Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Reexamination Certificate
2001-06-07
2002-11-05
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
C028S206000
Reexamination Certificate
active
06475429
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a heat sink substrate having a large area and a method of manufacturing the same and, more specifically, to a large area heat sink substrate which is mounted on a power semiconductor such as a metal oxide semiconductor field effect transistor (MOSFET), IGBT and the like and on a large capacity rectifier used to an electric railcar, electric automobile and so on and a method of manufacturing the heat sink substrate.
(2) Description of the Related Art
Semiconductors have been widely used and, in particular, so-called power semiconductors including MOSFETs, IGBTs and so on which generate heat have been extensively used in various fields expanding from industrial equipment to household equipment. As the power semiconductors are applied to electric cars and automobiles including hybrid vehicles, their output power and size are outstandingly increased and the amount of heat generated by them is inevitably increased.
Power semiconductors, from which a current of several hundreds of amperes flows, are different from MPUs (microprocessor units) conventionally used in so-called personal computers and so on in the materials constituting them and the design of their structure and they may substantially generate an amount of heat of several kilowatts. Their size is, for example, about 98-375 cm
2
which is at least ten times the size of the MPUs which is about 2.2-25 cm
2
. Thus, the power semiconductors are often used under severe conditions as to vibration, humidity, temperature, strength and so on. As a result, when they are repeatedly used many times under the very severe conditions, cracking, exfoliation and the like are caused to them and their life is ended regardless of that no cracking and exfoliation are seemingly caused thereto.
Further, the power semiconductor is required for reliability which is more severe than that required to the MPU. In particular, the power semiconductor must pass a life test of several hundreds to several thousands of times in terms of a heat cycle as a parameter, as to deformation caused by the warp and the like of a substrate mounted on it and the occurrence of cracking, in spite of that the power semiconductor has a large area. Accordingly, when a heat sink substrate having a large area of, for example, about 100-400 cm
2
is warped or when a heat sink substrate, on which a plurality of semiconductor elements are mounted, generates heat and the semiconductor elements are differently expanded by the heat, cracking and exfoliation are caused to the heat sink substrate due to the warp of the heat sink substrate or the different thermal expansion of the components and straining resulting from the expansion, even if the heat sink substrate has passed a life test executed in the state that it outputs a considerably large power (10-50 W) in a severe environment. From the above-mentioned, difference in size of the heat sink substrate is an important technical problem.
It is preferable for the heat sink substrate of the power semiconductor to have thermal conductivity of at least 200 W/m·K and more preferably at least 230 W/m·K. In addition, the heat sink substrate must have suitably small thermal expansion and strength which is larger than that of a copper material. Further, more important is that the thermal conductivity of the material of the heat sink substrate is not lowered, even if heat is generated thereto, to such a degree as to injure the operation thereof when the heat sink substrate is practically used.
On the other hand, there is generally a problem that an increase in the size of a metal heat sink material makes the characteristic anisotropy thereof more outstanding. The inventors have studied and developed a single-layer composite material by mixing a copper power with a molybdenum power and sintering and rolling them. The single-layer composite material is considerably uniform as an entire body and has a small amount of characteristic anisotropy. The single-layer composite material has not any void as well as the thermal conductivity and thermal expansion coefficient thereof are very closely analogous to the values which are prescribed from a mixing ratio of copper and molybdenum, even if the composite material does not contain a sintering assistant agent. Thus, it is supposed that the single-layer composite material can be effectively used to a heat sink substrate for a device on which semiconductor elements are mounted.
However, when a usual plate is rolled in an ordinary process, it is economically difficult to make the characteristic anisotropy thereof to zero. When the composite material is, for example, cross rolled, the size thereof is regulated by the work rolls used in the rolling as well as it is difficult to finish the composite material without leaving straining which take places in the rolling in the interior of the composite material. Accordingly, the single-layer composite material has been not suitable as a material as a heat sink substrate which constitutes a power semiconductor having a large area and high reliability. That is, even if the joint shape of the composite material and a joint agent used to it are changed, there cannot be obtained a heat sink substrate for the power semiconductor device.
It is possible to prepare a plate member having a length of at least 200 mm in one direction and an amount of warping of 200 &mgr;m. However, this plate member is inconvenient because the warping thereof is increased by residual straining while it is annealed or subjected to surface processing such as plating and the like. That is, it is required that a heat sink substrate which will be assembled to a large area power semiconductor has substantially no residual straining or a minimal possible amount of residual straining.
Further, a material that satisfies the following performances is required to a heat sink substrate used to the power semiconductor.
First, the material has thermal conductivity of at least 200 W/m·K (at a room temperature−200° C.), preferably at least 230 W/m·K, and most preferably at least 300 W/m·K as a temperature increases. In this state, however, the thermal expansion coefficient of copper (=370 W/m·K), for example, is 16-17×10
−6
/K and the Young's modulus thereof is also low (13×10
3
kgf/mm
2
). In practical use, it is impossible to devise an arrangement having reliability from the material, since the heat sink substrate made of the material is cracked and exfoliated as well as the thermal expansion coefficient and Young's modulus thereof are excessively different from the elements mounted thereon and the peripheral material thereof.
To cope with the above problem, a semiconductor element is mounted on a material which is mainly composed of multilayer material, such as Cu/Al
2
O
3
/Cu, Cu/AlN/Cu, Cu/AlN, AlN, Al
2
O
3
and so on. It is essential that the heat sink substrate uses the material which has a thermal conductivity superior to that of AlN having the maximum thermal conductivity of 200 W/m·K among the materials which at least regulate a heat sink property and can be practically put into market. An Al/SiC composite material is said to be light in weight and to have high thermal conductivity and is such that when it is heated to about 120-150° C., the thermal conductivity thereof is lowered by about 20%. Further, although some materials, which are put into market in the state that they are subjected to a melting and impregnating process, have thermal conductivity of 200 W/m·K at an ordinary temperature, they are not sufficiently satisfied in practical application because their thermal conductivity is lowered to 160 W/m·K at 120° C.
The thermal expansion coefficient is 12×10
−6
/K or less and preferably 9×10
−6
/K or less. It can be said that a material which is most affected by other material in the restricting relationship with it, warping and the like is ceramic. When only this point is taken into consideration, the thermal expansion coefficient i
Arikawa Tadashi
Asai Kiyoshi
Hirayama Norio
Ichida Akira
Maesato Hidetoshi
Jenkins Daniel J.
Sughrue & Mion, PLLC
Tokyo Tungsten Co., Ltd.
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