Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1998-12-15
2001-02-27
Picard, Leo P. (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S704000, C361S705000, C361S707000, C361S709000, C257S706000, C257S712000, C257S717000, C257S718000, C165S080200, C165S080300, C174S016300, C174S050510
Reexamination Certificate
active
06195256
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of heat sinks, and, more particularly, to a technique of dissipating heat generated inside a surface mounted integrated power device for a printed circuit board.
BACKGROUND OF THE INVENTION
A problem of dissipating heat generated within an integrated circuit and of maintaining an internal temperature of a package below a maximum tolerable value is particularly relevant when assembling power device packages on the surface of a printed circuit board. Such power device packages are typically formed from a plastic material. These power device packages include a body of resin that partially encapsulates a metal heat sink which acts as an internal heat sink. The metal heat sink is usually made of copper.
A silicon chip or die of an integrated circuit is mounted on the surface of the internal heat sink. According to a typical geometry of many of these power device packages, the presence of the metal heat sink on a bottom face of the integrated device makes it impossible to install an external heat sink that simply rests on the device when mounted on the printed circuit board because of the relatively poor thermal coupling formed.
To increase heat dissipation during operation, several approaches are known. A first approach is to pattern a copper pad on the printed circuit board onto which the internal heat sink of the power device package is soldered. In this manner, the heat generated inside the device may be effectively transferred through the copper pad. Accordingly, the heat is spread out over the printed circuit board.
Often, a plurality of holes are formed within the copper pad. These holes have their inner wall covered with a film of copper, and comprise as many conductive vias as necessary for transferring heat across the thickness of the board to the other side where another dedicated copper pad may be patterned. On the copper pad, an external metal sink may be placed or contact is made by the metal chassis of the apparatus.
According to a similar approach, the metal coated holes do not cross the entire thickness of the board, but reach a ground metallization layer sandwiched therein. The ground metalization layer is typically in the form of a copper film, which disperses the heat throughout the board. Yet another approach includes using printed circuit boards specially formed on a relatively thick metal substrate of a thermally conductive material, such as aluminum.
All of these approaches imply the use of specially configured printed circuit boards, or the use of printed circuit boards having special composite structures for heat dissipation. Therefore, these known approaches are rather costly, and often require special printed circuit boards that may not be compatible with a particular manufacturer's requirements.
SUMMARY OF THE INVENTION
An external metal heat sink may be installed on a power device to be assembled on a printed circuit board. The external metal heat sink may be soldered to an internal heat sink of the power device despite being customarily located on a bottom side of the device, i.e., slug down. The bottom side of the device is surface mounted on the printed circuit board.
Therefore, according to the invention, it is possible to place an external heat sink over the power device and solder it to the internal heat sink of the package during normal soldering operations. This is applicable to all devices and components installed on the printed circuit board that have a normal structure. A normal structure is one that does not have specially designed characteristics, other than having an internal metal heat sink placed at the bottom of the package for dissipating the heat generated in the power device. The internal metal heat sink placed at the bottom of the package is on the assembling side, i.e., slug down.
The invention applies to any power device where the internal metal heat sink or slug of the power device package has at least a portion projecting out of a surface of at least one side of the package. The package is commonly a molded body of encapsulating resin. An external metal heat sink has a shape suited to couple with a particular package of the power device, and has at least one surface abutting the package. The at least one surface of the external heat sink leaves a gap between the protruding part of the internal metal heat sink incorporated in the package and a surface of the external metal heat sink spacingly opposed to it. The power device is for surface-mounting on a printed circuit board. The gap is readily filled by molten eutectic tin/lead alloy during a normal soldering step of the devices and components installed on the printed circuit board.
According to a first embodiment of the invention, the internal metal heat sink has a portion that extends from a plane of the bottom encapsulating resin body of the power device package. The external metal heat sink has a base that is substantially shaped as a flat frame. Dimensions of the flat frame correspond with the package dimensions so that the protruding part of the internal metal heat sink of the package remains contained in an aperture of a bottom frame portion of the external heat sink. The sides of the frame portion abut the recessed perimeter part of the bottom of the molded body of encapsulating resin of the package that surrounds the protruding part of the internal metal heat sink.
In this manner, a gap remains between the outer perimeter surface of the internal metal heat sink and the inner perimeter surface of the bottom flat frame of the external metal heat sink. This gap may be filled at least partially by the molten soldering eutectic alloy of a patterned layer of soldering paste. The patterned layer of soldering paste is preventively applied on the assembly areas of the devices and components. Upon solidification of the solder alloy between the inner perimeter of the projecting part of the internal metal heat sink of the package and the inner surface of the aperture of the flat frame of the external metal heat sink, an effective thermally conducting bridge is formed. The thermally conducting bridge favors heat transfer from the internal metal heat sink to the external heat sink.
According to an alternative embodiment of the invention, the internal heat sink protrudes from two opposed sides of the molded body of resin of the power device package. The bottom of one of the sides rests on the patterned layer of soldering paste defined on the component assembly side of the board. The external metal heat sink is shaped to be placed over the power device. The external metal heat sink has two legs that extend toward the surface of the printed circuit board along the above mentioned sides of the package. The two legs rest on the printed layer of soldering paste, thus defining a gap between each laterally extending part of the internal metal heat sink and the inner face side of the relative vertical legs of the external metal sink opposed to it. The small vertical gap is at least partially filled by liquefied eutectic alloy during the soldering phase. This permanently solders each of the two laterally protruding parts of the internal metal heat sink to the opposed legs of the external heat sink placed over the device.
REFERENCES:
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patent: 5216283 (1993-06-01), Lin
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Patent Abstracts of Japan, vol. 006, No. 211, Oct. 23, 1982, and JP 57 117263 A (Mitsubishi Denki KK), Jul. 21, 1982.
Patent Abstracts of Japan, vol. 011, No. 292, Sep. 19, 1987, and JP 62 092347 A (Mitsubishi Electric Corp.), Apr. 27, 1987.
Patent Abstracts of Japan, vol. 008, No. 139, Jun. 28, 1984, and JP 59 047750 A (Hitachi Seisakusho KK), Mar. 17, 1984.
Patent Abstracts of Japan, vol. 018, No. 299, Jun. 8, 1994, and JP 06 061635 A (Rohm Co Ltd), Mar. 4, 1994.
G.L. Bond et al., “Wrap-Around Heat Sink,” IBM Technical Disclosure Bulletin, vol. 20, No. 4, Sep. 1977, pp. 1434-1435.
“Technique for H
Rossi Roberto
Tiziani Roberto
Villa Claudio Maria
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Datskovsky Michael
Galanthay Theodore E.
Picard Leo P.
STMicroelectronics S.r.l.
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