Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
1998-10-26
2001-02-06
Turner, Archene (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C428S457000, C428S469000, C428S698000, C428S701000, C428S702000
Reexamination Certificate
active
06183875
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for producing ceramic substrates having metals joined thereto. More particularly, the invention relates to a process for producing aluminum or aluminum alloy-ceramics composite electronic circuit boards suitable for mounting highpower electronic devices such as power modules.
This invention further concerns a process for producing rugged metal-bonded-ceramic (MBC) material or components. Particularly the present invention is directed to an industrial process for producing MBC material or components of metals with oxide, nitride or carbide ceramics that are suitable for use in automotive parts, electronic parts, etc. The invention also is directed to MBC plates produced by such process, as well as electronic circuit substrates fabricated from such MBC plates.
2. Background Information
Metal-ceramics composite electronic circuit boards which have metals joined to the surface of ceramic substrates are commonly used for mounting high-power devices such as power modules. To make such composite circuit boards, copper, which has high electrical and thermal conductivities, is joined to alumina or aluminum nitride ceramics.
MBC components are extensively used in automobiles, electronic equipment and other applications by taking advantage of the chemical stability, high melting points, insulating property and high hardness of ceramics in combination with the high strength, high toughness, free workability and conducting property of metals. Typical applications of the components include rotors for automotive turbochargers, as well as substrates and packages for mounting large-power electronic elements.
MBC material and MBC components are known to be produced by various methods including adhesion, plating, metallizing, thermal spraying, brazing, direct bonding or “DBC”, shrink fitting and casting.
Adhesion is a process in which a metal member is joined to a ceramic member with an organic or inorganic adhesive.
Plating is a process that comprises activating the surface of a ceramic member and dipping it in a plating bath to apply a metal plate.
Metallizing is a process that comprises applying a paste containing metal particles to the surface or a ceramic member and sintering it to form a metal layer.
Thermal spraying is a process in which molten metal (or ceramic) drops are sprayed onto the surface of a ceramic (or metal) member so that a metal (or ceramic) layer is formed on that surface.
The direct bonding process (DBC) has been developed for the particular purpose of joining copper to oxide ceramics. In the DBC method, an oxygen-containing copper member sheet is heated in an inert gas atmosphere while it is joined to a ceramic member or, alternatively, oxygen-free copper is heated in an oxidizing atmosphere, to join copper to a ceramic member, i.e., the surface of an oxygen-free copper member is first oxidized to form a copper layer then joined to a ceramic member. Thus in order to join copper to non-oxide ceramics by the DBC method, an oxide layer must first be formed on the surface of a non-oxide ceramic substrate. As disclosed in Unexamined Published Japanese Patent Application (Kokai) No. 3077/1984, an aluminum nitride substrate is first treated in air at a temperature of about 1000° C. to form an oxide on the face of the substrate and the direct bonding method is then applied to join copper to the aluminum nitride.
In brazing, copper is joined to ceramics with the use of an intermediary brazing material containing an active metal. The most common brazing material used in this method is based on a Ag—Cu—Ti system.
Brazing is a process in which metal and a ceramic member are joined with the aid of a low-melting point filler metal or alloy. To insure that the filler metal or alloy is securely joined to the ceramic member, a metal component highly reactive with ceramics is added or a metal layer is preliminarily formed on the joining surface of the ceramic member by a suitable method such as metallizing or thermal spraying.
In shrink fitting, the ceramic and metal members to be joined are provided with a projection and a cavity, respectively, in such a way that the outside diameter of the projection is equal to the inside diameter of the cavity and the metal member is heated to expand the cavity, into which the projection of the ceramic member is inserted; thereafter, the two members are cooled so that they form an integral assembly in which the projection on the ceramic member is nested in the cavity in the metal member.
Casting is similar to shrink fitting, except that a metal is cast around a ceramic member and cooled so that it shrinks to have the ceramic member nested as an integral part.
These prior art methods, however, have problems. Adhesion produces composites that are low in adhesion strength and heat resistance. The applicability of plating, metallizing and thermal spraying is usually limited to the case of forming thin metal (or ceramic) layers whose thickness ranges from a few microns to several tens of microns.
Shrink fitting and casting are applicable only to a special case in which at least part of a ceramic member is to be nested in a metal.
In DBC, copper is the only metal that can be joined and the temperature for joining must be within a narrow range close to the eutectic point of Cu—O and, hence, there is a high likelihood for the development of joining defects such as swelling and incomplete joining.
Brazing uses expensive filler metals or alloys and requires the joining operation to be performed in a vacuum and, hence, the operational cost is high enough to prevent the use of the method in a broad range of applications.
In spite of their widespread use, the copper-ceramics composite substrates have several problems associated with production and practical use. The most serious problem is that cracks can develop in the ceramic substrates during mounting electronic components and during their subsequent use, whereupon dielectric breakdown may occur across the thickness of the substrate.
In order to join copper to ceramic substrates, they are heated to almost 1,000° C. and, in addition, the copper-ceramics composite substrate is heated to almost 400° C. when mounting electronic components such as power modules. The thermal expansion coefficient of copper is higher than that of ceramics by a factor of about 10, so when the composite substrate is cooled to room temperature, the thermal expansion mismatch will cause a substantial thermal stress within the substrate.
Furthermore, on account of the environment in which the electronic components are used, as well as the heat generated during their use, the temperature of the composite substrate is subject to constant changes, which cause corresponding changes in the thermal stress on the substrate. Such thermal stresses contribute to cracking in the ceramic substrate. One of the important parameters for evaluation of ceramic electronic circuit boards is resistance to heat cycles, or the number of thermal swings between −40° C. and 125° C. that can be applied to the circuit board until cracking occurs. Copper-alumina composite substrates fabricated by the direct bonding method can withstand 20 such heat cycles but, on the other hand, similar substrates fabricated by brazing can withstand only 10 heat cycles or less.
Aluminum has comparable electrical and thermal conductivities to copper and the idea of using aluminum as a conductive circuit material has been known for many years (see, for example, Unexamined Published Japanese Patent Application (kokai) No. 121890/1984, which discusses such idea). Aluminum is softer than copper and its yield strength is about a quarter of the value for copper. Therefore, it is anticipated that the residual stress in the composite substrates could be markedly reduced by using aluminum as a circuit material. However, Unexamined Publication Japanese Patent Application (kokai) No. 121890/1984, supra, do not disclose a specific method of joining aluminum to ceramics.
Unexamined Published Japanese P
Kimura Masami
Nagata Choju
Ning Xiao-Shan
Sakuraba Masami
Suganuma Katsuaki
Dowa Mining Co. Ltd.
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Turner Archene
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