U.S. patent application number 10/263172 was filed with the patent office on 2004-04-08 for high performance thermal stack for electrical components.
Invention is credited to Eesley, Gary L., Elmoursi, Alaa A., Myers, Bruce A., Patel, Nilesh B., Smith, John R., Wang, Xiao-Gang.
Application Number | 20040065432 10/263172 |
Document ID | / |
Family ID | 32041952 |
Filed Date | 2004-04-08 |
United States Patent
Application |
20040065432 |
Kind Code |
A1 |
Smith, John R. ; et
al. |
April 8, 2004 |
High performance thermal stack for electrical components
Abstract
A thermal stack laminate and a process for producing the same
are disclosed. The thermal stack laminate includes a baseplate
formed from a heat sink material that has on a first surface a very
thin thermally sprayed alumina layer to serve as a dielectric and
attached to the alumina is a kinetic spray applied solderable
layer. An electrical component is attached to the thermal stack
laminate by solder. The thermal stack laminate optionally includes
a kinetic spray applied first and/or second metal matrix composite
layer between the baseplate and the alumina layer and between the
alumina layer and the solderable material. In addition, one other
optional layer comprises a first layer of the solderable material
applied via a thermal spray process followed by the remainder of
the solderable material applied by a kinetic spray process.
Inventors: |
Smith, John R.; (Birmingham,
MI) ; Elmoursi, Alaa A.; (Troy, MI) ; Wang,
Xiao-Gang; (Troy, MI) ; Eesley, Gary L.; (Lake
Orion, MI) ; Patel, Nilesh B.; (Macomb Township,
MI) ; Myers, Bruce A.; (Kokomo, IN) |
Correspondence
Address: |
SCOTT A. MCBAIN
DELPHI TECHNOLOGIES, INC.
P.O. Box 5052
Mail Code: 480-414-420
Troy
MI
48007-5052
US
|
Family ID: |
32041952 |
Appl. No.: |
10/263172 |
Filed: |
October 2, 2002 |
Current U.S.
Class: |
165/80.2 ;
257/E23.102 |
Current CPC
Class: |
H01L 23/367 20130101;
C23C 4/02 20130101; H01L 2924/0002 20130101; B05B 7/1626 20130101;
H01L 2924/0002 20130101; B05B 7/1486 20130101; C23C 4/11 20160101;
C23C 24/04 20130101; C23C 28/04 20130101; H01L 21/4871 20130101;
H01L 2924/00 20130101 |
Class at
Publication: |
165/080.2 |
International
Class: |
F28F 007/00 |
Claims
1. A method for formation of a thermal stack laminate for coupling
to an electronic component comprising the steps of: a) providing a
heat sink material having a first surface; b) applying a layer of
alumina onto the first surface of the heat sink material by a
thermal spray process; and c) applying a layer of a solderable
material onto the layer of alumina by a kinetic spray process,
thereby forming a thermal stack laminate.
2. The method of claim 1, wherein step a) comprises providing a
heat sink material comprising copper or aluminum.
3. The method of claim 1, wherein step b) comprises applying a
layer of alumina having a thickness of from 50.0 to 210.0
microns.
4. The method of claim 1, wherein step b) comprises applying a
layer of alumina having a thickness of from 75.0 to 130.0
microns.
5. The method of claim 1, wherein step b) comprises applying the
layer of alumina by one of a plasma thermal spray process or a
high-velocity oxyfuel thermal spray process.
6. The method of claim 1, wherein step c) comprises providing one
of copper or a copper alloy as the solderable material and applying
the solderable material onto the layer of alumina by a kinetic
spray process.
7. The method of claim 1, wherein step c) comprises applying a
layer of a solderable material having a thickness of from 250.0
microns to 1.0 centimeters onto the layer of alumina by a kinetic
spray process.
8. The method of claim 1, wherein step c) further comprises
applying a first layer of the solderable material by a thermal
spray process onto the layer of alumina and then applying the
remainder of the solderable material by a kinetic spray
process.
9. The method of claim 8, comprising applying the first layer of
the solderable material to a thickness of from 50.0 microns to 130
microns and the remainder of the solderable material to a thickness
of from 200.0 microns to 1.0 centimeters.
10. The method of claim 1, further comprising the step of applying
a metal matrix composite layer comprising a mixture of at least one
metal, or at least one alloy, or a combination of at least one
metal and at least one alloy with at least one ceramic by a kinetic
spray process onto the first surface of the heat sink material and
then applying the layer of alumina onto the metal matrix composite
layer by a thermal spray process.
11. The method of claim 10, comprising applying a metal matrix
composite layer comprising a mixture of aluminum and silicon
carbide.
12. The method of claim 10, comprising applying a metal matrix
composite layer having a thickness of from 500.0 microns to 1.1
centimeters.
13. The method of claim 10, comprising applying a metal matrix
composite layer having a thickness of from 500.0 microns to 5.0
millimeters.
14. The method of claim 10, comprising applying a metal matrix
composite layer having a thickness of from 500.0 microns to 2.1
millimeters.
15. The method of claim 10, further comprising the step of applying
a metal matrix composite layer comprising a mixture of at least one
metal, or at least one alloy, or a combination of at least one
metal and at least one alloy with at least one ceramic by a kinetic
spray process over the layer of alumina prior to step c).
16. The method of claim 1, further comprising the step of applying
a metal matrix composite layer comprising a mixture of at least one
metal, or at least one alloy, or a combination of at least one
metal and at least one alloy with at least one ceramic by a kinetic
spray process over the layer of alumina prior to step c).
17. The method of claim 16, comprising applying a metal matrix
composite layer comprising a mixture of aluminum and silicon
carbide.
18. The method of claim 16, comprising applying a metal matrix
composite layer having a thickness of from 500.0 microns to 1.1
centimeters.
19. The method of claim 16, comprising applying a metal matrix
composite layer having a thickness of from 500.0 microns to 5.0
millimeters.
20. The method of claim 16, comprising applying a metal matrix
composite layer having a thickness of from 500.0 microns to 2.1
millimeters.
21. The method of claim 1, comprising the further step after step
c) of soldering an electrical chip to the solderable material
layer.
22. A thermal stack laminate for attachment to an electrical
component, said thermal stack laminate comprising: a baseplate of a
heat sink material having a first surface; attached to said first
surface a thermal spray applied layer of alumina having a thickness
of from 50.0 to 210.0 microns; and attached to said layer of
alumina a kinetic spray applied layer of a solderable material.
23. A thermal stack laminate as recited in claim 22, wherein said
baseplate of a heat sink material comprises copper or aluminum.
24. A thermal stack laminate as recited in claim 22, wherein said
layer of alumina has a thickness of from 75.0 to 130.0 microns.
25. A thermal stack laminate as recited in claim 22, wherein said
solderable material comprises one of copper, aluminum, or a copper
alloy.
26. A thermal stack laminate as recited in claim 22, wherein said
layer of a solderable material has a thickness of from 250.0
microns to 1.0 centimeters.
27. A thermal stack laminate a recited in claim 22, further
including an electrical chip soldered to said layer of solderable
material.
28. A thermal stack laminate for attachment to an electrical
component, said thermal stack laminate comprising: a baseplate of a
heat sink material having a first surface; attached to said first
surface a kinetic spray applied layer of a metal matrix composite
layer comprising a mixture of at least one metal, or at least one
alloy, or a combination of at least one metal and at least one
alloy with at least one ceramic; attached to said layer of a metal
matrix composite a thermal spray applied layer of alumina having a
thickness of from 50.0 to 210.0 microns; and attached to said layer
of alumina a kinetic spray applied layer of a solderable
material.
29. The thermal stack laminate of claim 28, wherein said layer of a
metal matrix composite comprises a mixture of aluminum and silicon
carbide.
30. The thermal stack laminate of claim 28, wherein said layer of a
metal matrix composite has a thickness of from 500.0 microns to 1.1
centimeters.
31. The thermal stack laminate of claim 28, wherein said layer of a
metal matrix composite has a thickness of from 500.0 microns to 5.0
millimeters.
32. The thermal stack laminate of claim 28, wherein said layer of a
metal matrix composite has a thickness of from 500.0 microns to 2.1
millimeters.
33. The thermal stack laminate of claim 28, further comprising a
second layer of a metal matrix composite applied by a kinetic spray
process, said second layer of a metal matrix composite located
between said layer of alumina and said layer of a solderable
material.
34. The thermal stack laminate of claim 28, further comprising an
electrical chip soldered to said layer of a solderable
material.
35. A thermal stack laminate for attachment to an electrical
component, said thermal stack laminate comprising: a baseplate of a
heat sink material having a first surface; attached to said first
surface a thermal spray applied layer of alumina having a thickness
of from 50.0 to 210.0 microns; attached to said layer of alumina a
kinetic spray applied layer of a metal matrix composite layer
comprising a mixture of at least one metal, or at least one alloy,
or a combination of at least one metal and at least one alloy with
at least one ceramic; and attached to said layer of a metal matrix
composite a kinetic spray applied layer of a solderable
material.
36. The thermal stack laminate of claim 35, wherein said layer of a
metal matrix composite comprises a mixture of aluminum and silicon
carbide.
37. The thermal stack laminate of claim 35, wherein said layer of a
metal matrix composite has a thickness of from 500.0 microns to 1.1
centimeters.
38. The thermal stack laminate of claim 35, wherein said layer of a
metal matrix composite has a thickness of from 500.0 microns to 5.0
millimeters.
39. The thermal stack laminate of claim 35, wherein said layer of a
metal matrix composite has a thickness of from 500.0 microns to 2.1
millimeters.
40. The thermal stack laminate of claim 35, further comprising an
electrical chip soldered to said layer of a solderable
material.
41. A thermal stack laminate for attachment to an electrical
component, said thermal stack laminate comprising: a baseplate of a
heat sink material having a first surface; attached to said first
surface a thermal spray applied layer of alumina having a thickness
of from 50.0 to 210.0 microns; and attached to said layer of
alumina a thermal spray applied layer of a solderable material and
a kinetic spray applied layer of a solderable material applied to
said thermal spray applied layer of a solderable material.
42. A thermal stack laminate as recited in claim 41, wherein said
thermal spray applied layer of a solderable material has a
thickness of from 50.0 to 130.0 microns and said kinetic spray
applied layer of a solderable material has a thickness of from
200.0 microns to 1.0 centimeters.
Description
INCORPORATION BY REFERENCE
[0001] U.S. Pat. No. 6,139,913, "Kinetic Spray Coating Method and
Apparatus," and U.S. Pat. No. 6,283,386 "Kinetic Spray Coating
Apparatus" are incorporated by reference herein.
TECHNICAL FIELD
[0002] The present invention is related to thermal management of
high performance electronic components and, more particularly, to a
thermal stack and methods of forming the same for use in the
thermal management of high performance electronics.
BACKGROUND OF THE INVENTION
[0003] During the past 20 years the utilization of computer chips
has increased dramatically. With this progress has come a
subsequent decrease in the size of the chips and an increase in the
density of electrical circuits on a given chip. These high-density
chips may have power densities as high as 10 W/cm.sup.2. With the
increase in power density of modern chips has come a concomitant
increase in the need to thermally regulate the chips. These chips
and other such high-density electrical components generate a
tremendous amount of heat which must be dissipated to prevent
damage to the chip.
[0004] Initially, the heat was dissipated by securing the chip to a
heat sink material having high thermal conductivity. Examples of
such materials include copper, aluminum, and diamond. One
difficulty associated with such solutions is that typically the
heat sink material has a much higher thermal expansion coefficient
than the silicon chip. For example, the thermal expansion
coefficient of silicon is 4 ppm .degree. C..sup.-1 while the
expansion coefficient of aluminum is 24 ppm .degree. C.sup.-1.
Thus, during thermal cycling of the system the aluminum will expand
to a much greater extent than the silicon chip. This leads to
debonding of the chip from the heat sink material.
[0005] In an effort to address this difficulty the industry has
developed metal matrix composites formed from ceramic preforms that
have been infiltrated with molten metal under high temperature and
often high pressure to create a metal matrix composite. The metal
matrix composite, which is designed to have a thermal expansion
coefficient somewhere between that of the chip and the heat sink
material, is placed as a layer between the heat sink material and
the silicon chip to relieve the stress between them caused by
thermal cycling. The difficulty associated with this solution is
that the metal matrix composites made in that manner are extremely
costly to produce, can only be done with certain ceramic materials,
and require inclusion of various compounds such as silicon in the
infiltrating metal to prevent adverse reactions between the metal
and the ceramic. Because the infiltration temperatures are
generally in the range of 800.degree. C. or higher reactions
between the metal and the ceramic can occur that lead to
degradation in the thermal conductivity of the final metal matrix
composite. The goal of these metal matrix composites is to produce
a composite material that maintains the high thermal conductivity
of the metallic element while adding the low thermal expansion
coefficient of the ceramic to reduce differential expansion and
contraction of the heat sink material relative to the silicon
chip.
[0006] One other necessary layer between the heat sink material and
the silicon chip is a dielectric layer. This layer serves to
electrically isolate the chip from the heat sink material, which is
also typically electrically conductive. A typical dielectric layer
is formed from sintered alumina (Al.sub.2O.sub.3). The sintered
alumina is typically provided as a plate having a thickness of from
25 to 40 thousands of an inch (mils), this thickness is necessary
for structural stability of the plate during handling. The sintered
alumina plate is attached to a surface of the heat sink material
using a thermal grease or other bonding agent. One problem with the
dielectric layer, is that it and the thermal grease are typically
the most thermally resistant layers in the thermal stack. As
discussed above, the thickness of the sintered alumina can not be
reduced because then the plate is susceptible to cracking during
handling. Alternatively, more expensive dielectric layers having
higher thermal conductivity can be formed from aluminum nitride
(AlN) or beryllium oxide (BeO).
[0007] In a typical construction of a silicon chip with an attached
heat sink material the first step is formation of the thermal stack
laminate. Then the laminate is attached to the silicon chip,
usually by soldering. The first laminate layer is generally a
baseplate of a heat sink material formed from a pure metal having a
high thermal conductivity such as aluminum or copper. The baseplate
will often be placed in the flow of a water stream or an air stream
in its final environment. The second layer is typically a thermal
grease or a first metal matrix composite layer secured to the
baseplate. The third layer is some form of a dielectric material
such as alumina, aluminum nitride, or beryllium oxide. A second
metal matrix composite layer may be placed over the dielectric
layer. Finally, another layer formed from copper or some other
solderable material is attached to the dielectric layer or the
second metal matrix composite layer if used. The metalization of
the dielectric layer by the layer of solderable material is usually
accomplished by a screen printing process or a direct bond. The
layer of solderable material is typically quite thin. Once this
thermal stack laminate is formed the silicon chip can be soldered
to the layer of solderable material.
[0008] It would be advantageous to provide a dielectric layer
having a reduced thermal resistance, thereby lowering the thermal
resistance between the chip and the heat sink, that was less costly
than aluminum nitride or beryllium oxide and to provide a method
for forming it that could be utilized in current production
methods.
SUMMARY OF THE INVENTION
[0009] In one embodiment, the present invention is a method for
formation of a thermal stack laminate for coupling to an electronic
component comprising the steps of: providing a heat sink material
having a first surface; applying a layer of alumina onto the first
surface of the heat sink material by a thermal spray process; and
applying a layer of a solderable material onto the layer of alumina
by a kinetic spray process, thereby forming a thermal stack.
[0010] In another embodiment, the present invention is a thermal
stack laminate for attachment to an electrical component, the
thermal stack laminate comprising: a baseplate of a heat sink
material having a first surface; attached to the first surface a
thermal spray applied layer of alumina having a thickness of from
50.0 to 210.0 microns; and attached to the layer of alumina a
kinetic spray applied layer of a solderable material.
[0011] In another embodiment, the present invention is a thermal
stack laminate for attachment to an electrical component, the
thermal stack laminate comprising: a baseplate of a heat sink
material having a first surface; attached to the first surface a
kinetic spray applied layer of a metal matrix composite comprising
a mixture of a metal or an alloy with one or more ceramics;
attached to the layer of a metal matrix composite a thermal spray
applied layer of alumina having a thickness of from 50.0 to 210.0
microns; and attached to the layer of alumina a kinetic spray
applied layer of a solderable material.
[0012] In another embodiment, the present invention is a thermal
stack laminate for attachment to an electrical component, the
thermal stack laminate comprising: a baseplate of a heat sink
material having a first surface; attached to the first surface a
thermal spray applied layer of alumina having a thickness of from
50.0 to 210.0 microns; attached to the layer of alumina a kinetic
spray applied layer of a metal matrix composite comprising a
mixture of a metal or an alloy with one or more ceramics; and
attached to the layer of a metal matrix composite a kinetic spray
applied layer of a solderable material.
[0013] In another embodiment, the present invention is a thermal
stack laminate for attachment to an electrical component, said
thermal stack laminate comprising: a baseplate of a heat sink
material having a first surface; attached to the first surface a
thermal spray applied layer of alumina having a thickness of from
50.0 to 210.0 microns; and attached to the layer of alumina a
thermal spray applied layer of a solderable material and a kinetic
spray applied layer of a solderable material applied to the thermal
spray applied layer of a solderable material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The present invention will now be described, by way of
example, with reference to the accompanying drawings, in which:
[0015] FIG. 1 is a generally schematic layout illustrating a
kinetic spray system for performing the method of the present
invention;
[0016] FIG. 2 is an enlarged cross-sectional view of a kinetic
spray nozzle used in the system;
[0017] FIG. 3 is a schematic drawing of a thermal stack laminate
according to the present invention; and
[0018] FIG. 4 is a graph of the theoretical thermal resistance of a
series of thermal stack configurations plotted as a function of the
thermal conductivity of an alumina layer.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0019] The present invention comprises a method for formation of
thermal stack laminates and their use to cool high power electrical
components, such as silicon chips. The present invention also
discloses such a thermal stack laminate. The method combines the
use of a thermal spray process, which is known in the art, with the
relatively new technology of a kinetic spray process. The kinetic
spray process used is generally described in U.S. Pat. Nos.
6,139,913, 6,283,386 and the two articles by Van Steenkiste, et al.
entitled "Kinetic Spray Coatings", published in Surface and
Coatings Technology, Volume III, pages 62-72, Jan. 10, 1999 and
"Aluminum coatings via kinetic spray with relatively large powder
particles", published in Surface and Coatings Technology 154, pages
237-252, 2002, all of which are herein incorporated by
reference.
[0020] The thermal stack laminate of the present invention will be
described more fully below, but in simplest terms it generally
comprises a baseplate of a heat sink material having a first
surface; attached to the first surface is a thermal spray applied
layer of alumina having a thickness of from 50.0 to 210.0 microns;
and attached to the layer of alumina is a kinetic spray applied
layer of a solderable material. Optional layers include one or more
metal matrix composite layers applied by a kinetic spray process
and located either: between the baseplate and the layer of alumina;
between the layer of alumina and the layer of solderable material;
or both between the baseplate and the layer of alumina and between
the layer of alumina and the layer of solderable material. In
addition, another option is to apply a first layer of the
solderable material via a thermal spray process and then apply the
remainder via a kinetic spray process.
[0021] Referring first to FIG. 1, a kinetic spray system for use
according to the present invention is generally shown at 10. System
10 includes an enclosure 12 in which a support table 14 or other
support means is located. A mounting panel 16 fixed to the table 14
supports a work holder 18 capable of movement in three dimensions
and able to support a suitable substrate material to be coated. The
enclosure 12 includes surrounding walls having at least one air
inlet, not shown, and an air outlet 20 connected by a suitable
exhaust conduit 22 to a dust collector, not shown. During coating
operations, the dust collector continually draws air from the
enclosure 12 and collects any dust or particles contained in the
exhaust air for subsequent disposal.
[0022] The spray system 10 further includes an air compressor 24
capable of supplying air pressure up to 3.4 MPa (500 psi) to a high
pressure air ballast tank 26. The air ballast tank 26 is connected
through a line 28 to both a high pressure powder feeder 30 and a
separate air heater 32. The air heater 32 supplies high pressure
heated air, the main gas described below, to a kinetic spray nozzle
34. The temperature of the main gas varies from 100 to 3000.degree.
C., depending on the powder or powders being sprayed. The pressure
of the main gas and the powder feeder varies from 200 to 500 psi.
The powder feeder 30 mixes particles of a powder or a powder
mixture of particles with unheated high-pressure air and supplies
the mixture to a supplemental inlet line 48 of the nozzle 34. The
particles are described below and may comprise a metal, an alloy, a
ceramic, or mixtures thereof. As known to those of ordinary skill
in the art an alloy is defined as a solid or liquid mixture of two
or more metals, or of one or more metals with certain nonmetallic
elements, as in carbon containing steel. A computer control 35
operates to control both the pressure of air supplied to the air
heater 32 and the temperature of the heated main gas exiting the
air heater 32. As would be understood by one of ordinary skill in
the art, the system 10 can include multiple powder feeders 30, all
of which are connected to supplemental feedline 48. For clarity
only one powder feeder 30 is shown in FIG. 1. Having multiple
powder feeders 30 allows one to rapidly switch between spraying one
particle population to spraying a multiple of particle populations.
Thus, an operator can form zones of two or more types of particles
that smoothly transition to a single particle type and back
again.
[0023] FIG. 2 is a cross-sectional view of the nozzle 34 and its
connections to the air heater 32 and the supplemental inlet line
48. A main air passage 36 connects the air heater 32 to the nozzle
34. Passage 36 connects with a premix chamber 38 which directs air
through a flow straightener 40 and into a mixing chamber 42.
Temperature and pressure of the air or other heated main gas are
monitored by a gas inlet temperature thermocouple 44 in the passage
36 and a pressure sensor 46 connected to the mixing chamber 42.
[0024] The mixture of unheated high pressure air and coating powder
is fed through the supplemental inlet line 48 to a powder injector
tube 50 comprising a straight pipe having a predetermined inner
diameter. The predetermined diameter can range from 0.40 to 3.00
millimeters. Preferably it ranges from 0.40 to 0.90 millimeters in
diameter. The tube 50 has a central axis 52 which is preferentially
the same as the axis of the premix chamber 38. The tube 50 extends
through the premix chamber 38 and the flow straightener 40 into the
mixing chamber 42.
[0025] Mixing chamber 42 is in communication with the de Laval type
nozzle 54. The nozzle 54 has an entrance cone 56 that decreases in
diameter to a throat 58. Downstream of the throat is an exit end
60. The largest diameter of the entrance cone 56 may range from 10
to 6 millimeters, with 7.5 millimeters being preferred. The
entrance cone 56 narrows to the throat 58. The throat 58 may have a
diameter of from 3.5 to 1.5 millimeters, with from 3 to 2
millimeters being preferred. The portion of the nozzle 54 from
downstream of the throat 58 to the exit end 60 may have a variety
of shapes, but in a preferred embodiment it has a rectangular
cross-sectional shape. At the exit end 60 the nozzle 54 preferably
has a rectangular shape with a long dimension of from 8 to 14
millimeters by a short dimension of from 2 to 6 millimeters. The
distance from the throat 58 to the exit end 60 may vary from 60 to
400 millimeters.
[0026] As disclosed in U.S. Pat. Nos. 6,139,913 and 6,283,386 the
powder injector tube 50 supplies a particle powder mixture to the
system 10 under a pressure in excess of the pressure of the heated
main gas from the passage 36. The nozzle 54 produces an exit
velocity of the entrained particles of from 300 meters per second
to as high as 1200 meters per second. The entrained particles gain
kinetic and thermal energy during their flow through this nozzle.
It will be recognized by those of skill in the art that the
temperature of the particles in the gas stream will vary depending
on the particle size and the main gas temperature. The main gas
temperature is defined as the temperature of heated high-pressure
gas at the inlet to the nozzle 54. These temperatures and the
exposure time of the particles are kept low enough that the
particles are always at a temperature below their melting
temperature so even upon impact, there is no change in the solid
phase of the original particles due to transfer of kinetic and
thermal energy, and therefore no change in their original physical
properties. The particles exiting the nozzle 54 are directed toward
a surface of a substrate to coat it.
[0027] Upon striking a substrate opposite the nozzle 54 the
particles flatten into a nub-like structure with an aspect ratio of
generally about 5 to 1. When the substrate is a metal and the
particles include a metal, all the particles striking the substrate
surface fracture the oxidation on the surface layer and the metal
particles subsequently form a direct metal-to-metal bond between
the metal particle and the metal substrate. Upon impact the kinetic
sprayed particles transfer substantially all of their kinetic and
thermal energy to the substrate surface and stick if their yield
stress has been exceeded. As discussed above, for a given particle
to adhere to a substrate it is necessary that it reach or exceed
its critical velocity which is defined as the velocity where at it
will adhere to a substrate when it strikes the substrate after
exiting the nozzle 54. This critical velocity is dependent on the
material composition of the particle. In general, harder materials
must achieve a higher critical velocity before they adhere to a
given substrate. It is not known at this time exactly what is the
nature of the particle to substrate bond; however, it is believed
that a portion of the bond is due to the particles plastically
deforming upon striking the substrate.
[0028] In FIG. 3 a thermal stack laminate is shown generally at 100
attached to an electrical component 112, such as a silicon chip.
The thermal stack laminate 100 includes a baseplate of a heat sink
material 102 having a first surface 103. Optionally attached to the
heat sink material 102 is a first metal matrix composite layer 104
applied via a kinetic spray process. Attached to the first metal
matrix composite layer 104 is a thermal spray applied dielectric
alumina layer 106. If the first metal matrix composite layer 104 is
not used, then the alumina layer 106 is directly applied to heat
sink material 102. A second optional metal matrix composite layer
108 is applied over the alumina layer 106 via a kinetic spray
process. A solderable material layer 110 is applied to either the
alumina layer 106 or the second metal matrix composite layer 108
via a kinetic spray process. A third optional layer is layer 109
which comprises a thin layer of the solderable material applied via
a thermal spray process. This third optional layer has several
advantages, described below. Finally the electrical component 112
is soldered to the solderable material layer 110. Each of these
layers is more fully described below.
[0029] The heat sink material 102 is typically a pure metal or an
alloy having high thermal conductivity. Some suitable examples
include aluminum, copper, and copper alloys. The heat sink material
102 may have any required thickness, typically 5.0 to 10.0
millimeters in thickness.
[0030] As discussed the thermal stack laminate 100 according to the
present invention may include an optional first metal matrix
composite layer 104 applied via a kinetic spray process to the
first surface 103 of the heat sink material 102. Such a layer 104
is not necessary, but may be desirable when there is a large
thermal expansion coefficient mismatch between the dielectric
alumina layer 106 and heat sink material 102. While this first
metal matrix composite layer 104 does not significantly effect the
thermal performance of the thermal stack laminate 100, it does
lower the thermal stress from the heat sink material 102 and
alumina layer 106 interface. Typically the first metal matrix
composite layer 104 has thickness of from 500 microns to 1.1
centimeters, more preferably 500 microns to 5.0 millimeters, and
more preferably 500 microns to 2.1 millimeters. It can, however, be
thicker if necessary even up to several centimeters.
[0031] The kinetic spray system 10 is extremely versatile in
producing any of a variety of coatings. Utilizing a system 10 that
includes a plurality of powder feeders 30 enables one to produce an
endless variety of mixes of particles exiting the nozzle 54 to coat
a substrate such as the heat sink material 102. The system 10
permits one to create coatings that initially are composed of a
plurality of components and then as the coating layer is built up
supply of one or more of the particles may be stopped thus enabling
the coating to transition to a different composition from that
initially coated on the substrate. The size of particles utilized
in the powder feeders 30 generally can range from 1 to 250 microns,
more preferably from 60 to 250 microns, and most preferably from 60
to 150 microns. Utilizing the system 10 it is possible to produce
metal matrix composite layers that previously were only possible
utilizing the above-mentioned method of infiltrating a molten metal
into a preformed ceramic. The system 10 has been utilized to
produce metal matrix composite layers that comprise one or more
metals or alloys in combination with one or more ceramics as
disclosed in co-pending United States patent application Ser. No.
10/098,800, filed Mar. 15, 2002. Metals that have been utilized
include aluminum, copper, tin alloys, steel alloys and other
alloys. The ceramics that have been utilized include diamond,
silicon carbide, and aluminum nitride. As would be understood by
one of ordinary skill in the art, however, other metals, alloys,
and ceramic materials can be utilized to form the metal matrix
composite layers. Using the system 10 the first and second metal
matrix composite layers 104 and 108 are easily produced.
[0032] Since the particles are never melted in the kinetic spray
process, this dramatically reduces the thermal stress that occurs
in applying the metal matrix composite layers 102 and 108 relative
to previous metal matrix compositions. In addition, the overall
temperature during formation of the metal matrix composites 102 and
108 of the present invention is much lower than that utilized
during the prior art metal matrix compositions formed by
infiltration of a molten metal into a ceramic preform. Therefore,
the metal matrix compositions of the present invention do not
permit reactions between the metal and the ceramic of the metal
matrix composition.
[0033] The present invention can be utilized to coat any of a large
variety of heat sink materials 102 including substrates that are
formed from metal, alloys, ceramics, plastics, silicon, and other
substrate materials. The system 10 permits one to produce coatings
that have thicknesses ranging from several microns to several
centimeters in thickness. Typically, the amount of ceramic in the
mixture of metal and ceramic used to form the metal matrix
composition ranges from 30 to 70% by weight based on the total
weight of the mixture. The composition of the mixture utilized
depends on the identity of the heat sink material 102 and the
dielectric layer 106. The main gas temperature that is utilized for
accelerating the particles in the present invention can vary from
100.degree. C. to approximately 1700.degree. C. The main gas
temperature utilized depends on the identity of the metal or alloy
utilized to form the metal matrix composition. A typical metal
matrix composite is produced by combining 50% by weight aluminum
with 50% by weight silicon carbide. This particle mixture can then
be sprayed through the system 10 at a temperature of approximately
500.degree. C. at pressures of from 300 to 350 psi. Once the
thermal stack laminate 100 is fully formed and prior to attachment
of the electrical component 112 it can be beneficial to subject the
thermal stack laminate 100 to a post-coating treatment of heating
to 550.degree. C. in air for approximately one hour. It is also
possible to use any inert gas as the atmosphere during the heat
treatment. It is not necessary that the heat treatment occur for
all metal matrix composite 104 and 108 coatings of the present
invention but it can be useful depending on the identity of the
metal matrix composite 102 and 108.
[0034] As discussed above the alumina layer 106 is either directly
applied to the first surface 103 of the heat sink material 102 or
to the first metal matrix composite layer 104, when used. The
alumina layer 106 is applied using either a plasma gas thermal
spray process or a High Velocity Oxy-Fuel combustion (HVOF) thermal
spray process. These general processes are known in the art, but
have not been utilized to form thermal stack laminates 100. Either
is suitable for applying the very thin alumina layer 106. The
alumina layer 106 is preferably 50.0 to 210.0 microns in thickness,
and more preferably from 75.0 to 130.0 microns in thickness.
[0035] In general, the parameters useful in a plasma thermal spray
process using a Metco 3MB gun comprise: spherical alumina powder
having a particle size of 15 to 50 microns; a voltage of from 64 to
71 volts; a current of 500 to 550 amps; a primary gas of argon at
40 to 80 SLM; a secondary gas of hydrogen at 8 to 15 SLM; a carrier
gas of argon at 37 to 40 SLM; a standoff distance of 60 to 120
millimeters; feed rate of 20 to 30 grams/minute; particle
temperature of 2400.degree. C., plus or minus 300.degree. C.; and a
particle velocity of 650 meters/second, plus or minus 50
meters/second.
[0036] In general, the parameters useful in a HVOF thermal spray
process using a Praxair HV2000 gun comprise: nearly spherical
alumina powder having a particle size of 5 to 22 microns; propylene
at 75 SLM; oxygen at 273 SLM; nitrogen at 21 SLM; combustion
chamber of 22 millimeters; standoff distance of 150 millimeters;
feed rate of 20 to 30 grams/minute; particle velocity of 680
meters/second, plus or minus 50 meters/second; and a particle
temperature of 2100.degree. C., plus or minus 200.degree. C.
[0037] In a specific example an alumina layer 106 having a
thickness of 130 microns was applied directly to an aluminum heat
sink material 102 by a plasma thermal spray process using a Praxair
SG-100 gun and the following parameters: a Praxair 175 anode; a
Praxair 129 cathode; a Praxair 113 gas injector; primary gas argon
at 50 psi; secondary gas helium at 150 psi; a current of 800 amps;
a standoff distance of 100 millimeters; horizontal traverse speed
17 inches/second; vertical increment 0.2 inches; grit of alumina at
a grit size of 60; grit blast pressure 40 psi; powder Norton 153
alumina in a particle size range of 15 to 45 microns; a Praxair
powder hopper operating at 1.5 rounds/minute with a regular hopper
wheel; internal powder feed; carrier gas argon at 30 psi; and with
cooling jets. The alumina layer 106 had a thermal conductivity of
from 3 to 5 W/m.degree. K, by way of contrast the prior art thick
sintered alumina layer has a thermal conductivity of about 25
W/m.degree. K. Unexpectedly, despite the higher thermal
conductivity a prior art thermal stack laminate using sintered
alumina and thermal grease had a much larger thermal resistance
than did the thermal stack laminate 100 of the present invention.
The breakdown voltage of the alumina layer 106 was less than 310
V/mil. The strength of the alumina layer 106 was tested using a
stud pull out test. The force required to pull out the stud ranged
from 6.5 to 8.5 kpsi. In addition, subjecting the layer to six
thermal cycles of 5 minutes at minus 40.degree. C. then 5 minutes
at 100.degree. C. did not alter the bond strength in the stud pull
out test.
[0038] In an attempt to further evaluate the value of the present
invention a number of theoretical multi-layer stacks were proposed
and then the known values for the thermal conductivity of each
layer were used to calculate the thermal resistance of the entire
stack for a selected value of the thermal conductivity of the
alumina layer in the stack. The compositions of the three stacks
are shown below in tables 1-3, respectively. FIG. 4 is a graph of
the theoretical resistance of the whole stack plotted against the
thermal conductivity of the alumina layer. Stack 2, table 2, is a
prior art stack that uses a copper/aluminum nitride/copper layer
with thermal grease, thus on FIG. 4 its thermal resistance shown at
200 is constant versus the alumina layer thermal conductivity. In
theoretical stack 1, table 1, the copper/aluminum nitride/copper
layer is replaced by a copper/thin thermal spray applied alumina
layer. In FIG. 4 lines 202 and 204 represent the calculated values
for stack 1 with an alumina layer having a thickness of 5 thousands
of an inch or 3 thousands of an inch respectively. It can be seen
that this stack has a lower thermal resistance than stack 2 only if
the thermal conductivity of the alumina layer can be brought up to
above 15 W/m.degree. K, which is well above the currently measured
values for thermally sprayed alumina according to the present
invention. In FIG. 4 lines 206 and 208 represent the calculated
values for stack 3 wherein the thermal grease has been removed and
the copper layer is a thin thermally sprayed layer. In reference
line 206 the alumina layer is 5 thousands of an inch thick and in
reference line 208 it is 3 thousands of an inch thick. It can be
seen that by removing the thermal grease and replacing the highly
thermally conducting layer of aluminum nitride the thermal
resistance of the stack is dramatically lowered. Because the
present invention allows for removal of the thermal grease by
directly applying the alumina to the heat sink material a dramatic
reduction in the thermal resistance of the stack is achieved.
1TABLE 1 Thermal Outside Conductivity of Layer dimensions of the
Thickness of the the layer composition layer (mils.) layer (mils.)
(w/m .degree. K.) Silicon device 280 .times. 280 10 90 Solder 280
.times. 280 15 100 Copper 360 .times. 520 30 385 Alumina 360
.times. 520 3 or 5 See FIG. 4 Thermal grease 360 .times. 520 1
0.72
[0039]
2TABLE 2 Thermal Outside Conductivity of Layer dimensions of the
Thickness of the the layer composition layer (mils.) layer (mils.)
(W/m .degree. K.) Silicon device 280 .times. 280 10 90 Solder 280
.times. 280 15 100 Copper 360 .times. 520 30 385 Aluminum nitride
360 .times. 520 25 180 Copper 360 .times. 520 10 385 Thermal grease
360 .times. 520 1 072
[0040]
3 Thermal Outside Conductivity of Layer dimensions of the Thickness
of the the layer composition layer (mils.) layer (mils.) (W/m
.degree. K.) Silicon device 280 .times. 280 10 90 Solder 280
.times. 280 15 100 Copper 360 .times. 520 5 385 Alumina 360 .times.
520 3 or 5 See FIG. 4
[0041] As discussed above, the thermal stack laminate 100 may
include an optional second metal matrix composite layer 108. The
layer 108 is applied as described above for the first metal matrix
composite layer 104. When present, it has been found that the
second metal matrix composite layer has a preferred thickness of
from 5 to 7 millimeters, and more preferably is 6 millimeters
thick. This thickness provides the minimum peak temperature of the
electrical component 112.
[0042] Attached to either the second metal matrix composite layer
108 or the alumina layer 106 is a layer of kinetic spray applied
solderable material 110. The solderable material 110 may comprise
any solderable metal or alloy. Examples include copper and copper
alloys among others. The kinetic spray process is as described
above for the metal matrix composite layers. The main gas
temperature is chosen to accelerate the particles above their
critical velocity with out thermally softening them. The solderable
material 110 layer may range from 250 microns thick to 1.0
centimeters or more. The solderable material 110 can also function
as a heat sink type material.
[0043] As discussed above, a first layer of the solderable material
109 can be applied using a thermal spray process prior to deposit
of the remainder of the solderable material by kinetic spray.
Preferably the layer 109 has a thickness of from 2 to 5 thousands
of an inch. The composition of the solderable material used as this
first layer can be, but does not need to be identical to that of
the remainder of the solderable material in layer 110. For
instance, the layer 109 could comprise pure copper and layer 110
may comprise a copper alloy. The optional layer 109 provides
several benefits including better adhesion of the kinetically
sprayed solderable material and prevention of surface defects in
the subsequently applied solderable material by kinetic spray.
[0044] In an example copper was thermally sprayed onto a thermally
sprayed alumina layer, described above, using the following
parameters by a plasma thermal spray process: a Praxair SG-100 gun;
a Praxair 730 anode; a Praxair 129 cathode; a Praxair 112 gas
injector; primary gas argon at 40 psi; secondary gas helium at 100
psi; a current of 600 amps; a standoff distance of 150 millimeters;
horizontal traverse speed 17 inches/second; vertical increment 0.2
inches; powder X-Form Company SCM200RL copper in a particle size
range of greater than 15 microns; a Praxair powder hopper operating
at 2.5 rounds/minute; and the carrier gas argon at 30 psi.
[0045] An additional advantage of the present invention is that
prior art difficulties with kinetic spray processes have arisen
when the substrate surface has imperfections, these can cause
imperfections in the kinetic spray layer. It has been found,
however, that first applying a thin layer using a thermal spray
process corrects the surface defects and prevents them from
altering the subsequently applied kinetic spray layer. Thus the
alumina layer 106 and the optional layer 109 ensure that the
subsequent kinetically applied layers are free from defects. In the
final step the electrical component 112 is soldered to the
solderable layer 110.
[0046] While the preferred embodiment of the present invention has
been described so as to enable one skilled in the art to practice
the present invention, it is to be understood that variations and
modifications may be employed without departing from the concept
and intent of the present invention as defined in the following
claims. The preceding description is intended to be exemplary and
should not be used to limit the scope of the invention. The scope
of the invention should be determined only by reference to the
following claims.
* * * * *