High performance heat exchanger and method

[0002] The present invention relates to heat exchangers and, more specifically, relates to a metal foam heat exchanger having a surface for direct bonding to an electronic module.

[0003] Performance, reliability and packaging efficiency of advanced power electronics modules strongly depend on their ability to effectively remove dissipated energy. The effectiveness of the heat removal directly translates to lowering the junction temperatures of power semiconductor devices. Lower junction temperatures result in lower reverse saturation current of p-n junctions, higher switching speeds in MOSFET structures, increased reliability of metalized and bonded interfaces and a reduction in operating temperature.

[0004] Heat exchangers or heat sinks are metal heat radiators designed to remove heat from electronic components, particularly power transistor modules, by thermal conduction, convection, or radiation. Heat exchangers dissipate heat at a surface of an electronic module which may be generated by an internal junction of a semiconductor device within the module. Heat exchangers may also transfer heat from the location of device being cooled to another location by providing passages for a fluid coolant to flow through the heat exchanger.

[0005] In one known technique for cooling electronic modules, an oversized cold plate is attached to the module. The oversized cold plate reduces module temperatures by heat spreading and increasing the surface area for heat dissipation. However, the cold plate adversely increases the weight, volume, and cost of modules. Additionally, bonding the cold plate to the module requires the use of soft polymeric compounds which have high thermal resistance.

[0006] Accordingly, it is an object of the present. invention to provide a novel heat exchanger and method.

[0007] It is another object of the present invention to provide a novel integrated heat exchanger and thermal base plate assembly.

[0008] It is yet another object of the present invention to provide a novel metal foam for transferring heat from an electronic module.

[0009] It is still another object of the present invention to provide a novel metal foam having a surface for bonding directly to an electronic module where the metal foam provides a heat transfer path for the electronic module.

[0010] It is a further object of the present invention to provide a novel heat exchanging foam having a network of metal ligaments for cooling an electronic module by pulling heat away from the module through the metal ligaments and transferring heat in the metal ligaments to another location by directing a fluid coolant through interconnected voids which are formed by ligament surfaces.

[0011] It is yet a further object of the present invention to provide a novel heat exchanging metal foam having a network of metal ligaments forming elongated substantially ellipsoidal shaped voids oriented in one direction for directing a fluid coolant through the foam.

[0012] It is still a further object of the present invention to provide a novel heat exchanger having plural blocks of metal foam positioned in series on a surface of an electronic module where the blocks have variable thermal performance.

[0013] These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.

[0014] FIG. 1 is a pictorial of one embodiment of a heat exchanger of the present invention.

[0015] FIGS. 2a-2dare magnified photographic views of a representative cross-section of a block of metal foam.

[0016] FIG. 3 is a magnified photographic view of a representative cross-section of a block of metal foam having randomly distributed equiaxed cells.

[0017] FIG. 4 is a pictorial of a second embodiment of a heat exchanger of the present invention.

[0018] FIG. 5 is a pictorial of one embodiment of an integrated heat exchanger/thermal base plate assembly of the present invention.

[0019] With reference to FIG. 1, a heat exchanger may include a block of porous metal foam 10. An electronic module to be cooled by the heat exchanger may include thermal base plate 12 and a semiconductor device 14. The thermal base plate 12 may have metalized surfaces 18 for direct bonding with the block 10 and the semiconductor device 14. Direct bonding may be achieved through soldering, active brazing, or simply brazing.

[0020] The thermal base plate 12 may be made of any thermally conducting and electrically insulating substrate having a metalizable surfaces and having a coefficient of thermal expansion (CTE) that is substantially matching with the CTE of the silicon power semiconductor device 14 such as AlN, BeO, Metal Matrix Composites, layered or infiltrated all metal composites (e.g., CuMo).

[0021] The metal foam 10 may be made of aluminum, copper, or silver. The metal foam heat exchanger 10 may be directly bonded to a low expansion, thermal base plate 12. The CTE of the base plate and metal foam may be substantially different, any physical stresses due to mismatching levels of thermal expansion is minimized by the structural compliance of the foam. Physical stresses due to mismatching level of thermal expansion may be induced by the heat flux generated during the operation of the module or due to the temperature variations in the intended work environment of the module. The effective modulus of the metal foam structures, which scales the thermal mismatch stresses, is known to be proportional to the square of the foam relative density. To provide solderable metalization, a block of aluminum foam may be electroplated with copper and soldered to the thermal base 12 with paste 20.

[0022] The R_jb, junction-to-base thermal resistance of a power module with directly bonded foam based heat exchanger, may be as low as 0.07° OC./(Watt*in²). The foam may provide an R_ba, base-to-ambient thermal resistance, of up to about 0.01° C./Watt for a 1 inch surface contact.

[0023] The electronic module to be cooled may be any form of generic high density intelligent and dumb power module which may house a multiplicity of power semiconductor dies (MCT, MOSFET, IGBT, or alike) and diodes as the basic elements of single or multiple switches. The thermal base 12 and the semiconductor device 14 may be considered as being an electronic module or portions of an electronic module which is to be cooled.

[0024] In operation, heat 24 generated by the semiconductor device 14 radiates through the thermal base plate 12 into block 10. Heat in block 10 is dissipated through convection by directing a fluid coolant 25 through the block 10 in a direction perpendicular to heat flow 24.

[0025] FIGS. 2a-2dare magnified photographs of the metal foam which illustrate the internal structure of the metal foam. As illustrated the metal foam may include a network of ligaments or wires which form numerous open cells. The ligaments are aligned to be in a direction which is substantially parallel to the direction of heat flowing from an electronic module. The aligned metal ligaments provide a higher metal density in cross-sectional planes of the foam perpendicular to the intended direction of heat flow. Due to the higher ligament density, the foam has a lower thermal resistance in the direction in which the ligaments are aligned than in any other direction.

[0026] The alignment of the ligaments forces the cells to be aligned in the same direction as the ligaments. The open cells may also be considered pores or voids. The shape of the cells may be substantially ellipsoidal or be considered to be of an equiaxed shape which has been collapsed uniaxially or biaxially. The cells may have opening to permit fluid coolants to pass from cell to cell. The cells are randomly distributed throughout the foam. The cell density of the foam in terms of average number of cells or pores per inch and volumetric density of the foam defines the surface area per unit volume of the metal foam. The porosity of the metal foam is due to gaps in the ligaments which allows a fluid coolant to be passed through the foam in any direction.

[0027] The metal foam is fabricated by expanding and solidifying a pool of liquid metal saturated with an inert gas under pressure. The solidification takes place as the saturated gas expands to atmospheric pressure. Density of the porous metal is varied by applying different levels of pressure. The porosity of the foam after solidification may be in the range of 85% to 95% with the open cells generally being equiaxed voids that are randomly distributed throughout the foam.

[0028] Metal foam having randomly distributed equiaxed pores is commercially available (illustrated in FIG. 3). The commercially available porous aluminum foam provides a pore density of up to 40 pores per inch, a surface area of approximately 10 to 50 in²/in³(ligament surface area/metal foam volume) and theoretical mass density of 10% to 20%.

[0029] Since the heat flux of the module is one dimensional, the commercially available foam having randomly distributed equiaxed cells is not suitable for electronic module heat transfer applications. To significantly improve the thermal capability of the foam, the randomly distributed ligaments and cells are aligned in the direction parallel to the direction of heat flux to be removed. The alignment is accomplished by compressing and annealing the metallic foam in multiple steps until a desired volumetric density and level of alignment is obtained. The alignment process requires inelastic deformation of the foam through lateral compression. The deformation may be uniaxial or biaxial. After the alignment process, the randomness of the cells (i.e., in size, direction and radius) tends to disappear and the cells have a preferred orientation in a direction perpendicular to the surface of the module. The compression also increases the surface area of ligaments. The volumetric density of the foam may be varied from a before alignment density of 5% to 15% up to theoretically 100% (the pore volume density goes to zero as solid density goes to 100%. The deformation of the foam allows the tailoring of the flow resistance. (pressure drop), thermal performance, and structural compliance of the foam.

[0030] The aligned foam may provide 150 in²heat transfer area in a 2 by 1.5 by 0.25 inch sized cavity with a flow resistance of less than 10 psi for up to 2 gpm flow rate for a typical liquid coolants such as water and FC-40. To achieve the same surface area in the same volume with an extruded aluminum based heat exchanger 0.005 inch wide fins with 0.005 inch spacing is required.

[0031] In application, the foam may be about 0.05 to 1.00 inches thick. Preferably, the thickness of the block of metal foam is at least about 0.100 inches which may provide a junction to ambient thermal resistance of 0.07 C/Watt for a 1 inch base/foam contact. The foam primarily enhances the surface area and the forced convection coefficients. Further, the three dimensional pseudo-random structure of the foam eliminates macroscopic scale film boiling and provides a high rate of heat transfer through increased number of nucleation sites over the extended surface.

[0032] The porosity of the foam must be sufficient to permit continuous flow of fluid coolant. The foam may have a porosity of between about 5 and about 50 pores per inch. The foam may have a volumetric density of between about 2% and 50%. The foam may have a total surface area of between about 2 and about 300 inches squared per cubic inch of foam (in²/in³).

[0033] In packaging, the metallic foam may be a current carrying electrode as well as a heat exchanger. Further, because of the adjustable level of porosity, copper or silver foams can be deposited with CVD diamond to maximize conductivity and cooling capability.

[0034] With reference to FIG. 4, a heat exchanger may include several blocks of metal foam 40, 42, and 44 positioned in series. The blocks 40, 42, and 44 may be directly bonded to a thermal base 12. The blocks 40, 42, and 44 may have varying densities in order to match varying heat dissipation requirements on the surface of the module. Additionally, smaller blocks may have better structural compliance and are easier to bond.

[0035] With reference to FIG. 5, an integrated heat sink/thermal base assembly 60 may include a thermal base plate 12, and a casing 62, enclosing a block of metal foam (shown in FIGS. 1-3). The casing may include manifolds 64 and 66.

[0036] In operation, convective cooling is accomplished by directing a fluid coolant through the block. The coolant may be directed into the assembly through inlet manifolds 64 and directed out of the assembly through outlet manifolds 66.

[0037] While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.