U.S. patent application number 11/678391 was filed with the patent office on 2011-04-14 for composite material, having high thermal conductivity and low thermal expansion coefficient, and heat-dissipating substrate, and their production methods.
This patent application is currently assigned to HITACHI METALS, LTD.. Invention is credited to Hideko Fukushima.
Application Number | 20110086218 11/678391 |
Document ID | / |
Family ID | 33028409 |
Filed Date | 2011-04-14 |
United States Patent
Application |
20110086218 |
Kind Code |
A9 |
Fukushima; Hideko |
April 14, 2011 |
COMPOSITE MATERIAL, HAVING HIGH THERMAL CONDUCTIVITY AND LOW
THERMAL EXPANSION COEFFICIENT, AND HEAT-DISSIPATING SUBSTRATE, AND
THEIR PRODUCTION METHODS
Abstract
A composite material having a high thermal conductivity and a
small thermal expansion coefficient, which is obtained by
impregnating a porous graphitized extrudate with a metal; the
composite material having such anisotropy that the thermal
conductivity and the thermal expansion coefficient are 250 W/mK a
more and less than 4.times.10.sup.-6/K, respectively, in an
extrusion direction; and that the thermal conductivity and the
thermal expansion coefficient are 150 W/mK or more and
10.times.10.sup.-6/K or less, respectively, in a direction
perpendicular to the extrusion direction.
Inventors: |
Fukushima; Hideko;
(Saitama-ken, JP) |
Assignee: |
HITACHI METALS, LTD.
Tokyo
JP
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20080038535 A1 |
February 14, 2008 |
|
|
Family ID: |
33028409 |
Appl. No.: |
11/678391 |
Filed: |
February 23, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10845257 |
May 14, 2004 |
7282265 |
|
|
11678391 |
Feb 23, 2007 |
|
|
|
Current U.S.
Class: |
428/312.6 ;
428/312.8 |
Current CPC
Class: |
H01L 21/4878 20130101;
H01L 23/3736 20130101; Y10T 428/12493 20150115; Y10T 428/31678
20150401; H01L 2924/0002 20130101; C22C 47/06 20130101; Y10T
428/249957 20150401; C04B 41/89 20130101; C22C 2001/1021 20130101;
C04B 41/009 20130101; Y10T 428/249967 20150401; C22C 1/1036
20130101; H01L 23/373 20130101; H01L 2924/0002 20130101; C04B
41/4523 20130101; C04B 41/81 20130101; Y10T 428/249969 20150401;
C04B 41/52 20130101; C04B 41/52 20130101; H01L 23/3733 20130101;
C04B 41/009 20130101; C04B 41/4523 20130101; C04B 41/4523 20130101;
Y10T 428/24997 20150401; C04B 41/4523 20130101; C04B 41/4523
20130101; C04B 41/009 20130101; C04B 41/52 20130101; C04B
2111/00844 20130101; C04B 41/0072 20130101; Y10T 428/30 20150115;
H01L 2924/00 20130101; C04B 41/4521 20130101; C04B 41/515 20130101;
C04B 38/00 20130101; C04B 41/0072 20130101; C04B 41/5116 20130101;
C04B 41/4521 20130101; C04B 41/0072 20130101; C04B 41/4521
20130101; C04B 41/4521 20130101; C04B 35/522 20130101; C04B 41/4523
20130101; C04B 41/4521 20130101; C04B 41/51 20130101; C04B 41/5155
20130101; C04B 41/0072 20130101; C04B 41/4541 20130101; C04B
41/0072 20130101; C04B 41/5096 20130101; C04B 41/5127 20130101;
C04B 41/5144 20130101 |
Class at
Publication: |
428/312.6 ;
428/312.8 |
International
Class: |
B32B 3/26 20060101
B32B003/26 |
Foreign Application Data
Date |
Code |
Application Number |
May 16, 2003 |
JP |
2003-138121 |
Claims
1. A composite material having a high thermal conductivity and a
small thermal expansion coefficient, which is obtained by
impregnating a porous graphitized extrudate with a metal; said
composite material having such anisotropy that said thermal
conductivity and said thermal expansion coefficient are 250 W/mK or
more and less than 4.times.10.sup.-6/K, respectively, in an
extrusion direction; and that said thermal conductivity and said
thermal expansion coefficient are 150 W/mK or more and
10.times.10.sup.-6/K or less; respectively, in a direction
perpendicular to said extrusion direction.
2. (canceled)
3. The composite material having a high thermal conductivity and a
small thermal expansion coefficient according to claim 1, wherein
said composite material has a bulk density of 1.9 g/cm.sup.3 or
more; and wherein the amount of said metal is 10 to 30% by
volume.
4. The composite material having a high thermal conductivity and a
small thermal expansion coefficient according to claim 1, wherein
the resistivity of said composite material is 4 .mu..OMEGA.m or
less in said extrusion direction and 7 .mu..OMEGA.m or less in said
perpendicular direction.
5. The composite material having a high thermal conductivity and a
small thermal expansion coefficient according to claim 1, wherein
said metal is at least one metal selected from the group consisting
of aluminum, copper, chromium, silver, magnesium and zinc, or an
alloy comprising one or more of said metals.
6. The composite material having a high thermal conductivity and a
small thermal expansion coefficient according to claim 5, wherein
said metal is an aluminum alloy comprising 11 to 14% by mass of
silicon, the balance being substantially aluminum and inevitable
impurities, and wherein the percentage of a needle-shaped structure
having a length of 30 .mu.m or less and an aspect ratio
(length/diameter) of 10 or more is 10% or less among a silicon
(Si)-rich phase precipitated in said metal.
7. The composite material having a high thermal conductivity and a
small thermal expansion coefficient according to claim 5, wherein
said metal is copper or a copper alloy; and wherein the amount of
oxygen in said metal is 400 ppm or less.
8-17. (canceled)
18. A heat-dissipating substrate comprising the composite material
having a high thermal conductivity and a small thermal expansion
coefficient recited in claim 1, wherein the thickness direction of
said substrate is substantially in alignment with the extrusion
direction of said porous graphitized extrudate; and wherein said
substrate has a surface perpendicular to said extrusion direction,
onto which a heat-generating body is going to be bonded.
19. The heat-dissipating substrate according to claim 18, wherein a
metal layer having such sealability that the amount of a helium gas
leaked is 1.times.10.sup.-2 Pacm.sup.3/s or less is formed on at
least on a surface, onto which a heat-generating body is going to
be bonded.
20. The heat-dissipating substrate according to claim 19, wherein
said metal layer comprises a Ni--B plating layer and/or a Ni--P
plating layer each having a thickness of 0.5 to 20 .mu.m.
21. The heat-dissipating substrate according to claim 18, wherein
said substrate has throughholes, in each of which a reinforcing
pipe member is fitted.
22-24. (canceled)
25. The composite material having a high thermal conductivity and a
small thermal expansion coefficient according to claim 1, wherein a
ratio of the thermal expansion coefficient in the perpendicular
direction to that in the extrusion direction is 2 or more.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a composite material having
a high thermal conductivity and a small thermal expansion
coefficient, a heat-dissipating substrate formed therefrom, and
methods for producing them, particularly to a composite material
composed of a porous graphitized extrudate and aluminum or copper
and thus having a high thermal conductivity, a small thermal
expansion coefficient and small resistivity with substantially no
thermal hysteresis, which is suitable for heat sinks for electronic
equipment, etc., a heat-dissipating substrate such as a heat sink,
etc. formed from such composite material, and methods for producing
them.
BACKGROUND OF THE INVENTION
[0002] Because electronic parts have been increasing their heat
generation as their integration, volume, output, etc. are
increasing, demand is mounting on materials having a high thermal
conductivity and a small thermal expansion coefficient. Because
semiconductor devices such as CPUs, light-emitting diodes, etc.
generate large amounts of heat, heat sinks are usually mounted to
them. Heat transmitted from the semiconductor devices to the heat
sinks is dissipated by fans or cooling media, etc. The heat sinks
are usually made of aluminum, copper or their alloys having
excellent thermal conductivity.
[0003] Because CPU, for instance, is much smaller than the heat
sink, a high-thermal-conductivity body called "heat spreader" is
usually interposed therebetween. Materials for the heat spreader
preferably have a high thermal conductivity and as small a thermal
expansion coefficient as that of the CPU made of silicon. This is
true of light-emitting diodes made of compound semiconductors
(GaAs, GaN, etc.). For such purposes, a lot of substrates made of
composite materials of ceramics having small thermal expansion
coefficients such as silicon carbide, alumina, silicon nitride or
aluminum nitride and aluminum or copper have been proposed.
However, substrates made of these composite materials are
disadvantageously poor in workability because of containing
ceramics. Though composite material substrates made of metals
having small thermal expansion coefficients such as tungsten or
molybdenum and copper have also been proposed, these composite
material substrates are disadvantageously poor in workability.
[0004] Under the above circumstances, a lot of attempts have
recently been proposed to use composite materials of carbon
particles or fibers and metals for heat-dissipating substrates. For
instance, JP 10-168502 A discloses a high-thermal-conductivity
composite material obtained by mixing 1 to 200 parts by weight of
one or more crystalline carbon materials selected from the group
consisting of graphite, carbon fibers, carbon black, fullerene and
carbon nanotubes, and 100 parts by weight of metal powder selected
from the group consisting of Fe, Cu, Al, Ag, Be, Mg, W, Ni, Mo, Si,
Zn and these alloys and hot-pressing the resultant mixture.
However, because this composite material has a structure containing
crystalline carbon materials dispersed in a metal matrix, it has as
large a thermal expansion coefficient as that of the metal matrix,
though it has a high thermal conductivity.
[0005] JP 2000-203973 A discloses a carbon-based metal composite
material comprising a carbonaceous matrix impregnated with at least
one metal selected from the group consisting of aluminum,
magnesium, tin, zinc, copper, silver, iron, nickel and their
alloys, 90% by volume or more of voids in the carbonaceous matrix
being impregnated with the metal, and the amount of the metal being
35% by volume or less of the entire carbon-based metal composite
material.
[0006] JP 2001-58255 A discloses a carbon-based metal composite
material produced by impregnating carbon moldings comprising carbon
particles or fibers containing graphite crystals with aluminum,
copper, silver or these alloys at high pressure by a melt-forging
method, which has a thermal conductivity of 150 W/mK or more and a
thermal expansion coefficient of 4.times.10.sup.-6/K to
12.times.10.sup.-6/K in a thickness direction at room temperature.
This carbon-based metal composite material has a structure
comprising a graphite matrix having high rigidity, a high thermal
conductivity and a small thermal expansion coefficient as a
skeleton, with its voids impregnated with a metal. Accordingly, it
has both small thermal expansion coefficient inherent in graphite
and high thermal conductivity inherent in a metal.
[0007] Despite the above advantages, these carbon-based metal
composite materials are disadvantageous in having much larger
thermal expansion coefficients than those of silicon and compound
semiconductors. Large differences from silicon and compound
semiconductors in a thermal expansion coefficient undesirably exert
large thermal stress to CPUs or light-emitting diodes during
soldering or brazing heat-dissipating substrates to the CPUs or the
light-emitting diodes, or during the operation of the CPUs or the
light-emitting diodes. Accordingly, a stress-relieving member is
usually interposed between the CPU or the light-emitting diode and
the heat-dissipating substrate. However, because the
stress-relieving member does not necessarily have a sufficiently
large thermal conductivity, the use of the carbon-based metal
composite material having a high thermal conductivity for the
heat-dissipating substrate does not exhibit its effect
sufficiently.
[0008] Further, when the heat-dissipating substrate is bonded to
the CPU or the light-emitting diode, soldering is usually conducted
at about 200 to 300.degree. C. in the case of an
aluminum-impregnated graphite substrate, and brazing is usually
conducted at about 700 to 800.degree. C. in the case of a
copper-impregnated graphite substrate. It has been found, however,
that when exposed to such high temperature, the composite substrate
impregnated with aluminum or copper exhibits extremely different
sizes due to a residual stress before and after the heating. Such
thermal hysteresis causes warp in the heat-dissipating substrate
bonded to the CPU or the light-emitting diode, so that the
heat-dissipating substrate may finally be broken, or that the CPU
or the laser diode, etc. may also be damaged by thermal stress.
OBJECTS OF THE INVENTION
[0009] Accordingly, an object of the present invention is to
provide a composite material having a high thermal conductivity and
as small a thermal expansion coefficient as those of silicon and
compound semiconductors, with substantially no thermal
hysteresis.
[0010] Another object of the present invention is to provide a
heat-dissipating substrate formed from a composite material having
a high thermal conductivity and a small thermal expansion
coefficient.
[0011] A further object of the present invention is to provide a
method for producing such a composite material having a high
thermal conductivity and a small thermal expansion coefficient.
[0012] A still further object of the present invention is to
provide a method for producing such a heat-dissipating substrate
having a high thermal conductivity and a small thermal expansion
coefficient.
SUMMARY OF THE INVENTION
[0013] As a result of intense research in view of the above
objects, the inventor has found that by impregnating a porous
graphitized extrudate with a molten metal and then heat-treating
the resultant composite material, it is possible to (a) increase
the thermal conductivity and decrease the thermal expansion
coefficient to the same level as those of silicon and compound
semiconductors, and (b) substantially remove thermal expansion
hysteresis: thereby providing the composite material with good
dimensional stability when heated. The present invention has been
completed based on this finding.
[0014] Thus, the composite material of the present invention having
a high thermal conductivity and a small thermal expansion
coefficient is obtained by impregnating a porous graphitized
extrudate with a metal, having such anisotropy that the thermal
conductivity and the thermal expansion coefficient are 250 W/mK or
more and less than 4.times.10.sup.-6/K, respectively, in an
extrusion direction; and that the thermal conductivity and the
thermal expansion coefficient are 150 W/mK or more and
10.times.10.sup.-6/K or less, respectively, in a direction
perpendicular to the extrusion direction.
[0015] In a preferred embodiment of the present invention, the
thermal conductivity is 250 W/mK or more in an extrusion direction
and 150 W/mK, or more in a direction perpendicular to said
extrusion direction, and the thermal expansion coefficient is
0.1.times.10.sup.-6/K or more and less than 4.times.10.sup.-6/K in
the extrusion direction and 4.times.10.sup.-6/K or more and
10.times.10.sup.-6/K or less in the perpendicular direction.
[0016] In a preferred embodiment of the present invention, a
dimensional change ratio due to thermal hysteresis is within
.+-.0.1% in the extrusion direction and in a direction
perpendicular to the extrusion direction after a heat treatment,
meaning that the composite material has substantially no thermal
expansion hysteresis.
[0017] The heat-dissipating substrate of the present invention is
composed of the above composite material having a high thermal
conductivity and a small thermal expansion coefficient, and has a
thickness direction substantially in alignment with the extrusion
direction of the porous graphitized extrudate, and a surface
perpendicular to the extrusion direction, onto which a
heat-generating body is going to be bonded.
[0018] The method for producing a composite material having a high
thermal conductivity and a small thermal expansion coefficient
according to the present invention comprises the steps of (1)
graphitizing an extrudate of carbon particles and/or carbon fibers
and tar pitch by burning; (2) impregnating the resultant porous
graphitized extrudate with a molten metal at high temperature and
pressure; and (3) heat-treating the resultant graphite/metal
composite material.
[0019] The method for producing a heat-dissipation substrate having
a high thermal conductivity and a low thermal expansion according
to the present invention comprises the steps of producing a
composite material having a high thermal conductivity and a small
thermal expansion coefficient by the above method; and then cutting
the composite material along a surface substantially perpendicular
to the extrusion direction of the porous graphitized extrudate.
Said perpendicular surface is preferably used as a surface, onto
which a heat-generating body is going to be bonded.
[0020] The thus obtained composite material having a high thermal
conductivity and a small thermal expansion coefficient is cut to a
plate shape having a thickness of about 0.1 to 100 mm, desirable
for use as heat sinks, etc. The metal-impregnated composite
material is usually cut to a plate shape after the heat treatment.
However, when the heat-dissipating substrate is required to have a
shape with extremely high precision, the metal-impregnated
composite material is preferably heat-treated after cut to a plate
shape, and cut to the target shape again. In any case, the
heat-dissipating substrate of the present invention formed from the
porous graphitized extrudate has a dimensional change ratio of
within .+-.0.1% due to thermal hysteresis both in an extrusion
direction and in a perpendicular direction. Accordingly, even
though it is subjected to thermal stress during brazing, etc., it
is free from warp and peeling at bonding interfaces, etc. after
cooling.
[0021] The composite material of the present invention having a
high thermal conductivity and a small thermal expansion coefficient
has anisotropy in the thermal conductivity and the thermal
expansion coefficient. Specifically, its thermal conductivity is
250 W/mK or more and its thermal expansion coefficient is lower
than 4.times.10.sup.-6/K in the extrusion direction, and its
thermal conductivity is 150 W/mK or more and its thermal expansion
coefficient is 10.times.10.sup.-6/K or less in a direction
perpendicular to the extrusion direction. Accordingly, when used
for heat sinks or heat spreaders, etc. for semiconductor devices,
the influence of thermal stress is suppressed, and heat spreads in
a lateral direction and is well conducted in a thickness direction,
achieving efficient heat dissipation. In addition, it has a small
thermal expansion coefficient in the thickness direction, it
exhibits high dimensional precision in the thickness direction when
assembled to a package, making it possible to provide a
high-sealing package.
[0022] The preferred structure of the composite material of the
present invention having a high thermal conductivity and a small
thermal expansion coefficient will be explained below. The
composite material of the present invention having a high thermal
conductivity and a small thermal expansion coefficient preferably
has a bulk density of 1.9 g/cm.sup.3 or more, with the metal in an
amount of 10 to 30% by volume.
[0023] The resistivity of the composite material having a high
thermal conductivity and a small thermal expansion coefficient is
preferably 4 .mu..OMEGA.m or less in the extrusion direction, and 7
.mu..OMEGA.m or less in the perpendicular direction. The more
preferred resistivity is 2 .mu..OMEGA.m or less in the extrusion
direction, and 3.5 .mu..OMEGA.m or less in the perpendicular
direction.
[0024] The metal in the composite material having a high thermal
conductivity and a small thermal expansion coefficient is
preferably at least one selected from the group consisting of
aluminum, copper, chromium, silver, magnesium and zinc or an alloy
comprising one or more of said metals. In a preferred embodiment,
the metal is an aluminum alloy comprising 11 to 14% by mass of
silicon, the balance being substantially aluminum and inevitable
impurities, the percentage of a needle-shaped structure having a
length of 30 .mu.m or less and an aspect ratio (length/diameter) of
10 or more being preferably 10% or less, more preferably 5% or
less, in a silicon (Si)-rich phase precipitated in the metal. The
amount of oxygen in the aluminum alloy is preferably 400 ppm or
less. When the metal is copper or its alloy, the amount of oxygen
in the metal is preferably 400 ppm or less, more preferably 250 ppm
or less.
[0025] The porous graphitized extrudate used in the present
invention is preferably composed of carbon particles such as cokes,
etc. and tar pitch, and the carbon particles preferably have an
average particle size of 50 .mu.m or more. The ash content of the
extrudate is preferably 0.5% by mass or less, more preferably 0.3%
by mass.
[0026] The resistivity of the porous graphitized extrudate used in
the present invention is preferably less than 7 .mu..OMEGA.m in an
extrusion direction, and 7 .mu..OMEGA.m or more in a direction
perpendicular to the extrusion direction, and the ratio of the
resistivity in the extrusion direction to that in the perpendicular
direction is preferably 0.9 or less. More preferably, the
resistivity of the porous graphitized extrudate is 6 .mu..OMEGA.m
or less in the extrusion direction, and 8 .mu..OMEGA.m or more in
the direction perpendicular to the extrusion direction, and the
resistivity ratio (extrusion direction/perpendicular direction) is
0.6 or less.
[0027] The thermal expansion coefficient of the porous graphitized
extrudate used in the present invention is preferably
3.times.10.sup.-6/K or less in the extrusion direction, and
4.times.10.sup.-6/K or less in the direction perpendicular to the
extrusion direction, and the ratio of the thermal expansion
coefficient in the extrusion direction to that in the perpendicular
direction is 0.8 or less. More preferably, the thermal expansion
coefficient of the porous graphitized extrudate is
1.times.10.sup.-6/K or less in the extrusion direction, and
3.times.10.sup.-6/K or less in the direction perpendicular to the
extrusion direction, and the thermal expansion coefficient ratio
(extrusion direction/perpendicular direction) is 0.5 or less.
[0028] The thermal conductivity of the porous graphitized extrudate
used in the present invention is preferably 150 W/mK or more in the
extrusion direction, and 80 W/ml or more, more preferably 100 W/mK
or more, in a direction perpendicular to the extrusion direction.
The ratio of the thermal conductivity in the extrusion direction to
that in the perpendicular direction is preferably 1.3 or more, more
preferably 1.5 or more.
[0029] When the metal is aluminum or its alloy, the impregnation of
the porous graphitized extrudate with the molten metal is
preferably conducted at a temperature higher than the melting point
by 10.degree. C. or more and at a pressure of 10 MPa or more, and
the heat treatment of the graphite/metal composite material is
preferably conducted under the conditions of a temperature of (the
melting point -10.degree. C.) or less and 200.degree. C. or higher,
at a temperature-elevating speed of 30.degree. C./minute or less,
and a cooling speed of 20.degree. C./minute or less. More
preferably, the temperature-elevating speed is 10.degree. C./minute
or less, and the cooling speed is 10.degree. C./minute or less.
[0030] When the metal is copper or its alloy, the impregnation of
the porous graphitized extrudate with the molten metal is
preferably conducted at a temperature higher than the melting point
by 10.degree. C. or more and at a pressure of 10 MPa or more, and
the heat treatment of the graphite/metal composite material is
preferably conducted under the conditions of a temperature of (the
melting point -10.degree. C.) or less and 300.degree. C. or higher,
at a temperature-elevating speed of 30.degree. C./minute or less,
and at a cooling speed of 20.degree. C./minute or less. More
preferably, the temperature-elevating speed is 10.degree. C./minute
or less, and the cooling speed is 10.degree. C./minute or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1(a) is a schematic cross-sectional view showing a die
apparatus for melt-forging a porous graphitized extrudate, which
comprises a cavity in which a porous graphitized extrudate is
placed and into which a molten metal is poured;
[0032] FIG. 1(b) is a schematic cross-sectional view showing the
die apparatus of FIG. 1(a) for melt-forging the molten metal
containing the porous graphitized extrudate;
[0033] FIG. 1(c) is a schematic cross-sectional view showing the
die apparatus of FIG. 1(a), from which a melt-forged product is
taken out;
[0034] FIG. 1(d) is a cross-sectional view showing a
metal-impregnated porous graphitized extrudate cut out from the
melt-forged product;
[0035] FIG. 2 is a graph schematically showing a heat treatment
pattern conducted on the graphite/metal composite material of the
present invention;
[0036] FIG. 3 is a schematic cross-sectional view showing the
heat-dissipating substrate of the present invention, to which a
semiconductor device is bonded;
[0037] FIG. 4(a) is a SIM photograph showing the graphite/Al--Si
composite material before the heat treatment in Example 1;
[0038] FIG. 4(b) is a SIM photograph showing the graphite/Al--Si
composite material after the heat treatment in Example 1;
[0039] FIG. 5(a) is a graph showing the thermal expansion
hysteresis of the graphite/Al--Si composite material before the
heat treatment in its extrusion direction in Example 1;
[0040] FIG. 5(b) is a graph showing the thermal expansion
hysteresis of the graphite/Al--Si composite material after the heat
treatment in its extrusion direction in Example 1;
[0041] FIG. 6(a) is a graph showing the thermal expansion
hysteresis of the graphite/Al--Si composite material before the
heat treatment in the perpendicular direction in Example 1;
[0042] FIG. 6(b) is a graph showing the thermal expansion
hysteresis of the graphite/Al--Si composite material after the heat
treatment in the perpendicular direction in Example 1;
[0043] FIG. 7(a) is a SIM photograph showing the graphite/copper
composite material before the heat treatment in Example 2;
[0044] FIG. 7(b) is a SIM photograph showing the graphite/copper
composite material after the heat treatment in Example 2;
[0045] FIG. 8(a) is a graph showing the thermal expansion
hysteresis of the graphite/copper composite material before the
heat treatment in its extrusion direction in Example 2;
[0046] FIG. 8(b) is a graph showing the thermal expansion
hysteresis of the graphite/copper composite material after the heat
treatment in its extrusion direction in Example 2;
[0047] FIG. 9(a) is a graph showing the thermal expansion
hysteresis of the graphite/copper composite material before the
heat treatment in the perpendicular direction in Example 2;
[0048] FIG. 9(b) is a graph showing the thermal expansion
hysteresis of the graphite/copper composite material after the heat
treatment in the perpendicular direction in Example 2;
[0049] FIG. 10 is cross-sectional view showing an example of a
semiconductor module comprising a heat-dissipating substrate formed
by the graphite/metal composite material of the present
invention;
[0050] FIG. 11 is a perspective view showing an example of a
reinforcing pipe member used as a substrate with throughholes;
[0051] FIG. 12(a) is a cross-sectional view taken along the line
A-A in FIG. 11;
[0052] FIG. 12(b) is a cross-sectional view showing a state where a
metal pipe member is being fitted in a throughhole of the
heat-dissipating substrate shown in FIG. 11;
[0053] FIG. 12(c) is a cross-sectional view showing another example
of a metal pipe member fitted in the throughhole of the
heat-dissipating substrate;
[0054] FIG. 12(d) is a perspective view showing a further example
of a metal pipe member fitted in the throughhole of the
heat-dissipating substrate; and
[0055] FIG. 12(e) is a perspective view showing a still further
example of a metal pipe member fitted in the throughhole of the
heat-dissipating substrate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[1] Composite Material Having High Thermal Conductivity and Small
Thermal Expansion Coefficient
(A) Structure
(1) Porous Graphitized Extrudate
[0056] The porous graphitized extrudate used in the present
invention preferably has a bulk density of 2 g/cm.sup.3 or less,
particularly 1.6 to 1.95 g/cm.sup.3. When the bulk density is more
than 2 g/cm.sup.3, the impregnation of a molten metal is
insufficient, failing to obtain sufficient effects of improving its
thermal conductivity. On the other hand, when the bulk density is
less than 1.6 g/cm.sup.3, the graphite skeleton has insufficient
strength, so that the thermal expansion coefficient of the entire
composite material is largely influenced by the thermal expansion
coefficient of a metal. The more preferred bulk density of the
porous graphitized extrudate is 1.65 to 1.85 g/cm.sup.3.
(2) Impregnating Metal
[0057] A molten metal impregnated into the porous graphitized
extrudate is preferably at least one metal selected from the group
consisting of aluminum, copper, chromium, silver, magnesium and
zinc or an alloy comprising one or more of the metals.
Particularly, the aluminum alloy is preferably an aluminum-silicon
alloy containing 11 to 14% by mass of silicon. The reason therefor
seems to be that the inclusion of 11 to 14% by volume of silicon
lowers the melting point of a molten metal, thereby suppressing the
generation of aluminum carbide, and thus preventing decrease in
thermal conductivity. An additional reason seems to be that
needle-shaped silicon particles contained in the alloy are
spheroidized by the heat treatment, resulting in a decreased heat
resistance and thus improved thermal conductivity.
[0058] The copper alloy is preferably a chromium-copper alloy. The
reason therefor seems to be that chromium contained in the alloy
improves the strength of an interface between graphite and copper,
resulting in improvement in the strength of the composite material.
The content of chromium is 0.1 to 10% by mass, preferably 0.1 to 5%
by mass, more preferably 0.1 to 2% by mass.
[0059] The percentage of the metal in the composite material is
preferably 10 to 30% by volume. When the amount of the metal
impregnating the porous graphitized extrudate is less than 10% by
volume of the composite material, the bulk density of the composite
material is less than 1.9 g/cm.sup.3, resulting in insufficient
effects of improving the thermal conductivity by metal
impregnation. On the other hand, when the metal is more than 30% by
volume, the amount of the impregnating metal is too much to the
graphite skeleton, so that the thermal expansion coefficient of the
entire composite material is largely influenced by the thermal
expansion coefficient of the metal, resulting in too large
difference in a thermal expansion coefficient from silicon and
compound semiconductors. The more preferred percentage of the metal
in the composite material is 5 to 25% by volume.
[0060] Because the voids of the porous graphitized extrudate are
filled with the metal as densely as possible, the bulk density of
the composite material is preferably 1.9 g/cm.sup.3 or more. When
the bulk density of the composite material is less than 1.9
g/cm.sup.3, the composite material has too much a void ratio,
failing to sufficiently improve the thermal conductivity. On the
other hand, when the bulk density is more than 5 g/cm.sup.3, the
high-pressure impregnation process needs too strict conditions of
temperature and pressure, making the production of the composite
material difficult and thus costly. The more preferred bulk density
of the composite material is 1.9 to 4 g/cm.sup.3.
(B) Production Method
(1) Production of Porous Graphitized Extrudate
[0061] The porous graphitized extrudate per se can be produced by
known methods. Typically, carbon materials such as cokes, etc. are
pulverized and classified to a proper size, and then melt-blended
with pitch as a binder. The resultant blend is extruded from a die
orifice having a predetermined shape, cut to a predetermined length
and then burned for graphitization. In place of the carbon powder,
carbon fibers may be used, and a mixture of carbon powder and
carbon fibers may be used.
[0062] The average particle size of the carbon powder such as coke
powder, etc. is preferably 50 .mu.m or more. When the average
particle size of the carbon powder is less than 50 .mu.m, the
resultant porous graphitized extrudate has an insufficient thermal
conductivity. When the average particle size of the carbon powder
is more than 3 mm, the resultant porous graphitized extrudate
disadvantageously has insufficient mechanical strength. The more
preferred average particle size of the carbon powder is about 50
.mu.m to about 3 mm. The carbon fibers are preferably pitch carbon
fibers with an average length of about 50 .mu.m to about 5 mm.
[0063] A mixing ratio of carbon powder and/or carbon fibers to
pitch is preferably 10:1 to 10:4, more preferably 10:2 to 10:3, by
weight. When the mixing ratio is less than 10:1 or more than 10:4,
the resultant blend has improper viscosity, resulting in difficulty
in extrusion.
[0064] The melt blend of carbon powder and/or carbon fibers and tar
pitch is extruded from the die at a temperature of 0.degree. C. to
140.degree. C.
[0065] To obtain the graphite/metal composite material having
excellent thermal conductivity and thermal expansion coefficient,
the porous graphitized extrudate is preferably composed of
high-purity graphite. Specifically, an ash content in the porous
graphitized extrudate is preferably 0.5% by mass or less, more
preferably 0.3% by mass or less. The porous graphitized extrudate
is burned at 700.degree. C. to 1000.degree. C. after extrusion.
Because the bred molded body has many voids, pitch is introduced
into the voids of the burned molded body and burned again, to
achieve a bulk density of 1.65 g/cm.sup.3 or more. Thereafter, the
molded body is heat-treated at a temperature of 2600.degree. C. to
3000.degree. C. for graphitization, which turns carbon to graphite,
thereby providing a porous graphitized extrudate. To obtain the
graphite/metal composite material having a high thermal
conductivity and a small thermal expansion coefficient,
incombustible mineral materials (ash content) remaining after
burning the porous graphitized extrudate should be 0.5% by mass or
less, namely, the porous graphitized extrudate should be turned to
high-purity graphite.
(2) Impregnation of Metal
[0066] The impregnation of the porous graphitized extrudate with a
molten metal can be conducted by a melt-forging method. One example
of die apparatuses suitable for conducting the melt-forging method
is shown in FIG. 1. As shown in FIG. 1(a), the die apparatus 1
comprises an upper die portion 11 having a center cavity 11a, a
lower die portion 12 disposed under the upper die portion 11 and
having a center opening 12a, a lower punch 13 disposed in a cavity
11a of the upper die portion 11, a shaft 14 connected to the bottom
of the lower punch 13 and passing through an opening 12a of the
lower die portion 12, an upper punch 15 entering into the cavity
11a of the upper die portion 11, and a plunger shaft 16 connected
to an upper surface of the upper punch 15.
[0067] As shown in FIG. 1(a), the upper punch 15 is removed, and
the porous graphitized extrudate 20 is placed on the lower punch
13, which is lowered to the bottom of the cavity 11a of the upper
die portion 11. In this state, a molten metal M is poured from a
ladle 2 into the cavity 11a. At this time, it is preferable that
the upper and lower die portions 11, 12, the porous graphitized
extrudate, etc. are heated to a predetermined temperature in
advance, and that the molten metal M in a sufficient amount is
poured into the cavity 11a to prevent the solidification of the
molten metal M during the impregnation. To prevent the porous
graphitized extrudate 20 from floating while pouring the molten
metal, a weight made of iron, etc. is more preferably placed on the
porous graphitized extrudate 20.
[0068] As shown in FIG. 1(b), the upper punch 15 is caused to enter
into the cavity 11a to press the molten metal M via the plunger
shaft 16 at a high pressure, such that the high-pressure molten
metal M penetrates into the voids of the porous graphitized
extrudate 20. After the molten metal M entering into the porous
graphitized extrudate 20 is solidified, as shown in FIG. 1(c), the
upper punch 15 is removed, and the lower punch 13 is then elevated,
to take out the resultant metal-impregnated porous graphitized
extrudate 21. Finally, the metal-impregnated porous graphitized
extrudate 21 is cut out from the solidified metal M' as shown in
FIG. 1(d). To prevent the molten metal M from being solidified
before fully entering into the voids of the porous graphitized
extrudate 20 under high pressure, it is preferable to heat the
upper and lower die portions 11, 12 and the upper and lower punches
13, 15 at a predetermined temperature during the melt-forging.
[0069] The melt-forging temperature is preferably higher than the
melting point of the molten metal by 10.degree. C. or more, though
it may vary depending on the types of the molten metal.
Specifically, the melt-forging temperature of each metal or its
alloy is as shown in Table 1 below. When the melt-forging
temperature is lower than the lower temperature limit in any molten
metal, the intrusion of the molten metal into the voids of the
porous graphitized extrudate is insufficient. Effects obtained by
elevating the melt-forging temperature are substantially saturated
at the upper temperature limit, and further improvement in the
effects cannot be obtained even by elevating the melt-forging
temperature. The temperature of the porous graphitized extrudate is
preferably heated at a temperature equal to the melting point of
the molten metal, particularly at a temperature higher than the
melting point of the molten metal in advance, in any molten metal
before the impregnation, because such heating makes it possible to
fully impregnate the voids of the extrudate with the molten metal.
TABLE-US-00001 TABLE 1 Molten Melt-Forging Temperature (.degree.
C.) Metal.sup.(1) Preferable Range More Preferable Range Aluminum
600 to 900 700 to 900 Copper 1100 to 1400 1200 to 1400 Silver 1000
to 1300 1100 to 1300 Magnesium 700 to 900 750 to 900 Zinc 500 to
800 600 to 800 Note: .sup.(1)True of alloys of each metal.
[0070] The melt-forging pressure should be 10 MPa or more
regardless of the types of the molten metal. It is more preferably
50 MPa or more. If the melt-forging pressure were lower than the
lower temperature limit, the intrusion of the molten metal into the
voids of the porous graphitized extrudate would be insufficient in
any molten metal. Effects obtained by elevating the melt-forging
pressure are substantially saturated at the upper pressure limit,
and further improvement in the effects cannot be obtained by
elevating the melt-forging pressure.
[0071] The pressing time may generally be 1 to 30 minutes
regardless of the type, temperature and pressure of the molten
metal. When the pressing time is less than 1 minute, the porous
graphitized extrudate is not fully impregnated with the molten
metal. On the other hand, when it is more than 30 minutes, the
temperature of the molten metal becomes too low, failing to achieve
further impregnation.
(3) Heat Treatment
[0072] FIG. 2 shows a preferable heat treatment pattern for the
graphite/metal composite material of the present invention. The
temperature-elevating speed of the graphite/metal composite
material is preferably 30.degree. C./minute or less, more
preferably 10.degree. C./minute or less. When the
temperature-elevating speed is more than 30.degree. C./minute, the
temperature of the composite material does not become uniform. The
lower limit of the temperature-elevating speed may be about
0.5.degree. C./minute, taking into account the efficiency of the
heat treatment.
[0073] The holding temperature of the graphite/metal composite
material is preferably (the melting point of each metal -10.degree.
C.) or less and 200.degree. C. or higher. When the holding
temperature is higher than the melting point of each metal
-10.degree. C., the metal is softened or melted, so that it is
likely to elute from the porous graphitized extrudate. On the other
hand, when the holding temperature is lower than 200.degree. C.,
sufficient heat treatment effects cannot be obtained. The holding
time may be about 1 to 120 minutes.
[0074] Because the graphite/metal composite material held at the
above temperature is preferably cooled slowly, its cooling speed is
preferably 20.degree. C./minute or less, more preferably 10.degree.
C./minute or less. When the cooling speed is more than 20.degree.
C./minute, thermal hysteresis remains in the impregnating metal.
The lower limit of the cooling speed may be about 0.5.degree.
C./minute, taking into account the efficiency of the heat
treatment.
[0075] The heat treatment may be conducted on the metal-impregnated
porous graphitized extrudate 21, and the metal-impregnated
graphite/metal composite material cut along a surface perpendicular
to the extrusion direction may be heat-treated. The former is more
preferable from the aspect of the production process.
[0076] The preferred heat treatment conditions of each
metal-impregnated graphite/metal composite material are shown in
Table 2 below. TABLE-US-00002 TABLE 2 Heat Treatment
Conditions.sup.(1) of Graphite/Metal Composite Material
Temperature- Holding Cooling Impregnating Elevating Temperature
Speed Metal Speed (.degree. C.) (.degree. C.) (.degree. C.)
Aluminum 0.5 to 30 200 to 550 0.5 to 20 (2 to 10) (450 to 550) (2
to 1.0) Copper 0.5 to 30 300 to 1000 0.5 to 20 (2 to 10) (800 to
1000) (2 to 10) Silver 0.5 to 30 300 to 900 0.5 to 20 (2 to 10)
(700 to 900) (2 to 10) Magnesium 0.5 to 30 200 to 650 0.5 to 20 (2
to 10) (550 to 650) (2 to 10) Zinc 0.5 to 30 200 to 450 0.5 to 20
(2 to 10) (300 to 450) (2 to 10) Note: .sup.(1)Upper rows show
preferred ranges, and lower rows show more preferred ranges.
(C) Properties (1) Thermal Conductivity
[0077] Because the graphite/metal composite material of the present
invention has a structure, in which a high-thermal-conductivity
metal has entered into the voids of the porous graphitized
extrudate at a high pressure, it has an extremely high thermal
conductivity due to the graphite. In addition, because the graphite
skeleton per se has anisotropy because of its structure as an
extrudate, it has different thermal conductivities in the extrusion
direction and a direction perpendicular thereto. The porous
graphitized extrudate per se has a thermal conductivity of 150 W/mK
or more in the extrusion direction, and 80 W/mK or more in the
perpendicular direction. Accordingly, the composite material has a
thermal conductivity of 250 W/mK or more in the extrusion
direction, and 150 W/mK or more in the perpendicular direction,
regardless of the type of the impregnating metal. Further, the
feature of the present invention is that the thermal conductivity
of the graphite/metal composite material is further improved by a
heat treatment.
[0078] The thermal conductivity of each graphite/metal composite
material before and after the heat treatment is shown in Table 3
below. TABLE-US-00003 TABLE 3 Thermal Conductivity (W/mK).sup.(1)
of Graphite/Metal Impregnating Composite Material Before and After
Heat Treatment Metal In Extrusion Direction In Perpendicular
Direction Aluminum After: 280-340 After: 230-300 Before: 240-300
Before: 220-260 Copper After: 290-350 After: 240-300 Before:
240-300 Before: 230-270 Silver After: 290-350 After: 240-300
Before: 240-300 Before: 230-270 Magnesium After: 250-300 After:
190-250 Before: 210-240 Before: 140-180 Zinc After: 250-300 After:
180-250 Before: 200-230 Before: 130-170 Note: .sup.(1)Upper rows
show the thermal conductivity ranges after the heat treatment, and
lower rows show those before the heat treatment.
(2) Thermal expansion coefficient
[0079] Because the graphite/metal composite material of the present
invention has a skeleton constituted by a porous graphitized
extrudate, it has a whole a thermal expansion coeffiecient close to
that of graphite. Also, because the graphite skeleton is
constituted by the extrudate, there are differences in a thermal
expansion coefficient between the extrusion direction and the
direction perpendicular thereto. The thermal expansion coefficient
of the porous graphitized extrudate per se is
3.0.times.10.sup.-6/K. or less in the extrusion direction and
40.times.10.sup.-6/K. or less in the perpendicular direction.
Accordingly, though slightly different depending on the type of the
impregnating metal, the thermal expansion coefficient of the
composite material is as small as less than 4.0.times.10.sup.-6/K.
in the extrusion direction, and 10.times.10.sup.-6K. or less in the
perpendicular direction expansion direction, and
10.times.10.sup.-6/K. or less in the perpendicular direction
expansion coefficient. Further, the feature of the present
invention is that the thermal expansion coefficient of the
graphite/metal composite material is further lowered by a heat
threatment.
[0080] The thermal expansion coefficient and dimensional change
ratio of each graphite/metal composite material before and after
the heat treatment are shown in Table 4 below. TABLE-US-00004 TABLE
4 Thermal Expansion Coefficient (.times.10.sup.-6/K).sup.(1) of
Graphite/ Metal Composite Material Before and After Heat Treatment
In Impregnating In Extrusion Perpendicular Dimensional Metal
Direction Direction Change Ratio (%).sup.(2) Aluminum After:
0.7-3.9 After: 4.0-10.0 After: 0.1 or less Before: 4.1-10.0 Before:
10.5-20.0 Before: >0.1 Copper After: 0.7-3.5 After: 4.0-10.0
After: 0.1 or less Before: 5.0-9.0 Before: 10.5-15.0 Before:
>0.1 Silver After: 0.7-3.9 After: 4.1-10.0 After: 0.1 or less
Before: 4.1-12.0 Before: 10.5-21.0 Before: >0.1 Magnesium After:
1.0-3.9 After: 5.0-10.0 After: 0.1 or less Before: 4.1-15.0 Before:
10.5-25.0 Before: >0.1 Zinc After: 1.0-3.9 After: 5.0-10.0
After: 0.1 or less Before: 4.1-18.0 Before: 10.5-28.0 Before:
>0.1 Note: .sup.(1)Upper rows show the ranges of the thermal
expansion coefficient and the dimensional change ratio after the
heat treatment, and lower rows show those before the heat
treatment. .sup.(2)[(size after thermal hysteresis - size before
thermal hysteresis)/size before thermal hysteresis] .times. 100.
The thermal hysteresis is a cycle of heating # from room
temperature to a predetermined temperature (aluminum: 500.degree.
C., copper: 900.degree. C., silver: 850.degree. C., magnesium:
550.degree. C., zinc: 350.degree. C.) and then cooling to room
temperature.
(3) Thermal expansion hysteresis
[0081] The thermal expansion of the graphite/metal composite
material before the heat treatment has a thermal hysteresis.
Namely, the graphite/metal composite material before the heat
treatment has such poor dimensional stability that when it is
heated and then cooled to room temperature, it does not return from
a thermally expanded state to the original size. It has been found,
however, that the graphite/metal
[0082] composited material is provided with an extremely reduced
dimensional change ratio by the heat treatment according to the
present invention. The graphite/metal composite material with an
excellent dimensional stability suffers substantially no
dimensional change when it is used for a heat-dissipating substrate
and thus subjected to heat by soldering or brazing. Accordingly,
the heat-dissipating substrate does not warp, so that unnecessary
stress is not applied to heat-generating devices such as
semiconductor devices or laser devices.
(4) Other properties
[0083] The resistivity of the graphite/metal composite material
slightly decreases by the heat treatment. Decrease in the
resistivity is remarkable particularly in the extrusion direction.
The resistivity of the graphite/metal composite material is
preferably 4 .mu..OMEGA.m or less in the extrusion direction and 7
.mu..OMEGA.m or less in the perpendicular direction. The
resistivity of each graphite/metal composite material before and
after the heat treatment is shown in Table 5 below. TABLE-US-00005
TABLE 5 Resistivity (.mu..OMEGA.m) of Graphite/Metal Composite
Impregnating Material Before and After Heat Treatment.sup.(1) Metal
In Extrusion Direction In Perpendicular Direction Aluminum After:
1.0-4.0 After: 1.0-7.0 Before: 1.0-4.1 Before: 1.0-7.1 Copper
After: 1.0-3.0 After: 1.0-5.0 Before: 1.0-3.1 Before: 1.0-5.1
Silver After: 1.0-3.0 After: 1.0-5.0 Before: 1.0-3.0 Before:
1.0-5.1 Magnesium After: 1.0-4.0 After: 1.0-7.0 Before: 1.0-4.1
Before: 1.0-7.1 Zinc After: 1.0-3.0 After: 1.0-7.0 Before: 1.0-4.1
Before: 1.0-7.1 Note: .sup.(1)Upper rows show resistivity ranges
after the heat treatment, and lower rows show those before the heat
treatment.
[0084] It is presumed that the decrease of the resistivity of the
graphite/metal composite material by the heat treatment is caused
by the fact that the heat treatment lowers the amount of oxygen in
the impregnating metal, increasing the purity of the metal. The
amount of oxygen in the impregnating metal is different depending
on the type of the metal. In the graphite/metal composite material
before the heat treatment, the amount of oxygen is generally 200 to
400 ppm in aluminum or its alloy, 500 to 1000 ppm in copper or its
alloy, 200 to 600 ppm in silver or its alloy, 200 to 600 ppm in
magnesium or its alloy, and 500 to 2000 ppm in zinc or its alloy.
Particularly in the case of copper or its alloy, the amount of
oxygen extremely decreases by the heat treatment. Specifically, the
amount of oxygen is as small as 400 ppm or less in the
graphite/copper composite material after the heat treatment.
[0085] The Young's modulus of the graphite/metal composite material
does not substantially change before and after the heat treatment,
and it is generally 5 GPa or more in a surface direction, on a
necessary level for use in the heat-dissipating substrates. Also,
the bending strength of the graphite/metal composite material does
not substantially change before and after the heat treatment, and
it is generally 10 MPa or more, on a necessary level for use in the
heat-dissipating substrates. The increase of a thermal
conductivity, the reduction of a thermal expansion coefficient and
the improvement of dimensional stability by the heat treatment seem
to be mainly due to the fact that a residual strain at the time of
melt forging disappears by the heat treatment. Particularly in the
case of using an Al--Si alloy, the spheroidization of the
needle-shaped structure having high heat resistance by the heat
treatment seems to contribute to the improvement of the thermal
conductivity. Also, in the case of using copper or its alloy, the
reduction of the amount of oxygen functioning to increase heat
resistance by the heat treatment seems to contribute to the
improvement of the thermal conductivity.
[2] Heat-Dissipating Substrate
[0086] The heat-dissipating substrate is obtained by cutting the
heat-treated graphite/metal composite material to a predetermined
size. Though the heat-dissipating substrate is preferably used as a
heat sink or a heat spreader, etc., it may have a structure in
which a heat-dissipating fan and a heat spreader are integrated
because of excellent workability of the graphite/metal composite
material. A surface of the heat-dissipating substrate, onto which a
heat-generating device such as a semiconductor device or a laser
device, etc. is bonded, is preferably perpendicular to the
extrusion direction of the graphite/metal composite material,
though it may be in parallel with the extrusion direction.
[0087] As shown in FIG. 3, for instance, when the surface for
bonding a semiconductor device 3 is perpendicular to the extrusion
direction of the heat-dissipating substrate 4, heat is quickly
conducted from the semiconductor device 3 to a heat sink 5 bonded
to the other surface of the heat-dissipating substrate 4, because
the heat-dissipating substrate 4 has a thermal conductivity larger
in the thickness direction than in the surface direction. Because
the heat-dissipating substrate 4 has a thermal expansion
coefficient larger in the surface direction than in the thickness
direction, the thermal expansion coefficient of the
heat-dissipating substrate 4 in the surface direction is close to
those of both of the semiconductor device 3 and the heat sink 5.
Accordingly, no large thermal stress is applied to both interfaces
between the heat-dissipating substrate 4 and the semiconductor
device 3 and between the heat-dissipating substrate 4 and the heat
sink 5 during the operation of the semiconductor device 3.
[0088] The semiconductor device 3 is not subjected to a large
thermal stress by heating during soldering or brazing for bonding
the semiconductor device 3 to the heat-dissipating substrate 4.
Further, because the thermal expansion coefficient of the
heat-dissipating substrate in the thickness direction is as small
as half or less of the thermal expansion coefficient in the surface
direction, the heat-dissipating substrate preferably exhibits a
small expansion ratio in the height direction during heating for
producing a package, so that it is easily positioned in an
assembling process.
[0089] The method for producing the heat-dissipating substrate is
characterized in that the graphite/metal composite material is cut
after the heat treatment. If the graphite/metal composite material
were cut before the heat treatment, because of poor dimensional
stability, the heat-dissipating substrate would be subjected to
dimensional change by heating during soldering or brazing or by
temperature elevation during the operation. Accordingly, finish
working may be needed to achieve dimensional precision, or the
bonding interfaces of the heat-dissipating substrate may have poor
reliability.
[0090] The heat-dissipating substrate cut to a predetermined size
is preferably coated with a metal layer to secure the sealing of a
package. Though the metal layer is usually formed on the entire
surface of the heat-dissipating substrate, the sealing of the
package can be secured by forming the metal layer at least on a
surface for mounting a semiconductor device, etc. (and a rear
surface). The sealing is enough when the amount of a helium gas
leaked is 1.times.10.sup.-2 Pacm.sup.3/s or less.
[0091] The method of forming the metal layer may be a CVD method, a
vapor deposition method, a sputtering method, a metal paste
printing/baking method, a plating method, etc. To achieve enough
sealing, the thickness of the metal layer is preferably 0.5 .mu.m
to 10 .mu.m. In the case of plating, electroless plating is more
preferable than electroplating, because the electroless plating can
form a uniform metal layer on the heat-dissipating substrate.
[0092] The plating layer is preferably Ni--P, Ni--B, Cu, etc. When
the impregnating metal is copper or its alloy, requiring a heat
resistance of 700.degree. C. or higher, the Ni--B plating is
particularly preferable because it is less diffusion-reactive to
the metal and thus stable. The metal layer may be utilized not only
for sealing, but also as a primer for adhesion to other parts. Such
metal layer desirably improves the adhesion to heat-generating
bodies such as semiconductor de-vices and packages.
[0093] Because the heat-dissipating substrate comprising aluminum
or its alloy as an impregnating metal has a large thermal
conductivity, a thermal expansion coefficient close to those of
silicon and compound semiconductors, and good solderability, it is
suitable for heat spreaders, etc. for semiconductor devices using
soldering for bonding. In addition, because the graphite/aluminum
composite material has low melt-forging temperature and
heat-treating temperature, it is advantageous in low production
cost. Because the heat-dissipating substrate of the present
invention has a thermal expansion coefficient closer to those of
semiconductor devices and is lighter in weight than conventional
heat spreaders composed of copper or aluminum, it is desirable for
heat spreaders with grease.
[0094] The heat-dissipating substrate comprising copper or its
alloy as an impregnating metal has a large thermal conductivity, a
small thermal expansion coefficient, and good dimensional
stability. In addition, because it is impregnated with copper
having a relatively high melting point, it has high heat
resistance, undergoing no change at a brazing temperature.
Accordingly, it is suitable for applications such as optical
transmission packages, etc. including the heat dissipation of laser
devices brazed with silver brazing alloys.
[0095] In the case of a heat-dissipating substrate having
throughholes for fastening, a metal pipe member fitted in each
throughhole acts as a reinforcing member, preventing damage such as
cracking even when a high fastening torque is applied. Thus, a high
fastening torque can be obtained by this fastening structure. The
metal pipe member also acts as a heat-conductive member for
dispersing a thermal stress concentrated around each throughhole,
thereby improving the function of the heat-dissipating
substrate.
[0096] Because the heat-dissipating substrate comprising silver or
its alloy as an impregnating metal is impregnated with silver
having a large thermal conductivity and a melting point between
those of aluminum and copper, it is suitable for applications
requiring a heat resistance of about 900.degree. C.
[0097] Because the heat-dissipating substrate comprising magnesium
or its alloy as an impregnating metal is impregnated with magnesium
having a melting point between those of aluminum and silver, it is
suitable for parts requiring a higher heat resistance than that of
aluminum.
[0098] Because the heat-dissipating substrate comprising zinc or
its alloy as an impregnating metal has a larger thermal expansion
coefficient than those of substrates impregnated with aluminum,
copper, silver, magnesium, etc., it is suitable for bonding to
parts having large thermal expansion coefficients, for instance,
bonding to heat sinks of aluminum or copper. In addition, it is
advantageous in low production cost, because of its low
impregnation temperature.
[0099] The present invention will be explained in detail referring
to Examples below without intention of restricting the present
invention thereto.
Example 1
[0100] Using a porous graphitized extrudate having a specific bulk
density of 1.70, an ash content of 0.3% by mass, resistivity of 5.0
.mu..OMEGA.m and 8.5 .mu..OMEGA.m, respectively, in an extrusion
direction and in the direction perpendicular to the extrusion
direction, a thermal expansion coefficient of 0.6.times.10.sup.-6/K
and 2.0.times.10.sup.-6/K, respectively, in an extrusion direction
and in the direction perpendicular to the extrusion direction, a
thermal conductivity of 230 W/mK and 120 W/mK, respectively, in an
extrusion direction and in the direction perpendicular to the
extrusion direction, which was obtained by extruding and
graphitizing a melt blend of coke particles having an average
particle size of 500 .mu.m and pitch, and an Al--Si alloy
containing 12% by mass of Si, a graphite/Al--Si composite material
was produced under the following conditions.
[0101] After the above Al--Si alloy melt (750.degree. C.) was
poured into a cavity of a die apparatus (held at 750.degree. C.)
shown in FIG. 1(a), in which the above porous graphitized extrudate
was placed, an upper punch was lowered to conduct melt-forging at
100 MPa for 5 minutes. An excess portion of the Al--Si alloy was
cut off to obtain the graphite/Al--Si composite material. This
graphite/Al--Si composite material was subjected to a heat
treatment under the following conditions.
[0102] Temperature-elevating speed: 2.degree. C./minute
[0103] Holding conditions: 500.degree. C..times.60 minutes
[0104] Cooling speed: 2.degree. C./minute
[0105] The graphite/Al--Si composite material after the heat
treatment was cut to a size of 40.0 mm.times.20.0 mm.times.2.0 mm
as a sample for a heat-dissipating substrate. The thickness
direction of the heat-dissipating substrate was in alignment with
the extrusion direction of the composite material.
[0106] Samples obtained from the graphite/Al--Si composite
materials before and after the heat treatment were measured with
respect to the amount of the Al--Si alloy, the amount of a
needle-shaped structure in the Si-rich phase in the Al--Si alloy, a
bulk density, a thermal conductivity, a thermal expansion
coefficient, resistivity, a Young's modulus, a bending strength,
and a dimensional change ratio by the following methods. The
measurement results are shown in Table 6 below.
(1) The bulk density was an apparent weight per a unit volume.
(2) The thermal conductivity was measured by a thermal constant
analyzer TC-7000H by a laser flash method available from
ULVAC-RIKO, Inc. according to JIS R 1611.
(3) The thermal expansion coefficient and the dimensional change
ratio were measured by a termomechanical analyzer using a thermal
analysis system EXSTAR6000 available from Seiko Instruments
Inc.
(4) The resistivity was measured by a four-terminal method using
ZEM-2 available from ULVAC-RIKO, Inc.
(5) The Young's modulus was measured by a two-probe method
receiving an ultrasonic transmission wave using UVM-2 and a digital
oscilloscope.
(6) The bending strength was measured by a re-point bending test
method using an autograph AG-G available from Shimadzu Corporation
according to JIS R 1601.
Comparative Example 1
[0107] A graphite/Al--Si composite material was produced and
evaluated in the same manner as in Example 1 except for conducting
a heat treatment under the following conditions. The results are
shown in Table 6 below.
[0108] Temperature-elevating speed: 2.degree. C./minute,
[0109] Holding conditions: 150.degree. C..times.60 minute, and
[0110] Cooling speed: 2.degree. C./minute. TABLE-US-00006 TABLE 6
Properties of Graphite/Al--Si Composite Material Measure-
Comparative ment Example 1 Example 1 Direc- Before After Before
After Items Measured tion HT.sup.(1) HT HT HT Al--Si Alloy -- 14 14
14 14 (vol. %).sup.(2) Amount of Needle- -- 90 90 90 5 Shaped
Structure (%) Bulk Density -- 2.2 2.2 2.2 2.1 (g/cm.sup.3) Thermal
ED.sup.(3) 300 300 300 340 Conductivity PD.sup.(4) 220 220 220 250
(W/mK) Thermal Expansion ED 8.1 8.0 8.1 3.5 Coefficient PD 17.8
17.0 17.8 6.9 (.times.10.sup.-6/K) Dimensional ED 0.18 0.18 0.18
0.01 Change Ratio (%) PD 0.32 0.32 0.32 0.01 Resistivity
(.mu..OMEGA.m) ED 1.4 1.4 1.4 1.2 PD 1.7 1.7 1.7 1.8 Young's
Modulus ED 18 18 18 18 (GPa) PD 11 11 11 11 Bending Strength ED 47
46 47 42 (MPa) PD 29 29 29 26 Amount of Oxygen -- 250 250 250 200
in Al--Si alloy (ppm) Note: .sup.(1)Heat treatment. .sup.(2)The
volume percentage of the Al--Si alloy per the entire
graphite/Al--Si composite material (100% by volume).
.sup.(3)Extrusion direction. .sup.(4)Direction perpendicular to the
extrusion direction.
[0111] As is clear from Table 6, the heat treatment increased the
thermal conductivity of the graphite/Al--Si composite material,
while extremely decreasing the thermal expansion coefficient and
dimensional change ratio thereof. There was substantially no change
before and after the heat treatment in the resistivity, the Young's
modulus and the bending strength. The above results indicate that
the heat treatment provided the graphite/Al--Si composite material
with desirable properties for heat-dissipating substrates. In
Comparative Example 1, on the other hand there was substantially no
change before and after the heat treatment in the thermal
conductivity, the thermal expansion coefficient and the dimensional
change ratio.
[0112] With respect to samples of the graphite/Al--Si composite
materials before and after the heat treatment, the structures of
their Al--Si regions were observed by a scanning ion microscope
(SIM) FB-2000A available from Hitachi, Ltd. The SIM photographs are
shown in FIGS. 4(a) and 4(b). As is clear from FIG. 4(a), a
needle-shaped structure composed of a Si-rich phase was
precipitated in a sample of the graphite/Al--Si composite material
before the heat treatment. In a sample composed of the
graphite/Al--Si composite material after the heat treatment, on the
other hand, the needle-shaped structure became spheroidal as is
clear from FIG. 4(b). In this Example, the percentage (surface area
ratio in the photomicrograph) of the needle-shaped structure having
a length of 30 .mu.m or less and an aspect ratio (length/diameter)
of 10 or more among the silicon-rich phase decreased to 5%. It is
presumed that spheroidization decreases the heat resistance of the
low-thermal-conductivity Si-rich phase, contributing to increase in
the thermal conductivity of the Si-rich phase. It has been found
that when the surface area ratio of the needle-shaped structure
having a length of 30 .mu.m or less and an aspect ratio of 10 or
more among the silicon-rich phase becomes 10% or less, particularly
5% or less, the thermal conductivity of the graphite/Al--Si
composite material extremely increases.
[0113] Samples composed of the graphite/Al--Si composite materials
before and after the heat treatment were heated to temperatures
ranging from room temperature to 500.degree. C., and left to cools
their hysteresis of thermal expansion was measured in the extrusion
direction and a perpendicular direction. The results are shown in
FIGS. 5 and 6. As is clear from FIGS. 5(a) and 6(a), the
graphite/Al--Si composite material before the heat treatment
exhibited a dimensional change ratio of 0.18% in the extrusion
direction and 0.32% in the perpendicular direction after the
thermal hysteresis. In the graphite/Al--Si composite material after
the heat treatment, on the other hand, as is clear from FIGS. 5(b)
and 6(b), there was as small a dimensional change ratio as 0.01% in
any of the extrusion direction and the perpendicular direction
after the thermal hysteresis, indicating that there was
substantially no dimensional change. The above results indicate
that the graphite/Al--Si composite material of the present
invention heat-treated after the metal impregnation suffered from
little dimensional change even after the thermal hysteresis,
excellent in dimensional stability.
Example 2
[0114] Using the same porous graphitized extrudate as in Example 1
and pure copper purity 99.9% or more), a graphite/copper composite
material was produced as follows. After a melt (1350.degree. C.) of
the above pure copper poured into a cavity of the die apparatus
(held at 1100.degree. C.) shown in FIG. 1(a), in which the above
porous graphitized extrudate was placed, an upper punch was lowered
to conduct melt-forging at 100 MPa for 5 minutes. Excess pure
copper was cut away to obtain the graphite/copper composite
material. This graphite/copper composite material was subjected to
a heat treatment under the following conditions.
[0115] Temperature-elevating speed: 5.degree. C./minute,
[0116] Holding conditions: 900.degree. C..times.120 minutes,
and
[0117] Cooling speed: 5.degree. C./minute.
[0118] The graphite/copper composite material after the heat
treatment was cut to a size of 40.0 mm.times.20.0 mm.times.2.0 mm
to provide a heat-dissipating substrate sample. The thickness
direction of the heat-dissipating substrate was in alignment with
the extrusion direction of the composite material. Each sample was
measured with respect to the amount of copper, a bulk density, a
thermal conductivity, a thermal expansion coefficient, a
dimensional change ratio (after heating and heat dissipation
described below), resistivity, Young's modulus, a bending strength
and the amount of oxygen in copper in the same manner as in Example
1. The measurement results are shown in Table 7 below.
Comparative Example 2
[0119] A graphite/copper composite material was produced and
measured in the same manner as in Example 2 except for using a
porous graphitized extrudate having a specific bulk density of
1.58, an ash content of 0.7% by mass, resistivity of 9.0
.mu..OMEGA.m and 9.5 .mu..OMEGA.m, respectively, in an extrusion
direction and in the direction perpendicular to the extrusion
direction, a thermal expansion coefficient of
4.10.times.10.sup.-6/K and 4.2.times.10.sup.-6/K, respectively, in
an extrusion direction and in the direction perpendicular to the
extrusion direction, and a thermal conductivity of 130 W/mK and 80
W/mK, respectively, in an extrusion direction and in the direction
perpendicular to the extrusion direction. The measurement results
are shown in Table 7 below. TABLE-US-00007 TABLE 7 Properties of
Graphite/Copper Composite Material Measure- Comparative ment
Example 2 Example 2 Direc- Before After Before After Items Measured
tion.sup.(2) HT.sup.(1) HT HT HT Amount of Copper -- 35 35 16 16
(vol. %).sup.(1) Bulk Density -- 4.5 4.5 3.2 3.1 (g/cm.sup.3)
Thermal ED.sup.(3) 160 180 300 350 Conductivity PD.sup.(4) 130 150
220 250 (W/mK) Thermal Expansion ED 12.0 7.0 6.1 1.7 Coefficient PD
13.5 8.0 11.9 5.8 (.times.10.sup.-6/K) Dimensional Change ED 0.60
0.05 0.35 0.02 Ratio (%) PD 0.50 0.03 0.40 0.01 Resistivity
(.mu..OMEGA.m) ED 10.0 10.0 1.6 1.1 PD 11.0 11.0 1.7 1.8 Young's
Modulus ED 25 25 15 16 (GPa) PD 20 20 10 10 Bending Strength ED 45
44 38 30 (MPa) PD 35 32 24 20 Amount of Oxygen -- 1000 400 850 150
in Copper (ppm) Note: .sup.(1)Heat treatment. .sup.(2)The volume
percentage of copper per the entire graphite/Al--Si composite
material (100% by volume). .sup.(3)Extrusion direction.
.sup.(4)Direction perpendicular to the extrusion direction.
[0120] As is clear from Table 7, the heat treatment provided the
graphite/copper composite material with a slightly increased
thermal conductivity and an extremely decreased thermal expansion
coefficient. With respect to the resistivity, the Young's modulus
and the bending strength, there was substantially no change before
and after the heat treatment. The above results indicate that the
graphite/copper composite material got desirable properties for
heat-dissipating substrates by the heat treatment. On the other
hand, the graphite/copper composite material of Comparative Example
2 had a low thermal conductivity and a large thermal expansion
coefficient even after the heat treatment, though its dimensional
change ratio decreased by the heat treatment.
[0121] With respect to samples composed of the graphite/copper
composite materials before and after the heat treatment, a copper
region was observed by a scanning ion microscope (SIM). The SIM
photograph is shown in FIG. 7. As is clear from FIG. 7(a), a sample
composed of the graphite/copper composite material before the heat
treatment had a copper phase with a typical structure. On the other
hand, as is clear from FIG. 7(b), a sample composed of the
graphite/copper composite material after the heat treatment had a
copper phase changed to an equiaxial crystal, with oxygen in an
extremely reduced amount. It is presumed that decrease in the
amount of oxygen reducing heat resistance results in the
improvement of a thermal conductivity.
[0122] Samples composed of the graphite/copper composite materials
before and after the heat treatment were heated to temperatures
ranging from room temperature to 900.degree. C. and then left to
cool to measure a thermal expansion hysteresis in both extrusion
and perpendicular directions. The results are shown in FIGS. 8 and
9. As is clear from FIGS. 8(a) and 9(a), the graphite/copper
composite material before the heat treatment had a dimensional
change ratio of 0.35% and 0.40%, respectively, in an extrusion
direction and a perpendicular direction after cooling. And as is
clear from FIGS. 9(b) and 9(b), the graphite/copper composite
material after the heat treatment had a dimensional change ratio of
0.02% and 0.01%, respectively, in an extrusion direction and a
perpendicular direction, with substantially no dimensional change.
This reveals that the graphite/copper composite material of the
present invention has a small dimensional change and thus excellent
dimensional stability even after heating.
Example 3
[0123] Using the same porous graphitized extrudate as in Example 1
having a specific bulk density of 1.70, an ash content of 0.3% by
mass, resistivity of 5.0 .mu..OMEGA.m and 8.5 .mu..OMEGA.m,
respectively, in an extrusion direction and in the direction
perpendicular to the extrusion direction, a thermal expansion
coefficient of 0.6.times.10.sup.-6/K and 2.0.times.10.sup.-6/K,
respectively, in an extrusion direction and in the direction
perpendicular to the extrusion direction, and a thermal
conductivity of 230 W/mK and 120 W/mK, respectively, in an
extrusion direction and in the direction perpendicular to the
extrusion direction, and brass comprising 70% by mass of Cu and 30%
by mass of Zn, a graphite/brass composite material was produced
under the following conditions.
[0124] After a melt (1350.degree. C.) of the above brass was poured
into a cavity of the die apparatus (held at 1000.degree. C.) shown
in FIG. 1(a), in which the above graphite was placed, an upper
punch was lowered to conduct melt-forging at 100 MPa for 5 minutes.
Excess pure copper was cut away to obtain the graphite/brass
composite material. This graphite/brass composite material was
subjected to a heat treatment under the following conditions.
[0125] Temperature-elevating speed: 5.degree. C./minute,
[0126] Holding conditions: 900.degree. C..times.120 minutes,
and
[0127] Cooling speed: 5.degree. C./minute.
[0128] The resultant graphite/brass composite material sample was
measured with respect to the amount of brass, a bulk density, a
thermal conductivity, a thermal expansion coefficient, a
dimensional change ratio (after heating and heat dissipation),
resistivity, a Young's modulus, a bending strength, and the amount
of oxygen in brass in the same manner as in Example 1. The
measurement results are shown in Table 8 below.
Comparative Example 3
[0129] A graphite/brass composite material was produced and
measured in the same manner as in Example 3 except for conducting a
heat treatment under the following conditions. The results are
shown in Table 8.
[0130] Temperature-elevating speed: 5.degree. C./minute,
[0131] Holding conditions: 250.degree. C..times.120 minutes,
and
[0132] Cooling speed: 5.degree. C./minute. TABLE-US-00008 TABLE 8
Properties of Graphite/Brass Composite Material Measure-
Comparative ment Before Example 3 Example 3 Items Measured
direction HT.sup.(1) (After HT) (After HT) Amount of Brass 22 22 22
(vol. %).sup.(2) Bulk Density (g/cm.sup.3) -- 3.2 3.2 3.1 Thermal
Conductivity ED.sup.(3) 280 280 330 (W/mK) PD.sup.(4) 180 180 220
Thermal Expansion ED 7.1 7.1 2.0 Coefficient (.times.10.sup.-6/K)
PD 13.1 13.0 6.5 Dimensional Change ED 0.32 0.30 0.02 Ratio (%) PD
0.38 0.36 0.01 Resistivity (.mu..OMEGA.m) ED 1.8 1.9 1.7 PD 1.9 1.9
1.8 Young's Modulus ED 15 15 16 (GPa) PD 10 10 10 Bending Strength
ED 38 38 36 (MPa) PD 24 24 23 Amount of Oxygen -- 850 850 150 in
Brass (ppm) Note: .sup.(1)Heat treatment. .sup.(2)The volume
percentage of brass per the entire graphite/brass composite
material (100% by volume). .sup.(3)Extrusion direction.
.sup.(4)Direction perpendicular to the extrusion direction.
Example 4
[0133] A heat-dissipating substrate of 40.0 mm.times.20.0
mm.times.2.0 mm cut out from the graphite/Al--Si alloy composite
material (after the heat treatment) of Example 1 shown in Table 6
was subjected to electroless Ni--P plating after a zincate
treatment. A heat-dissipating substrate of 40.0 mm.times.20.0
mm.times.2.0 mm cut out from the graphite/Cu composite material
(after the heat treatment) of Example 2 shown in Table 7 was also
subjected to electroless Ni--B plating. To evaluate the correlation
of the presence of a plating layer and sealability, the amount of a
helium gas passing through each plated heat-dissipating substrate
was measured by a helium leak detector DLMS-33 available from
ULVAC, Inc. according to JIS C 7021 A-6. The amount of a helium gas
passing through the plated heat-dissipating substrate, as the
amount of a leaked helium gas, was used as a parameter of
sealability. The results are shown in Table 9 below. TABLE-US-00009
TABLE 9 Type of Thickness Amount of Leaked Helium Type of Substrate
Plating (.mu.m) Gas (.times.10.sup.-2 Pa cm.sup.3/s)
Graphite/Al--12Si Non -- 1.0 Graphite/Al--12S1 Ni--P 2 <0.1
Graphite/Cu Non -- 30 Graphite/Cu Ni--B 4 0.50 Graphite/Cu Ni--B 6
<0.10 Graphite/Cu Cu 5 0.35
[0134] It has been found that any of the graphite/Al-12Si and the
graphite/Cu was extremely improved in sealability by plating.
Incidentally, when the plating thickness is less than 0.5 .mu.m,
sufficient sealability cannot be obtained. On the other hand, when
the plating thickness is more than 20 .mu.m, the remaining stress
increases, resulting in peeling of the plating layer. The
graphite/Al-12Si composite material, etc. are preferable in high
adhesion to a vapor-deposited Al layer. The graphite/Cu composite
material, etc. may be printed with an Ag paste and then sintered at
900.degree. C. In this case, because of a low film stress, the film
thickness may be as large as about 30 .mu.m.
Example 5
[0135] FIG. 10 shows an example of a module for mounting
semiconductor devices, which comprises a heat-dissipating substrate
4a of 100 mm.times.100 mm.times.2 mm formed by the graphite/metal
(Cu) composite material of the present invention) an insulating
substrate 6 formed by a silicon nitride plate of 30 mm.times.30
mm.times.0.8 mm, and a heat sink 5. The insulating substrate 6 was
brazed to the heat-dissipating substrate 4a, and after plating the
insulating substrate 6 with Ni, semiconductor chips 3 of 10
mm.times.10 nm were soldered to the insulating substrate 6. The
heat-dissipating substrate 4a and the heat sink 5 were mechanically
fastened by bolts, etc. via a high-thermal-conductivity grease to
provide a module.
[0136] The graphite/Cu composite material used in this Example to
form the heat-dissipating substrate 4a was impregnated with an
copper-chromium alloy containing 1.0% by mass of Cr. The
heat-dissipating substrate 4a had a thermal conductivity of 250
W/mK or more in a thickness direction and 150 W/mK or more in a
direction perpendicular to the thickness direction (on the side of
the device-mounting surface), and a thermal expansion coefficient
of more than 0.1.times.10.sup.-6/K and less than
4.times.10.sup.-6/K in the thickness direction and
4.times.10.sup.-6/K or more and 10.times.10.sup.-6/K or less in the
perpendicular direction.
[0137] The heat-dissipating characteristics of the semiconductor
module in this Example were evaluated by measuring the surface
temperature of each semiconductor chip 3 and thermal resistance
(.degree. C./W) between the semiconductor chip 3 and the rear
surface of the heat sink 5 during supplying current, and further by
measuring thermal resistance between the semiconductor chip 3 and
the rear surface of the heat sink 5 after 3000 cycles of a
heating/cooling test from -40.degree. C. to 125.degree. C. The
thermal resistance after 3000 cycles of the temperature cycle test
is expressed by an increment (%) relative to the thermal resistance
before the temperature cycle test.
[0138] It was thus found that the surface temperature of the
semiconductor devices was 52.1.degree. C., that the thermal
resistance was 0.23.degree. C./W, and that the increment after the
cycle test was 2.5%, indicating that the surface temperature of the
semiconductor devices and the thermal resistance were lower, and
the increment after the cycle test was smaller than those of
conventional ones.
[0139] When the graphite/metal composite material of the present
invention is used for a heat-dissipating substrate 4a, the
heat-dissipating substrate 4a may have throughholes for fastening a
heat sink 5 as in the above embodiment. In this case, a reinforcing
metal pipe member 7 is preferably fitted in each throughhole 40.
Though not particularly restrictive, the pipe member 7 may have
such a shape as shown in FIGS. 12(a) and (b), or may have a flange
70 as shown in FIG. 12(c), as long as it is fitted in the
throughhole 40. Flanges 70 provided on both ends preferably further
prevent cracks from generating from the throughholes 40. When the
metal pipe member 7 has a flange 70 only on one end, the flange 70
is positioned such that it is in contact with a bolt head.
[0140] As shown in FIG. 12(d), the metal pipe member 7 may have a
slit 71. Further, as shown in FIG. 12(e), the metal pipe member 7
may have a notch 72. The slit 71 and the notch 72 enable the metal
pipe member 7 to elastically deform in a circumferential direction,
thereby making it easy to fit the metal pipe member 7 in the
throughhole 40. The slit 71 and the notch 72 have a function to
buffer a load applied to the substrate due to the expansion of the
pipe member 7, which is caused by the heat generation of the
substrate. Accordingly, they provide the metal pipe member 7 with
sufficient durability to a cooling/heating cycle, a soldering
reflow process, etc. The metal bonding such as brazing between the
pipe member 7 and the substrate preferably improves their adhesion,
resulting in improved heat dissipation.
[0141] As described above in detail, the graphite/metal composite
material of the present invention has (a) both properties of
graphite (small thermal expansion coefficient) and properties of a
metal (large thermal conductivity), because it comprises a graphite
skeleton and a high-thermal-conductivity metal entering into the
voids of the graphite skeleton, and (b) a thermal conductivity
slightly improved and a thermal expansion coefficient extremely
decreased by the heat treatment. In addition, because the
graphite/metal composite material of the present invention has a
skeleton of a graphitized extrudate, there are differences in
properties between an extrusion direction and a perpendicular
direction. Accordingly, with a cutting direction in parallel with
the extrusion direction or the perpendicular direction depending on
applications, it is possible to obtain the heat-dissipating
substrate having desired thermal conductivity and thermal expansion
coefficient. Further, because the thermal expansion hysteresis is
reduced to substantially zero by the heat treatment, the
graphite/metal composite material of the present invention
advantageously has excellent dimensional accuracy even after
soldering or brazing.
* * * * *