Thermal conductivity promoter for electrically conductive compositions

Abstract

This disclosure relates to a thermal conductivity promoter used in electrically conductive compositions, such as film adhesives.

Description

THERMAL CONDUCTIVITY PROMOTER FOR ELECTRICALLY CONDUCTIVE COMPOSITIONSBACKGROUNDFIELD

[0001] This disclosure relates to a thermal conductivity promoter used in electrically conductive compositions, such as film adhesives.BRIEF DESCRIPTION OF RELATED TECHNOLOGY

[0002] With the rapid development of high functioning electronics devices, heat management becomes more vital, which leads to a demand on the development of conductive assembly film adhesives with higher thermal conductivity. Especially for electronics in the aerospace industry, high frequency communication systems use GaAr / GaN chips. These chips generate heat that current assembly film products can no longer dissipate.

[0003] The conductive assembly film is usually used in second level interconnect (package to board). Typically, a highly complex printed circuit board (with a dimension of in2) is adhered using a conductive assembly film adhesive to a heat sink to effectively dissipate heat. Many current conductive assembly films in the market provide a thermal conductivity of less than 10 W / m-k.

[0004] Some of the current solutions to increase thermal conductivity include:1. Increasing the silver loading, which largely decreases the tensile lap shear adhesion.2. Curing the adhesive at a temperature over 200’C to get silver sintering effect, which is too harsh for 2ndlevel chip assembly.3. Adding carbon nanotubes to boost thermal performance.

[0005] But these solutions also come with drawbacks. For instance, increasing the silver loading decreases the tensile lap shear adhesion, which may also affect the film integrity (e g., high silver loading may also cause unqualified films to satisfy an industrial standard); curing the adhesive at a temperature over 200’C to promote a silver sintering effect is too harsh an environment for 2ndlevel chip assembly as the chip(s) can become compromised at such temperatures; and adding carbon nanotubes.to boost thermal performance reduces adhesion, and at a minimum the dispersion of CNT is very challenging at a commercial level.

[0006] In one approach to improving thermal conductivity, U.S. Patent No. 10,388,423 (Van Veen), which is directed to and claims an electrically conductive composition, provides(i) 2 - 50 wt% of a binder selected from polyurethane elastomers, polyesters, phenolic resins, acrylic polymers, acrylic block copolymers, acrylic polymers having tertiary-alkyl amide functionality, polysiloxane polymers, polystyrene copolymers, polyvinyl polymers, divinylbenzene copolymers, polyetheramides, polyvinyl acetals, polyvinyl butyrals, polyvinyl acetols, polyvinyl alcohols, polyvinyl acetates, polyvinyl chlorides, methylene polyvinyl ethers, cellulose acetates, styrene acrylonitriles, amorphous polyolefins, thermoplastic urethanes, polyacrylonitriles, ethylene vinyl acetate copolymers, ethylene vinyl acetate terpolymers, functional ethylene vinyl acetates, ethylene acrylate copolymers, ethylene acrylate terpolymers, ethylene butadiene copolymers and / or block copolymers, styrene butadiene block copolymers, phenolics, urethanes, phenoxy resins, epoxies, melamines and mixtures thereof;(ii) a conductive filler comprising(a) particles having a core plated with silver, wherein the core is selected from copper, nickel, palladium, carbon black, carbon fiber, graphite, aluminum, indium tin oxide, glass, polymers, antimony doped tin oxide, silica, alumina, fiber, clay, and mixtures thereof, and(b) a filler material selected from silver, copper, gold, palladium, platinum, nickel, gold or silver-coated nickel, carbon black, carbon fiber, graphite, aluminum, indium tin oxide, silver coated copper, silver coated aluminum, metallic coated glass spheres, metallic coated filler, metallic coated polymers, silver coated fiber, silver coated spheres, antimony doped tin oxide, conductive nanospheres, nano silver, nano aluminum, nano copper, nano nickel, carbon nanotubes or mixtures thereof; where the conductive filler makes up 20 - 70 wt.% of the composition; and(iii) a solvent comprising one or more dibasic esters, butylglycolacetate, and / or propylacetate;wherein said composition has a sheet resistivity of less than about 0.100 Ohm / square at 25 microns.

[0007] Other approaches include U.S. Patent No. 4,356,505 (Lovinger et al.) and U.S. Patent No. 8,398,898 (Dershem); Lu, Daoqiang, Quinn K. Tong and C.P. Wong, “A study of lubricants on silver flakes for microelectronics conductive adhesives”, IEEE Transactions on Components and Packaging Technologies 22.3, pgs. 365-371 (1999);Ogunjimi, A.O. et al., “A review of the impact of conductive adhesive technology on interconnection", Journal of Electronics Manufacturing 2,03, pgs. 109-118 (1992).

[0008] Notwithstanding the state of the art, alternative approaches to improving thermal conductivity of electrically conducive composition would be desirable, particularly where the adhesive performance and electrically conductive performance of such compositions is not comprised by the improved thermal conductivity.SUMMARY

[0009] The present invention provides an electrically conductive composition comprising:(a) an epoxy component;(b) a silver component;(c) a cure system; and(d) a thermal conductivity promoter; and(e) optionally, a solvent.

[0010] The conductive composition should have a proper balance of physical properties including having at least one, desirably at least two and even more desirably all, of a thermal conductivity of greater than 17 W / m-k, a volume resistivity of 10'4Ohm- cm or less, and tensile lap shear strength on aluminum substrates of greater than 1500 psi.

[0011] Another embodiment provides electronic devices manufactured using the electrically conductive composition so described.

[0012] Still another embodiment is directed to a process of making or forming an electronic device using the electrically conductive composition of the invention. The process comprises coating the composition onto a release liner for subsequent use as afilm. In use as a film, the electrically conductive composition may be applied onto a desired substrate, and then cured to obtain the desired performance, in terms of thermal conductivity, electrical conductivity and adhesion to the substrate,

[0013] Exemplary electronic devices that might use the so described compositions include computers and computer equipment, such as printers, fax machines, scanners, keyboards and the like; household appliances; medical sensors; automotive sensors and the like; and personal electronic devices, such as mobile phones, remote controls, cameras, music and video players, and the like,

[0014] Moreover, the so-formed electrically conductive compositions for use as conductive assembly adhesive films may be used in aerospace electronics, radar / sonar, and satellites due to the need for higher performing and reliability.DETAILED DESCRIPTION[0001 S] As noted above, the present invention provides an electrically conductive composition comprising:(a) an epoxy component;(b) a silver component;(c) a cure system; and(d) a thermal conductivity promoter; and(e) optionally, a solvent.

[0016] The conductive composition should have at least one, desirably at least two and even more desirably all, of a thermal conductivity of greater than 17 W / m~k, a volume resistivity of lO^ Ohm-om or less, and tensile lap shear strength on aluminum substrates of greater than 1500 psi.

[0017] The epoxy component of the conductive composition should comprise at least one epoxy resin, such as at least two epoxy resins, desirably at least three epoxy resins.

[0018] The epoxy component comprises one or more epoxy resins selected from aliphatic epoxy resins, cycloaliphatic epoxy resins, aromatic epoxy resins, hydrogenated epoxy resins and combinations thereof. For instance, the epoxy component may comprise one or more monofunctional epoxy resins selected from monofunctionalaliphatic epoxy resins, monofunctional cycloaliphatic epoxy resins, monofunctional aromatic epoxy resins, monofunctional hydrogenated aromatic epoxy resins and combinations thereof.

[0019] Or, the epoxy component may comprise one or more difunctionai epoxy resins selected from difunctional aliphatic epoxy resins, difunctionai cycloaliphatic epoxy resins, difunctionai aromatic epoxy resins, difunctionai hydrogenated aromatic epoxy resins and combinations thereof.

[0020] The epoxy component may also comprise one or more polyfunctional epoxy resins selected from polyfunctional aliphatic epoxy resins, polyfunctional cycloaliphatic epoxy resins, polyfunctional aromatic epoxy resins, polyfunctional hydrogenated aromatic epoxy resins and combinations thereof,

[0021] The epoxy component should be present in an amount of about 15 percent by weight to about 30 percent by weight, based on the total weight of the composition,

[0022] The silver component may comprise silver in flake form, particle form, sphere form, platelet form and / or powder form,

[0023] That portion of the silver component in particle form should have a size in the range of about 0.1 um to about 50 urn, such as about 0.1 urn to about 25 urn.

[0024] That portion of the silver component in particle form should have about 50 percent of the silver particles of a size in the range in the range of about 1 um to about 25 um, such as about 3 um to about 10 um,

[0025] The silver component should be present in the range of about 50 percent by weight to about 90 percent by weight, such as about 70 percent by weight to about 85 percent by weight, based on the total weight of the composition.

[0026] The conductive composition comprises a cure system, such as one including imidazole or imidazole derivatives. Particularly desirable examples of the cure system include those available commercially from Evonik Corporation, sold under the CUREZOL tradename. For instance, the OR series, the CN series, the Azine series, the OK series, the HZ series and the ZL series, are all imidazole based and according to the manufacturer may be used among other things as an epoxy resin hardener for electronic and electric applications or as an accelerator for epoxy resinhardeners combined with phenolic resins, acid anhydrides or dicyandiamide, and indicates good characteristics for electronic and electronic application,

[0027] The cure system should be present in the range of about 0.1 percent by weight to about 5 percent by weight, based on the total weight of the composition.

[0028] The thermal conductivity promoter ordinarily contains a hydroxyl terminated aromatic ether. The addition of the promoter to a silver filled electrically conductive adhesive, such as one in film form, can improve thermal conductivity significantly without compromising the adhesion strength. The cured product of the electrically conductive composition shows good thermal dissipation capability and is useful in electrical, thermal, and mechanical assembly applications, particularly in assembling circuit board materials, metal backplanes, and heat sinks.

[0029] The thermal conductivity promoter may be represented by;wherein X and Y are each independently H, OH or Ar~A, wherein Ar is an aromantic ring or an aromatic ring system; A occurs 1 to 4 times on the aromatic ring and is selected from halogen, OH, OR, wherein R is CM alkyl, COOH, COOR, wherein R is CM alkyl, NOs, and NRR1, wherein R and R1are each independently H or CM alkyl: and n is 1-4.

[0030] More specifically , the thermal conductivity promoter is selected from propylene glycol phenyl ethers.

[0031] The thermal conductivity promoter should be present in the range of about 0.1 percent by weight to about 5 percent by weight, such as about 0.2 percent by weight to about 1.5 percent by weight, based on the total weight of the composition. Used in such an amount, the thermal conductivity promoter improves thermal conductivity of the electrically conductive composition though surprisingly without compromising adhesion strength.

[0032] A particularly desirable example of a thermally conductivity promoter for use herein is available commercially from Dow Chemical under the tradename DOWANOL™ PPh glycol ether. That DOWANOL product is indicated by the manufacturer to be propylene glycol phenyl ether. According to the manufacturer DOWANOL™ PPh Glycol Ether is a slow-evaporating, very hydrophobic glycol ether ideal in coalescing and carrier solvent applications. DOWANOL™ PPh Glycol Ether is a slow evaporating, very hydrophobic glycol ether - more hydrophobic than would be expected based simply on its molecular weight. This product has low odor. With its aromatic structure, DOWANOL™ PPh Glyco! Ether is an excellent match for phenolic coatings and linings; it is also an excellent coalescent for acrylic-based latexes.DOWANOL™ PPh Glycol Ether is also used extensively in dyeing applications, where it can function as both a dye solubilizer and as a dye carrier. DOWANOL™ PPh Glycol Ether has superior viscosity reduction properties in metalworking fluids.

[0033] DOWANOL™ PPh Glycol Ether is represented by:

[0034] As a result of the thermal conductivity enhancement enabled by DOWANOL, conductive film adhesives, which ordinarily would rely upon increasing filler loading or increased cure temperature to render high thermal conductivity, can be made to achieve a given level of conductivity using less of the conductive filler than would otherwise be required. The adverse effects of high filler loading on the physical properties of the film adhesive can be minimized while maintaining a high level of thermal conductivity.

[0035] The viscosity of the composition can be adjusted downward if desired through the use of solvent to assist in dispensing onto a substrate or coating onto arelease substrate for film formation. Exemplary solvents that may be utilized, either separately or in combination, are glycidyl ethers, for example 1 ,4-butanediol diglycidyl ether; methyl ethyl ketone, methyl iso-butyl ketone, p-tert-butyl-phenyl glycidyl ether, allyl glycidyl ether, glycerol diglycidyl ether, butyldiglycol, 2-(2-butoxyethoxy)~ethy tester, butylglycolacetate, acetic acid, 2-butoxyethylester, butylglycol, 2-butoxyethanol, isophorone, 3,3,5 trimethyl-2-cyclohexene-1-one, dimethylsuccinate, dimethylglutarate, dimethyladipate, water, acetic acid, dipropylene glycol (mono)methyl ether, propylacetate, glycidyl ether of alkyl phenol (commercially available from Cardolite Corporation as CARDOLITE NC513), although other solvents may be used as well.

[0036] When present, the solvent desirably is selected from methyl ethyl ketone, methyl iso-butyl ketone, and combinations thereof.

[0037] Additional components may be included in the composition to provide desired properties, such as surface active agents, surfactants, wetting agents, antioxidants, thixotropes, reinforcement materials, silane functional perfluoroethers, phosphate functional perfluoroethers, silanes, titanates, wax, phenol formaldehyde, air release agents, flow additives, adhesion promoters, rheology modifiers, surfactants, spacer beads and mixtures thereof. The components are specifically chosen to obtain the desired balance of properties for the use of the composition in a particular application.

[0038] The additional component(s) may be used in an amount up to about 20 percent by weight, such as up to about 10 percent by weight, based on the total weight of the composition.

[0039] The composition is made by mixing together in any order the components described herein for a time sufficient to ensure substantially uniform dispersion.

[0040] The composition so formed may be applied onto a release layer to form a B staged film (once any solvent is eliminated) for subsequent use. In the form of B staged films, the electrically conductive compositions offer a clean, hazard-free operation with minimal waste and excellent bondline control due to predetermined film thickness(es) of the B staged film.

[0041] In B staged film form, an electrically conductive composition is provided that comprises:~an epoxy component in the range of about 5 percent by weight to about 15 percent by weight, based on the total weight of the composition in film form; a silver component in the range of about 80 percent by weight to about 90 percent by weight, based on the total weight of the composition in film form; a cure system in the range of about 0.25 percent by weight to about 1 percent by weight, based on the total weight of the composition in film form; and a thermal conductivity promoter in the range of about 0.5 percent by weight to about 1.5 percent by weight, based on the total weight of the composition in film form.

[0042] Typically, the composition may be cured after dispensing onto the desired substrate(s) at a temperature of about 150°C for a period of time of about 1 hour.Ordinarily, the substrate(s) are aluminum.

[0043] Once cured, the composition ordinarily demonstrates as noted above at least one, desirably at least two and even more desirably all, of a thermal conductivity measured according to ASTM E1461 of greater than 17 W / m-k, a volume resistivity measured according to ASTM D2739 of 10-4Ohm-cm or less, and tensile lap shear strength measured according to ASTM D3528 on aluminum substrates of greater than 1500 psi. In a particularly desirable embodiment, the composition once cured demonstrates at least one, desirably at least two and even more desirably all, of a thermal conductivity of 20 W / m-k or greater, a volume resistivity of 10-4Ohm-cm or less, and tensile lap shear strength on aluminum substrates of greater than 1500 psi.

[0044] The invention is further illustrated by the following non-limiting examples.EXAMPLES

[0045] Compositions were prepared with the components listed in the amounts noted in Table 1. The solid components were added to solvent at room temperature with mixing until a homogenous mixture was formed.

[0046] Sample preparation:

[0047] 4 mil B-staged films were hot laminated into 15-20 mil films and cured under 50 psi at 150°C for 1 hour. After curing, samples were punched into 0.5” diameter round samples on a hot plate,

[0048] Sample measurement:

[0049] Sample thickness was measured with a thickness gauge, diameter was measured with a caliper, density was measured with a density kit installed on a standard analytical balance.

[0050] Prior to testing:

[0051] The samples were spray coated with approximately 5 pm of graphite. A correction was applied to account for the effect of the graphite coating.

[0052] Table 2 shows the performance measured from the compositions when subjected to the respective ASTM test method, noted above.Table 1

[0053] Each of Compositions 1-3 was formulated such that when solvent was eliminated silver filler(s) loading of 87% was achieved. Before conducting any of the ASTM test methods, the compositions were B-staged to eliminate solvent and form a film.

[0054] In Table 1A, the percent by weight range of constituents for B staged films is noted (after elimination of any solvent):Table 1A

[0055] Composition 1 contains no thermal conductivity promoter; Composition 1 shows 15 W / m-k thermal conductivity and 1600 psi adhesion.

[0056] Composition 2 was developed from Composition 1 to which was added Glycol Ether PPh; Composition 2 shows an increase in thermal conductivity to 21 W / m-k and 1600 psi adhesion.

[0057] For comparison, PEG 200 was added to Composition 1 to form Composition 3. Composition 3 shows an increase in thermal conductivity from 15 to 17.3 W / m-k. However, that increase in thermal conductivity comes at a sacrifice: adhesion drops to 1100 psi.

[0058] Composition 4 was formulated with silver filler(s) in an amount sufficient to bring the thermal conductivity measurement to match that of Composition 2 and volume resistivity measurement to be consistent with that observed for Composition 2.However, in doing so, the adhesion drops to 900 psi, which represents a 600 psi reduction compared to Composition 2 (or a 40% reduction in adhesion). Silver filler loading here was 88%.

[0059] Composition 5 was formulated with a higher amount of silver f iller(s) to boost conductivity. Here, an amount of silver fillers) was used that translated to a thermal conductivity of 28 W / m-k and a volume resistivity consistent with Composition 2. However, adhesion drops to 800 psi, which represents a 700 psi reduction compared to Composition 2 (or a 46.7% reduction in adhesion). Silver filler loading here was 90%.Table 2

Claims

What is Claimed Is:1 . An electrically conductive composition comprising:(a) an epoxy component;(b) a silver component;(c) a cure system; and(d) a thermal conductivity promoter; and(e) optionally, a solvent.

2. The conductive composition of claim 1 , having at least one of a thermal conductivity of greater than 17 VWm-k, a volume resistivity of 10'40hm-cm or less, and tensile lap shear strength on aluminum substrates of greater than 1500 psi.

3. The conductive composition of claim 1 , having at least two of a thermal conductivity of greater than 17 W / m~k, a volume resistivity of 10"4Ohm-cm or less, and tensile lap shear strength on aluminum substrates of greater than 1500 psi.

4. The conductive composition of claim 1 , having a thermal conductivity of greater than 17 W / m-k, a volume resistivity of 10~4Ohm-cm or less, and tensile lap shear strength on aluminum substrates of greater than 1500 psi.

5. The conductive composition of claim 1 , wherein the epoxy component comprises at least one epoxy resin.

6. The conductive composition of claim 1 , wherein the epoxy component comprises at least two epoxy resins.

7. The conductive composition of claim 1, wherein the epoxy component comprises at least three epoxy resins.

8. The conductive composition of claim 1 , wherein the epoxy component comprises one or more epoxy resins selected from aliphatic epoxy resins, cycloaliphatic epoxy resins, aromatic epoxy resins, hydrogenated epoxy resins and combinations thereof,9. The conductive composition of claim 1 , wherein the epoxy component comprises one or more monofunctional epoxy resins selected from monofunctional aliphatic epoxy resins, monofunctional cycloaliphatic epoxy resins, monofunctional aromatic epoxy resins, monofunctional hydrogenated aromatic epoxy resins and combinations thereof.

10. The conductive composition of claim 1 , wherein the epoxy component comprises one or more difunctional epoxy resins selected from difunctional aliphatic epoxy resins, difunctional cycloaliphatic epoxy resins, difunctional aromatic epoxy resins, difunctional hydrogenated aromatic epoxy resins and combinations thereof.11 . The conductive composition of claim 1 , wherein the epoxy component comprises one or more polyfunctional epoxy resins selected from aliphatic epoxy resins, cycloaliphatic epoxy resins, aromatic epoxy resins, hydrogenated epoxy resins and combinations thereof.

12. The conductive composition of claim 1 , wherein the epoxy component is present in an amount of about 15 percent by weight to about 30 percent by weight, based on the total weight of the composition.

13. The conductive composition of claim 1 , wherein the silver component comprises silver in flake form, particle form, sphere form, platelet form and / or powder form.

14. The conductive composition of claim 1 , wherein the silver component has a particle size in the range of about 0.1 urn to about 50 um.

15. The conductive composition of claim 1 , wherein about 50 percent of the silver component has a particle size in the range in the range of about 0.1 um to about 25 um.

16. The conductive composition of claim 1 , wherein the silver component is present in the range of about 50 percent by weight to about 90 percent by weight, based on the total weight of the composition.

17. The conductive composition of claim 1 , wherein the cure system comprises imidazole or an imidazole derivative.

18. The conductive composition of claim 1 , wherein the cure system is present in the range of about 0.1 percent by weight to about 5 percent by weight, based on the total weight of the composition.

19. The conductive composition of claim 1 , wherein the thermai conductivity promoter is represented by:wherein X and Y are each independently H, OH or Ar-A, wherein Ar is an aromantic ring or an aromatic ring system; A occurs 1 to 4 times on the aromatic ring is selected from halogen, OH, OR, wherein R is CM alkyl, COOH, COOR, wherein R is CM alkyl, NO2, and NRR1, wherein R and R1are each independently H or CM alkyl; and n is 1-4.

20. The conductive composition of claim 1 , wherein the thermal conductivity promoter is a propylene glycol phenyl ether.21 . The conductive composition of claim 1 , wherein the thermal conductivity promoter is22. The conductive composition of claim 1 , wherein the thermal conductivity promoter is present in the range of about 0.5 percent by weight to about 1.5 percent by weight, based on the total weight of the composition.

23. The conductive composition of claim 1 , wherein the solvent is present and is selected from the group consisting of methyl ethyl ketone, methyl iso-butyl ketone, and combinations thereof.

24. An electronic device comprising the conductive composition of claim 1 .

25. The conductive composition of claim 1 , having at least one of a thermal conductivity of 2,0 W / m-k or greater, a volume resistivity of 10'4Ohm-cm or less, and tensile lap shear strength on aluminum substrates of greater than 1500 psi.

26. An electrically conductive composition in B staged film form the comprising:(a) an epoxy component in the range of about 5 percent by weight to about 15 percent by weight, based on the total weight of the composition in film form;(b) a silver component in the range of about 80 percent by weight to about 90 percent by weight, based on the total weight of the composition in film form;(c) a cure system in the range of about 0.25 percent by weight to about 1 percent by weight, based on the total weight of the composition in film form; and(d) a thermal conductivity promoter in the range of about 0.5 percent by weight to about 1 .5 percent by weight, based on the total weight of the composition in film form.

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