High-performance conformal coating

Abstract

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.

Description

Technical Field

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

Background Art

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

[0003] Once assembled, such electrical assemblies are relatively fragile and delicate and can be damaged if not handled carefully, or are subject to malfunctions or performance degradation due to exposure to and interaction with moisture, particulates, and other environmental factors. One way to protect such fragile and delicate electrical assemblies is to apply a thin polymer coating or film that conforms to the contours of the traces and other components and seals the traces and components to reduce or eliminate harmful interactions with damage and environmental factors. Such coatings and films are generally referred to as "conformal coatings". Some novel compositions useful as conformal coatings that provide certain advantages over prior art compositions and methods of forming such compositions are disclosed herein.

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 OF THE INVENTION

[0005] The compositions and methods disclosed herein are described in detail by way of example. It will be understood that modifications may be made to the examples, arrangements, configurations, components, elements, devices, methods, materials, etc. disclosed and described, which may be desirable for a particular application. In this disclosure, any identification of a particular technique, configuration, method, etc. is related to the particular example presented or is merely a general description of such technique, configuration, method, etc. The identification of a particular detail or example is not intended to be construed as essential or limiting, and should not be so construed, unless so specifically designated. Selected examples of conformal coatings and methods for 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 curing methods that result in coatings useful as protective coatings for electronic assemblies, among other applications.

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

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

[0009] Examples of two-component compositions use a thiol-ene polymerizable resin in combination with an epoxy resin or combination of epoxy resins, or an initiator sufficient to cure by applying heat within 5 minutes (e.g., an epoxy-imidazole adduct curing agent). In another example, the two-component composition uses a thiol-ene polymerizable resin in combination with an epoxy resin or combination of epoxy resins or a sufficient initiator (e.g., a phosphonium ionic liquid catalyst) to enable curing by application of heat and produce a cured coating having 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. Such embodiments can be formed by a plurality of defined options. In a first example, a one-component coating composition contains a reactive liquid acrylic liquid elastomer containing a silyl functional group and an initiator sufficient to enable the elastomer to polymerize by condensation curing upon exposure to moisture at room temperature or elevated temperature. In another example, a one-component coating composition contains a reactive liquid acrylic liquid elastomer containing acrylate functional groups and, optionally in combination with applied heat, an initiator sufficient to enable the elastomer to polymerize upon exposure to UV light or electron beam ("EB") radiation. In another example, a one-component coating composition contains a combination of a reactive liquid acrylic liquid elastomer containing silyl and acrylate functional groups and an initiator sufficient to polymerize the elastomer upon exposure to UV light or EB radiation, moisture, or heat. In another example, a one-component coating composition contains a reactive liquid acrylic liquid elastomer containing a silyl functional group, combined with an epoxy resin or blend of epoxy resins, and an initiator sufficient to enable the elastomer to polymerize by condensation curing upon exposure to moisture at room temperature or elevated temperature. In another example, a one-component coating composition contains a reactive liquid acrylic liquid elastomer containing acrylate functional groups, combined with an epoxy resin or blend of epoxy resins, an optionally applied heat, and an initiator sufficient to polymerize the elastomer upon exposure to UV or EB radiation. In another example, a one-component coating composition contains a combination of a reactive liquid acrylic liquid elastomer containing silyl and acrylate functional groups, combined with an epoxy resin or blend of epoxy resins and an initiator sufficient to polymerize the elastomer upon exposure to UV or EB radiation, moisture, or heat.

[0011] In another general embodiment, the conformal coating is formed using epoxy resin chemistry that results in a coating having useful physical properties such as chemical resistance, adhesion, abrasion resistance, heat resistance and moisture resistance, as well as electrical properties. Such embodiments can be formed by a plurality of defined 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 of 1% to 10%. The first two components can be mixed prior to application to the electrical assembly to be protected, and the volatile diluent can be added to adjust the viscosity of the coating before or during application of the conformal coating to the electrical assembly. Such a conformal coating can include a first part comprising an epoxy resin or combination of epoxy resins with a reactive and / or volatile diluent, and a second part comprising a single imidazole or combination of imidazoles with a reactive and / or volatile diluent. Alternatively, such a conformal coating can include a first part comprising an epoxy resin or combination of epoxy resins with a reactive and / or volatile diluent, and a second part comprising a single amine or combination of amines, polyamine with a reactive and / or volatile diluent. Alternatively, the conformal coating can include a first part comprising an epoxy resin or combination of epoxy resins with a reactive and / or volatile diluent, and a second part comprising a single mercaptan curing agent or combination of mercaptan curing agents with a reactive and / or volatile diluent. In yet another alternative embodiment, the conformal coating can include a first part comprising an epoxy resin or combination of epoxy resins with a reactive and / or volatile diluent, and a second part comprising a single anhydride or combination of anhydrides with a reactive and / or volatile diluent.

[0013] In one embodiment of the two-component epoxy system, the diluent used can be an organic solvent such as 10 - 15% methyl ethyl ketone (MEK), or other suitable solvents such as acetone. The epoxy resin used can be bisphenol A-co-epichlorohydrin, and the imidazole curing agent used can be, for example, 1-cyanoethyl-2-ethyl-4-methylimidazole. In another embodiment, the bisphenol A epoxy resin is combined with a high molecular weight epoxy such as Olin 684-EK40 (with MEK at a concentration of 50 - 70%, a combination of MEK with phenol, 4,4'-(1-methylethylidene)bis-polymer, and 2,2'-[(1-methylethylidene)bis(4,1-phenyleneoxymethylene)]bis[oxirane]). Such a combination provides good film-forming properties, flexibility, and impact resistance.

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

[0015] Polymer epoxies in organic solvents such as Olin 684-EK40 form a final coating film that can be easily reprocessed or removed. In one embodiment of the polymer epoxy solution, Olin 684-EK40 is combined at 0.01 to 1.0 wt% with a thermosetting catalyst, such as K-Pure CXC-1612 Blocked Acid Catalyst (available from King Industries), and combined with 3% of an alicyclic epoxide resin, such as ERL-4221 (3,4-epoxycyclohexane methyl 3,4-epoxycyclohexanecarboxylate). Such a combination results in a higher glass transition temperature (T g ).

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

[0017] In another example for forming a conformal coating with useful physical properties, the conformal coating is formulated using a solution of a thermoplastic phenoxy (also known as polyhydroxyether) resin in a volatile organic solvent / diluent. High molecular weight polyhydroxyl ethers (e.g., Gabriel PKHH) are, among others, an example of such a phenoxy resin. Such a combination produces a coating with an appropriate viscosity for application by dipping, brushing, or spraying methods. The formulation deposited on the substrate can be dried at room temperature or elevated temperature to remove the carrier solvent, leaving an adhesive, flexible, transparent epoxy film. Such a combination provides good electrical, mechanical, and environmental properties.

[0018] In one embodiment, the phenoxy resin is used in combination with bisphenol F type epoxy (e.g., Araldite GY282), UV curable epoxy polymer (e.g., ERL 4221), flexible resin (e.g., poly BD650E), and cationic photoinitiator (e.g., UVI 6976). Such combinations produce good electrical, mechanical, and environmental properties. In other embodiments, the UV cationic curing agent can be replaced with a thermosetting catalyst such as K-Pure CXC-1612. The above embodiments can also be used in combination with latent curing agents. The above embodiments can also be used in combination with 0.01 to 1.0 wt% of a blocked acid type thermal initiator.

[0019] In another embodiment for forming a conformal coating having useful physical properties, the conformal coating is a one-component 100% solids epoxy conformal coating containing a 100% solids epoxy resin (or a combination of epoxy resins), a reactive diluent, and a UV curing agent with a sufficient waiting time 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 alicyclic epoxy resins such as 30 to 50 wt% of 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate (e.g., Syna 21), 0.1 to 2.0 wt% of one or more cationic photoinitiators, and 40 to 50 wt% of a cyclic ether cationic monomer.

[0020] The foregoing description of the examples has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the forms described. Numerous modifications are possible in light of the above teachings. Some of these modifications have been discussed, while others will be understood by those skilled in the art. The examples were chosen and described in order to best explain the principles of the various examples for a particular intended application. Of course, the scope of the invention is not limited to the examples described herein and may be used by those skilled in the art in any number of applications and equivalent devices.

Claims

1. A resin capable of thiol-ene polymerization. A crosslinking agent comprising at least one compound having a carbon-carbon double bond selected from diallylphthalate and ethoxylated bisphenol A dimethacrylate, A stabilizing compound selected from triphenyl phosphite and 4-methoxy-1-naphthol, and Photoinitiator, Includes, A conformal coating for protecting electronic assemblies, which can be cured by exposure to ultraviolet light having a wavelength of approximately 365 nm for approximately 30 to 120 seconds to form a cured conformal coating on the electronic assembly.

2. The conformal coating according to claim 1, wherein the photoinitiator initiates thiolene polymerization upon exposure to ultraviolet light.

3. The conformal coating according to claim 1, wherein the crosslinking agent comprises ethoxylated bisphenol A dimethacrylate.

4. The conformal coating according to claim 1, wherein the crosslinking agent comprises diallyl phthalate.

5. The conformal coating according to claim 1, wherein the cured conformal coating provides electrical insulation properties and moisture resistance while conforming the surface features of the electronic assembly.

6. The conformal coating according to claim 1, wherein the stabilizing compound comprises triphenylphosphite.

7. The conformal coating according to claim 1, wherein the stabilizing compound comprises 4-methoxy-1-naphthol.

8. Reactive acrylic liquid elastomer, and Initiator Includes, A conformal coating wherein the reactive acrylic liquid elastomer contains silyl functional groups, and the reactive acrylic liquid elastomer can be cured upon exposure to moisture at room temperature or high temperature to form a flexible and electrically insulating conformal coating on an electronic assembly.

9. The conformal coating according to claim 8, wherein the initiator enables curing by exposure to moisture.

10. Thermoplastic linear epoxy polymer, and Contains volatile organic solvents, A conformal coating, which, after application to an electronic assembly and evaporation of the volatile organic solvent, forms an adhesive and flexible reworkable epoxy film that can be removed by dissolving in the organic solvent, allowing access to the electronic assembly.

11. The conformal coating according to claim 10, wherein the volatile organic solvent comprises methyl ethyl ketone.

12. Applying a coating composition to an electronic assembly, wherein the coating composition is A resin capable of thiol-ene polymerization. A crosslinking agent comprising at least one compound having a carbon-carbon double bond selected from diallylphthalate and ethoxylated bisphenol A dimethacrylate, and A stabilizing compound selected from triphenyl phosphite and 4-methoxy-1-naphthol, and Photoinitiator, The above applies, The coating composition is cured by exposing it to ultraviolet light having a wavelength of approximately 365 nm for approximately 30 to 120 seconds, thereby forming a cured conformal coating on the electronic assembly. A method for forming a conformal coating on an electronic assembly, including the method described above.

13. The method according to claim 12, wherein the photoinitiator initiates thiolene polymerization upon exposure to ultraviolet light.

14. The method according to claim 12, wherein the crosslinking agent comprises ethoxylated bisphenol A dimethacrylate.

15. The method according to claim 12, wherein the crosslinking agent comprises diallylphthalate.

16. The method according to claim 12, wherein the cured conformal coating provides electrical insulation properties and moisture resistance while conforming the surface features of the electronic assembly.

17. The method according to claim 12, wherein the stabilizing compound comprises triphenylphosphite.

18. The method according to claim 12, wherein the stabilizing compound comprises 4-methoxy-1-naphthol.

19. Applying a coating composition comprising a thermoplastic linear epoxy polymer dissolved in a volatile organic solvent to an electronic assembly, The process involves evaporating the volatile organic solvent to form an adhesive, flexible epoxy film on the electronic assembly, wherein the epoxy film can be removed by dissolving in the organic solvent, allowing access to the electronic assembly. A method for forming a reworkable conformal coating on an electronic assembly, including the method described above.

20. The method according to claim 19, wherein the volatile organic solvent comprises methyl ethyl ketone.