THERMAL INTERFACE MATERIALS AND METHODS FOR APPLICATION

Abstract

A thermal interface material delivered as a single-component precursor mixture which reacts to form a soft, solid material. Thermally conductive particles are dispersed in the reactive polymer matrix resulting in a composite material with high thermal conductivity. A reaction inhibitor is provided so that the one-component system is stable in storage and handling at room temperature, and curable at an elevated temperature. The uncured precursor material is easily dispensed using conventional single-component automated pumping equipment, and subsequently cured in place.

Claims

1. A precursor mixture for forming a thermally conductive material having a thermal conductivity of at least 0.5 W/m*K, said precursor mixture comprising: a first reactant composition including silicone; a second reactant composition that is reactive with the first reactant composition to form a siloxane; a reaction inhibitor effective to slow a reaction rate between the first and second reactant compositions at a storage temperature below 40° C., wherein an initial viscosity of the mixture maintained at the storage temperature increases by less than 100% over 14 days; and thermally conductive particles dispersed in at least one of the first and second reactant compositions.

2. The precursor mixture as in claim 1 wherein the second reactant composition is reactive with the first reactant composition to form a polydimethylsiloxane.

3. The precursor mixture as in claim 2 wherein the polydimethylsiloxane includes a terminal vinyl group, a pendant vinyl group, a terminal silicon hydride, or a pendant silicon hydride.

4. The precursor mixture as in claim 1, including a reaction catalyst selected from the group consisting of platinum, rhodium, palladium, osmium, and complexes and organometallic compounds thereof.

5. The precursor mixture as in claim 4 wherein the reaction inhibitor includes one or more of a maleate, an acetylenic alcohol, and a fumarate.

6. The precursor mixture as in claim 1 wherein the initial viscosity is less than 500 Pa*s at 100 s.sup.−1 at 25° C.

7. The precursor mixture as in claim 1 wherein the initial viscosity is less than 3500 Pa*s at 1.0 s.sup.−1 at 25° C.

8. The precursor mixture as in claim 7 being thixotropic.

9. The precursor mixture as in claim 1 wherein the thermally conductive material is curable from the precursor mixture to exhibit a cured durometer of between Shore 00=5 and Shore 00=90 at 25° C.

10. The precursor mixture as in claim 10 wherein the thermally conductive particles include one or more of aluminum oxide, aluminum nitride, silicon oxide, zinc oxide, and boron nitride.

11. A package for dispensing a curable mixture to form a thermally conductive body, said package comprising: a vessel defining a chamber in fluid communication with an orifice, the curable mixture being disposed in the chamber and including: a first reactant composition including silicone; a second reactant composition reactive with the first reactant composition to form a siloxane; a reaction catalyst; a reaction inhibitor effective to inhibit the catalyzed reaction between the first reactant composition and the second reactant composition at temperatures below 40° C., wherein an initial viscosity of the curable mixture maintained at a storage temperature below 40° C. increases by less than 100% over 14 days; and thermally conductive particles dispersed in at least one of the first and second reactant compositions;

12. The package as in claim 11 wherein the thermally conductive body exhibits a thermal conductivity of at least 0.5 W/m*K.

13. The package as in claim 12 wherein initial viscosity is between 100-3,500 Pa*s at 1.0 s.sup.−1 at 25° C.

14. The package as in claim 13 wherein the initial viscosity is between 50-500 Pa*s and 100 s.sup.−1 at 25° C.

15. The package as in claim 14 wherein the curable mixture is curable to a durometer hardness of between Shore 00=5 and Shore 00=90.

16. The package as in claim 11 wherein the curable mixture is dispensable through the orifice at a flow rate of 5-200 g/min under 90 Psi pressure for at least 14 days after initial combination of the curable mixture into the chamber when maintained at the storage temperature of less than 40° C.

17. The package as in claim 16 wherein the orifice is 2 mm or less in diameter.

18. A method for applying a thermal interface material to a surface, said method comprising: (a) providing a curable mixture including: (i) a first reactant composition including silicone; (ii) a second reactant composition reactive with the first reactant composition to form a siloxane; (iii) a reaction catalyst (iv) a reaction inhibitor effective to interact with the reaction catalyst to slow a reaction rate between the first and second reactant compositions; and (v) thermally conductive particles dispersed in at least one of the first and second reactant compositions; (b) storing the curable mixture in a vessel for more than 24 hours; and (c) dispensing the curable mixture from the vessel through an orifice onto the surface.

19. The method as in claim 18, including, subsequent to dispensing, heating the curable mixture to above 40° C. for a period of time sufficient to cure the curable mixture.

20. The method as in claim 18 wherein the thermal interface material exhibits a thermal conductivity of at least 0.5 W/m*K.

21. The method as in claim 18 wherein the surface is part of a heat-generating electronic component.

22. The method as in claim 21, including dispensing the curable mixture between the surface and a heat dissipation member.

23. The method as in claim 18 wherein the orifice is 2 mm or less in diameter.

24. A method for applying a thermal interface material to a surface for filling a thermal gap between a heat-generating electronic component and a heat dissipation member, said method comprising: (a) providing a curable mixture having a viscosity of less than 500 Pa*s at 100 s.sup.−1 at 25° C.; (b) storing the curable mixture in a vessel for more than 24 hours; (c) dispensing the curable mixture from the vessel to the surface of at least one of the heat-generating electronic component and the heat dissipation member; and (d) heating the curable mixture to above 40° C. for a period of time sufficient to form the thermal interface material from only the curable mixture, wherein said thermal interface material exhibits a durometer hardness of at least 5 shore 00 and a thermal conductivity of at least 0.5 W/m*K.

25. The method as in claim 24 wherein said thermal interface material includes a siloxane.

26. The method as in claim 25 wherein the siloxane includes a polydimethylsiloxane with a terminal vinyl group, a pendant vinyl group, a terminal silicon hydroxide, or a pendant silicon hydride.

27. The method as in claim 24, including storing the curable mixture in the vessel at less than 40° C.

28. The method as in claim 24, including sandwiching the thermal interface material between the heat-generating electronic component and the heat dissipation member.

29. The method as in claim 28 wherein the thermal interface material is in physical contact with each of said heat-generating electronic component and said heat dissipation member.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 is a schematic illustration of a precursor mixture being dispensed from a vessel onto a surface.

[0016] FIG. 2 is a cross-sectional view of an electronic package incorporating a thermally conductive interface material of the present invention.

[0017] FIG. 3 is a chart plotting flow rate of a precursor mixture over time.

[0018] FIG. 4 is a chart plotting durometer hardness against mass concentrations of the polymer components of a thermal interface material of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The thermally conductive interface material of the present invention includes a highly conformable silicone polymer filled with thermally conductive particles. Generally, the silicone may be an organosiloxane having the structural formula:

##STR00001##

wherein “R.sub.1” represents hydrogen, hydroxyl or methyl groups, and wherein “X.sub.1” and “X.sub.2” represents an integer ranging from between 1 and 1,000 and do not need to be equal. The thermally conductive interface material may be prepared as a reaction product of the organosiloxane together with a chain extender/cross-linker such as a hydride terminated polydimethyl siloxane having the structural formula:

##STR00002##

wherein “R.sub.2” represents either hydrogen, methyl or hydroxyl groups, and wherein “Y” represents an integer having a value of between 1 and 1,000.

[0020] Generally, the thermally conductive interface material is a curable composition formed from a precursor mixture of a first reactant composition including silicone, a second reactant composition that is reactive with the first reactant composition to from a siloxane, and a reaction catalyst. Organosiloxanes useful in a first reactant composition may include at least two aliphatic unsaturated organic groups such as vinyl, allyl, butenyl, hexenyl, ethenyl, and propenyl. The unsaturated functional groups may be located at terminal or pendant positions.

[0021] An example first reactant composition for a curable mixture of the present invention includes polydiorganosiloxanes, such as various vinyl or siloxy-terminated polydimethylsiloxanes (PDMS). Example commercially-available PDMS materials include Nusil PLY-7500, 7905, 7924, and 7925 available from Avantor, Inc.; Evonik VS 100, 200, 500, 10000, 20000, and 65000 available from Evonik Industries AG; and Gelest DMS-V21, V22, V41, V42, and V43 available from Gelest, Inc. The first reactant composition may include one or more polymers that differ in, for example, molecular weight, viscosity, and molecular structure.

[0022] The second reactant composition that is reactive with the first reactant composition may include a cross-linker for a hydrosilylation reaction. The second reactant composition may include a dihydroxy aliphatic chain extender such as a hydride-terminated polydimethylsiloxane. The silicon-bonded hydrogen atoms may be located at terminal, pendant, or at both terminal and pendant positions. The second reactant composition may include one or more organohydrogenpolysiloxanes that may differ in at least one of molecular weight, viscosity, and molecular structure. Example commercially-available methylhydropolydimethylsiloxanes useful as a second reactant composition reactive with the first reactant composition include Nusil XL-173, 176, and 177 available from Avantor, Inc.; Gelest HMS-071, 082, and 991 available from Gelest, Inc.; and Andisil XL-1B and 1340 available from AB Specialty Silicones.

[0023] In some embodiments, the precursor mixture for forming a thermally conductive material includes a reaction catalyst, such as catalyst effective in a hydrosilylation curable composition. Suitable hydrosilylation catalysts are known in the art and commercially available. Hydrosilylation catalysts may include, for example, platinum, rhodium, palladium, osmium, and complexes and organometallic compounds thereof. Example commercially-available catalysts include Nusil Catalyst 50 from Avantor, Inc.; Gelest SIP6030.3 from Gelest, Inc.; Evonik Catalyst 512 from Evonik Industries AG; and Sigma Aldrich 479519.

[0024] In order to enhance the thermal conductivity of the thermally conductive material, the compositions of the present invention may include thermally conductive particles dispersed therein. The particles may be both thermally conductive and electrically conductive. Alternatively, the particles may be thermally conductive and electrically insulating. Example thermally conductive particles include aluminum oxide, silicon oxide, aluminum trihydrate, zinc oxide, graphite, magnesium oxide, aluminum nitride, boron nitride, metal particulate, and combinations thereof. The thermally conductive particles may be of various shape and size, and it is contemplated that a particle size distribution may be employed to fit the parameters of any particular application. In some embodiments, the thermally conductive particles may have an average particle size of between about 0.1-250 micrometers, and may be present in the thermally conductive material at a concentration by weight of between about 20-95%.

[0025] The thermally conductive particles may be dispersed in at least one of the first and second reactant compositions at a loading concentration of about between about 20-95% by weight. It is desirable that sufficient thermally conductive particles are provided so that the thermally conductive material formed from the precursor mixture exhibits a thermal conductivity of at least 0.5 W/m*K.

[0026] A reaction inhibitor is preferably provided in the precursor mixtures of the present invention that is effective to inhibit the reaction between the first reactant composition and the second reactant composition. An aspect of the present invention is to permit storage of the precursor mixture in a vessel as a single form-factor preparation that is stable at room temperature for at least 14 days. For the purposes hereof, the preparation or precursor mixture that is stable at room temperature is one in which an initial viscosity of the precursor mixture maintained at a storage temperature below 40° C. increases by less than 100% over the course of 14 days. This stability of the precursor mixture permits packaging of the mixture into a vessel and storage for an extended period prior to dispensation. The extended term of stability permits the manufacture and packaging of thermally conductive material to be performed at a place and/or time that is different than the place and/or time of dispensation such as at an electronic package assembler.

[0027] In some embodiments, the reaction inhibitor may be effective to interact with the reaction catalyst to slow the reaction rate between the first and second reactant compositions. Generally, the reaction inhibitor may include one or more of a maleate, and acetylenic alcohol, and a fumarate. Example reaction inhibitors include dimethyl maleate, diallyl maleate, bis(1-methoxy-2-propyl)maleate, dibutyl maleate, 1-ethynyl-cyclohexanol, 2-methyl-3-butyn-2-ol, 3,7,11-trimethyl-1-dodecyn-3-ol, 3,5-dimethyl-1-hexyn-3-ol, 1-ethynyl-1-cyclopentanol, 3-methyl-1-dodecyn-3-ol, 4-ethyl-1-octyn-3-ol, 1,1-diphenyl-2-propyn-1-ol, 2,3,6,7-tetramethyl-4-octyne-3,6-diol, 3,6-diethyl-1-nonyn-3-ol, 3-methyl-1-pentadecyn-3-ol, 2,5-dimethyl-3-hexyne-2,5-diol, 2,7-dimethyl-3,5-octadiyne-2,7-diol, 3-methyl-1-pentyn-3-ol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 1,4bis(1′-hydroxycyclohexyl)-1,3-butadiyne, 3,4-dimethyl-1-pentyne-3,4-diol, 1-(1-butynyl)cyclopentanol, 2,5-dimethyl-5-hexen-3-yn-2-ol, 5-dimethylamino-2-methyl-3-pentyl-2-ol, 3,6-dimethyl-6-hepten-4-yn-3-ol, 3-methyl-1-octyn-3-ol, 3,4,4-trimethyl-1-pentyn-3-ol, 3-isobutyl-5-methyl-1-hexyn-3-ol, 2,5,8-trimethyl-1-nonen-3-yn-5-ol, 1-(1-propynyl)cyclohexanol.

[0028] Various other components are contemplated as being optionally included in the compositions of the present invention, such as adhesion promoters, surfactants, stabilizers, fillers, and combinations thereof.

[0029] The precursor mixtures of the present invention are preferably stable at room temperature and will react at elevated temperature, such as above 40° C., to cure to a solid body as a form-in-place interface. The rate of this reaction can be controlled by the concentration of reactive functional groups, the catalyst, and the reaction inhibitor. Rheology of the dispersion may be further controlled by the sizes, shapes, and loading concentration of thermally conductive particles dispersed therein.

[0030] FIG. 1 illustrates an example application of the present invention, wherein a curable mixture 10 is contained in a vessel 12 having an orifice 14 through which the curable mixture may be dispensed. In the illustrated embodiment, curable mixture 10 is dispensed to a surface 22 of a member 20. As is known in the art, one or both of vessel 12 and member 20 may be moved relative to one another, such as along direction arrows 8 to apply the curable mixture 10 as needed at surface 22. Member 20 may, for example, be a heat-generating electronic component or a heat dissipation member. FIG. 2 illustrates curable mixture 10 disposed between the heat-generating electronic component 30 and a heat dissipation member 40. The curable mixture 10 may be heated to above 40° C. for a period of time sufficient to form a thermal interface material from only the curable mixture 10. Heating of the curable mixture in situ may be performed by known heating means, such as a heat oven or the like.

Examples

[0031] An example precursor mixture is sets forth in Table 1 below:

TABLE-US-00001 TABLE 1 Constituent Concentration (wt %) Vinyl terminated PDMS  5-15 Methylhydro PDMS 1-5 Catalyst <0.1 Dimethyl maleate <0.1 Aluminum oxide powder 50-95

[0032] The precursor mixture exhibits an inhibited reaction rate illustrated by the small change in viscosity over an extended working time of at least 14 days with the material at room temperature. FIG. 3 plots a flow rate of the precursor mixture through a 2 mm orifice under 90 Psi pressure at 25° C. over time. This enables automated processing with tight control on dispensed quantity and patterning. The long working time also provides flexibility in the electrical component handling, transportation, and assembly processes.

[0033] The final cured thermal interface material exhibits a hardness that is designed to be soft with good adhesion to common metal and plastic substrates found in electronic devices. FIG. 4 illustrates how various hardness levels can be tuned and controlled by the reactivity and concentration of the first and second reactant compositions. Softness of the thermal interface material allows the material to flex and resist cracking as the device goes through thermal cycling during operation. The following Table 2 sets forth physical property parameters of the precursor mixture, as well as hardness for the cured thermally conductive material:

TABLE-US-00002 TABLE 2 Parameter Range EFD Flow rate - 2 mm orifice, 90 psi 5-200 g/min Hardness - Shore 00 5-90 Thermal conductivity - ASTM D5470 0.5-5 W/m-K Low shear viscosity - parallel plate 100-3500 Pa*s rheometer, 25 mm platen, 1 mm gap, shear rate = 1.0 s.sup.−1 High shear viscosity - capillary rheometer, 50-500 Ps*s 16 × 2 mm die, shear rate = 100 s.sup.−1

[0034] The invention has been described herein in considerable detail in order to comply with the patent statutes, and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use embodiments of the invention as required. However, it is to be understood that various modifications can be accomplished without departing from the scope of the invention itself.