Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2000-05-26
2001-10-30
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S209000, C428S901000, C174S258000, C174S259000
Reexamination Certificate
active
06309737
ABSTRACT:
The present invention relates to a highly reliable circuit substrate useful for e.g. power modules.
Conventionally, for semiconductor devices useful for e.g. power modules, circuit substrates having a ceramic substrate of e.g. alumina, beryllia, silicon nitride or aluminum nitride, and a circuit and a heat-radiating plate made of e.g. Cu, Al or an alloy of such a metal component, formed respectively on the front and on the back side of the ceramic substrate, have been developed (U.S. Pat. No. 5,354,415) and used practically. These circuit substrates have such a merit that high insulating properties will be obtained stably as compared with resin substrates or composite substrates of a resin substrate and a metal substrate.
Methods for bonding the circuit and the heat-radiating plate to the ceramic substrate are classified roughly into brazing employing a bonding material (i.e. brazing material) and a method of not employing a bonding material. As a representative method of the latter, DBC method of bonding alumina to a tough pitch copper plate by utilizing the Cu—O eutectic point has been known.
However, in the case where the circuit is made of Cu, the thermal stress resulting from difference in thermal expansion between the circuit and the ceramic substrate or the solder is inevitable, and thus cracks tend to form on the ceramic substrate or the solder due to repeated heat history, and no adequately high reliability will be obtained. On the other hand, when Al is selected for the material for the circuit, although it is somewhat poor in thermal conductivity and electrical conductivity as compared with Cu, the Al circuit will easily undergo plastic deformation even if thermal stress is applied thereto, whereby the stress to be applied to the ceramic substrate or the solder will be moderated, and the reliability will significantly improve.
However, the Al circuit has such a problem that it is expensive. To form an Al circuit, there are following methods: (1) a melting method in which a molten aluminum is contacted with a ceramic substrate, followed by cooling to produce a bonded product, and the thickness of the Al plate is adjusted by machine grinding, followed by etching (JP-A-7-193358, JP-A-7-27262) and (2) a method of brazing an Al foil or an Al alloy foil, followed by etching (JP-A-3-125463). Both the methods will cost from about twice to about five times as much as the case of forming a Cu circuit, and thus there is a little possibility that these methods are used widely, except for a special purpose.
Not to speak of the melting method wherein the production efficiency is poor, the major cause why the Al circuit by brazing is more costly than a Cu circuit, is that the bonding is carried out under severe conditions. Namely, the melt temperature of Al (660° C.) and the bonding temperature (from about 630 to about 650° C. in the case of an Al—Si type which is the commonest bonding material) are close, whereby Al is likely to melt locally to cause soldering defects (moth-eaten phenomenon formed on the Al circuit), and thus considerable skill and labor are required to produce the Al circuit while preventing such defects.
Under these circumstances, the present inventors have found that an Al circuit can be easily formed on a ceramic substrate by bonding under specific conditions by using, as a bonding material, a low-priced Al—Cu type alloy which has attracted no attention, and they have further conducted extensive studies to accomplish the present invention.
It is an object of the present invention to provide a circuit substrate which has a ceramic substrate and an Al or Al alloy circuit formed on the ceramic substrate, at a low cost while keeping its high reliability. Particularly, it is to provide a highly reliable circuit substrate wherein not only cracks on a solder or the ceramic substrate but also peeling of a bonding wire or plating is significantly prevented.
Another object of the present invention is to produce such a highly reliable circuit substrate easily.
Namely, the present invention resides in a circuit substrate and a process for producing the circuit substrate, which have the following essential features.
1. A circuit substrate which has a ceramic substrate and an Al circuit comprising Al or an Al alloy bonded to said ceramic substrate via a layer comprising Al and Cu.
2. A circuit substrate which has a ceramic substrate and an Al circuit comprising Al or an Al alloy bonded to said ceramic substrate by using, as a bonding material, an Al—Cu type alloy or a mixture containing Al and Cu.
3. The circuit substrate according to Item 2, wherein the bonding material is an Al—Cu type alloy foil.
4. The circuit substrate according to Item 1, 2 or 3, wherein the ceramic substrate is an aluminum nitride substrate or a silicon nitride substrate.
5. The circuit substrate according to any one of Items 1 to 4, wherein the ceramic substrate is an aluminum nitride substrate having a thermal conductivity of at least 130 W/mK and having such a X-ray diffraction peak intensity ratio that 2≦Y
2
O
3
.Al
2
O
3
×100/AlN≦17 and 2Y
2
O
3
.Al
2
O
3
×100/AlN≦2 on the surface.
6. The circuit substrate according to any one of Items 1 to 5, wherein the Al circuit comprising Al or an Al alloy is formed by using Al having a purity of at least 99.85 wt %.
7. The circuit substrate according to any one of Items 1 to 6, wherein the Al circuit comprising Al or an Al alloy is formed by using a rolled Al having a purity of at least 99.99 wt %.
8. The circuit substrate according to Item 1, wherein the proportion of Cu in the layer comprising Al and Cu is from 1 to 6 wt %.
9. The circuit substrate according to Item 2 or 3, wherein the bonding material comprises at least 86 wt % of Al, from 1 to 6 wt % of Cu and at most 3 wt % of Mg (not including O).
10. The circuit substrate according to any one of Items 1 to 9, wherein the Al circuit comprising Al or an Al alloy has a thickness of at least 100 &mgr;m and a Vickers hardness of at most 15 kgf/mm
2
.
11. The circuit substrate according to any one of Items 1 to 10, which has a heat-radiating plate comprising Al or an Al alloy formed on the ceramic substrate on the side (back side) opposite to the side on which the Al circuit is formed.
12. The circuit substrate according to Item 11, wherein the volume ratio of the Al circuit to the heat-radiating plate is from 0.80 to 1.2.
13. The circuit substrate according to Item 11 or 12, wherein the Al circuit has a Vickers hardness of at most 16 kgf/mm
2
, and the heat-radiating plate has a Vickers hardness of from 19 to 30 kgf/mm
2
.
14. A process for producing a circuit substrate, which comprises disposing an Al or Al alloy plate, pattern or both on a ceramic substrate by means of, as a bonding material, an Al—Cu type alloy or a mixture containing Al and Cu, and heating the resulting assembly at a temperature of from 540 to 640° C. while applying a pressure of from 1 to 100 kgf/cm
2
thereto in a direction perpendicular to the ceramic substrate to bond the Al or Al alloy plate, pattern or both to the ceramic substrate, followed by etching as the case requires.
15. The process for producing a circuit substrate according to Item 14, wherein the bonding material is an Al—Cu type alloy foil.
16. The process for producing a circuit substrate according to Item 15, wherein the bonding material is an Al—Cu type alloy foil having a thickness of from 15 to 35 &mgr;m, the Al or Al alloy plate, pattern or both, having a thickness of at least 100 &mgr;m, is disposed on either side of the ceramic substrate by means of said bonding material, and the resulting assembly is held under heating at a temperature of at least 590° C. for at least 20 minutes while applying a pressure of from 8 to 50 kgf/cm
2
thereto in a direction perpendicular to the ceramic substrate.
17. The process for producing a circuit substrate according to Item 15 or 16, wherein the Al—Cu type alloy foil is an Al—Cu—Mg type alloy foil comprising at least 86 wt % of Al, from 1 to 6 wt % of Cu and at most 3 wt % of Mg (not including O).
Now
Gotoh Takeshi
Hirashima Yutaka
Hushii Yasuhito
Miyai Akira
Sugimoto Isao
Denki Kagaku Kogyo Kabushiki Kaisha
Lam Cathy
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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