U.S. patent application number 11/956024 was filed with the patent office on 2009-06-18 for compliant thermal interface design and assembly method.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to John P. Karidis, Mark D. Schultz.
Application Number | 20090151907 11/956024 |
Document ID | / |
Family ID | 40751675 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090151907 |
Kind Code |
A1 |
Karidis; John P. ; et
al. |
June 18, 2009 |
COMPLIANT THERMAL INTERFACE DESIGN AND ASSEMBLY METHOD
Abstract
A method for producing a compliant thermal interface device for
cooling an integrated circuit includes steps of: cutting a
plurality of high thermal conductivity sheets according to at least
one selected pattern, the sheets made up of a first material;
forming spring elements in at least one of the plurality of sheets,
such that the sheets include both flat areas and spring elements;
coating the sheets with a second material, wherein the second
material is different from the first material; stacking the high
thermal conductivity sheets; and bonding at least a portion of at
least one of the stacked sheets using thermo-compression
bonding.
Inventors: |
Karidis; John P.; (Ossining,
NY) ; Schultz; Mark D.; (Ossining, NY) |
Correspondence
Address: |
MICHAEL BUCHENHORNER, P.A.
8540 SW 83 STREET, SUITE 100
MIAMI
FL
33143
US
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
40751675 |
Appl. No.: |
11/956024 |
Filed: |
December 13, 2007 |
Current U.S.
Class: |
165/104.33 ;
29/890.03 |
Current CPC
Class: |
H01L 21/4882 20130101;
Y10T 29/4935 20150115; H01L 23/473 20130101; H01L 2924/0002
20130101; H01L 2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
165/104.33 ;
29/890.03 |
International
Class: |
F28D 15/00 20060101
F28D015/00; B21D 53/02 20060101 B21D053/02 |
Claims
1. A method for producing a compliant thermal interface device for
cooling an integrated circuit; the method comprising steps of:
cutting a plurality of high thermal conductivity sheets according
to at least one selected pattern; the high thermal conductivity
sheets comprising a first material; forming spring elements in at
least one of the plurality of high thermal conductivity sheets,
such that the sheets comprise flat areas and spring elements;
coating the high thermal conductivity sheets with a second
material, wherein the second material is different from the first
material; stacking the high thermal conductivity sheets; and
bonding at least a portion of at least one of the stacked sheets
using thermo-compression bonding.
2. The method of claim 1 further comprising a step of: machining
the bonded at least one stacked sheet.
3. The method of claim 1 wherein the first material comprises
copper.
4. The method of claim 1 further comprising a step of attaching at
least one end cap to the at least one sheet.
5. The method of claim 1 wherein cutting the high thermal
conductivity sheets comprises etching the high thermally conductive
sheets.
6. The method of claim 1 wherein cutting the high thermal
conductivity sheets comprises stamping the high thermal
conductivity sheets.
7. The method of claim 1 wherein cutting the high thermal
conductivity sheets comprises cutting at least one slot to allow
relative movement of flat portions of the sheet.
8. The method of claim 1 wherein coating the stacked sheets
comprises applying a second material comprising silver.
9. The method of claim 1 wherein coating comprises using a sputter
method.
10. The method of claim 1 wherein coating comprises using a plating
method.
11. The method of claim 1 wherein the pattern comprises holes for
carrying fluid to or from a neighboring stacked sheet.
12. The method of claim 1 wherein the first material comprises
silver-bearing copper.
13. The method of claim 1 further comprising: inserting at least
one restricting strip into at least one slot to direct coolant
flow.
14. A compliant thermal interface device comprising: a plurality of
stacked sheets of a high thermally conductive material, wherein at
least one stacked sheets is cut according to at least one pattern;
a plurality of spring elements formed on at least one sheet; a
coating applied over at least two of the sheets; and bonding of at
least a portion of the coatings of the at least two sheets.
15. The compliant thermal interface device of claim 14, wherein the
coating comprises silver.
16. The compliant thermal interface device of claim 14, further
comprising an integrated circuit to be cooled.
17. The compliant thermal interface device of claim 16, wherein the
at least one pattern comprises cooling fins for fluid micro
channels.
18. The compliant thermal interface device of claim 16, wherein at
least one of the plurality of spring elements comprise a cooling
fin structure.
19. The compliant thermal interface device of claim 16, wherein at
least one of the plurality of spring elements is formed according
to a bend different than another at least one of the plurality of
spring elements.
20. A compliant thermal interface device comprising: a plurality of
stacked sheets of a high thermally conductive material, wherein at
least one stacked sheets is cut according to at least one pattern;
a plurality of spring elements formed on at least one sheet; and a
plurality of cooling fin elements separate from the spring elements
formed on the at least one sheet
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] None.
STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT
[0002] None.
INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT
DISC
[0003] None.
FIELD OF THE INVENTION
[0004] The invention disclosed broadly relates to the field of
designing cooling devices for integrated circuits and more
particularly relates to the field of compliant thermal interface
design.
BACKGROUND OF THE INVENTION
[0005] Compliant thermal interface solutions are desirable for a
number of reasons. Many of them use dense thermally conductive
spring structures. These structures have proven difficult to
execute in a way that is easy to manufacture. In particular,
bonding sheet elements formed to provide the necessary spacing with
angled mating surfaces can be accomplished to create structures.
However, the process demonstrated with such an approach is not
readily given to manufacturing.
[0006] "Potting and plating" is a method that requires the creating
of a platable surface from elements which may not be in good
electrical contact and it involves multiple potting steps.
Soldering or any liquid bonding process presents difficulties in
keeping the spring elements, which may be in contact, from bonding.
Additional difficulties arise because the high temperatures
(>200 C) required for these methods can destroy the temper of
the work-hardened copper sheets. In addition, these methods fail to
account for issues such as thermal shorting between fluid flow
paths, the need for sensors in certain applications, and the torque
exerted on the membrane by the compression of the bent springs.
[0007] Solders are difficult to restrain to areas where bonding is
required and oxidation control is difficult; glues are not a
reasonable option due to strength requirements and the same
location restraint requirements; and copper compression bonding
requires very high temperatures.
[0008] Hardened copper has been the material of choice for creating
the spring structures as it combined the desired high thermal
conductivity with reasonable deflections/loads to yield. Raw or
annealed copper yields at very low loads. Other cooling
(non-compliant) structures composed of copper currently use copper
to copper thermo-compression bonding. However, the temperatures
required for this bonding fully anneal the copper, making such an
approach useless for these structures (and often undesirable even
in non-compliant assemblies).
[0009] Several conflicting requirements led to the invention of the
disclosed assembly process. Hardened copper was the material of
choice for creating the spring structures as it combined the
desired high thermal conductivity with reasonable deflections/loads
to yield. Raw or annealed copper yields at very low loads. Other
cooling (non-compliant) structures composed of copper currently use
copper to copper thermo-compression bonding. Unfortunately, the
high temperatures required fully anneal the copper, making such an
approach useless for these structures.
[0010] Therefore, there is a need for an improved method of bonding
membrane and support areas of stacked sheets in order to overcome
the shortcomings of the prior art.
SUMMARY OF THE INVENTION
[0011] Briefly, according to an embodiment of the invention, a
method for producing a compliant thermal interface device for
cooling an integrated circuit includes steps or acts of: cutting a
plurality of high thermal conductivity sheets according to at least
one selected pattern, the sheets made up of a first material;
forming spring elements in at least one of the plurality of sheets,
such that the sheets include both flat areas and spring elements;
coating the sheets with a second material, wherein the second
material is different from the first material; stacking the high
thermal conductivity sheets; and bonding at least a portion of at
least one of the stacked sheets using thermo-compression
bonding.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] To describe the foregoing and other exemplary purposes,
aspects, and advantages, we use the following detailed description
of an exemplary embodiment of the invention with reference to the
drawings, in which:
[0013] FIG. 1 is a flowchart of a method for forming a compliant
thermal interface according to an embodiment of the present
invention;
[0014] FIG. 2a is an illustration of an etched or stamped sheet,
according to an embodiment of the present invention;
[0015] FIG. 2b is an illustration of the sheet of FIG. 2a with the
springs formed, according to an embodiment of the present
invention;
[0016] FIG. 2c is an illustration of the sheets after they are
stacked and bonded, according to an embodiment of the present
invention;
[0017] FIG. 2d is an illustration of the sheet stack after final
cutting, according to an embodiment of the present invention;
[0018] FIG. 3 is an illustration of the basic embodiment, according
to an embodiment of the present invention;
[0019] FIG. 4 shows a close-up view of a staggered fin microchannel
design on a 20-sheet stack, according to an embodiment of the
present invention;
[0020] FIGS. 5 is an oblique view of a sheet stack with end caps,
according to an embodiment of the present invention;
[0021] FIG. 6a is a close-up view of the fluid channel, according
to an embodiment of the present invention;
[0022] FIG. 6b shows the fluid channel of FIG. 6a with a
restricting strip, according to an embodiment of the present
invention;
[0023] FIG. 6c shows a close-up view of the restricting strip,
according to an embodiment of the present invention;
[0024] FIG. 6d shows a restricting strip in the sheet stack of FIG.
5, according to an embodiment of the present invention;
[0025] FIG. 6e shows a restricting strip with varying height,
according to an embodiment of the present invention;
[0026] FIG. 6f shows a flow blocker inserted into an end cap,
according to an embodiment of the present invention;
[0027] FIG. 7 is an illustration of an embodiment comprising
transverse between spring restricting elements; and
[0028] FIG. 8 is an illustration of an embodiment comprising an
invertible paired sheet design.
[0029] While the invention as claimed can be modified into
alternative forms, specific embodiments thereof are shown by way of
example in the drawings and will herein be described in detail. It
should be understood, however, that the drawings and detailed
description thereto are not intended to limit the invention to the
particular form disclosed, but on the contrary, the intention is to
cover all modifications, equivalents and alternatives falling
within the scope of the present invention.
DETAILED DESCRIPTION
[0030] We discuss a thermal interface assembly approach that
utilizes predominantly flat sheet elements with formed spring
elements, allowing for the use of a range of bonding approaches,
such as the thermo-compression bonding of silver-plated hard copper
elements. Thermo-compression bonding is ideal because it allows for
only the flat areas to bond under pressure, as the spring elements
are only intermittently in contact with each other and cannot carry
a bonding load. Utilizing silver on copper thermo-compression
bonding produces integrated coolers that are solid metal, with no
gaskets. A further advantage to this method is that it bonds the
membrane and support areas of stacked sheets without bonding the
springs together and without utilizing temperatures that would
destroy the temper of the work-hardened copper sheets (>200
C).
[0031] Several improved design considerations are also presented
including: a staggered fin microchannel design (as shown in FIG. 4)
which maintains the sheet integrity necessary to avoid columnar
collapse under a high bonding load. This staggered fin microchannel
design also includes some of the following features: fluid path
thermal isolation features; a transverse between spring restricting
elements to direct the flow from inlet channels to outlet channels
as desired, including customizable flow restricting elements to
address load non-uniformity issues; and an invertible paired sheet
design that allows for the assembly of what are essentially
alternating elements from a single parent sheet part; alternating
bend springs to prevent net torque on the membrane due to spring
compression; alternatingly positioned crosstie elements to maintain
sheet stability under bond load while allowing adequate flow. The
listed features not shown in FIG. 4 can be seen in FIGS. 5 and the
following, which detail a design which incorporates microchannel
cooling at the back of the membrane rather than at the back of the
spring structure as shown in FIG. 4. The microchannel is formed in
the space between two sheets (separated by a third sheet).
[0032] The design further features flow blockers constructed
either: in place utilizing heat-cured silicone or some similar
substance; or pre-made (molded) of ideally rubber or other
compliant material, although solid material or features constructed
into the end caps could be used. The blocker/blocking material, as
shown in FIG. 6f fills the space between the end cap surface and
the nearest sheet element in order to prevent flow between the end
cap and the nearest sheet element.
[0033] In a preferred embodiment, we use silver-plated hard copper
elements which are thermo-compression bonded. This provides high
performance heat transfer for non-flat chip surfaces utilizing an
interface that will conform repeatedly to any chip surface. Plating
the copper with a thin layer of silver (nominally 2.5 micrometers)
and placing it under loading pressures near or, in some cases,
above the yield strength of silver and/or copper, forms a bond
approaching the strength of the bulk material at temperatures as
low as 170 C (200 C typical). This is below the softening
temperature of normal copper, and well below the softening
temperature of a more ideal base material than normal hardened
copper, hardened silver bearing copper.
[0034] Silver bearing copper is preferable to copper because it has
a higher softening (annealing) temperature and is thus more robust
against other high temperature processing. Softening (annealing) of
the copper material greatly reduces the effectiveness of the
springs, reducing both their initial spring constant and the range
of operation before plastic (permanent) deformation of the springs.
The resulting structures retain the hardness of the copper spring
elements while bonding the flat sheet surfaces together in a
structure that provides a solid thin membrane when cut.
[0035] Referring now in specific detail to the drawings, and
particularly FIG. 1, there is illustrated a flow chart 100 of a
method for flat sheet assembly, according to an embodiment of the
present invention. The method begins at step 110 by patterning
sheets of copper or a copper alloy. The sheets may be cut by
etching or stamping them or by any other suitable method. The
sheets are cut according to a pattern appropriate for the desired
spring structures and/or coolant flow paths.
[0036] Next, in step 120 the patterned sheets are plated or
otherwise coated with silver or an alloy containing silver. The
coating may be accomplished by sputter or chemical vapor deposition
(CVD) method, but the preferred method is electroplating. In step
130 spring elements are formed from portions of each sheet. The
sheet includes a pair of slots to allow relative movement of the
two flat portions separated by the spring elements when the spring
elements are actually formed. Note that the holes formed by these
slots when the sheets are stacked must be filled or otherwise
sealed in order for the spring area carrying coolant to be
leak-proof.
[0037] In step 140, the sheets are stacked. As an example, we show
in FIGS. 2c, 2d and FIG. 4 the stacking of 20 sheets which produces
a stack approximately 1 mm high. Next, in step 150, the membrane
and support areas of the stacked sheets are assembled utilizing
silver on copper thermo-compression bonding. Bonding occurs at high
pressure and a temperature of 150-250 degrees C. The pressure may
typically reach 30,000 pounds per square inch (PSI). The
temperature may be held at 200 degrees C. for eight hours or
higher.
[0038] Lastly, in step 160 the bonded stack is machined to final
form. Once the bonded stack has been cut, it forms a single solid
thin membrane. Note that the order of the steps may be altered. For
example, coating may occur before or after cutting or forming.
[0039] FIGS. 2a through 2d provide exemplary illustrations of the
process as described above. FIG. 2a shows an etched sheet. FIG. 2b
shows a sheet after the springs are formed. FIG. 2c shows an
example of a bonded 20-sheet stack, while FIG. 2d shows the bonded
stack after machining. FIG. 3 shows an assembled interface after
alignment. FIG. 4 shows a close-up of a 20 sheet bonded stack after
membrane machining. The sheets include holes 420 for providing a
flow path for coolant from one sheet to the next sheet in the stack
(or into or out of the stack if the sheet is the last in the
stack). These holes provide integrated manifolding. The sheets also
include cooling fins 430, and alternating bend springs 440.
[0040] The resulting integrated cooler 400 is solid metal, with no
gaskets of any kind. This device 400 demonstrates substantially
improved uniformity in thermal resistance over typical
non-compliant heat sinks. Compliant thermal interface (CTI)
uniformity in air is better than standard heat sink uniformity in
helium (a much better thermal interface gas). This compliance
produces a much more uniform gap. The lack of uniform gap seen in
standard heat sinks is often mitigated when using such sinks by
utilizing helium rather than air in the gap to reduce both the
overall thermal resistance and the variation in thermal resistance
associated with that gap. Compliant heat sinks as described herein
provide comparable to superior results without the helium.
[0041] Thermo-compression bonding allows for only the flat areas to
bond under pressure, as the spring elements 440 are only
intermittently in contact with each other and cannot carry a
bonding load.
[0042] FIG. 3 shows the basic embodiment after alignment.
[0043] Referring again to FIG. 4 we see the stack of the sheets
shown in FIG. 2 wherein a staggered fin microchannel design
maintains the sheet integrity necessary to avoid columnar collapse
under a high bonding load. This design (unlike that shown in FIGS.
5 and following) incorporates cooling fins 430 on both sides of the
sheet. These fins 430 are located such that on consecutive layers,
the fins 430 do not overlap each other, producing a staggered fin
cooler 400 which is highly stable under the loading.
[0044] FIG. 5 is an oblique view of two of what would be a stack of
sheets (one shown pre-forming for information purposes. All sheets
520 would have springs 440 in the actual stack) along with a pair
of end caps 530 and 540. The inlet flow path 550, and outlet flow
path 555 are shown. This serves to illustrate how the coolant flows
into and through the stack of sheets 520. In this design, both
inlet and outlet ports (the holes 420) are isolated using thermal
isolation features (shown in FIG. 7). In this illustration only two
sheets 520 are shown (one bent, one straight), for clarity.
[0045] Referring to FIG. 5, two end caps 530 and 540 guide the flow
of coolant into and through the stack 520. Note that such end caps
530 and 540 are not required if the coolant is brought in through
the side of the cooler opposite the membrane. The darker arrow 550
shows the path of the coolant through the fluid inlet and through
the sheets 520. The lighter arrow 555 represents the path of the
now heated coolant flowing through the fluid outlet and away from
the stack 520.
[0046] FIG. 6a is a close-up view of the fluid channel. Note that
in this illustration one flat sheet 620 and one bent sheet 620 are
shown. Here more detail of the flow path is shown, including the
heat transfer zone 630.
[0047] In FIG. 6b we show the same fluid channel of FIG. 6a with
the addition of a transverse between spring restricting element
(restricting strip) 680 designed to direct the coolant flow from
inlet channels to outlet channels as desired. The flow restricting
strips 680 are designed to prevent flow through the springs and
structure above the springs in order that all or nearly all of the
flow passes over the cooling fins 430 placed in close proximity to
the contact membrane. The restricting strips 680 may be inserted in
slots after or during stacking to force flow at the bottom.
[0048] FIG. 6c shows a close-up view of the restricting strip 680.
As shown in FIGS. 6d, 6e, and 6f, the restricting strips 680 may
have varying height along their length and extend down into the gap
between the cooling fin portions of the spring elements to further
restrict flow in areas where less cooling is needed and divert flow
to areas where more coolant is needed. Varying the height of the
restricting strips 680 can also be done to address load
non-uniformity issues. Areas where the strip 680 is shorter allow
for more flow (and greater cooling capacity). Areas where the strip
680 is taller (as shown where the strip intersects the heat
transfer flow zone) allow less flow (less cooling capacity).
[0049] Appropriately designed strips 680 optimally direct the most
cooling capacity (for a given total flow) to the portions of the
device to be cooled which produce the most heat. Areas which
produce little heat can then also consume little coolant flow. Note
that the design shown provides for parallel flow of coolant to
short segments of the high flow resistance heat transfer flow
zones. If the fluid had to flow from one end of the sheet 520 to
the other (through, in this case, 16 coolant fins 430 on each
sheet), the flow resistance would be much higher than the described
embodiment.
[0050] FIG. 6d shows the restricting strip 680 utilized in the
sheet stack of FIG. 5 and FIG. 6e is a close-up view of the
restricting strip 680 showing the variations in height.
[0051] FIG. 6f shows a flow blocking element 690 inserted or formed
into one of the end caps. The flow blockers 690, which would
generally be inserted into both end caps, may be constructed
either: in place utilizing heat-cured silicone or some similar
substance; or pre-made (molded) of ideally rubber or other
compliant material. In the alternative, solid material or features
constructed into the ends caps 530 and 540 could be used to fulfill
the same flow blocking function.
[0052] FIG. 7 shows an illustration of an embodiment comprising an
invertible paired sheet design (only one sheet shown in this
figure) that allows for the assembly of what are essentially
alternating elements from a single parent sheet part. In this
figure one can clearly see how the thermal isolation feature 715
serves to separate the cooling fluid inlets 735 from the cooling
fluid outlets 745. The restricting strips 680 are positioned to
force flow at the bottom.
[0053] FIG. 8 shows the sheet of FIG. 7 with the overlapped
alternate sheet section 820. Offset cross-tie elements 815 allow
flow with good sheet stability. Alternating bend springs prevent
net torque on the membrane due to spring compression. Alternating
positioned crosstie elements 815 maintains sheet stability under
bond load while allowing adequate flow.
[0054] Therefore, while there has been described what is presently
considered to be the preferred embodiment, it will understood by
those skilled in the art that other modifications can be made
within the spirit of the invention. The above descriptions of
embodiments are not intended to be exhaustive or limiting in scope.
The embodiments, as described, were chosen in order to explain the
principles of the invention, show its practical application, and
enable those with ordinary skill in the art to understand how to
make and use the invention. It should be understood that the
invention is not limited to the embodiments described above, but
rather should be interpreted within the full meaning and scope of
the appended claims.
* * * * *