Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2001-01-30
2002-11-26
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S209000
Reexamination Certificate
active
06485816
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a laminated radiation member, a power semiconductor apparatus, and a method for making the same.
A known power semiconductor apparatus is, for example, one which is composed of the main part as shown in FIG.
4
. In
FIG. 4
,
101
indicates a power semiconductor apparatus,
102
indicates a semiconductor chip comprising IGBT or the like,
103
indicates a metal base plate for radiating the heat generated from the semiconductor chip
102
,
104
indicates a ceramic plate comprising aluminum nitride or the like for insulating the semiconductor chip
102
from the metal base plate
103
,
105
a
indicates a first metal electrode provided above the upper surface of the ceramic plate
104
,
105
b
indicates a second metal electrode provided below the lower surface of the ceramic plate
104
,
106
a
indicates a first hard solder paste for bonding the ceramic plate
104
to the first metal electrode
105
a
,
106
b
indicates a second hard solder paste for bonding the ceramic plate
104
to the second metal electrode
105
b
,
107
a
indicates a first solder for bonding the semiconductor chip
102
to the first metal electrode
105
a
,
107
b
indicates a second solder for bonding the metal base plate
103
to the first metal electrode
105
b
,
108
a
indicates a first metal wire comprising aluminum to be connected to the semiconductor chip
102
,
108
b
indicates a second metal wire comprising aluminum to be connected to the first metal electrode
105
a
, and
109
indicates a silicone gel which covers the semiconductor chip
102
, the ceramic plate
104
, the first metal electrode
105
a
and the second metal electrode
105
b
and seals them.
The conventional power semiconductor apparatus having the above construction is usually made by the following method. In the case of making the conventional power semiconductor apparatus
101
, first, hard solder pastes, which are the first and second hard solder pastes
106
a
and
106
b
, are printed at a given thickness on both surfaces of the ceramic plate
104
. Then, two metal electrodes, which are the first and second metal electrodes
105
a
and
105
b
, are put on the hard solder pastes printed on both the surfaces of the ceramic plate
104
and heat treated at a given temperature, for example, about 850° C., thereby bonding the first and second metal electrodes to both surfaces of the ceramic plate
104
.
Thereafter, the ceramic plate
104
, to both surfaces of which the metal electrodes are bonded is bonded, to the metal base plate
103
with a high-temperature solder (melting point: about 260° C.) which is the second solder
107
b
, and the semiconductor chip
102
is bonded with a low-temperature solder (melting point: about 150° C.), which is the first solder
107
a
, to both surfaces of the ceramic plate
104
on which the metal electrodes are bonded. A metal wire, which is the first metal wire
108
a
, is connected to the semiconductor chip
102
by wire bonding, and a metal wire, which is the second metal wire
108
b
, is connected to the metal electrode which is the first metal electrode
105
a
by wire bonding.
Usually, the metal base plate
103
on which the semiconductor chip
102
, the ceramic plate
104
, the first metal electrode
105
a
and the second metal electrode
105
b
, and the like are mounted is contained in a package. Silicone gel
109
is vacuum injected into the package and cured by heating, whereby the semiconductor chip
102
, the ceramic plate
104
, the first metal electrode
105
a
and the second metal electrode
105
b
, and the like are covered with the silicone gel
109
and sealed. In this way, the conventional power semiconductor apparatus
101
is made.
However, since the insulation substrate (
104
) and the metal electrodes (
105
a,b
) are bonded with the hard solders (
106
a,b
), cracks occur due to the difference in expansion coefficient between the insulation substrate, which has a low thermal expansion coefficient, and the hard solders and metal electrodes, which have high thermal expansion coefficients. Furthermore, since the insulation substrate (
104
) and the radiation plate (
103
) are connected with solder, there is the problem of high thermal resistance.
On the other hand, as an example of using no solder for bonding of an insulation plate and a radiation plate, JP-A-11-269577 proposes a method of forming a metal base composite material having a heat sink function by a chemical process utilizing a reaction between a ceramic dispersion material and a molten metal. This method suffers from the problem that since the molten metal is high-pressure injected into the ceramic dispersion material, expensive facilities are required, causing an increase of cost. There may be considered a means to carry out the reaction under impregnating the ceramic dispersion material with molten metal, but in this case, there is the problem that the penetrating speed is slow. In this method, the insulation substrate and the metal base composite material as a radiation plate are connected with a metal film or are connected with disposing a compound containing a firing aid for the insulation substrate at the bonded surface between the insulation substrate and the metal film, and therefore the thermal conductivity is better than the case of connecting with a solder. However, occurrence of cracks caused by difference in thermal expansion coefficient between the insulation plate of low thermal expansion coefficient and the metal film of high thermal expansion coefficient or the metal film provided with a compound containing firing aid for the insulation substrate cannot sometimes be avoided.
As an example of using no metallic radiation plate, there is, for example, an aluminum-silicon carbide composite material known as a metal ceramics composite material. This composite material is generally prepared by making a molded body (preform) of ceramic particles, ceramic fibers, whiskers, etc., then impregnating the preform with a molten metal and cooling it. As the method for impregnating with molten metal, there are various known methods such as a method based on powder metallurgy, a method according to high-pressure casting, e.g., die casting (JP-A-5-508350), a melt forging method (“Material,” Vol.36, No. 1, 1997, pages 40-46), spontaneous penetrating method (JP-A-2-197368), etc.
On the other hand, as power semiconductor apparatuses, there are known, for example, those which comprise a semiconductor chip comprising IGBT or the like, a metal base plate of about 4 mm thick comprising copper or the like for radiating heat generated from the semiconductor chip, and a ceramic plate of about 0.6 mm thick comprising aluminum nitride or the like for insulating the semiconductor chip from the metal base plate. A first metal electrode of about 0.4 mm thick comprising copper or the like is bonded to the upper surface of the ceramic plate with a first hard solder of a given thickness. A semiconductor chip is bonded to the upper surface of the metal electrode with a solder of about 0.2 mm thick. A second metal electrode of about 0.2 mm thick comprising copper or the like is bonded to the under surface of the ceramic plate with a second solder of a given thickness. The under surface of the ceramic plate and the second metal electrode are bonded to the radiation plate with a solder or a hard solder.
However, there is a problem of low radiation property because the insulating ceramic substrate and the radiation plate are connected with a solder. Moreover, in the case of bonding the insulating ceramic substrate and the metallic heat sink material with a hard solder by active metal method or the like, cracks caused by thermal stress at the time of bonding occur on the side of the insulating ceramic substrate because of the great difference in thermal expansion coefficient between both the materials. Furthermore, a multi-layer type bonded body, which is bonded with a solder and provided with a stress relaxing layer by a means other than soldering, is low in endurance when
Araki Kiyoshi
Bessyo Yuki
Ishikawa Takahiro
Kida Masahiro
Makino Takuma
Burr & Brown
Lam Cathy
NGK Insulators Ltd.
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