Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With provision for cooling the housing or its contents
Reexamination Certificate
2000-02-10
2004-05-04
Picardat, Kevin M. (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With provision for cooling the housing or its contents
C257S713000, C257S717000, C257S721000
Reexamination Certificate
active
06730998
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to heat sinks used to dissipate heat from semiconductor devices during normal operation thereof. Particularly, the present invention pertains to the use of stereolithographic techniques to fabricate heat sinks for use on semiconductor devices, to heat sinks so fabricated, and to semiconductor devices including stereolithographically fabricated heat sinks.
2. State of the Art
Heat Sinks
During normal use, semiconductor devices generate heat. Adequate dissipation of the heat generated during normal use of a semiconductor device is necessary for the safe and reliable operation of an electronic appliance that includes the semiconductor device. If the semiconductor device reaches an excessively high temperature, the integrated circuits of the semiconductor device can fail or a circuit board fire can result, damaging the electronic system of which the semiconductor device is a part.
While some semiconductor devices are able to dissipate sufficient amounts of heat without an additional heat sink or heat spreader, state of the art semiconductor devices with increased speed, circuit complexity, and circuit density often require added heat sinks.
In particular, as semiconductor devices have become more dense in terms of electrical power consumption per unit volume, heat generation has greatly increased, requiring package construction which dissipates the generated heat much more rapidly. As the state of the art progresses, the ability to adequately dissipate heat is often a severe constraint on the size, speed, and power consumption of an integrated circuit design.
In this application, a heat sink will be distinguished from a “heat spreader,” the former pertaining to a structure with a heat transfer portion or element positioned proximate to a semiconductor device and a heat dissipation portion or element relatively more remote from the semiconductor device, the latter pertaining to a member which channels heat from a semiconductor die to leads which exit the die package. However, a heat sink and a heat spreader may together be used to cool a device.
Typically, heat sinks are fabricated from materials with good thermal conductivity, such as metals (e.g., aluminum, copper alloys, etc.), ceramic materials, and glass. The heat transfer portion of a heat sink is configured to absorb heat from the semiconductor device proximate thereto and, therefore, generally contours to at least a portion of a surface of the semiconductor device. The heat dissipation portion of a heat sink may include a series of small protrusions, which are typically referred to as “fins,” that receive heat from the heat transfer portion of the heat sink and are configured to dissipate the heat away from the semiconductor device as air flows between the fins. The shapes, sizes, arrangement, spacing, and numbers of fins on a heat sink are configured so as to optimize the heat dissipation capabilities of the heat sink with respect to the particular heat dissipation needs of a specific type of semiconductor device.
Heat sinks are typically manufactured separately from the semiconductor devices to which they are subsequently secured.
Conventionally, metal heat sinks have been manufactured by extrusion or casting processes. When extruded, molten metal is forced through an extrusion die to produce an elongated extrusion of a cross-section taken transverse to the length thereof of a desired heat sink configuration. The elongate extrusion is then sectioned transverse to the length thereof to provide the heat sinks. Cast heat sinks are manufactured by disposing a molten quantity of heat conductive material into a refractory mold.
Heat sinks can also be machined from blocks of material. As conventional heat sinks have spaced apart fins, however, machining processes waste a considerable amount of material. In addition, due to the small size and high complexity of conventional heat sinks, the use of machining processes can be very time consuming and expensive. For these reasons, the use of machining processes to manufacture heat sinks is somewhat undesirable.
The use of extrusion, casting, and machining processes to manufacture heat sinks are also somewhat undesirable since each of the processes limit the possible configurations of the manufactured heat sinks. For example, when extrusion is used, the transverse cross-section taken along the entire length of each heat sink has the same two-dimensional shape, being that imparted by the two-dimensional configuration of the extrusion die. When heat sinks are cast, the configurations thereof are determined by the casting molds. Typically, molds have two parts, and may include additional inserts to facilitate the formation of more complex features. State-of-the-art machining processes are limited to, at most, seven axes. Typically, however, less complex three-axis or five-axis machines are used. Nonetheless, certain types of features, such as internally confined cavities and non-linear channels cannot be formed easily when casting or state-of-the-art machining equipment is used.
An alternative method for manufacturing heat sinks is disclosed in U.S. Pat. No. 5,814,536, issued to Rostoker et al. on Sep. 29, 1998 (hereinafter “the '536 Patent”). The '536 Patent discloses the use of powder metallurgy techniques to form a heat sink. Thus, the heat sink is formed from a mixture of powdered metal (e.g., copper, aluminum, tungsten, titanium, and alloys thereof) and a suitable binder. The mixture is placed into a mold, where the metal particles are bonded to adjacent particles, or sintered together, under appropriate pressure and at an appropriate temperature. The binder, if any, is removed (i.e., burned off) during the sintering process. The sintered heat sink can then be machined to provide features that may not be readily obtained or possible to obtain by the sintering process alone. Since the sintering process of the '536 Patent employs a mold, it is somewhat undesirable due to the previously mentioned conformational limitations that are present when a mold is used.
As noted above, a prefabricated heat sink is conventionally assembled with a semiconductor device. The assembly can then be packaged by known techniques, such as by transfer molding of a particle-filled polymer, as known in the art. If such an assembly is packaged, however, the packaging mold must usually be configured so as to receive at least a portion of the heat sink to permit its projection beyond the polymer packaging. The manufacture of molds configured to receive heat sinks is somewhat undesirable due to the complexity of the mold designs and the high costs of machining such molds.
The art does not teach a method of fabricating heat sinks on semiconductor devices or of fabricating heat sinks by stereolithography, or layered manufacturing, processes.
Stereolithography
In the past decade, a manufacturing technique termed “stereolithography,” also known as “layered manufacturing,” has evolved to a degree where it is employed in many industries.
Essentially, stereolithography as conventionally practiced involves utilizing a computer to generate a three-dimensional (3-D) mathematical simulation or model of an object to be fabricated, such generation usually effected with 3-D computer-aided design (CAD) software. The model or simulation is mathematically separated or “sliced” into a large number of relatively thin, parallel, usually vertically superimposed layers, each layer having defined boundaries and other features associated with the model (and thus the actual object to be fabricated) at the level of that layer within the exterior boundaries of the object. A complete assembly or stack of all of the layers defines the entire object, and surface resolution of the object is, in part, dependent upon the thickness of the layers.
The mathematical simulation or model is then employed to generate an actual object by building the object, layer by superimposed layer. A wide variety of approaches to stereolithography by differ
Street Bret K.
Williams Vernon M.
Picardat Kevin M.
TraskBritt
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