Metal fusion bonding – Process – Metal to nonmetal with separate metallic filler
Reexamination Certificate
2000-08-16
2002-03-12
Jones, Deborah (Department: 1775)
Metal fusion bonding
Process
Metal to nonmetal with separate metallic filler
C228S122100, C228S123100, C228S124500, C228S177000, C228S903000, C029S428000, C029S458000
Reexamination Certificate
active
06354484
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to metal-ceramic composite substrates, and more specifically to brazing materials that can effectively be used in joining metals to ceramic substrates.
2. Description of the Prior Art
Several methods are known in the art of joining metal such as copper plates to ceramic substrates such as aluminum nitride (AlN) and two typical examples are the active metal brazing process described in Japanese Patent Laid-Open No.166165/1985 entitled “A method of Joining Nitride-Base Ceramics to Metals” and the process in which a copper plate is directly joined to a modified surface of an aluminum nitride substrate (as typically described in Japanese Patent Laid-Open No. 163093/1981).
The active metal brazing process provides a higher strength of joint than the direct joining process and the resulting joint has the desired characteristics such as high durability to repeated heat cycles. Hence, the active metal brazing process currently finds extensive use in the joining of copper plates to non-oxide-base ceramic substrates, say, nitride ceramic substrates.
Two brazing materials are commercially used in the practice of the active metal brazing process; one of them is composed of Ag, Cu a nd an active metal selected from among Ti, Zr and Hf (as described in Japanese Patent Laid-Open No.166165/1985) and the other is an active metal paste material composed of Ag, Cu and titanium hydride (as described in Japanese patent Laid-Open No.101153/1991).
Circuit substrates produced by joining copper plates to both sides of an aluminum nitride substrate with the aid of these brazing materials have already been commercialized.
In many brazing materials, an active metal s added to a structure comprising 72% of Ag and 28% of Cu, and accordingly the joining process is carried out at a temperature higher than the eutectic point of 780° C., preferably, at a temperature of around 850° C.
However, most recent circuit substrates are required to perform on larger electrical power and to meet this requirement, it has been desired to develop circuit substrates that are not only provided with good heat dissipating and electrical insulating properties but which also exhibit higher strength and thermal impact resistance.
SUMMARY OF THE INVENTION
An object of the invention is to provide a process for producing metal-ceramic composite substrates using the brazing material.
The present inventors conducted intensive studies in order to attain the stated objects and found that joining processes could be carried out at a temperature of 780° C.~870° C. lower than the melting point of 875° C. by using active metal brazing materials added with 90% of Ag and 9.5% of Cu or 0.5% of an active metal Ti, and that the jointed composite substrates had no non-contact portions.
The present invention has been accomplished on the basis of this finding.
According to a first aspect of the invention, there is provided a metal-ceramic composite substrate produced by joining a metal plate to a ceramic substrate by using a brazing material of which the solid content comprises 90.0~99.5% of Ag, 0~9.5% of Cu, 0.5~4.0% of an active metal on a weight basis.
Said ceramic substrate is at least one kind of ceramic substrate selected from a group consisting of Al
2
O
3
, AlN and Si
3
N
4
.
Said active metal is at least one active metal selected from a group consisting of Ti, Zr and Hf.
There is provided a process for producing a metal-ceramic composite substrate by joining a metal plate to a ceramic substrate,which comprises the steps of:
adding 10-14 parts by weight of a vehicle to 100 parts by weight of a powder of which the solid content comprises 90.0~99.5% of an Ag powder, 0~9.5% of a Cu powder and 0.5~4.0% of an active metal powder on a weight basis;
blending the respective ingredients to form a brazing material in a paste form;
applying the brazing material to the ceramic substrate;
superposing a metal plate on the applied brazing material and heating the assembly at a temperature not higher than the melting point of the brazing material to form a joint of the metal plate and the ceramic substrate; and
applying an etching resist on the metal plate of the joint to form a circuit pattern and etching the metal plate to form a metalized circuit.
Said heating temperature of the assembly is in the range of 780° C. to 870° C. which is not higher than the temperature of the melting point of the brazing material.
There is provided a brazing material for use in joining a metal plate to a ceramic substrate, which is in a paste form prepared by blending 10-14 parts by weight of a vehicle with 100 parts by weight of a powder of which the solid content comprises 90.0~99.5% of an Ag powder, 0~9.5% of a Cu powder and 0.5~4.0% of an active metal powder on a weight basis.
There is provided a brazing material for use in joining a metal plate to a ceramic substrate, which is in a paste form prepared by blending 10-14 parts by weight of a vehicle with 100 parts by weight of a powder of which solid content comprises 90.0~99.5% of an Ag powder,0~9.5% of a Cu powder, 0.5~4.0% of an active metal powder and 0.0~0.9% of a titanium oxide powder on a weight basis.
The brazing material to be used in the invention has an Ag content of 90.0~99.5 wt %. If the Ag content is less than 90 wt % unsuitable composite substrates having non-contact portions will be produced.
The Cu content of the brazing material is less than 9.5 wt %. The reason why the Cu content is smaller than that of the conventional Ag—Cu brazing material is that when the assembly is heated the Ag component and the contact surface of the copper plate are reacted to create the eutectic crystalization.
The active metal that can be used in the invention is at least one element of group IVa of the periodic table as exemplified by Ti, Zr or Hf.
These active metals may be added either in an elemental form or as a hydride and the preferred amount of their addition ranges from 0.5 to 4.0 wt %.
Below 0.5 wt %, a nitride layer will form in an insufficient amount to provide the necessary adhesive strength; beyond 4.0 wt %,the adhesive strength is increased, but, on the other hand, cracking is prone to occur after joining a metal plate to a ceramic substrate.
Titanium oxide is added as TiO or TiO
2
in an amount ranging from 0.0 to 0.9 wt %. The titanium oxide ma y be amorphous or crystalline.
The present inventors confirmed by experiments that adding these amount of titanium oxide to a brazing material of the composition described above contributed to improvements in various characteristics of the resulting composite substrates such as resistance to repeated heat cycles, flexural strength, deflection and resistance to the passage through a furnace. A probable reason for these contributions would be that the TiO
2
or TiO added to the brazing material is dispersed uniformly to reduce the concentration of stresses.
An alloy having the above-defined composition may be directly used as the brazing material of the invention. Alternatively, a powder comprising the particles of the respective Ingredients may be mixed with an organic solvent to form a paste of the brazing material. If a metal member is to be simply joined to a ceramic member, the brazing alloy material in the form of a plate or a foil may be used. If an electronic circuit is to be formed on a ceramic substrate, a paste of the brazing material is preferably used.
In order to prepare the paste of brazing material, 55-75 parts by volume of an organic solvent such as terpineol, toluene, methyl cellosolve or ethyl cellosolve may be mixed with 25-45 parts by volume of an organic binder such as PMMA, methyl cellulose or ethyl cellulose to form a vehicle, which is then added in an amount of 10-14 parts by weight to 100 parts by weight of the powder comprising the particles of the respective ingredients for the brazing material.
If less than 10 parts by weight of the vehicle is used, the resulting paste is so viscous that blurred prints will tend to occur. If more than 14 parts by wei
Kimura Masami
Nakamura Junji
Sakuraba Masami
Takahara Masaya
Dowa Mining Co. Ltd.
Jones Deborah
McNeil Jennifer
Nilles & Nilles SC
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