than water . these coolants are clear, colorless per-fluorinated liquids with a relatively high density and low viscosity. they also exhibit a high dielectric strength and a high volume resistivity. the boiling points for the commercially available "fluorinert" liquids manufactured by the 3m company, range from 30 to 253 °c.
|boiling point @ 1 atm (°c)
|density x 10-3 (kg/m3)
|specific heat x 10-3 (w-s/kg-k)
|thermal conductivity (w/m-k)
|dynamic viscosity x104 (kg/m-s)
|heat of vaporization x10l-4 (w-s/kg)
|surface tension x103 (n/m)
|thermal coefficient of expansion x 103 (k-1)
table 1. comparison of thermophysical properties of some fluorocarbon
coolants and water.
these liquids should not be confused with the "freon" coolants which are chlorofluorocarbons (cfcs). although some of the "freons" (e.g. r-113) exhibit similar cooling characteristics, concern over their environmental effect on the ozone layer preclude their use.
modes of heat transfer
the convective heat transfer processes upon which liquid immersion cooling depends may be classified as natural convection, forced convection, or boiling modes. the relative magnitude of heat fluxes which can be accommodated by each mode is shown in figure 3, as a function of "wall superheat" or surface-to-liquid temperature difference for a typical fluorocarbon coolant.
figure 3: typical heat transfer regimes for immersion cooling with a
natural convection: as in the case of air cooling, natural convection is a heat transfer process in which mixing and fluid motion is induced by coolant density differences caused by the heat transferred to the coolant. as depicted in figure 3, this mode of heat transfer offers the lowest heat flux or cooling capability for a given "wall superheat". nonetheless, the heat transfer rates attainable with liquid natural convection can easily match or exceed those attainable with forced convection of air. natural convection would typically be employed within a closed container to transfer heat from chips or modules to liquid, and then from the liquid to the walls of the container. heat could then be transferred from the walls to outside air by natural or forced convection.
forced convection: higher heat transfer rates may be attained by utilizing a pump to provide forced circulation of the liquid coolant over the chip or module surfaces. this process is termed forced convection; and as with air cooling, the allowable heat flux for a given surface-to-liquid temperature difference can be increased by increasing the velocity of the liquid over the heated surface. depending upon the surface geometry and the nature of the flow (i.e. laminar or turbulent), the heat transfer coefficient will be proportional to the velocity to a power between 0.5 and 0.8. the price to be paid for increasing cooling performance in this way, will be a higher pressure drop. this can mean a larger pump and higher system operating pressures. although forced convection requires the use of a pump and the associated piping, it offers the opportunity to remove heat from high power modules in a confined space; and then transport the heat via the liquid coolant to a remote heat exchanger to reject the heat to air or water.
boiling: boiling is a complex convective heat transfer process depending upon liquid-to-vapor phase change by the formation of vapor bubbles at the heated surface. it is commonly characterized as either pool boiling (occurring in a stagnant liquid) or flow boiling. the pool boiling heat transfer rate usually follows a relationship of the form,
q = c'sf a (twall - tsat)n
where q = c'sf is a constant depending on each fluid-surface combination, a is the heat transfer surface area, twall is the temperature of the heated surface, and tsat is the saturation temperature (i.e. boiling point) of the liquid. the value of the exponent n is typically about 3.
the boiling curve for a particular surface and fluid of interest (e.g. silicon and fc-72) is usually obtained experimentally. an example of a boiling curve depicting the cooling path from natural convection to film boiling is shown in figure 3. if chip power is gradually increased in small steps, cooling occurs first by natural convection (a - b). eventually a power level is reached at which sufficient superheat is available to initiate the growth of vapor bubbles on the surface and boiling starts (b).
as power is increased, more nucleation sites become active and the frequency of bubble departure increases. the region between b and c is termed the nucleate boiling regime. vigorous agitation of the hot boundary along the heated surface, and gross fluid circulation caused by the motion of the vapor bubbles, provide the ability to accommodate substantial increases in heat flux with minimal increases in surface temperature. as power is increased to point c, the critical heat flux condition is reached. so many bubbles are generated at this point that they begin to form a vapor blanket inhibiting fresh liquid from reaching the surface. further increases in power will result in a transition to film boiling (d - e).
in this regime heat transfer from the surface to the liquid is dependent on thermal conduction through the vapor and it is very poor. in most electronic cooling applications, transition to film boiling will result in failure due to high temperatures. to take advantage of boiling to cool electronic devices, it is desirable to operate in the nucleate boiling regime (b - c).
a problem often associated with pool boiling of fluorocarbon liquids is that of temperature overshoot. this behavior is characterized by a delay in the inception of nucleate boiling (i.e. beyond point b), such that the heated surface continues to be cooled by natural convection; with increased surface temperatures unless a sufficient superheat is reached for boiling to occur.
this behavior is a result of the good wetting characteristics of the fluorocarbon liquids and the smooth nature of silicon chips. although much work  has been done in this area, it is still a potential problem which the thermal engineer must consider. there is usually little or no temperature overshoot associated with flow boiling cooling applications.
the typical critical heat fluxes encountered in saturated (i.e. liquid temperature ' saturation temperature) pool boiling of fluorocarbon liquids ranges from about 10 to 15 w/cm2, depending upon the nature of the surface (i.e. material, finish, geometry). the allowable critical heat flux may be extended by subcooling the liquid below its saturation temperature. for example, experiments conducted at ibm demonstrated that it is possible to increase the critical heat flux to as much as 25 w/cm2 by dropping the liquid temperature to -25 °c.
higher critical heat fluxes may be achieved using flow boiling. for example, heat fluxes from 25 to over 30 w/cm2 have been reported for liquid velocities of 0.5 to 2.5 m/s over the heated surface . heat fluxes in excess of 100 w/cm2 have been obtained with a fc-72 liquid jet impinging upon a 6.5 mm x 6.5 mm chip at a flow rate of 2.2 cm3/s .
in spite of prolonged interest in direct immersion liquid cooling as a means to cool high heat flux micro-electronics, there have been only a limited number of applications. as with indirect liquid cooling, these applications have been almost exclusively in the large mainframe and supercomputer arena. this is not surprising, since this has been the microelectronics technology sector with the highest packaging densities and concentration of heat.
the liquid encapsulated module (lem) developed at ibm in the 1970s provides an example of a package utilizing pool boiling. as shown in figure 4, a substrate with integrated circuit chips (100) was mounted within a sealed module-cooling assembly containing a fluorocarbon coolant (fc-72). boiling at the exposed chip surfaces provided high heat transfer coefficients (1700 to 5700 w/m2-k) to meet chip cooling requirements. internal fins provided a means to condense the vapors and remove heat from the liquid. either an air-cooled or water cooled cold-plate could be used to cool the module.
using this approach, it was possible to cool 4 w chips (4.6 mm x 4.6 mm) and module powers up to 300 w. direct liquid immersion cooling has been used within ibm for over 20 years, as a means to cool high powered chips on multi-chip substrates during electrical testing prior to final module assembly.
figure 4. air or water-cooled liquid encapsulated module (lem) packages.
an example of a large scale forced convection fluorocarbon cooling system is provided by the cray-2 supercomputer . as shown schematically in figure 5, stacks of electronic module assemblies were cooled by a forced flow of fc-77 in parallel across each module assembly. each module assembly consisted of 8 printed circuit boards on which were mounted arrays of single chip carriers. a total flow rate of 70 gpm was used to cool 14 stacks containing 24 module assemblies each. the power dissipated by a module assembly was reported to be 600 to 700 watts.
coolant was supplied to the electronics frame by two separate frames containing the required pumps and water-cooled heat exchangers to reject the total system heat load to customer supplied chilled water.
figure 5: cray-2 liquid immersion cooling system.
although this discussion has concentrated on the merits of immersion cooling, coolant selection, and possible modes of heat transfer; several other considerations should be kept in mind when considering direct liquid immersion for cooling electronics. since fluorocarbon liquids are expensive they should only be considered for use in closed systems.
whether the application is in a self-contained module like the lem or a forced flow scheme, care must be taken to ensure that the seal materials chosen are compatible with the liquid. information or guidance in this regard may sometimes be obtained from the manufacturer of the coolant. if boiling is to take place, then the design must incorporate a means to condense the resulting vapors. a finned surface may be designed for this purpose as in the lem example, or a remote finned condenser surface cooled by air or water might be used. in flow systems, care must be taken in selecting a pump.
the relatively high vapor pressure of the low boiling point fluorocarbons generally require that a higher suction head be provided to prevent cavitation in the pump. whether using a self-contained boiling module or a circulating flow system, care should be taken to make sure all internal surfaces in contact with the coolant are clean. this will ensure that manufacturing process residues or unclean surfaces do not introduce a contaminant into the liquid which could be carried to the heated chip surfaces and interfere with the boiling process. in forced circulating liquid systems, it may be desirable to add a particulate and a chemical filter to ensure the long-term purity of the coolant.
by selecting the appropriate liquid coolant and the mode of heat transfer, and by giving appropriate attention to these other considerations; direct liquid immersion cooling can be used successfully to provide an effective solution for cooling high heat flux chips and packages.
robert e. simons
electronics cooling applications,
16 shamrock circle, poughkeepsie, ny
1. danielson, r.d., tousignant, l., and bar-cohen, a., saturated pool boiling characteristics of commercially available perfluorinated liquids, proc. of asme/jsme thermal engineering joint conference, 1987.
2. bergles, a.e., and bar-cohen, a., immersion cooling of digital computers, cooling of electronic systems , kakac, s., yuncu, h., and hijikata, k., eds, kluwer academic publishers, boston, ma, pp. 539-621, 1994.
3. mudawar, i., and maddox, d.e., critical heat flux in subcooled flow boiling of fluorocarbon liquid on a simulated chip in a vertical rectangular channel, intl. j heat and mass transfer, 32, 1989.
4. chrysler, g.m., chu, r.c., and simons, r.e., jet impingement boiling of a dielectric coolant in narrow gaps, ieee trans. chmt-part a, vol. 18 (3), pp. 527-533, 1995.
5. danielson, r.d., krajewski, n., and brost, j., cooling a superfast computer, electronic packaging and production, pp. 44-45, july 1986.