COMPLIANT THERMAL INTERFACE DESIGN AND ASSEMBLY METHOD
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.
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None.
STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENTNone.
INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISCNone.
FIELD OF THE INVENTIONThe 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 INVENTIONCompliant 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 INVENTIONBriefly, 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.
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:
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 DESCRIPTIONWe 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
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
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
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
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.
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.
Referring again to
Referring to
In
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.
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 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
Type: Application
Filed: Dec 13, 2007
Publication Date: Jun 18, 2009
Applicant: International Business Machines Corporation (Armonk, NY)
Inventors: John P. Karidis (Ossining, NY), Mark D. Schultz (Ossining, NY)
Application Number: 11/956,024
International Classification: F28D 15/00 (20060101); B21D 53/02 (20060101);