Patterned Design for Thermal Management of Two-Phase Immersion Cooling Systems for Electronic Devices

Abstract

A direct-tip cooling film for two-phase cooling. The film includes a dielectric layer having a first surface for attachment to a cold plate or circuit and a second surface. A metal layer is on the second surface of the dielectric layer and has a pattern of features on the opposite side of the dielectric layer. The surface pattern provides increased surface area and multiple nucleation sites for bubble formation for two-phase cooling. The features can also include metal nodules to further enhance nucleation.

Description

[Background technology]

[0001] High density semiconductor chips generate high temperatures and require thermal management to keep the chip operating temperature intact. Immersion cooling technology is being considered as part of the thermal management for new data centers. Immersion cooling technology involves a phase change in the heat transport process, which reduces the interface resistance when in contact with heated semiconductor devices. Summary of the Invention

[0002] Submerged two-phase cooling systems use dielectric fluids such as fluorocarbon chemicals. However, due to low surface tension and wetting properties, the dielectric fluid penetrates deep into the cavity, which promotes the accumulation of overheat on the chip surface. This accumulation results in localized hot spots on the chip surface and leads to thermal stresses that ultimately degrade chip performance and increase chip failure rates. An embodiment of the present invention includes a film that can mitigate such accumulation of localized hot spots and improve two-phase cooling of electronic devices.

[0003] The first film with features for two-phase cooling includes a dielectric layer having a first surface for attachment to a cold plate or circuitry and having a second surface, and a metal layer is on the second surface of the dielectric layer and has a pattern of features on the opposite side of the dielectric layer.

[0004] A second film with features for dual phase cooling includes a metal layer having a first surface for attachment to a cold plate or circuit and having a second surface. Metal stands are on the first surface of the metal layer with a bonding material between the metal stands. A plurality of metal nodules are on the second surface of the metal layer. [Brief description of the drawings]

[0005] [Figure 1A] FIG. 2 is a cross-sectional side view of a chip cooling film with a micropattern. [Figure 1B] FIG. 1 is a cross-sectional side view of a chip cooling film with a micropattern having angled sidewalls. [Diagram 2] FIG. 1 is a cross-sectional side view of a chip cooling film with a micropattern and multiple cavities. [Figure 3A] FIG. 1 is a cross-sectional side view of a chip cooling film with a micropattern having angled sidewalls. [Figure 3B] FIG. 1 is a cross-sectional side view of a micropatterned chip cooling film having angled sidewalls. [Figure 4] FIG. 1 is a cross-sectional side view of a chip cooling film with a micropattern and an etched dielectric. [Figure 5A] FIG. 2 is a cross-sectional side view of a chip cooling film with nodules. [Figure 5B] FIG. 5B is a top-down image of the chip cooling film of FIG. 5A. [Figure 5C] FIG. 5B is a top-down image of the chip cooling film of FIG. 5A. [Figure 6] FIG. 13 is a cross-sectional side view of a chip cooling film with nodules on an angled micropattern. [Figure 7] FIG. 1 is a cross-sectional side view of a chip cooling film with a uniform pattern of microstructures. [Figure 8] FIG. 2 is a cross-sectional side view of a chip cooling film with a uniform pattern of features having nodules. [Figure 9] FIG. 2 is a cross-sectional side view of a chip cooling film with nodules and multiple cavities. [Figure 10A] FIG. 1 is a diagram of a two-phase immersion cooling system. [Figure 10B] FIG. 1 is a diagram of a two-phase immersion cooling system using an internal heat spreader. [Figure 10C] FIG. 1 is a diagram of a two-phase immersion cooling system using piping for cooling fluid. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0006] The embodiments include a flexible or rigid heat transfer nucleation film with features such as patterns and nodules to enhance a dual phase immersion cooling system for electronic devices. The film includes a dielectric and a thin metal layer formed on the dielectric, on which a uniform metal pattern is formed. The uniform metal pattern includes a three-dimensional microstructure and is formed with a specific trace width and thickness. One or more cavities extend through the bottom surface of the dielectric, and a solder or metal paste is filled inside the cavity of the dielectric, which is useful for attaching the film to an electronic device with a thick cold plate for cooling or directly to the electronic device without a thick cold plate.

[0007] Figure 1A is a cross-sectional side view of a chip cooling film attached to a cold plate 18 and comprising a micropattern having metal features 10, thin metal layer 12, dielectric 14, and solder 16 arranged as shown. Figure 1B is a cross-sectional side view of a chip cooling film attached to a cold plate 28 and comprising a micropattern with angled sidewalls having sloped metal microfeatures 20, thin metal layer 22, dielectric 24, and solder 26 arranged as shown.

[0008] The microstructure of the metal features 10 and 20 can be formed either by an additive electroplating process using a photomask, or using a subtractive process using a photomask. The development of a micropattern provides greater surface area and nucleation sites for bubble formation during heating. Also, uniformity of the metal microstructure is important for uniform heat dissipation across the surface.

[0009] These micropatterns serve to nucleate gas bubbles to transport the heat load from the electronics. The space and feature sizes of the micropatterns can be tailored depending on the thermal performance required.

[0010] Additionally, the dielectric may be etched to create one or more cavities, which are then filled with solder 35, as shown in Figure 2. Figure 2 is a cross-sectional side view of a chip cooling film with a micropattern and multiple cavities attached to a cold plate 36 and having metal features 30, a thin metal layer 32, a dielectric 34, and cavities containing solder 35, arranged as shown. The cavities may help provide structural integrity for the circuitry attached to the cold plate when solder reflow occurs. The microstructure of the metal features 30 may be formed as described above.

[0011] 3A and 3B show metal features with different angles. FIG. 3A is a cross-sectional side view of a chip cooling film attached to a cold plate 46 and comprising a micropattern with angled sidewalls, with angled sidewalls 38, metal features 40, thin metal layer 42, dielectric 44, and cavities containing solder 45, arranged as shown. FIG. 3B is a cross-sectional side view of a chip cooling film attached to a cold plate 56 and comprising a micropattern with angled sidewalls, with cone-shaped pattern 48, metal features 50, thin metal layer 52, dielectric 54, and cavities containing solder 55, arranged as shown. The angled metal features provide a larger surface area to facilitate liquid flow between the metal features. The microstructure of the metal features 40 and 50 can be formed as described above. Additionally, the space is reduced at the bottom of the sidewalls to facilitate a rapid nucleation process. The gap and sidewall angle can be adjusted according to the thermal performance requirements. Figure 3B shows a uniform cone-shaped structure, which is conducive to faster nucleation and formation of small bubbles due to narrow nucleation sites, enhancing the heat dissipation performance.

[0012] To provide structural integrity and allow for easy handling of the thin, flexible chip cooling film, the dielectric can be further etched and filled entirely with metal by plating techniques, as shown in Figure 4. Figure 4 is a cross-sectional side view of a chip cooling film with a micropatterned and etched dielectric having microfeatures 58, metal filled vias 60, and solder or metal paste 62 arranged as shown. This filled etch provides rigidity and allows for easy handling during solder reflow or other bonding processes. The etch also improves the thermal performance of the system.

[0013] FIG. 5A illustrates the formation of nodular features on a uniform metal layer. FIG. 5A is a cross-sectional side view of a chip cooling film with nodules attached to a cold plate 72 and having metal nodules 64, thin metal layer 66, supporting metal stands 68, and vias 70 arranged as shown. The uniform nodular features can be formed through electroplating techniques. The vias 70 extend between the metal stands 68, and the vias can be formed by patterning the dielectric layer followed by chemical etching. These vias can be filled with solder or metal bonding paste, either by screen printing or dispensing, and then attached to a thick rigid cold plate. These fine patterns help nucleate bubbles to transport the heat load from the electronics. The size of the nodular features can be adjusted depending on the required thermal performance.

[0014] FIG. 5B shows a micrograph of metal nodule grains and the size of these nodules, and the distance between the nodules can be adjusted depending on the thermal performance requirements. FIG. 5C shows uniform roughness on the metal surface. Instead of nodules, uniform roughness can be created to improve the heat transport in the boiling system. Uniform roughness ranging from nano-level to micrometer level can help develop a large amount of bubbles due to the large surface area and nucleation sites.

[0015] FIG 6 shows nodules on a graded uniform metal pattern. FIG 6 is a cross-sectional side view of a chip cooling film with nodules on an angled micropattern attached to a cold plate 80 and having nodules 74, a thin metal layer 76, a dielectric 78, and vias 80 filled with a metallic bonding material, arranged as shown. This surface pattern provides an additional increase in surface area and provides multiple nucleation sites for bubble formation. The microstructure of the metal layer 76 can be formed as described above.

[0016] Figure 8 illustrates another embodiment of nodule features on the uniform metal pattern shown in Figure 7. Figure 7 is a cross-sectional side view of a chip cooling film attached to a cold plate 92 and comprising a uniform pattern of microstructures having a uniform pattern of features 84, a thin metal layer 86, supporting metal stands 88, and vias 90 filled with a metallic bonding material, arranged as shown. Figure 8 is a cross-sectional side view of a chip cooling film attached to a cold plate 104 and comprising a uniform pattern of nodule-bearing features having metal nodules 94, uniform metal features 96, a thin metal layer 98, supporting metal stands 100, and vias 102 filled with a metallic bonding material, arranged as shown.

[0017] Multiple cavities can be formed under a single patterned cold plate, helping to provide structural integrity of the circuit when solder reflow occurs, as shown in Figures 7 and 8. The uniform metal microfeatures of the chip cooling film shown in Figures 7 and 8 can be formed either by an additive electroplating process using a photomask, or using a subtractive process using a photomask. Additionally, plating of nodules can be performed using electroplating to form uniform nodules throughout the chip cooling film. The development of micropatterns and nodules on the patterned metal provides more surface area and nucleation sites for bubble formation during heating. The uniformity of the metal microfeatures is important for uniform heat dissipation across the surface.

[0018] FIG. 9 shows metal pillars underneath the thin metal features. These pillars can improve thermal performance due to their larger surface area and thermal capacity, and the pillars can provide structural integrity when circuitry is attached using a thick metal cold plate. FIG. 9 is a cross-sectional side view of a chip cooling film with nodules and multiple cavities attached to a cold plate 114 and having metal nodules 106, thin metal layer 108, supporting metal stands 110, and solder or metal bonding material 112 arranged as shown. Once the metal nodules are formed as described above, a photomask is used to pattern the dielectric side of the film and chemical etching of the dielectric is performed to form the cavities. After the chemical etching process, the photoresist is removed. Electroplating from inside the cavities is used to fill the etched portions with metal to form the metal pillars. Finally, the remaining exposed dielectric is removed by a chemical etching process to form cavities adjacent to the metal pillars. These cavities can be filled with metal bonding material to bond directly to a thick metal cold plate or electronics.

[0019] The above features can also be formed using thick metal embossing techniques. A thick metal foil or metal plate can be embossed with different dimensions and features at the bottom, and then electroplating techniques can be used to create uniform nodules. Finally, the embossed metal cavities are filled with a bonding material.

[0020] Below are example materials, dimensions and configurations of the chip cooling films described herein.

[0021] Suitable dielectric materials include polyimide, liquid crystal polymer, PEEK polymer, polyethylene terephthalate, flexible FR4 sheet, or polyolefin. Suitable metals used to create the features and layers are copper, steel, and silver. Suitable cladding plating materials include gold, silver, and tin. Suitable bonding materials include solder, metal paste, silver paste, and anisotropic conductive film (ACF) bonding solutions.

[0022] Suitable bonding materials include lead-free solder, copper paste, silver paste, ACF film or paste, and thermally conductive adhesives.

[0023] The suitable thickness of the dielectric layer is in the range of 25 μm to 125 μm. The thickness of the metal filled vias may be in the range of 25 μm to 125 μm. The micropatterned features may have a height from 5 μm to 100 μm above the underlying metal layer. The nodules may have a diameter in the range of 0.1 μm to 5 μm.

[0024] Non-tilted metal features formed on the top metal surface have a 90 degree angle, within a certain tolerance, relative to the underlying metal layer.

[0025] The slanted or angled metal features formed on the upper metal surface can have an angle of about 30 to 90 degrees relative to the underlying metal layer.

[0026] The etched dielectric cavity can have a slope angle of about 25 to 90 degrees relative to the underlying cold plate or circuitry. This trapezoidal slope angle helps to retain the solder or metallic bonding paste, such as silver, in the cavity.

[0027] 10A-10C are diagrams illustrating the use of a tip cooling film in a two-phase cooling system.

[0028] 10A is a diagram of a two-phase immersion cooling system having a printed circuit board (PCB) 116, an integrated circuit (IC) 118, and a chip cooling film 120. The IC 118 can be attached to the chip cooling film 120 using a metal bonding material such as solder or material in a cavity or via, without a separate thick metal cold plate.

[0029] 10B is a diagram of a two-phase immersion cooling system using an internal heat spreader and having a PCB 122, an IC 124, an internal heat spreader (IHS) 126, and a chip cooling film 128. The chip cooling film 128 can be attached to the IHS 126 using a metal bonding material such as solder or material in the cavities or vias without a separate thick metal cold plate.

[0030] 10C is a diagram of a two-phase immersion cooling system using conduits for cooling fluid and having IC 130, conduits 132 for cooling fluid, and an internal baffle with a chip cooling film, such as baffle 134 with chip cooling film 136. Chip cooling film 136 can be attached to baffle 134 using a metal bonding material, such as solder or material in the cavity or via, without a separate thick metal cold plate. Each of the baffles can include a chip cooling film attached in a similar manner.

[0031] The chip cooling films 120, 128, and 136 can be implemented with any of the chip cooling films described herein, and in some cases with an associated metal cold plate. The chip cooling films can be used in two-phase cooling systems with flexible or rigid circuits. The chip cooling films can be attached in physical contact directly to the circuit without a separate metal cold plate, or alternatively can be used with a metal cold plate between and in physical contact with the circuit and the chip cooling film. In addition to the respective embodiments, the following aspects are noted. (Appendix 1) 1. A film with features for two-phase cooling, comprising: a dielectric layer having a first surface for attachment to a cold plate or circuitry and having a second surface; a metal layer on the second surface of the dielectric layer, the metal layer having a pattern of features thereon opposite the dielectric layer and for use in two-phase cooling when the film is attached to the cold plate or circuit; A film comprising: (Appendix 2) 2. The film of claim 1, wherein the feature has a sloping sidewall. (Appendix 3) 2. The film of claim 1, wherein the feature comprises a cone-shaped pattern. (Appendix 4) 2. The film of claim 1, wherein the pattern is uniform. (Appendix 5) 2. The film of claim 1, further comprising a nodule on the feature. (Appendix 6) 6. The film of claim 5, wherein the nodules are composed of copper, steel, or silver. (Appendix 7) 2. The film of claim 1, wherein the dielectric layer comprises a cavity. (Appendix 8) 8. The film of claim 7, wherein the dielectric layer is etched to form the cavity. (Appendix 9) 8. The film of claim 7, further comprising a bonding material in the cavity. (Appendix 10) 2. The film of claim 1, wherein the dielectric layer includes vias that accommodate a bonding material. (Appendix 11) 2. The film of claim 1, wherein the metal layer is composed of copper, steel, or silver. (Appendix 12) 2. The film of claim 1, wherein the dielectric layer is composed of a polyimide, a liquid crystal polymer, a PEEK polymer, a polyethylene terephthalate, a flexible FR4 sheet, or a polyolefin. (Appendix 13) 1. A film with features for two-phase cooling, comprising: a metal layer having a first surface for attachment to a cold plate or circuitry and having a second surface; a metal stand on the first surface of the metal layer; a bonding material on the first surface of the metal layer between the metal stands; a plurality of metal nodules on the second surface of the metal layer, the plurality of metal nodules being for use in two-phase cooling when the film is attached to the cold plate or circuit; A film comprising: (Appendix 14) 14. The film of claim 13, further comprising a pattern of features, the nodules being on the features. (Appendix 15) 15. The film of claim 14, wherein the pattern is uniform. (Appendix 16) 14. The film of claim 13, wherein the metal layer is composed of copper, steel, or silver. (Appendix 17) 14. The film of claim 13, wherein the nodules are comprised of copper, steel, or silver. (Appendix 18) 1. A two-phase cooling system for an electronic device, comprising: A printed circuit board; an integrated circuit on the printed circuit board; a chip cooling film on the integrated circuit opposite the printed circuit board, the chip cooling film comprising any one of the films described in appendix 1-17; A two-phase cooling system comprising: (Appendix 19) 1. A two-phase cooling system for an electronic device, comprising: A printed circuit board; an integrated circuit on the printed circuit board; an internal heat spreader over the integrated circuit and on the printed circuit board; a chip cooling film on the internal heat spreader opposite the integrated circuit, the chip cooling film comprising any one of the films described in appendix 1-17; A two-phase cooling system comprising: (Appendix 20) 1. A two-phase cooling system for an electronic device, comprising: An integrated circuit; a conduit containing a cooling fluid in contact with the integrated circuit; a baffle inside the conduit; a tip cooling film on one or more of the baffles, the tip cooling film comprising any of the films described in appendix 1-17; A two-phase cooling system comprising:

Claims

1. 1. A film with features for two-phase cooling, comprising: a dielectric layer having a first surface for attachment to a cold plate or circuitry and having a second surface; a metal layer on the second surface of the dielectric layer, the metal layer having a pattern of features thereon opposite the dielectric layer and for use in two-phase cooling when the film is attached to the cold plate or circuit; Equipped with the dielectric layer has a cavity with a trapezoidal inclination angle with respect to the cold plate or the circuit; A film, the film being attached to the cold plate or circuitry by a metallic bonding material filled into the cavity.

2. The film of claim 1 , wherein the dielectric layer is etched to form the cavity.

3. 1. A film with features for two-phase cooling, comprising: a metal layer having a first surface for attachment to a cold plate or circuitry and having a second surface; a metal stand on the first surface of the metal layer; a bonding material on the first surface of the metal layer between the metal stands; a plurality of metal nodules on the second surface of the metal layer, the plurality of metal nodules being for use in two-phase cooling when the film is attached to the cold plate or circuit; Equipped with the metal stand has a cavity with a trapezoidal inclination angle with respect to the cold plate or the circuit; The bonding material filled into the cavity attaches the film to the cold plate or circuitry.

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