US20090151907A1

US20090151907A1 - Compliant thermal interface design and assembly method

Info

Publication number: US20090151907A1
Application number: US11/956,024
Authority: US
Inventors: John P. Karidis; Mark D. Schultz
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 2007-12-13
Filing date: 2007-12-13
Publication date: 2009-06-18
Also published as: US20090151893A1

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.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT

None.

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

None.

FIELD OF THE INVENTION

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

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.
“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.
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.
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).
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.
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

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

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:

FIG. 1 is a flowchart of a method for forming a compliant thermal interface according to an embodiment of the present invention;

FIG. 2 a is an illustration of an etched or stamped sheet, according to an embodiment of the present invention;

FIG. 2 b is an illustration of the sheet of FIG. 2 a with the springs formed, according to an embodiment of the present invention;

FIG. 2 c is an illustration of the sheets after they are stacked and bonded, according to an embodiment of the present invention;

FIG. 2 d is an illustration of the sheet stack after final cutting, according to an embodiment of the present invention;

FIG. 3 is an illustration of the basic embodiment, according to an embodiment of the present invention;

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;

FIGS. 5 is an oblique view of a sheet stack with end caps, according to an embodiment of the present invention;

FIG. 6 a is a close-up view of the fluid channel, according to an embodiment of the present invention;

FIG. 6 b shows the fluid channel of FIG. 6 a with a restricting strip, according to an embodiment of the present invention;

FIG. 6 c shows a close-up view of the restricting strip, according to an embodiment of the present invention;

FIG. 6 d shows a restricting strip in the sheet stack of FIG. 5, according to an embodiment of the present invention;

FIG. 6 e shows a restricting strip with varying height, according to an embodiment of the present invention;

FIG. 6 f shows a flow blocker inserted into an end cap, according to an embodiment of the present invention;

FIG. 7 is an illustration of an embodiment comprising transverse between spring restricting elements; and

FIG. 8 is an illustration of an embodiment comprising an invertible paired sheet design.

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

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).
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).
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. 6 f 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.
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.
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.
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.
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.
In step 140, the sheets are stacked. As an example, we show in FIGS. 2 c, 2 d 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.
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.
FIGS. 2 a through 2 d provide exemplary illustrations of the process as described above. FIG. 2 a shows an etched sheet. FIG. 2 b shows a sheet after the springs are formed. FIG. 2 c shows an example of a bonded 20-sheet stack, while FIG. 2 d 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.
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.
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.
FIG. 3 shows the basic embodiment after alignment.
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.
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.
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.
FIG. 6 a 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.
In FIG. 6 b we show the same fluid channel of FIG. 6 a 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.
FIG. 6 c shows a close-up view of the restricting strip 680. As shown in FIGS. 6 d, 6 e, and 6 f, 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).
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.
FIG. 6 d shows the restricting strip 680 utilized in the sheet stack of FIG. 5 and FIG. 6 e is a close-up view of the restricting strip 680 showing the variations in height.
FIG. 6 f 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.
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.
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.
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.

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 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