Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2000-03-09
2002-01-22
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S840000, C029S832000, C029S740000
Reexamination Certificate
active
06339875
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable.
FIELD OF THE INVENTION
The present invention relates generally to heatsinks, and more particularly, to materials for securing heatsinks to integrated circuits (ICs).
BACKGROUND OF THE INVENTION
As is known in the art, integrated circuits may be manufactured using a so-called encapsulation process in which an integrated circuit chip or die (i.e. an unpackaged functional element manufactured by subdividing a wafer of semiconductor material) is packaged for mounting and/or protective purposes. During the encapsulation process, a lead frame is disposed in a lead frame holder. A body of the lead frame has projecting therefrom a plurality of pins which form the electrical contact points of a complete integrated circuit. One or more dies may be disposed on a flag portion of the lead frame and wires are bonded to the die(s) and to corresponding ones of the lead frame pins to thus provide a lead frame assembly.
The lead frame assembly is disposed in a mold. An encapsulating material such as plastic, for example, is injected into the mold to enclose the lead frame assembly including the semiconductor die. The resultant package thus corresponds to an encapsulated integrated circuit.
The pins project through the encapsulating housing and thus provide electrical connection points for the integrated circuit. The encapsulating material is proximate to and often physically contacts the semiconductor die.
As is also known, there is a trend to reduce the size of semiconductor devices, integrated circuits and microcircuit modules while having the devices, circuits and modules perform more functions. As a result of the increased functionality, such devices, circuits and modules thus use increasingly more power than heretofore. Such power is typically dissipated as heat generated by the devices, circuits and modules.
This increased heat generation coupled with the need for devices, circuits and modules having relatively small sizes has led to an increase in the amount of heat which must be transferred away from the devices in order to prevent the devices, circuits and modules from becoming destroyed due to exposure to excessive heat. Such devices circuits and modules, are presently limited with respect to the amount of the self-generated heat which they can successfully expel and prevent from building up as they are caused to operate at higher powers.
The proximity of the semiconductor die and the encapsulating housing results in a path between the die and the housing through which heat flows from the semiconductor die to the housing. Heat paths also exist through the bond wires which lead from the semiconductor dies to the pins of the integrated circuit device, however, such heat paths are relatively ineffective due to the relatively small size of the bond wires which are typically provided having a diameter in the range of 0.001 to 0.005 inch. Thus, in most cases it is desirable to extract heat through the surface of the encapsulating housing of the integrated circuit.
The encapsulating material has a mold release characteristic which prevents the encapsulating material from adhering to the mold and thus allows the complete integrated circuit to be separated from the mold in a relatively easy manner and without causing damage to any portion of the integrated circuit. One problem with such encapsulating material, however, is that the mold release characteristic of the encapsulating material prevents other circuit components from adhering to the integrated circuit housing. Thus it is relatively difficult to reliably attach a heatsink to the encapsulating material of the integrated circuit package.
One approach to attaching heatsinks, therefore, has been to mechanically attach the heatsink with a clamp for example. In this approach, a thermal grease or oil is applied to the heatsink, for example, and the heatsink is then placed on the integrated circuit package. A clamp is then used to secure the heatsink to the integrated circuit. One problem with this approach, however, is that the clamps take up space on the printed circuit board to which the clamp is attached. Furthermore, it is relatively time consuming to attach heatsinks to integrated circuits using such clamps.
Moreover, the clamp generates a relatively large compression force between the IC package and the heatsink. The compression force can thus bend and/or distort both the heatsink and the IC package thereby damaging either one or both of the heatsink and IC.
Also, it is relatively difficult to interface such a clamp to an IC package because the clamp attachment points represent areas of very high local stress. If the IC package is plastic, the clamp can locally deform, or even crack the plastic.
Furthermore, the electrical pins of the IC are often located in the regions most desirable for mechanical clamp attachment means. Thus, a relatively complex clamp is often required to properly secure the heatsink to the IC.
Another approach for adhering a heatsink to an IC is to use double sided tape to secure the heatsink to a surface of the integrated circuit. In this approach, a first surface of a strip of double sided tape is placed on the integrated circuit and a second surface of the double sided tape is left exposed. A heatsink is placed on the integrated circuit with at least a portion of the heatsink attached to the second surface of the double sided tape. In this manner the heatsink can be attached to the integrated circuit. One problem with each of these approaches, however, is that due to heating of the integrated circuit, the tape tends to separate from either the integrated circuit package or the heatsink. Thus the heatsink can separate from or fall off the integrated circuit.
In some instances, the environments in which the devices, circuits and modules are used permit complex forced-fluid cooling systems to be employed. Such forced-fluid cooling systems while effective for cooling the devices, circuits and modules are relatively expensive and bulky and require a relatively large amount of space.
Another more economical approach involves the attachment of relatively simple heatsinks having fins provided by metal extrusion or stamping techniques. Such finned heatsinks help to conduct and radiate heat away from the thermally vulnerable regions of the integrated circuit component. For such purposes, it is important that the thermal impedance between a semiconductor or microcircuit device and its associated heatsink structure be kept to a minimum and that it be of uniformity which will prevent build-up of localized hot spots on the device, circuit or module. Such characteristics are not always realized to a satisfactory extent by simply abutting some part of the heat-generating unit with complementary surfaces of its heatsink because, despite appearances, the respective mating surfaces of the heat-generating unit and heatsink will generally have only a relatively small percentage of surface area in actual physical contact.
Such limited contact between a heatsink and an IC component and the attendant poor transmission of heat is due, at least in part, to relatively gross imperfections in the contacting surfaces of the heatsinks and the devices, circuits and modules with which the heatsinks are used. The contact area may be increased somewhat by machining the mating surfaces of the heatsink and the device, circuit or module to relatively precise tolerances. Alternatively, other surface shaping techniques may also be used.
The contact area may also be enlarged by tightly clamping together the heatsink and the device circuit or module. A relatively large clamping force between the heatsink and IC forces irregular surfaces of the heatsink into contact with irregular surfaces of the IC and thus can improve heat transfer characteristics between the heatsink and IC, but the effect is non-linear with a fractional exponent. Thus relatively large increases in force are needed to achieve small improvements in thermal transfer characteristics. As mentioned above, exposure to such la
Arbes Carl J.
Daly, Crowley & Mofford LLP
Heat Technology, Inc.
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