Electronics mounting plate with heat exchanger and method for manufacturing same

A combined electronics mounting plate and heat exchanger and method of manufacturing same. The present invention provides an electronics mounting plate formed from a metal matrix composite and a metal heat exchanger in order to combine the low coefficient of thermal expansion and high heat dissipation characteristics of a metal matrix composite, with the cost effectiveness and relatively easy manufacturability of metal. In the preferred embodiment, the metal matrix composite is provided in the form of aluminum and silicon carbide, and the metal heat exchanger is provided in the form of aluminum fins. The method of manufacturing the present invention includes the steps of cleaning the metal matrix composite mounting plate and aluminum heat exchanger in a nitric acid fluoride salt before brazing the elements together. In one embodiment of the present invention the metal matrix composite mounting plate includes pin fins extending toward the aluminum heat exchanger fins, and an additional aluminum braze sheet is provided between the mounting plate and heat exchanger fins to enhance the brazement therebetween and improve the thermal conductivity of the resulting mounting plate and heat exchanger.

TECHNICAL FIELD 
The present invention generally relates to the substrates for mounting 
electronic components thereon, and more particularly relates to 
electronics mounting plates having heat exchanger capabilities formed 
integrally therewith. 
BACKGROUND 
Modern electronics are often comprised of a plurality of individual 
components mounted onto a substrate and electrically interconnected for 
operation of a controlled device. Thousands of electronically controlled 
applications are currently in existence, and although the present 
invention is primarily designed for use on aircraft, it is to be 
understood that the principles, devices and methods disclosed herein could 
be employed with other electronic applications with equal success. 
The electronic components involved, such as power supplies, IGBTs, 
transistors, and other power switching components, are generally referred 
to as power devices. Such power devices consume large amounts of energy 
and consequently generate large amounts of heat. If the heat is not 
properly dissipated, the efficacy of the electronic components will 
necessarily suffer and the controlled device can potentially fail. Since 
one application of the present invention is for use onboard modern 
aircraft, failure of the electronic controls of the aircraft could prove 
disastrous. Proper heat dissipation is therefore of an utmost concern. 
The substrates used are normally selected to maximize heat dissipation for 
the reasons indicated above, while minimizing cost. Metals, such as 
aluminum, can be used because they can be relatively easily formed into 
fins and other intricate heat exchanging structures at relatively low 
cost, and are particularly good conductors of heat. However, metals such 
as aluminum have relatively poor coefficients of thermal expansion, and 
therefore tend not to maintain their shape at elevated temperatures. On 
the other hand, the semiconductor devices mounted to the substrate 
typically have very low coefficients of thermal expansion, and thus when 
subjected to high operaturing temperatures, relative movement betwen the 
semiconductor devices and the metal substrate causes the semiconductor 
devices to become disconnected or otherwise damaged. 
In recent years, therefore, new materials have been employed to provide 
high levels of thermal conductivity, while also providing low coefficients 
of thermal expansion to thereby maintain the structural integrity of the 
unit even at high operating temperatures. This area of the art has greatly 
benefitted from the advent of metal matrix composites (MMCs). MMCs are a 
mixture of metal and ceramic fibers which provide a material with high 
thermal conductivity at a low coefficient of thermal expansion. For 
example, a known MMC includes a porous matrix of silicon carbide which is 
then impregnated with aluminum. Semiconductor devices can then be mounted 
on the MMC, and since the MMC and the semiconductor devices have nearly 
identical coefficients of thermal expansion, little or no relative 
movement between the semiconductor devices and MMC occurs, even at the 
high operating temperatures of many applications. 
While such materials do provide relatively high thermal conductivity and 
relatively low coefficients of thermal expansions, they come at a 
relatively high monetary expense. Moreover, if it is often difficult and 
therefore additionally expensive to form intricate heat dissipating 
shapes, such as lanced, offset fins and the like, from the MMC. This 
results from the fact tat MMCs are typically cast or hot iso-statically 
processed into form, and as such typically have a high modulus of 
elasticity, making them unsuitable for cold-working into intricate heat 
exchanging shapes. 
The relevant industry has therefore attempted to merge the heat dissipation 
and thermal expansion benefits of MMC technology, with the cost 
effectiveness, structural integrity and malleability of conventional 
metals to form a combined electronics mounting plate and heat exchanger. 
For example, U.S. Pat. No. 5,533,257, discloses a method for forming a 
heat dissipation apparatus formed from an MMC mounting plate and aluminum 
heat exchanger fins. While such an apparatus touches upon the benefits of 
a metal/MMC combination, it employs an epoxy to bond the aluminum fins to 
the MMC mounting plate. The thermal conductivity between the mounting 
plate and heat exchanger fins therefore suffers as a result of the 
relatively low thermal conductivity of the epoxy. Moreover, such a bonding 
epoxy severely curtails the shape of the heat exchanger surface which can 
be bonded to the MMC mounting plate due to the relatively high amount of 
surface area required from both the heat exchanger element and MMC 
mounting plate in order to provide a sufficiently strong bond. The harsh 
vibrational environment of an aircraft requires greater structural 
integrity than the epoxy can sometimes provide. 
Other attempts have been made to provide more intricate heat exchanger 
shapes with enhanced structural integrity. For example, U.S. Pat. No. 
5,402,004 discloses a MMC mounting plate with which an aluminum sponge, 
similar to "steel wool", is in thermal communication. In one embodiment, 
the MMC plate has an aluminum layer formed thereon by ion vapor 
deposition. A layer of aluminum/silicon alloy is then disposed on the 
aluminum layer, and the sponge is then brazed to the aluminum/silicon 
alloy. While such a system may adequately dissipate the heat generated by 
its electronic components, a fluid coolant is required to pass through the 
sponge given the relatively small cross-sectional size of each strand of 
the "steel wool". Considering the huge heat dissipation demands of 
aircraft electronics, such a system cannot be sized to operate effectively 
under the present application. Moreover, its complex, multilayered 
attachment method substantially negates the cost benefit of using aluminum 
as a heat exchanger in the first place. 
SUMMARY 
It is therefore a primary aim of the present invention to provide a 
combined electronics mounting plate and heat exchanger which provides high 
heat dissipation with low thermal expansion in a cost-effective package. 
It is an objective of the present invention to provide a combined 
electronics mounting plate and heat exchanger which provides the 
aforementioned heat dissipation, thermal expansion and cost benefits while 
providing high structural integrity adapted to withstand the harsh 
environmental conditions of modern aircraft. 
It is another objective of the present invention to provide a method for 
attaching the mounting plate to the heat exchanger which maintains the 
heat dissipation, thermal expansion, structural integrity and cost 
benefits indicated above, while enabling a wide range of actual heat 
exchanger shapes and elements to be reliably secured together and thereby 
enhance the manufacturability of the device. 
In accordance with these objectives, it is a feature of the present 
invention to provide a combination electronics mounting plate and heat 
exchanger in the form of a metal matrix composite mounting plate and an 
aluminum heat exchanger. The mounting plate is provided with a first 
planar surface adapted to have electronic components mounted thereon, and 
a second, opposed, planar surface adapted to be joined to the aluminum 
heat exchanger. The aluminum heat exchanger, in one embodiment, is 
provided with a plurality of fins which are brazed to the second planar 
surface of the mounting plate. 
It is another feature of the present invention to provide a combination 
electronics mounting plate and heat exchanger wherein the mounting plate 
is formed from an aluminum and silicon carbide metal matrix composite, and 
the aluminum heat exchanger fins are brazed to the metal matrix composite 
mounting plate. In one embodiment of the present invention, the metal 
matrix composite includes a plurality of pins extending from the second 
planar side in order to increase the surface area available for thermal 
conduction and heat dissipatoin. 
It is another feature of the present invention to provide a combination 
electronics mounting plate and heat exchanger including a metal matrix 
composite mounting plate having a plurality of pin fins mounted integrally 
therewith and a plurality of aluminum heat exchanger fins brazed to the 
plurality of pin fins. Alternatively, the pin fins could be brazed to one 
side of an aluminum plate, and a plurality of aluminum fins could be 
brazed to the opposite side of the aluminum plate. 
It is still another feature of the present invention to provide a method 
for manufacturing a combination electronics mounting plate and heat 
exchanger wherein the mounting plate is formed from an aluminum and 
silicon carbide metal matrix composite and the heat exchanger is formed 
from aluminum. The method includes the steps of cleaning the metal matrix 
composite mounting plate and aluminum heat exchanger in a nitric acid 
fluoride salt bath before brazing the elements together. 
It is a still further feature of the present invention to provide a method 
for manufacturing a combination electronics mounting plate and heat 
exchanger wherein the mounting plate and heat exchanger are cleaned in 
nitric acid salt bath less than twenty-four (24) hours prior to the 
brazing step. In an alternative embodiment, the method further includes 
the step of brazing the mounting plate to the heat exchanger when the 
elements are within an approximate temperature range of between 1070-1080 
F..degree.. Moreover, the method can further include the step of using an 
additional braze foil or paste between the metal matrix composite mounting 
plate and aluminum heat exchanger to further enhance the structural 
integrity and thermal conductivity of the resulting mounting plate and 
heat exchanger.

While the present invention is described below with reference to certain 
preferred embodiments, it is to be understood that such embodiments are 
chosen for the express purpose of disclosing the best mode of the present 
invention, and should in no way be construed to limit the scope of the 
invention to such specifically disclosed embodiments. Rather, the present 
invention is intended to cover all embodiments of the present invention as 
specifically described herein, reasonably taught thereby, and falling 
within the scope of the claims appended hereto. 
DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, the present invention is generally designated as 
combined electronics mounting plate and heat exchanger 20. As stated 
above, the present invention does not endeavor to claim the broad concept 
of combining a metal matrix composite mounting plate for electronics with 
an aluminum heat exchanger. The prior art shows that such devices exist. 
However, what the prior art lacks, and what the present invention 
provides, is a combination metal matrix composite mounting plate and heat 
exchanger wherein the heat exchanger includes a plurality of individual 
fins brazed to the metal matrix composite. Moreover, the present invention 
provides a method for manufacturing a combination electronics mounting 
plate and heat exchanger which enables a variety of heat exchanger shapes 
and structures to be reliably secured to the mounting plate and which are 
sized to dissipate the large amounts of heat generated by the type of 
aircraft electronics to which the present invention is directed. 
As shown in FIG. 1, combination mounting plate and heat exchanger 20 
includes a planar mounting plate 22 joined to heat exchanger 24 in the 
form of a plurality of individual fins 26. In the embodiment shown in 
FIGS. 1 and 2, mounting plate 22 includes first planar side 28, second 
opposed planar side 30, and four orthogonally disposed sides 32. It is to 
be understood that in alternative embodiments, the actual shape of plate 
22 could vary greatly, and that the import of plate 22 is that it is 
formed form a metal matrix composite having one side adapted to have 
electronic components mounted thereon, and another side adapted to be 
brazed to a heat exchanger. In the preferred embodiment, first planar side 
28 is adapted to have electronic components 34 mounted thereon. Any type 
of electronic component is envisioned as being mountable to plate 22, 
including transistors, processing chips, memory chips, IGBTs, power 
modules, and the like. 
Similarly, heat exchanger 24 is adapted to formed in any number of 
different shapes and configurations for augmenting the surface area over 
which the heat generated by components 34 is communicated and dissipated. 
With that being said as a preface, heat exchanger 24 is shown as being 
comprised of a plurality of fins 26 extending away from second planar 
surface 30 of mounting plate 22. It is to be understood that a variety of 
other shapes including offset or lanced fins could be used with equal 
efficacy assuming the fins are sized adequately to dissipate the sizable 
heat output of components 34. Moreover, the present invention could also 
include the provision of a liquid coolant circuit which would pass between 
fins 26 for added cooling capacity. 
Turning now with specific reference to FIG. 2, the present invention is 
shown in cross-section to more effectively depict the structure and 
formation of the combined electronics mounting plate and heat exchanger 
20. As shown therein, second planar surface 30 of mounting plate 22 is 
brazed to ends 40 of heat exchanger fins 26. In order to effect this braze 
a thin metal skin 42 is formed on second planar surface 30, which, in the 
preferred embodiment, is formed from aluminum. Fins 26 are then brazed to 
skin 42 according to the method of the present invention, which method 
will be described in greater detail herein. In the preferred embodiment 
skin 42 has a thickness within the range of 0.001 to 0.006 inches and an 
average thickness of 0.004 inches. Also in the preferred embodiment, the 
metal matrix composite is composed of 6061 or 356 aluminum matrix with 68 
percent SiC particulate reinforcement. 
For the sake of clarity, it is important to understand at this point of the 
disclosure that the present invention as now described diverges from, and 
improves upon, the prior art by brazing aluminum fins 26 directly to an 
aluminum/silicon carbide metal matrix composite mounting plate 22. By 
doing so, the present invention provides a mounting plate 22 with a low 
coefficient of thermal expansion which will allow the plate 22 to maintain 
its shape and not allow components 34 to migrate from their proper 
position and alignment even when subjected to high operating temperatures. 
Moreover, the present invention provides high heat dissipation 
capabilities due to the large amount of surface area provided by fins 26, 
high structural integrity due to the use of brazing for the attachment 
method, and a multitude of heat exchanger shapes given the relatively easy 
manufacturability and malleable nature of aluminum. 
In an alternative embodiment of the present invention, shown in FIG. 3, 
mounting plate 22 is formed with a plurality of pin fins 50, which can be 
economically manufactured on an MMC plate and formed integrally with 
second planar side 30. Pin fins 22 extend toward aluminum fins 50 to 
provide an even more structurally sound brazement with enhanced heat 
conductivity and dissipation. Alternatively, pin fins 22 could extend 
toward another aluminum or MMC plate to which additional fins could 
brazed. 
In an alternative embodiment, an additional brazing sheet is provided 
between second planar surface 30 and aluminum fins 26. The brazing sheet 
can be provided in the form of a foil or paste. The additional brazing 
material is provided to enhance the thermal conductivity and communication 
between mounting plate 22 and heat exchanger 24, while further fortifying 
the structural integrity of the brazed joint therebetween. 
The method by which the present invention is formed will now be described 
in detail. As stated, supra, mounting plate 22 is formed from aluminum and 
silicon carbide, while heat exchanger 24 is formed from aluminum. In the 
most basic form of the present invention, fins 26 are brazed to second 
planar surface 30, meaning the components are heated to a temperature 
which allows a brazing agent to melt and join the fins 26 to the second 
planar surface 30. In the preferred embodiment, additional steps and more 
specific parameters are followed to ensure proper formation of the 
brazement, while in alternative embodiments other methods of joining the 
elements together can be used effectively. 
With reference to the preferred embodiment, the aforementioned additional 
steps include the step of chemically cleaning the mounting plate 22 and 
heat exchanger 24 prior to brazing. The specific parameters include the 
requirement that the cleaning step is performed in a nitric acid salt bath 
to thoroughly remove any particulates or impurities which would otherwise 
detrimentally affect the efficacy of the brazement. In the most preferred 
embodiment of the present invention, the elements are cleaned in the 
nitric acid salt bath less than twenty-four (24) hours before brazing to 
most effectively assure the contaminants are not only removed, but removed 
at the time of brazing. If manufacturing criterion dictate otherwise, 
however, the cleaning window can be extended for up to one hundred and 
twenty (120) hours if the elements are stored in sealed containers, such 
as plastic bags having reclosable strips, with a desiccant immediately 
after cleaning to assure condensate does not form on the elements. 
The method of the present invention is further optimized if the brazing is 
performed when mounting plate 22 and heat exchanger 24 are within the 
temperature range of 1070-1080 F..degree.. In order to assure this 
temperature range, it may therefore be necessary to equip the furnace or 
kiln within which the elements are brazed with a kill or disable switch to 
prevent the temperature within the furnace from exceeding the 
aforementiond desired temperature range. If temperatures in excess of the 
stated range are used, excessive brazing material will be formed which 
will necessarily decrease the thermal conductivity between mounting plate 
22 and heat exchanger 24. 
In order to most effectively remove oxygen prior to brazing and immediately 
after brazing, the method of the present invention can further include the 
steps of adding nitrogen to the furnace or kiln when it is within a 
temperature range of approximately 500-600 F..degree. prior to brazing, 
and again adding nitrogen to the furnace or kiln when it is within a 
temperature range of approximately 1000-1050 F..degree. immediately after 
brazing. Furthermore, the method can include the step of adding magnesium 
powder or chips to the furnace to remove any remaining oxygen. It is 
desirable to remove all oxygen so as to prevent oxidation of the surfaces 
to be brazed together, which oxidation would detrimentally affect the 
integrity of the resulting brazement. 
From the foregoing, it can be appreciated by one of ordinary skill in the 
art that the present invention brings to the art a new, useful, and 
non-obvious combination electronics mounting plate and heat exchanger and 
method for manufacturing such a device. By brazing an aluminum heat 
exchanger to an aluminum and silicon carbide mounting plate, the thermal 
expansion of the mounting plate is decreased, the heat dissipation 
capability of the heat exchanger is increased, the structural integrity of 
the combination is increased, and the cost of the combination is 
decreased. The present invention lies not only within the apparatus 
disclosed herein, but the method by which the apparatus is manufacuted. 
Prior art devices fully fail to disclose, suggest, teach, or provide a 
compelling motivation to create such an invention as described above and 
claimed below. 
While the invention has been described in terms of its preferred 
embodiment, it should be understood that numerous modifications may be 
made thereto without departing from the spirit and scope of the present 
invention. It is intended that the present invention should include not 
only the specific embodiments disclosed, supra, but also any embodiments 
equivalent thereto, reasonably taught thereby, or falling within the scope 
of the appended claims.