Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
1999-11-12
2002-09-10
Copenheaver, Blaine (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S545000, C428S317900, C428S318400, C428S319100, C428S313300, C428S313900, C428S315900
Reexamination Certificate
active
06447894
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a highly thermal-conductive silicon carbide composite material having excellent thermal conductive characteristics, having a light weight, and suitable as a heatsink for semiconductor parts such as a ceramic substrate or an IC package, a method for producing it and a heat dissipation device employing it.
BACKGROUND OF THE INVENTION
Along with enlargement of the capacity of semiconductor elements and high-integration of semiconductor elements in the semiconductor field in recent years, it has been an important theme how to discharge heat energy generated from a semiconductor element to the exterior effectively. A semiconductor element is usually loaded on an insulating substrate such as a ceramic substrate. In such a case, generated heat from the semiconductor element is discharged to the exterior by means of a heatsink provided on e.g. the back side of the substrate, to secure performance characteristics of the semiconductor element.
Conventionally, copper (Cu) has been mainly used for the material of the heatsink. Although copper has a coefficient of thermal conductivity of as high as 390 W/mK at a temperature in the vicinity of room temperature, it has a high coefficient of thermal expansion of 17×10
−6
/K, and accordingly, cracks or fractures may form on a ceramic substrate due to a difference in thermal expansion between the ceramic substrate (coefficient of thermal expansion: 7-8×10
−6
/K) and the heatsink, by addition of thermal cyclings. Conventionally, when the ceramic substrate for a heat dissipation device is used in such a field that reliability is required, e.g. Mo/W having a small difference in the coefficient of thermal expansion with the ceramic substrate, has been used as the heatsink.
Although the above-described Mo/W heatsink has an excellent reliability, it has a coefficient of thermal conductivity of as low as 150 W/mK, such being problematic in view of heat dissipation characteristics, and further, such a heatsink is expensive. Under these circumstances, an attention has been drawn to a metal-ceramic composite which comprises ceramic fibers or particles and copper or aluminum alloy reinforced by the ceramic fibers or particles, which is referred to simply as MMC (Metal Matrix Composite) in recent years. Such a composite is usually made in such a manner that ceramic fibers or particles as a reinforcing material are preliminarily formed to prepare a preform, and a metal as a base material (matrix) is infiltrated into the fibers or particles of the preform. As the reinforcing material, a ceramic such as alumina, silicon carbide, aluminum nitride, silicon nitride, silica or carbon may be employed. However, wettability of the ceramic as the reinforcing material and the alloy as the matrix, and the reaction layer at the interface therebetween, significantly affect the coefficient of thermal conductivity of the composite.
For the above-mentioned composite, to increase the coefficient of thermal conductivity, it is necessary to select a reinforcing material and an alloy having a high coefficient of thermal conductivity, and to decrease the coefficient of thermal expansion, it is necessary to select a reinforcing material having a low coefficient of thermal expansion. Accordingly, a composite of silicon carbide with aluminum alloy has been mainly studied.
However, with respect to the heat dissipation device comprising a conventional ceramic substrate and heatsink bonded to each other, as mentioned above, when a heavy-metal material such as Mo or W is employed for the heatsink, the heat dissipation device will be heavy, and heat dissipation properties will not be adequate. On the other hand, when e.g. Cu or Al, being relatively light and having excellent heat dissipation properties, is used as the heatsink, the difference in thermal expansion with the ceramic substrate will be large, and in order to obtain a structure with high reliability, the bonding structure itself will be extremely complicated, thus leading to increase in production costs and increase in thermal resistance as the heat dissipation device. Accordingly, with respect to the conventional heat dissipation device having bonding structure of the ceramic substrate and the heatsink, it has been objects to simplify the bonding structure, and to improve reliability and heat dissipation properties.
On the other hand, to overcome the above-mentioned problems, a metal-ceramic composite has been studied. However, in order to obtain a coefficient of thermal expansion close to the ceramic substrate, it is necessary to increase the ratio of the ceramic as a reinforcing material having a low coefficient of thermal expansion. To increase the ratio of the ceramic component, it is necessary to form a preform under a high forming pressure, whereby the cost will be high, and subsequent infiltration of the alloy may not be adequately carried out. Accordingly, it has been an object to develop techniques to provide a metal-ceramic composite having a coefficient of thermal expansion close to the ceramic substrate and having a high coefficient of thermal conductivity with a low cost.
Further, when such a composite is used as the heat dissipation device, the composite will be soldered to a circuit substrate, and accordingly, if the warpage of the composite is too large, the soldering will be difficult. Accordingly, when such a composite is used as the heat dissipation device, it is required to control the warpage within a certain amount. On the other hand, a device having such a heat dissipation device incorporated therewith, such as a power module, is usually fixed on e.g. a heat dissipation fin by screws. In such a case, the bonding surface between the device such as a power module and the heat dissipation fin is preferably convex so that a stress is applied on the bonding surface, in view of heat dissipation property since screw force after screwing will be high. However, with respect to the conventional metal-ceramic composite, in order to optionally add a shape such as warpage, as mentioned above, there is no way except adjustment by mechanical processing. In such a case, the metal-ceramic composite will be extremely hard, the mechanical processing cost will be high, and the device itself will be extremely expensive.
SUMMARY OF THE INVENTION
Under these circumstances, the present invention has been made to provide a composite which has a high thermal conductivity, a small specific gravity and a coefficient of thermal expansion close to the ceramic substrate, which has warpage, and which can be tightly bonded to e.g. a heat dissipation device, and a heat dissipation device employing it, with a low cost.
The present inventors have been made extensive studies to achieve the above-mentioned objects, and as a result, they have found that characteristics such as the coefficient of thermal expansion and the shape of the composite can be controlled by adjusting the composition and the structure of the composite, and the present invention has been accomplished.
Namely, the present invention provides a silicon carbide composite which is a flat composite comprising a porous preform of silicon carbide and a metal containing aluminum as the main component, infiltrated into the porous preform, said composite having a warpage of at most 250 &mgr;m per 10 cm of the principal plane length of the composite.
The present invention further provides a silicon carbide composite which is a flat composite having at least 4 holes in its plane, and having a relation of 50≦Cx≦250 and −50≦Cy≦200, where Cx is a warpage (&mgr;m) per 10 cm in a hole-to-hole direction (X direction) and Cy is a warpage (&mgr;m) per 10 cm in a direction perpendicular thereto (Y direction).
The present invention further provides a silicon carbide composite having both front and back sides covered with a metal layer containing aluminum as the main component with an average thickness of from 10 to 150 &mgr;m, with a difference in the average thickness between the front and bac
Hirotsuru Hideki
Hiruta Kazuyuki
Miyai Akira
Nomura Kenji
Saito Mitsuaki
Copenheaver Blaine
Denki Kagaku Kogyo Kabushiki Kaisha
Vo Hai
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