Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-09-11
2001-01-30
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S832000, C228S056300, C228S124600
Reexamination Certificate
active
06178628
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an apparatus and method for thermally coupling a heat sink directly to a surface mount heat generating device package in a manner which provides a more efficient thermal path between the heat sink and the device package, and which allows for a simplified assembly process. More particularly, it relates to a heat sink having a reservoir of thermal binder which allows the heat sink to be secured to a device package prior to or at the same time the device package is secured to the surface mount substrate, thereby simplifying the assembly process while providing a very efficient thermal interface between the heat sink and the heat generating device package.
BACKGROUND OF THE INVENTION
Many electronic devices and electrical systems, such as transistors, integrated circuits, power controls, switches, microprocessors, and the like, generate heat during operation. The capability of some electronic devices is limited by their ability to remove or expel internally generated heat. This heat should be removed from the device to avoid general or localized thermal degradation or failure of the device. In some devices, the heat generated is sufficiently dissipated to the surrounding environment by the enclosure, package, header, or leads. Other devices require additional heat dissipating apparatus, such as heat sinks, heat exchangers, etc., for removing and dissipating excess thermal energy. For purposes of the present invention, a heat sink is any body of thermally conductive material such as metal or other like material which is placed in thermal communication with an electronic device package or other heat generating component for transferring internally generated heat from the device and rapidly dissipating this heat to the surrounding environment by conduction, convection, and/or radiation. Heat sinks may be extruded, machined, molded, sawed, or formed of sheet metal bodies.
It is instructive to consider the dissipation of heat from a heat generating device in terms of heat transfer, that is, the movement or transfer of heat from the device to the heat dissipating apparatus where the heat may be efficiently dissipated. The term “thermal path” will be used herein to refer to the path along which the heat is transferred from the heat generating device through the heat dissipating device (generally a heat sink) to the surrounding environment. A typical thermal path for a heat dissipating assembly would be as follows: the heat is generated by a heat generating electronic device package; the heat travels from the device package through a first thermal interface between the device package and a heat sink; the heat travels through the heat sink; the heat travels through a second thermal interface between the heat sink and the surrounding environment; and the heat is then dissipated into the surrounding environment. In order to ensure that the heat can be dissipated from the heat generating device at a sufficient rate, the heat must be able to travel from the heat generating device to the dissipating environment at a rate commensurate with the rate at which the heat is being generated. Accordingly, the heat must be able to travel along the thermal path as efficiently as possible. Thus, each step in the thermal path from the heat generating device to the dissipating environment must be designed to maximize the efficient transfer of heat. For example, to maximize the rate at which heat can be transferred through the heat sink itself, heat sinks are generally made of materials having high coefficients of thermal conduction such as aluminum, copper, and alloys thereof. Similarly, since a typical heat sink for electrical applications functions by conducting heat away from the heat generating component and dissipating the heat into the surrounding air, heat sinks are typically shaped to maximize surface area by incorporating fins or pins. Increasing the heat sink's surface area increases the physical size of the thermal interface between the heat sink and the surrounding atmosphere (the second thermal interface referenced above), thereby increasing the heat sink's ability to dissipate heat to the surrounding atmosphere.
Of particular interest to the invention at hand, is the first thermal interface, i.e., the thermal interface between the heat sink and the heat generating device package. In order for the heat generated to efficiently travel from the heat generating device to the heat sink, the heat sink must be placed in efficient thermal communication with the heat generating device package. Generally, the most efficient thermal communication can be achieved by securing the heat sink directly to the heat generating device package. Various means have been used to attach heat sinks in efficient thermal communication with heat generating device packages. A known practice is to glue, solder, or otherwise adhere a heat sink directly to a heat dissipating surface of the body of a heat generating device package with heat-conductive epoxy, solder paste, thermally enhanced adhesives, or the like. Heat sinks may also be mechanically attached to heat generating device packages with resilient metal clips mounted on the heat sink or with screws, bolts, clamps, or other connective means which urge the heat sink and electronic device package into mutual physical contact. Although typically not as efficient, heat sinks may also be remotely located but thermally coupled to a heat generating device via a heat spreader device, a heat pipe, or any other means of transferring heat from the source of the heat to the heat sink.
Recently, technological advances have allowed electronic components to decrease in size while significantly increasing in power and speed. This miniaturization of electronic components with increased capability has resulted in the generation of more heat in less space. As a result, the electronic device packages have less physical structure for dissipating heat and less surface area for attaching a heat sink to dissipate the heat. The reduction of surface area available to attach a heat sink or other heat dissipating device reduces the effective thermal path for the heat to move from the heat generating device to the heat dissipating device. A smaller thermal path means less heat can move from the heat generating device to the heat sink; thus, the heat is dissipated at a slower rate and ultimately less heat can be dissipated.
Further complicating these general thermal management issues is the growing preference for surface mounting electronic components on printed circuit boards (PCBs) or other substrates. The use of surface mount PCBs or substrates has become increasingly popular because such substrates allow for a less costly and less time consuming process of fabricating and populating the PCB. As opposed to the manufacturing assembly process of older substrates which required insertion of components through holes in the circuit board for subsequent soldering operations, surface mount PCBs allow for the increased use of automated manufacturing and assembly techniques. In particular, surface mountable devices are typically robotically picked and placed on the PCB and then soldered to the PCB in one automated manufacturing process. In addition to reducing assembly costs, however, the surface mount technology has also allowed for even greater miniaturization of the electronic device packages used on the boards. These smaller surface mount device packages further reduce the device's ability to dissipate its own heat, thus increasing the need for separate heat sinks. In addition, the smaller packages make it increasingly difficult to attach a heat sink directly to the device package. Finally, even when a heat sink can be attached directly to the heat generating device package, the efficiency of the thermal path is limited by the available contacting surface area on the smaller device package.
Several methods have been suggested to effectively dissipate heat from these smaller surface mount electronic device packages. One common ap
Clemens Donald L.
Kuzmin Gary
Mellinger Mark
Aavid Thermalloy LLC
Arbes Carl J.
Cohen & Pontani, Lieberman & Pavane
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