Circuit system, a composite material for use therein, and a method of making the material

Discrete powder particles of copper 14 and INVAR 12 are mixed together in a container 16 and packed into a powder metal article. This article is hot vacuum degassed and vacuum sealed and then heated to temperature well below the sintering temperature of copper or INVAR. Immediately after heating the article, it is subjected to a high pressure, high strain force such as extrusion through a die thereby yielding a fully dense, strong composite material 10 with excellent combined thermal expansion and conductivity properties.

BACKGROUND OF THE INVENTION 
This invention relates to a composite material for thermal management and 
method of manufacture and more particularly to the material and method for 
producing a composite metal material for use in mounting semiconducting 
devices in electronic circuit systems. 
Conventional electronic systems employ a variety of circuit board 
substrates and the like for mounting semiconductor devices such as 
integrated circuit chips and the like. In such systems, it is know to be 
desirable to provide substrate materials which have coefficients of 
thermal expansion corresponding to those of the semiconductor devices to 
be mounted thereon, whereby the semiconductor devices can remain securely 
mounted and electrically connected to circuit on the substrates during 
thermal cycling of the systems. It is also known to be desirable to 
provide substrate materials with substantial thermal conductivity 
properties for dissipating heat from the semiconductor devices during 
operation of the devices, thereby to improve operating and reliability 
characteristics of the devices and systems. It is also known to employ 
composite metal materials in such substrates to combine relatively low 
coefficient of thermal expansion characteristics of one metal material 
with relatively high thermal conductivity characteristics of another metal 
material, thereby to provide composite materials having desirable 
combinations of coefficient of expansion and thermal conductivity 
properties. 
For example, U.S. Pat. No. 4,894,293 provides for taking a plurality of 
discrete particles of a ferrous metal alloy having a relatively low 
coefficient of thermal expansion and coating them with a copper material 
having a relatively high thermal conductivity. These coated particles are 
then mixed with pure copper particles and pressed together and heated to 
provide for diffusion bonding of the particles together to form a 
continuous copper matrix with the discrete ferrous particles contained 
therein. 
U.S. Pat. No. 3,399,332 provides a grid of a metal material of relatively 
low coefficient of thermal expansion having openings in the grid filled 
with a copper material or the like of relatively higher thermal 
conductivity to provide a mounting for a semiconductor device having a 
desired combination of thermal expansion and conductivity properties. In 
another embodiment, the patent suggests that particles of a ferrous alloy 
can be impregnated into a copper material for providing an alternate 
material having selected thermal expansion and conductivity properties. 
U.S. Pat. No. 4,283,464 provides two grids of a metal material of 
relatively low coefficient of thermal expansion on either side of an inner 
layer of copper metal for providing a composite substrate material having 
another described combination of thermal expansion and conductivity 
properties. U.S. Pat. No. 4,472,672 shows layer combinations of ferrous 
metal materials of relatively low thermal expansion properties with layer 
materials of relatively high thermal conductivity where the layer 
thicknesses are regulated to be within selected ranges for providing 
composite metal materials with coefficients of thermal expansion 
substantially corresponding to those of semiconductor devices to be 
mounted thereon. U.S. Pat. Nos. 3,097,329 and 4,158,719 show composite 
metal materials formed by powder metallurgy techniques or the like either 
by compacting mixtures of metal powders of relatively low coefficient of 
thermal expansion with metal powders of relatively high thermal 
conductivity materials and then heating the compacted powders for 
diffusion bonding the particles to each other or by compacting and 
sintering one of the metal powders to form a porous sintered compact and 
by then filling the pores of that sintered compact with a melt of the 
other metal material. 
However, each of such previously known composite metal substrate materials 
has been subject to some objection. Thus, the composite metal materials 
shown in U.S. Pat. Nos. 4,894,293, 3,097,329 and 4,158,719 are not found 
to consistently and economically provide desirable combinations of thermal 
expansion and conductivity properties because of the manner in which they 
are made; the composite metal materials shown in U.S. Pat. Nos. 3,399,332 
and 4,283,464 are difficult to manufacture and to apply to specific 
circuit system applications; and the composite metal material shown in 
U.S. Pat. No. 4,472,762 does not provide desirably high thermal 
conductivity in all directions. 
Lastly, composite powdered metal systems such as copper tungsten, copper 
molybdenum and silver invar can provide good thermal conductivity and 
thermal expansion characteristics; but such combinations are expensive to 
produce, and in the case of copper tungsten and copper molybdenum, 
difficult to machine and electroplate. 
BRIEF SUMMARY OF THE INVENTION 
Accordingly, the present invention provides for a powder metal composite 
material and method of manufacture typically for a thermal management 
material in a semiconductor circuit system which is capable of being 
formed in various shapes, and having a superior combination of thermal 
expansion and conductivity properties. Further, the composite material is 
easily and economically manufactured while being characterized by good 
strength, machinability and electroplatability. 
Briefly described, in accordance with the invention, a plurality of 
discrete particles of a ferrous metal alloy having a relatively low 
coefficient of thermal expansion are mixed with a plurality of discrete 
particles of a metal having a relatively high thermal conductivity and 
packed together in accordance with conventional powder metallurgy 
techniques, preferably forming an article having a porosity of less than 
about 40% or the like. These packed particles are then heated below the 
sintering temperature for them and subjected to a high pressure imparting 
high strain to them for a short duration of time such as an extrusion 
process having an area reduction ratio of about 8 to 1 or greater. This 
processing yields a completely dense composite material with excellent 
combined thermal expansion and conductivity properties and good strength 
capable of being economically produced in various shapes and which is 
readily platable and machinable. The relative ratio of these starting 
materials will provide the predictable composite material with needed 
thermal expansion and conductivity properties. The high strain exerted 
upon the particles causes the formation of a strong bond between them and 
a fully dense material. The fact that any heating of the combined 
particles is below the sintering temperature provides that no real 
diffusion of ferrous, nickelous or other materials from the low 
coefficient of thermal expansion component diffuse into the high 
conductivity material component to greatly reduce the conductivity of said 
high conductivity component and, thus, the composite. Further, this 
process works well with a copper and nickel/iron system which is 
economical, machinable and easy to plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, a composite material 10 in accordance with the 
invention comprises a mixture of metal powders, the first metal powder 12 
of a low thermal expansivity and a second metal powder 14 of a high 
thermal conductivity. The selection of the low-expansivity metal and 
proportion in the final composite 10 depends in part to the expansivity of 
the high conductivity metal used in the composite and also on the desired 
expansivity in the final composite article. Typically, a low expansivity 
metal suitable for use in this invention should have a thermal expansion 
coefficient of less than about 10 ppm/.degree.C. at room temperature. A 
class of alloys which provides the desired expansion coefficients for the 
low-expansivity metal are the iron-nickel and iron-nickel-cobalt 
controlled expansion alloys. The predominant elements of these alloys are 
iron and nickel which coact to provide an austenitic microstructure, in 
which the cobalt may be substituted for some of the nickel. A preferred 
composition for alloys of this type comprises, in weight percent (w/o), 
about 45 to 70% iron, about 20 to 55% nickel, up to 25% cobalt and up to 
about 5% chromium, which is balanced to provide an alloy with desired low 
expansivity. A typical choice of material is INVAR, an alloy containing 
about 36% nickel, balance essentially iron, together with usual amounts of 
impurities and incidental elements. 
The high thermal conductivity material for use in this invention has an 
electrical/thermal conductivity ranging from about 60 to 105% of pure 
copper (thermal and electrical conductivity values for these high 
conductivity metals are directly proportional to one another and 
electrical conductivity values will be used throughout). The suitable high 
conductivity metals which are preferred for use in this invention are 
substantially elemental copper or aluminum (at least 99% pure) and high 
copper/silver alloys. A number of other elemental metals such as silver, 
gold, and platinum also have suitable conductivities but are considered to 
be too expensive for general use. 
Metal powders 12 and 14 in accordance with this invention are produced by 
standard techniques such as electrochemically produced copper powder and 
gas atomized iron-nickel alloyed powder. These powders of a size typically 
between 44 and 425 microns (although other sizes could be used) are 
blended together to get a homogenous mixture which is packed to form a 
powder article having a porosity of about less than 40%. This powder 
article is heated to a temperature below the sintering temperatures of its 
constituents 12 and 14 and subjected to a high pressure, rapid 
consolidation of the particles to provide an essentially 100% dense 
article which has essentially no interdiffusion between the metal 
constituents of the article while still providing a strong article which 
does not need a post compaction sintering operation. As used herein, the 
sintering temperature is the temperature equal to 75% to 80% of the 
absolute melting temperature of the lower melting point component of the 
composite material. It is the high pressure, rapid consolidation with the 
resulting high strain imparted to the powder materials which causes strong 
bonding between the particles and yields a 100% dense article. That is, 
the high strain on the individual particles causes stretching and 
elongation of them creating nascent surfaces which more readily bond to 
one another. Equally important, the high pressure, rapid consolidation has 
shown to alleviate the need for a post bonding sintering step in order to 
obtain a 100% dense and strong article. This fact is important because the 
diffusion associated with sintering greatly reduces the conductivity of 
the resulting composite article. Also, any heating of the powder/composite 
material should be as rapid as possible. 
With reference to a preferred embodiment of this invention (see FIG. 4) the 
final composite material 10 is made from first metal power 12 (20, FIG. 4) 
of a low thermal expansivity such as INVAR and second metal powder 14 (22, 
FIG. 4) of a high thermal conductivity such as essentially pure copper. A 
typical mixture ratio for component 10 is 40 volume percent copper and 60 
volume percent Invar with a powder size for both powders of approximately 
300 microns or less. It is to be understood, however, that other ratios 
can be used in which the copper (high conductivity constituent) can vary 
from 10 volume percent to 60 volume percent with the balance being INVAR 
(low thermal expansivity constituent). These powders are blended in a 
standard V-blender or the like to provide a homogenous mixture (24, FIG. 
4) and then poured (24, FIG. 4) in a container 16 (see FIG. 2). This 
mixture is packed (26, FIG. 4) by tapping, or the like, preferably to 
densify the powders to a density of greater than 60% as compared to 
theoretical density although lesser densities could be used. The powder 
article (container with packed powders) is preferably sealed and evacuated 
(30, FIG. 4) and then heated to a temperature of between 250.degree. F. 
and 500.degree. F. for a period of about 30 minutes to remove any moisture 
and absorbed vaporable impurities yielding a final vacuum level of 
10.sup.-4 Torr or better. The vacuum sealed container 16 is then heated 
again at a temperature well below that of the sintering temperature of the 
powders, typically about 1200.degree. F., for sufficient time to assure 
uniform temperature of the article typically one hour or less and then 
extruded (32, FIG. 4) through a die in a high tonnage direct extrusion 
press with an extrusion reduction ratio of greater than 8 to 1 and 
typically about 16 to 1, with an extrusion ram speed ranging typically 
from 1 to 10 feet per minute. This extrusion process of the powder imparts 
a high true strain on the article (individual particles) in the range of 
1.75 to 3 and thus yields a fully dense composite material with strong 
inter-particle strength so that a post sintering operation would not be 
needed. If desired, the extruded material can be stress relieved at a 
temperature between 800.degree. F. to 1000.degree. F. for from typically 1 
minute to 30 minutes. This stress relieving operation is once again well 
below the sintering temperature of the powders and does not provide for 
diffusion between the two powder constituents. The final extruded 
composite material can be cut or otherwise formed (34, FIG. 4) into the 
desired shape for use as a heat sink or the like in an electronic circuit 
system. 
In another embodiment of this invention, a fully dense composite article 
with selected thermal coefficient expansion and thermal conductivity 
properties forms the core 10' of a final combination product 20 which 
includes an outer coating 22 of a material such as copper as shown in FIG. 
3. One convenient manner to make such a material is to replace the 
standard container 16, as shown in FIG. 2, with a heavy wall container 16' 
(see FIG. 4) made from a material such as copper so that the final 
composite extruded article has the solid outer coating of the material of 
the container. The ratio of the container wall thickness area to the 
overall area of the container with particles can be typically between 2 
and 30 percent. It is to be understood that other methods are available 
such as brazing, force fitting, etc., for providing an outer layer around 
the core 10'. This material combination 20 is especially useful in 
applications (see FIG. 7) where the core portion 10' is joined to a 
semiconductor device by conventional means and outer coating 22 is 
contacted by a package encapsulation material 23 such as plastic. Each of 
the parts of the material combination has a coefficient of thermal 
expansion to better match with the materials they are contacting (i.e., 
semiconductor material and plastic). 
In order to give greater appreciation of the advantages of the invention, 
the following examples are given: 
EXAMPLE I 
Elemental copper powder (-100 mesh) was blended with INVAR powder (-100 
mesh) in a V-blender in a ratio of 40 volume percent copper and 60 volume 
percent INVAR. The blended powder mixture was poured into a two inch 
diameter copper container of 1/16 inch wall thickness (extrusion can) 
filling the container. The blended powder was packed by tapping to a 
density of approximately sixty percent. The container was then sealed and 
evacuated and heated to about 300.degree. F. to hot vacuum degas and then 
the container was totally vacuum sealed with a vacuum level of 
1.times.10.sup.-5 Torr. Next, the sealed container was heated to 
1200.degree. F. for one hour, and then immediately extruded through a 1/2 
inch diameter circular die in a standard 300 ton direct extrusion press 
with an extrusion ram speed of 45 inches per minute to yield a fully dense 
circular rod. If desired, the rod can have any surface layer removed by 
appropriate etching or abrading. 
The composite material had the following properties: 
______________________________________ 
1. Density 8.4 gm/cc 
2. TCE (ppm/.degree.C.) 7.3 
3. Thermal Conductivity (watts/M.K.) 
126.0 
4. Electrical Conductivity (IACS) 
35% 
______________________________________ 
EXAMPLE II 
Copper powder was blended with INVAR powder in a V-blender in the same 
ratio as in Example I and processed identically except a 4 inch diameter 
container was used and the extrusion was done with a 1400 ton press 
through an one inch diameter die. 
The composite material had the following properties: 
______________________________________ 
1. Density 8.4 gm/cc 
2. TCE (ppm/C.) 6.8 
3. Thermal Conductivity (watts/M.K.) 
120.0 
4. Electrical Conductivity (IACS) 
35% 
______________________________________ 
EXAMPLE III 
This example was carried out identical to Example I above except that after 
forming the circular rod, the rod was further heated to 1650.degree. F. 
for 20 minutes (sintering operation). After this heating the electrical 
conductivity was 15% (IACS). 
EXAMPLE IV 
This example was carried out identical to Example I above except that after 
forming the rod, the rod was further heated to 950.degree. F. for 20 
minutes (stress relieving anneal below sintering temperature). After this 
heating the electrical conductivity was 35% (IACS). 
Thus, comparing the results of the electrical conductivity, of Examples III 
and IV clearly shows the problem that takes place if there is diffusion of 
the two components into one another which was practiced in the prior art 
to obtain the desired strength and density of the composite. Hot roll 
compaction has also not proven to provide for the fully dense, strong 
article due to the fact that the true strain values in this processing 
operation are not significantly high (less than 1.5). The present 
invention with the rapid, high true strain, high pressure consolidation as 
by extrusion provides for the novel composite material with greatly 
enhanced properties. Further, since there is no real diffusion between the 
individual particles of the composite material, it is possible to 
preferentially etch one of the constituents thereby leaving a porous 
surface layer. This porous surface layer can be very beneficial in 
improving adhesion to other materials as part of an encapsulation process 
with a plastic or the like when the composite is used in certain 
electronic device applications. Also, the use of the outer coating 
material 22 of FIG. 3 would be beneficial in these applications for 
enhanced thermal expansion matching to plastic encapsulants 23. 
The composite material 10 provided by the process of the invention is thus 
adapted to be used as a heat-transferring composite material for an 
electronic circuit to withdraw and dissipate heat from the circuit. For 
example, the composite material 10b is provided with a thin insulating 
coating 28 provided on one side of the composite and has an electronic 
circuit 30 diagrammatically indicated by circuit paths 32 and circuit 
components 34 in FIG. 6 mounted on the insulating coating. The substrate 
10b is adapted to withdraw heat from the circuit through the thin 
insulating coating 28 to distribute that heat throughout the substrate as 
indicated by the arrow 25 and transfer that heat to a support 36 or the 
like as indicated by the arrow 27. The composite material is also adapted 
for use in lead frame, pin grid array devices, etc. 
The novel article and method of the present invention provides for a 
composite material with high strength and improved combination of 
coefficient of expansion and thermal conductivity properties while still 
being economical to produce and easily machinable and able to be plated. 
While the invention has been described in combination with the specific 
embodiments thereof, it is evident that many alternatives, modifications, 
and variations will be apparent to those skilled in the art in light of 
the foregoing description such as rapid compaction by forging or by 
explosive compaction to achieve the rapid high pressure, high true strain 
consolidation yielding a fully dense material.