High-performance conformal coating

Abstract

Disclosed herein are various embodiments and embodiments of conformal coatings formed by various formulations and curing methods. In one embodiment, a resin capable of thiol-ene polymerization is used. In another embodiment, a reactive acrylic liquid elastomer is used. In yet another embodiment, an epoxy resin is used in combination with other chemicals to produce physical properties useful for conformal coatings.

Description

Field of the Invention

[0001] The present disclosure relates generally to conformal coatings and methods of forming such conformal coatings. More specifically, the present disclosure relates to conformal coatings useful as protective coatings for electronic components and electronic assemblies and methods of forming such conformal coatings. Background Art

[0002] Many modern advanced industrial consumer products and systems rely on small and delicate electrical assemblies such as printed circuit boards, printed wiring boards, and other such circuit combinations. Such electrical assemblies typically include one or more insulating substrates sandwiched together into a thin assembly, with a series of electrically conductive and selectively interconnected lines (such as traces) formed on the surface of the substrate. The traces can be formed by various processes, such as chemically etching a conductive layer applied to the surface of the substrate or printing conductive material directly onto the substrate. Additional electrical components can be added to the substrate by soldering and other such processes.

[0003] Once assembled, such electrical components are relatively fragile and delicate, and if not handled with care, can become damaged, or can fail or perform poorly due to exposure to and interaction with moisture, particulate matter, and other environmental factors. One method of protecting such fragile and delicate electrical components is to apply a thin polymer coating or film that conforms to the contours of traces and other components and seals the traces and components to reduce or eliminate damage and harmful interactions with environmental factors. Such coatings and films are often referred to as "conformal coatings." Several novel compositions useful as conformal coatings and methods of forming such compositions are disclosed herein that provide certain advantages over prior art compositions. Summary of the invention

[0004] Various embodiments and examples of conformal coatings formed by various formulations and curing methods are disclosed herein. In one embodiment, a resin capable of thiol-ene polymerization is used. In another embodiment, a reactive acrylic liquid elastomer is used. In yet another embodiment, an epoxy resin is used in combination with other chemicals to produce physical properties useful for conformal coatings. DETAILED DESCRIPTION

[0005] The compositions and methods disclosed in this document are described in detail by way of example. It should be understood that modifications may be made to the disclosed and described examples, arrangements, configurations, components, elements, devices, methods, materials, etc., and may be desired for specific applications. In this disclosure, any identification of specific techniques, arrangements, methods, etc. is either related to the specific examples presented or is merely a general description of such techniques, arrangements, methods, etc. Unless otherwise specified, the identification of specific details or examples should not be intended to be and is not to be construed as mandatory or restrictive. Selected examples of conformal coatings and methods of forming such coatings are disclosed and described in detail below.

[0006] The conformal coatings described herein are formed by a combination of resin chemistries and a curing method that produces a coating that can be used as a protective coating for electronic components as well as coatings for other applications.

[0007] In a general embodiment, the conformal coating includes a resin capable of thiol-ene polymerization (i.e., a reaction between a thiol and an olefin to form a thioether). This embodiment can be formed by a variety of formulation options, including a one-component formulation and a two-component formulation. For example, a thiol-ene polymerizable resin can be used with sufficient initiator to form a one-component coating composition to allow polymerization by ultraviolet (UV) light or visible light. In another example, a thiol-ene polymerizable resin can be used in combination with sufficient initiator to form a one-component coating composition to allow curing by reaction with UV light or visible light and / or in the presence of moisture. In another example, a thiol-ene polymerizable resin can be used in combination with sufficient initiator to form a one-component coating composition to allow curing by reaction with UV light and / or under the application of heat.

[0008] In one embodiment, the thiol-ene resin includes a suitable crosslinking agent containing carbon-carbon double bonds, such as diallyl phthalate (DAP) or any of a variety of acrylic monomers, such as 2-mole ethoxylated bisphenol A dimethacrylate (sold as Sartomer SR348). The addition of such a crosslinking material can improve electrical insulation properties, dimensional stability, and heat resistance. In addition, such an embodiment can be combined with a stabilizing compound such as triphenyl phosphite, 4-methoxy-1-naphthol, or similar materials. Such an embodiment can be cured by exposure to a full spectrum mercury vapor UV lamp (UV-LED with a wavelength of 365 nm) for 30 to 120 seconds.

[0009] An example of a two-component composition uses a thiol-ene polymerizable resin in combination with an epoxy resin or a combination of epoxy resins, or in combination with sufficient initiator (e.g., an epoxy-imidazole adduct curing agent) to allow curing by application of heat in 5 minutes or less. In another example, a two-component composition uses a thiol-ene polymerizable resin in combination with an epoxy resin or a combination of epoxy resins, or sufficient initiator (e.g., a phosphonium ionic liquid catalyst) to allow curing by application of heat to produce a cured coating with a high glass transition temperature.

[0010] In another general embodiment, the conformal coating comprises a reactive acrylic liquid elastomer. The resulting conformal coating is highly flexible and provides electrical insulation properties. This embodiment can be formed by multiple formulation options. In a first example, the one-component coating composition comprises a reactive liquid acrylic liquid elastomer containing a silyl functional group and sufficient initiator to allow the elastomer to polymerize by condensation curing when exposed to moisture at room temperature or elevated temperature. In another example, the one-component coating composition comprises a reactive liquid acrylic liquid elastomer containing an acrylate functional group and sufficient initiator to allow the elastomer to polymerize when exposed to UV light or electron beam ("EB (electron beam)") radiation and optionally in combination with applied heat. In another example, the one-component coating composition comprises a combination of a reactive liquid acrylic liquid elastomer containing a silyl functional group and an acrylate functional group and sufficient initiator to allow the elastomer to polymerize when exposed to UV light or EB radiation, moisture or heat. In another example, a one-component coating composition includes a reactive liquid acrylic liquid elastomer containing silyl functional groups in combination with an epoxy resin or a mixture of epoxy resins and sufficient initiator to allow the elastomer to polymerize by condensation cure when exposed to moisture at room temperature or elevated temperature. In another example, a one-component coating composition includes a reactive liquid acrylic liquid elastomer containing acrylate functional groups in combination with an epoxy resin or a mixture of epoxy resins and sufficient initiator to allow the elastomer to polymerize when exposed to UV or EB radiation and optionally under applied heat. In another example, a one-component coating composition includes a reactive liquid acrylic liquid elastomer containing silyl functional groups and acrylate functional groups in combination with an epoxy resin or a mixture of epoxy resins and sufficient initiator to allow the elastomer to polymerize when exposed to UV or EB radiation, moisture or heat.

[0011] In another general embodiment, the conformal coating is formed using epoxy chemistries that impart useful physical properties to the coating, such as chemical resistance, adhesion, abrasion resistance, temperature and moisture resistance, and electrical properties. This embodiment can be formed through a variety of formulation options.

[0012] In one example of forming a conformal coating having useful physical properties, the conformal coating uses a two-component epoxy system in combination with a volatile diluent at a concentration between 1% and 10%. The first two components can be mixed prior to application to the electrical component to be protected, and the volatile diluent can be added before or during application of the conformal coating to the electrical component to adjust the coating viscosity. Such a conformal coating can include a first component and a second component, the first component including an epoxy resin or a combination of epoxy resins and a reactive and / or volatile diluent, and the second component including a single imidazole or a combination of imidazoles and a reactive and / or volatile diluent. Alternatively, such a conformal coating can include a first component and a second component, the first component including an epoxy resin or a combination of epoxy resins and a reactive and / or volatile diluent, and the second component including a single amine or a combination of amines, a single polyamine or a combination of polyamines and a reactive and / or volatile diluent. Alternatively, such conformal coatings may include a first component and a second component, the first component including an epoxy resin or a combination of epoxy resins and a reactive and / or volatile diluent, and the second component including a single thiol curing agent or a combination of thiol curing agents and a reactive and / or volatile diluent. In yet another alternative, such conformal coatings may include a first component and a second component, the first component including an epoxy resin or a combination of epoxy resins and a reactive and / or volatile diluent, and the second component including a single anhydride or a combination of anhydrides and a reactive and / or volatile diluent.

[0013] In one embodiment of the two-component epoxy system, the diluent used may be an organic solvent such as 10%-15% methyl ethyl ketone (MEK) or other suitable solvents such as acetone. The epoxy resin used may be bisphenol A-co-epichlorohydrin, and an imidazole curing agent such as 1-cyanoethyl-2-ethyl-4-methylimidazole may be used. In another embodiment, a bisphenol A epoxy resin is combined with a polymeric epoxy resin such as Olin 684-EK40 (a combination of MEK and 4,4'-(1-methylethylene)bisphenol and 2,2'-[(1-methylethylene)bis(4,1-phenyleneoxymethylene)]bis[ethylene oxide], wherein the concentration of MEK is 50%-70%). This combination provides good film-forming properties, flexibility, and impact resistance.

[0014] In another example of forming a conformal coating with useful physical properties, a conformal coating is formulated using a solution of a thermoplastic epoxy resin in a volatile organic solvent / diluent. This combination produces a coating with a suitable viscosity for easy application by dipping, brushing or spraying methods. Once deposited on a substrate, the formulation can be dried at room temperature or elevated temperature to remove the carrier solvent, leaving an adhesive, flexible, transparent epoxy film. The coating can then be removed by dissolving in an organic solvent, allowing access to the protected device for inspection or repair. This conformal coating can combine a thermoplastic linear epoxy polymer (such as an epoxy resin based on Olin 684) with methyl ethyl ketone. Alternatively, this conformal coating can combine a thermoplastic linear epoxy polymer (as described above) with methyl ethyl ketone and an amount of isocyanate resin to allow further crosslinking by applying heat. This alternative can increase the adhesion and barrier properties of the epoxy film, which is useful for harsh environments. Alternatively, such conformal coatings may combine a thermoplastic linear epoxy polymer (as described above) with methyl ethyl ketone and a cationic UV light curing catalyst to allow the applied coating to further react by exposure to UV light. This approach may improve the adhesion and barrier properties of the epoxy film.

[0015] Polymerized epoxy resins in organic solvents, such as Olin 684-EK40, form a final coating film that can be easily reworked or removed. In one embodiment of the polymerized epoxy solution, Olin 684-EK40 is combined with 3% of a cycloaliphatic epoxy resin such as ERL-4221 (3,4-epoxycyclohexane methyl 3,4-epoxycyclohexane carboxylate), and with 0.01-1.0 wt.% of a thermal curing catalyst such as K-Pure CXC-1612 blocked acid catalyst (available from King Industries). This combination produces a higher glass transition temperature (Tg).

[0016] In another example of forming a conformal coating having useful physical properties, the conformal coating is a one-component 100% solid epoxy conformal coating that includes a 100% solid epoxy resin (or combination of epoxy resins), a reactive diluent, and a curing agent with sufficient latency to provide a stable formulation that does not react until subjected to an appropriately elevated temperature. When exposed to this temperature, the composition cures and has significant and useful mechanical, adhesive, and electrical properties.

[0017] In another example of forming a conformal coating with useful physical properties, a conformal coating is formulated using a solution of a thermoplastic phenoxy (aka polyhydroxy ether) resin in a volatile organic solvent / diluent. High molecular weight polyhydroxy ethers such as Gabriel PKHH are examples of such phenoxy resins, among others. This combination produces a coating with a suitable viscosity for application by dipping, brushing, or spraying methods. Once deposited on a substrate, the formulation can be dried at room temperature or elevated temperature to remove the carrier solvent, leaving behind an adherent, flexible, transparent epoxy film. This combination produces good electrical, mechanical, and environmental properties.

[0018] In one embodiment, a phenoxy resin is used in combination with a bisphenol F type epoxy resin (e.g., Araldite GY282), a UV curable epoxy polymer (e.g., ERL 4221), a flexible resin (e.g., poly BD650E), and a cationic photoinitiator (e.g., UVI6976). This combination produces good electrical, mechanical, and environmental properties. In other embodiments, the UV cationic curing agent can be replaced with a thermal curing catalyst (e.g., K-Pure CXC-1612). The above embodiments can also be used in combination with a latent curing agent. The above embodiments can also be used in combination with 0.01 wt.%-1.0 wt.% of a blocked acid type thermal initiator.

[0019] In another embodiment of forming a conformal coating having useful physical properties, the conformal coating is a one-component 100% solid epoxy conformal coating comprising a 100% solid epoxy resin (or combination of epoxy resins), a reactive diluent, and a UV curing agent with sufficient latency to provide a stable formulation that does not react until exposed to UV light. An example of such a coating composition is a mixture of 30 wt.% to 50 wt.% of a cycloaliphatic epoxy resin such as 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (e.g., Syna 21), 0.1 wt.% to 2.0 wt.% of one or more cationic photoinitiators, and 40 wt.% to 50 wt.% of a cyclic ether cationic monomer.

[0020] For the purpose of illustration and description, the foregoing description of examples has been given. It is not intended to be exhaustive or to limit the form described. In accordance with the above teachings, many modifications can be made. Some of these modifications have been discussed, and those skilled in the art will understand other modifications. These examples are selected and described in order to best illustrate the principles of various examples suitable for the intended specific use. Of course, the scope is not limited to the examples set forth herein, but can be adopted by those of ordinary skill in the art in any number of applications and equivalent devices.

Claims

1. A conformal coating, comprising: Resins capable of undergoing thiol-ene polymerization; as well as Initiator.

2. The conformal coating according to claim 1, wherein: The initiator allows polymerization by ultraviolet light.

3. The conformal coating of claim 1 , further comprising a crosslinking agent containing carbon-carbon double bonds.

4. The conformal coating according to claim 3, wherein: The crosslinking agent is diallyl phthalate.

5. The conformal coating according to claim 3, wherein: The crosslinking agent is ethoxylated bisphenol A dimethacrylate.

6. The conformal coating of claim 1 further comprising a stabilizing compound.

7. The conformal coating according to claim 6, wherein: The stabilizing compounds are triphenyl phosphite, 4-methoxy-1-naphthol.

8. A conformal coating comprising: Reactive acrylic liquid elastomer; as well as Initiator.

9. The conformal coating according to claim 8, wherein: The initiator allows polymerization by ultraviolet light.

10. The conformal coating according to claim 8, wherein: The reactive acrylic liquid elastomer includes acrylate functional groups.

11. The conformal coating of claim 8, wherein: The reactive acrylic liquid elastomer includes a silyl functional group and an acrylate functional group.

12. A conformal coating, comprising: Two-component epoxy system; as well as Volatile diluent.

13. The conformal coating of claim 12, wherein: Compared with the two-component epoxy system, the added amount of the volatile diluent is between 1% and 10%.

14. The conformal coating of claim 12, wherein: The first component of the epoxy system includes an epoxy resin, and the second component of the epoxy system includes an imidazole.

15. The conformal coating of claim 12, wherein: The first component of the epoxy system includes an epoxy resin, and the second component of the epoxy system includes an amine.

16. The conformal coating of claim 12, wherein: The first component of the epoxy system includes an epoxy resin, and the second component of the epoxy system includes a polyamine.

17. The conformal coating of claim 12, wherein: The first component of the epoxy system includes an epoxy resin, and the second component of the epoxy system includes an anhydride.

18. A conformal coating comprising: Thermoplastic epoxy resin; as well as Volatile organic solvents.

19. The conformal coating of claim 18, wherein: The thermoplastic epoxy resin is a thermoplastic linear epoxy polymer.

20. The conformal coating of claim 18, wherein: The volatile organic solvent is methyl ethyl ketone.