Power module cooling system

Abstract: The semiconductor wafer (22) of a power module (10) is mounted directly, separated only by thermal expansion control layers and insulator layers, on a metal base (14) which has fins (18, 20) or pins integrally formed thereon. This module (10) is mounted on a coolant fluid flow chamber so that the module (10) mounting is not a thermal interface. In this way, maximum power module cooling is achieved with minimum fluid flow.

Description: This invention is directed to the structure of an electronic power module which has its wafer mounted directly (except for thermal expansion and insulating layers) on the base, and the base has extended cooling surface thereon. Semiconductor electronic devices have internal losses which generate heat in use. When the electronic device is configured for a substantial amount of power, cooling is necessary in order to limit the semiconductor device to a sufficiently low temperature to provide a reasonable life. Power modules containing this type of semiconductor component are traditionally attached to a cold plate. The cold plate may have fins thereon with circulation of fluid therepast. The fluid is usually liquid when greater amounts of heat must be dissipated. As long as there is sufficient power to provide more flow of liquid, such a cooling system is satisfactory. A commonly used current method for cooling high-density hybrid power modules is by clamping them onto a typical internally finned flow-through cold plate. Characteristics of the thermal interface between the module and the cold plate are dependent on numerous variables such as surface finish and roughness, bolt torque, pressure uniformity, interface material conductivity and consistency, and the type of interface material such as gasket, grease or foil. In addition, thermal expansions and contractions, vibration, shock and handling will often vary the initial parameters and cause performance degradation. Typical finned flow-through cold plates are constructed of aluminum material for manufacturing reasons. The mismatch in thermal expansion rates between the aluminum cold plate and the module base plate creates additional degradation at the thermal interface. The dissipated power density of present-day power hybrid modules has reached a nominal 500 watts per square inch (1 inch = 2,54 cm) at the semiconductor chip. Current package power modules are clamped or bolted with the module base plate against the cold plate. The cold plate may have internal passages for fluid flow and internal fins for increased cold plate-liquid coolant interface area. This structure requires the cold plate of minimum thermal resistance and an interface between the power module base plate and the cold plate also of minimum thermal resistance. Achieving flatness is difficult, and tight bolting causes distortion. A liquid heat exchanger for an electronic component is known from EP-A-0 353 437. The heat exchanger comprises a body with a bottom. The bottom is mated against an electronic component such as a chip. The body includes a cavity and a plurality of substantially parallel spaced fins integrally formed with the bottom and extending into the cavity of the body, thereby abutting an opposite surface of the body. Another wafer cooling system is known from FR-A-2 500 215. The cooling system system comprises a semiconductor wafer and a metallic base on which the electronic wafer is mounted. A cold plate has a channel therethrough and connections on the cold plate for delivering coolant fluid through the channel. An opening is provided in the channel and the base is mounted on said opening so that an extended surface of said base extends into said channel, said base closing said opening into said channel so that said extended surface on said base is directly cooled by said coolant fluid. It is further suggested in this document to either provide a channel bottom formed as a slanted deflection surface so as to enhance flow through the rips forming the extended surface of the metallic base. However, the rips do not reach to the slanted bottom surface. As an alternative, it is suggested to arrange a joint part below the rips so as to urge the cooling fluid to flow in the space between the rips. Thus, there is a need for an improved power module cooling system by which the semiconductor devices of the power module can be cooled with a minimum amount of coolant power. This object is achieved by a power module cooling system comprising a power module having a semiconductor wafer and a metallic base, said metallic base having a first side, a second side and a metallic extended surface on said second side of said metallic base, and said electronic wafer being mounted on said first side of said metallic base, a cold plate, said cold plate having a channel therethrough and connections on said cold plate for delivering coolant fluid through said channel, an opening in said channel, said base being mounted on said opening so that said extended surface on said base extends into said channel, and said base closing said opening into said channel so that said extended surface on said base is directly cooled by said coolant fluid, wherein said channel and said extended surface are sized such that the extended surface reaches to the bottom of the channel and that the extended surface is attached to the bottom of the channel so as to prevent fluid flow beyond the end of the extended surface. It is thus a purpose and advantage of this invention to provide a power module which has a metallic base plate therein with an electronic semiconductor wafer mounted on the base plate, and the base plate carries integrally formed fins or pins which are immersed in liquid in a cold plate so that thermal resistance is minimized and liquid coolant can cool the fins or pins with a minimum expenditure of pumping power. Other purposes and advantages of this invention will become apparent from a study of the following portion of the specification, the claims and the attached drawings wherein: FIG. 1 is an isometric view of a cooling system including a power module having integral fins on its base plate mounted on a cold plate, with the fins inserted in the fluid passage. FIG. 2 is an enlarged section taken generally along line 2-2 of FIG. 1. FIG. 3 is a section taken generally along line 3-3 of FIG. 2. FIG. 4 is a section similar to FIG. 3 showing pins as providing the extended surface. A power module configured in accordance with this invention is generally indicated at 10 in FIGS. 1, 2 and 3. In FIG. 3, the near side of the cover 12 is broken away to schematically illustrate the equipment therein. The power module 10 comprises a base 14 which has an upper mounting surface 16 and which has a plurality of fins integrally formed therewith and extending out of the base on the side opposite the mounting surface. Fins 18 and 20 are specifically identified in FIG. 3, but it is seen that the fins are positioned under the active area of the upper mounting surface. The base 14 is sufficiently wide so that it can serve as an overlap mounting flange against the cold plate, as is described hereafter. The cover 12 is preferably of a dielectric material or has dielectric feedthroughs therethrough. The cover is secured to the outer edges of the base 14 at the sides, as seen in FIGS. 1 and 2, but is short of the ends of the base to expose a securing flange, as seen in FIGS. 1 and 2. If desired, the cover could be narrower, in the direction seen in FIG. 3, to leave an exposed securing flange all the way around. Mounted on top of the active area of the upper surface is the electronic stack indicated at 22 in FIG. 3. As previously indicated, the lowermost layer is the base 14. This is preferably a copper layer to achieve high thermal conductivity in a relatively inexpensive metal. Appropriate intermediate layers are provided between a semiconductor wafer in the stack and the copper base layer 14 in order to provide security of mounting, maximize heat transfer, minimize stresses in the wafer caused by differential expansion of the materials, and provide electrical insulation where necessary. The wafer in the stack has semiconductor electronic components therein, and particularly high-power electronics together with control components for controlling the main power flow. Principal electronic connections 26 and 28 are shown as the power connections to the power junctions in the module. These electrical connections are shown as extending out of the top of the cover for external connection. In addition, the control connections are brought out of the cover in a similar way, but with suitably smaller conductors. The semiconductor devices in the silicon wafer generate the heat which must be extracted to maintain the silicon wafer at a sufficiently low temperature that it remains operative over a long life. The heat is conducted downward through the stacks to the base layer 14. The base layer 14 has the plurality of fins thereon under the active area of the upper mounting surface. Cold plate 40, see FIGS. 1, 2 and 3, has a fluid inlet 42 and a fluid outlet 44, see FIG. 1. The inlet and outlet are respectively connected to inlet passage 46 and outlet passage 48. There are one or more channels in the cold plate depending on how many power modules 10 are to be mounted on each cold plate. As illustrated in FIG. 1, there are two channels in the cold plate 40. The first is channel 50 on which the module 10 is mounted, and there is also a channel 52 on which no power module is mounted for illustration purposes. In use, all channels would be covered. Furthermore, FIG. 1 is shown as being broken away so that the cold plate 40 may be longer in the upper right direction so as to provide the desired number of channels to accommodate the desired number of power modules. Since the cold plate 40 has no need for providing low resistance thermal connections, flatness is not a problem, and it can be made out of any convenient material, such as die-cast aluminum or molded synthetic polymer composition material. The modules are mounted on the cold plate with a gasket 54 therebetween to avoid coolant leakage out of the face of the cold plate. There is no need for the extra special flat surfaces required for thermal interface, but the surfaces need only be flat enough for sealing with the gasket. The power modules are held in place on the face of the cold plate by any convenient means. Machine screws 56 and 58 are illustrated in FIG. 1, and spring clamps 60 and 62 are illustrated in FIG. 2 as additional and/or alternate means for securing the power module on the cold plate. The fins and channels are sized so that the fins reach all the way to the bottom of the channel, as seen in FIGS. 2 and 3. The fins are attached by any convenient means, such as adhesive 64. As they are attached, the back wall 66 is supported better and fluid flow beyond the ends of the fins is eliminated. FIG. 4 illustrates the same structure except that pins are used instead of fins. Pins are employed as an alternate way of extending the surface. The parts are the same as in FIG. 3, except for the presence of fins instead of pins. Pins 68 and 70 are particularly identified. The pins are preferably integrally formed with the base 14 for maximum thermal transfer, but may be separate pins pressed into holes in the base. The pins are simply a special configuration of the fins. When a power module is attached to each channel and secured and gasketed in place, coolant liquid delivered into fluid inlet 42 travels through inlet passage 46 and thence up the channels 50 and 52 between the fins or pins integrally formed on base 14. The spent coolant liquid is collected in outlet passage 48 and discharged at fluid outlet 44. The coolant fluid is preferably air-cooled and pumped back into inlet 42. The liquid flow is controlled so that it all passes through the channels 50 and 52 and related channels. The fluid in each channel directly cools the base plate of the associated power module. A minimum thermal resistance is achieved, and this minimizes the coolant fluid pumping requirement. To summarize, the semiconductor wafer 22 of the power module 10 is mounted directly, separated only by thermal expansion control layers and insulator layers, on the metal base 14 which has fins 18, 20 or pins 68, 70 integrally formed thereon. This module 10 is mounted on the coolant fluid flow chamber so that the module 10 mounting is not a thermal interface. In this way, maximum power module cooling is achieved with minimum fluid flow. Means for mounting said wafer 22 on said base 14 comprise such plural layers to electrically insulate said wafer 22 from said base 14 and to accommodate for differences in thermal coefficient of expansion between said wafer 22 and said base 14. This invention has been described in its presently contemplated best mode, and it is clear that it is susceptible to numerous modifications, modes and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

Claims: A power module cooling system comprising: a power module (10) comprising a semiconductor wafer (22) and a metallic base (14), said metallic base (14) having a first side (16), a second side, and a metallic extended surface (18, 20; 68, 70) on said second side of said metallic base (14), and said semiconductor wafer (22) being mounted on said first side (16) of said metallic base (14); a cold plate (40), said cold plate (40) having a channel (50, 52) therethrough and connections (42, 44) on said cold plate (40) for delivering coolant fluid through said channel (50, 52); an opening in said channel (50, 52); said base (14) being mounted on said opening so that said extended surface (18, 20; 68, 70) on said base (14) extends into said channel (50, 52), and said base (14) closing said opening into said channel (50, 52) so that said extended surface (18, 20; 68, 70) on said base is directly cooled by said coolant fluid, wherein said extended surface (18, 20; 68, 70) is extended by cooling projections (18, 20; 68, 70) sized such that each projection (18, 20; 68, 70) reaches to the bottom of the channel (50, 52) and that each projection (18, 20; 68, 70) is attached to the bottom of the channel, so as to prevent fluid flow beyond the end of the projection (18, 20; 68, 70). The system of claim 1, characterized in that said base (14) is removably secured to said cold plate (40) for interchangeability of said base (14) and said semiconductor wafer (22) carried thereon. The system of claim 1 or claim 2, characterized in that a portion of said base (14) engages the face of said cold plate (40) and there is a gasket (54) therebetween to prevent coolant fluid leakage from said opening (50, 52) in said face of said cold plate (40). The system of any of claims 1 - 3, characterized in that a cover (12) is mounted on said base (14), said cover (12) being positioned to extend over said semiconductor wafer (22) in means for mounting said semiconductor wafer (22) on said base (14). The system of claim 4, characterized in that said cover (12) is dielectric. The system of any of claims 1 - 5, characterized in that said projections (18, 20; 68, 70) are fins (18, 20) or pins (68, 70) integrally formed with said base (14). The system of any of claims 1 - 6, characterized in that means for mounting said wafer (22) on said base (14) comprises plural layers to electrically insulate said wafer (22) from said base (14) and to accomodate for differences in thermal coefficient of expansion between said wafer (22) and said base (14).

Inventors: GOODARZI GHOLAM A, OLAVESON RICHARD J, TANZER HERBERT J, GOODARZI, GHOLAM A., OLAVESON, RICHARD J., TANZER, HERBERT J.
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1993
Main classification: H01L23
IPC codes: H01L23