WO2010141432A1

WO2010141432A1 - Thermally conductive circuit subassemblies, method of manufacture thereof, and articles formed therefrom

Info

Publication number: WO2010141432A1
Application number: PCT/US2010/036848
Authority: WO
Inventors: Sankar K. Paul
Original assignee: Rogers Corporation
Priority date: 2009-06-02
Filing date: 2010-06-01
Publication date: 2010-12-09

Abstract

A circuit subassembly comprises a dielectric layer formed from a composition comprising a thermosetting or thermoplastic polymer composition, and a surface coated boron nitride particulate filler, wherein the surface coating comprises a ceramic, a metal oxide, or a metal hydroxide. The subassembly can be disposed on a conductive layer, and is useful in the manufacture of circuits and multilayer circuits.

Description

THERMALLY CONDUCTIVE CIRCUIT SUBASSEMBLIES, METHOD OF MANUFACTURE THEREOF, AND ARTICLES FORMED THEREFROM

BACKGROUND

[0001] This invention generally relates to materials useful in the formation of thermally conductive circuit subassemblies, methods of manufacture of the circuit subassemblies, and articles formed therefrom, including circuits and multi-layer circuits.

[0002] Circuit subassemblies are used in the manufacture of circuits and multi- layer circuits, and include, for example, circuit laminates, bond plies, resin-coated conductive layers, and cover films, as well as packaging substrate laminates and build-up materials. Each of the foregoing subassemblies contains a layer of a dielectric material. For example, a circuit laminate has a conductive layer, e.g., copper, fixedly attached to a dielectric substrate layer. Double clad laminates have two conductive layers, one on each side of the dielectric layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit. Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multilayer circuits are formed by laminating one or more circuit layers together using bond plies and, in some cases, resin coated conductive layers, in proper alignment using heat and/or pressure. Both bond plies and the resin that coats the conductive layer are dielectric materials. Build-up materials are also based on dielectric layers. A build-up material is a layer of dielectric organic material used to separate circuitized conductive layers of a multi-layer circuit assembly, where the multiple circuitized layers are sequentially built up from a core layer in a process that includes laminating, patterning, drilling, and other steps.

[0003] As electronic devices and the features thereon become smaller, thermal management of the resulting dense circuit layouts manufacture becomes increasingly important. A number of efforts have been made to improve the thermal conductivity of the circuit laminates by incorporating thermally conductive particulate fillers in the dielectric layer, generally an inorganic material such as boron nitride, and the like. It has been found, however, that the presence of thermally conductive inorganic fillers results in unacceptable property tradeoffs such as poor adhesion between the dielectric layer and any conductive layer disposed on the dielectric layer, poor cohesive strength and generally difficulty in modifying the filler/resin interface in a way to give good wetting by the resin matrix. Boron nitride powder, for example, although desirable for its thermal conductivity, has been found to result in unacceptable inter-laminar or inter-ply adhesion and poor copper bonding in circuit laminates. Boron nitride has also been found in many cases to be difficult to disperse in resin varnishes used to produce dielectrics.

[0004] Accordingly, there remains a need in the art for improving thermal conductivity of the circuit laminates without suffering unacceptable tradeoffs in other properties.

SUMMARY

[0005] In one embodiment, a circuit subassembly comprises a dielectric layer formed from a composition comprising a thermosetting or thermoplastic polymer composition, and a surface coated boron nitride particulate filler, wherein the surface coating comprises a ceramic, a metal oxide, or a metal hydroxide.

[0006] In one specific embodiment, the dielectric layer is disposed on a conductive layer. The conductive layer can be patterned to form a circuit.

[0007] A method for the manufacture of a dielectric layer for a circuit subassembly comprises forming a dielectric layer comprising a thermosetting polymer composition and a surface coated boron nitride particulate filler, wherein the surface coating comprises a ceramic, a metal oxide, or a metal hydroxide; and at least partially curing the dielectric layer.

[0008] In an embodiment, the above circuit subassembly is a circuit laminate, a resin- coated conductive layer, a bond ply, or a buildup material. A circuit and a multilayer circuit is disclosed, comprising the circuit laminate, a resin-coated conductive layer, a bond ply, and/or a buildup material.

[0009] Another method for the manufacture of a circuit subassembly comprises melt blending a composition comprising a thermoplastic polymer composition and a surface coated boron nitride particulate filler, wherein the surface coating comprises a ceramic, a metal oxide, or a metal hydroxide; and forming a dielectric layer from the melt-blended composition.

[0010] Still another method for the manufacture of a dielectric layer for a circuit subassembly comprises forming a dielectric layer comprising a thermosetting polymer composition, and a surface coated boron nitride particulate filler, wherein the surface coating comprises a ceramic, a metal oxide, or a metal hydroxide; and at least partially curing the dielectric layer. The layer can be formed on another layer, for example a conductive layer such as copper, or disposed on another layer, for example a conductive layer such as copper after partial cure. [0011] A method for the manufacture of a circuit subassembly comprises forming a layered structure comprising a dielectric layer and at least one other layer, wherein the dielectric layer is formed from a composition comprising a thermosetting or thermoplastic polymer composition and a surface-coated boron nitride particulate filler, wherein the surface coating comprises a ceramic, a metal oxide, or a metal hydroxide; and laminating the layered structure at a temperature and pressure effective to adhere the dielectric layer to the at least one other layer to form the circuit subassembly. In one embodiment, the at least one other layer is a conductive layer, for example copper.

[0012] The invention is further illustrated by the following drawings, detailed description, and examples.

BRIEF DESCRIPTION OF DRAWINGS

[0013] Referring now to the exemplary drawings wherein like elements are numbered alike:

[0014] Figure 1 is a schematic cross-sectional diagram of an exemplary circuit subassembly with a thermosetting or thermoplastic dielectric layer disposed on a conductive layer, e.g., a copper foil.

[0015] Figure 2 is a schematic cross-sectional diagram of an exemplary circuit laminate comprising a thermosetting or thermoplastic dielectric layer disposed between two conductive layers, e.g., a copper foil.

DETAILED DESCRIPTION

[0016] The circuit subassemblies disclosed herein have high thermal conductivity and improved inter-laminar bond strength, ascribed to a dielectric layer comprising a coated boron nitride (BN) filler. The coated BN filler has been surface treated with an inorganic coating material such as a ceramic, a metal oxide, or a metal hydroxide. The dielectric layer is made from a blend of the coated BN filler and a thermosetting or thermoplastic polymer composition. The coated BN filler improves bond strength between the dielectric layer and an adjacent layer of the circuit subassembly, while surprisingly retaining at a minimum 90% of the Z-direction thermal conductivity compared to an otherwise similar circuit subassembly prepared from the BN filler without the surface treatment. "Z-direction thermal conductivity" refers to thermal conductivity in the direction perpendicular to the plane of the dielectric substrate layer. The disclosed dielectric layers help to overcome thermal management challenges in designing multi-layer circuits striving for further component miniaturization and integration, and higher operating frequencies. The adhesion strength between the dielectric substrate layer and conductive layer is at a minimum 3-5 pli depending, e.g. on copper and dielectric thickness as measured by IPC-TM-650-2.4.8. The Z-direction thermal conductivity is at a minimum about 0.8 WmK, as measured by ASTM C- 518, Unitherm 2021.

[0017] The coated BN filler is prepared from untreated particulate BN powder before combining it with any other components of the thermosetting or thermoplastic composition. The untreated BN powder comprises particles typically in the form of hexagonal platelets. The coating material, or if desirable a precursor material for the coating material, is deposited to a surface of the untreated BN powder. Thermal and/or chemical treatment can be used to convert the precursor material into the coating material disposed on a surface of the BN particles. The coating material can be disposed on the entire surface of the untreated BN powder or a portion thereof. The coating material can be disposed on all BN particles or a portion thereof.

[0018] Before the coating treatment, the untreated BN powder can be pre-treated thermally and/or chemically. For example, the untreated BN powder can be dried at about 100 to about 250 ⁰C, more specifically at about 150 to about 200 ⁰C for at least about 6 hours. Specifically, the untreated BN powder can be dried, for example in a forced air oven, at about 149 ⁰C (300° F) for at least 6 hours and then kept at 49 ⁰C (120° F) until the surface treatment to form a pre-treated BN powder. In another thermal pre-treatment, the untreated BN powder is sintered at a temperature of at least 1800 ⁰C for about 1 to 4 hours, either in an inert atmosphere such as nitrogen or argon or in a vacuum to form a pre-treated BN powder. In an exemplary chemical pre-treatment, the untreated BN powder, in an amount of 5-10 wt. % BN powder in water, is first washed in 2% glacial acetic acid/de-ionized water solution to remove possible residual surface contaminants. The mixture is then stirred at 80 to 100 ⁰C for about 2 to 3 hours and then vacuum filtered. The BN powder is then washed again with fresh deionized water before being dried in an air-circulating oven at 110 ⁰C to produce a pre- treated BN powder. The above-described pre-treatments are illustrative and not meant to be limiting.

[0019] The coated BN particles can form agglomerates in the blend or in the dielectric layer. The agglomerates have an average agglomerate size distribution (ASD), or diameter, of about 1 to about 200 micrometers, more specifically about 2 to about 125 micrometers, and most specifically about 3 to about 40 micrometers. The coated BN filler can be present as a mixture of agglomerates and/or non-agglomerated BN particles, in the blend and/or in the dielectric layer. In particular, about 50% or less, about 30% or less, or about 10% or less of the coated BN filler particles are agglomerated in the dielectric substrate layer, as determined from transmission electron micrographs of the substrate layer or the blend.

[0020] The coating material generally comprises certain ceramics, metal oxides, or metal hydroxides, in particular a silicon-containing material such as silica (Siθ₂), silicon carbide (SiC), and silicon oxycarbide. Other exemplary metal oxides and metal hydroxides include alumina, silica, soda, calcia, potassia, titania, iron oxide, zinc oxide, lead oxide, lithium oxide, sodium oxide, potassium oxide, magnesia, boria, zirconia, magnesium oxide, magnesium hydroxide, boehmite, manganese oxide, molybdenum trioxide, vanadium trioxide, barium oxide, cuprous oxide, rare earth oxides, combinations thereof, and the like. Other silicon-containing materials include silicates, aluminum silicate, borosilicates, lithium aluminum silicate, and the like.

[0021] More particularly, the coating material is silica, alumina, boehmite, magnesium hydroxide, titania, silicon carbide, silicon oxycarbide, or a combination thereof. Even more specifically, the coating material consists essentially of silica, alumina, boehmite, silicon carbide, silicon oxycarbide, or a combination thereof. In one embodiment, the coating material consists of silica, alumina, boehmite, magnesium hydroxide, titania, silicon carbide, silicon oxycarbide, or a combination thereof. In still another embodiment no transition metals other than platinum, palladium, or titanium are present in the coating material, hi still another embodiment no transition metals are present in the coating material. The coating material is used in an amount effective to provide the desired properties to the BN and to the dielectric layer containing the BN. In generally, the coating material is present in an amount of about 1 to about 30 wt.%, based on the weight of the uncoated BN filler, hi a specific embodiment, the coated boron nitride filler comprises an alumina/bo ehmite surface coating in an amount of about 1 to about 20 wt.%, based on the total weight of the uncoated boron nitride filler. In another specific embodiment, the coated boron nitride filler comprises a silica coating in an amount of about 1 to about 10 wt.%, based on the weight of the uncoated BN filler.

[0022] Precursor compounds for the coating material can be used. Metal oxide precursor compounds generally comprise a metal ion complexed with substituents comprising carbon, hydrogen, nitrogen, oxygen, or a combination thereof. Exemplary metal oxide precursor compounds include metal ion alkoxides of general structure (M^z)_n(0R)_m, where M is a metal ion of valence z, z is an integer of 1 to 6, n is an integer of 1 to 4, m equals n times z, and R is a an alkyl or aryl group having 1 to 20 carbons. Other metal oxide precursor compounds comprise metal ions complexed with beta-dicarbonyl compounds such as beta- diketones, beta-keto esters, malonic esters, and the like. In an embodiment the coating material is alumina prepared from a precursor compound aluminum acetylacetonate, Al(AcAc)₃.

[0023] Exemplary precursor compounds for silicon-containing coating materials include polysilazanes, polycarbosilanes, siloxanes, polysiloxanes, polycarbosiloxanes, polycarbosilazanes, silsesquioxanes, and organic substituted derivatives of the foregoing silicon-containing precursor materials, and combinations thereof.

[0024] Each of these silicon-containing precursor materials is generally known in the art. Polysilazanes generally contain units of the type [R₂S1NH], [RSi(MTh ₅] and/or [R₃Si(NH)i_/J wherein each R is independently a hydrogen, an alkyl or haloalkyl group containing 1 to 20 carbon atoms, alkenyl group containing 2 to 20 carbon atoms, or aryl group containing 6 to 20 carbon atoms. Mixtures of polysilazanes can also be used. Examples of polycarbosilanes are disclosed in U.S. Pat. No. 4,414,403, U.S. Pat. No. 4,761,458, and U.S. Pat. No. 5,087,685. An exemplary polycarbosilane is POLYRAMIC RD212A from Starfire Systems.

[0025] Polycarbosilanes contain units of [R2Si-(R2SiCR'2)] where each R and R' is independently a hydrogen, an alkyl or haloalkyl group containing 1 to 20 carbon atoms, alkenyl group containing 2 to 20 carbon atoms, or aryl group containing 6 to 20 carbon atoms. Exemplary R and R' groups include methyl, trifluoromethyl, and phenyl. R' can further comprise a halide, ether, alkoxy, vinyl group, or combinations thereof.

[0026] Siloxanes have the general structure (RO)₄Si where R is independently a hydrogen, an alkyl or haloalkyl group containing 1 to 20 carbon atoms, alkenyl group containing 2 to 20 carbon atoms, or aryl group containing 6 to 20 carbon atoms. Exemplary R groups include methyl, ethyl, propyl, butyl, and phenyl.

[0027] Polysiloxanes contain units of the type [R₂SiO] where R is independently a hydrogen, an alkyl or haloalkyl group containing 1 to 20 carbon atoms, alkenyl group containing 2 to 20 carbon atoms, or aryl group containing 6 to 20 carbon atoms. Exemplary R groups include methyl, ethyl, trifluoromethyl, and phenyl. More particularly, the polysiloxane is polydimethylsiloxane.

[0028] Polycarbosiloxanes contain units of the type [(R2SiO)-(R2SiCR'2)] where each R and R' is independently a hydrogen, an alkyl or haloalkyl group containing 1 to 20 carbon atoms, alkenyl group containing 2 to 20 carbon atoms, or aryl group containing 6 to 20 carbon atoms. Exemplary R and R' groups include methyl, trifluoro methyl, and phenyl. R' can further comprise a halide, ether, alkoxy, vinyl group, or combinations thereof. Examples of polycarbosiloxanes are disclosed in U.S. Pat. No. 6,147,243. An example of a commercially available polycarbosiloxane is POLYRAMIC® RD684a, which is sold as an about 40-55 wt.% mixture with 1080 E-glass fibers, by Starfire Systems.

[0029] A silsesquioxane is a compound having the empirical chemical formula RSiOi 5, where R is hydrogen or a group comprising 1-20 carbons, such as an alkyl, alkene, aryl, arylene group, or a substituted derivative thereof. Silsesquioxanes can have the form of random structures, caged structures, partial caged structures, or ladder structures.

[0030] Any of the foregoing and other silicon-containing precursor materials can contain reactive groups such as hydride, halide, alkoxy, or vinyl groups. An example of a commercially available vinyl-substituted siloxane is POLYRAMIC® RD684 by Starfire Systems Inc. Exemplary silicon-containing polymers are described in WO 2008/036657, WO 2008/36657, and WO 2008/036662; U.S. Publication No. 2007/093587; U.S. Pat. No. 4,289,720; U.S. Pat. No. 5,153,295 to Whitmarsh.

[0031] The silicon-containing precursors for the coating material can accordingly optionally also include a catalyst to promote crosslinking of the silicon-containing polymer. For example, commercially available polycarbosilanes and polycarbosiloxane polymers can be rendered photo-curable by the addition of vinyl or ethynyl side groups to the polymer. The polymer is crosslinked upon high intensity photo-radiation. A di-functional silane precursor (e.g., dichlorodimethylsilane) and tri-functional silane precursor (e.g., trichlorophenylsilane) can be utilized separately or in combination to enhance cross-linking and/or increase ceramic or glass yield. Further, the silicon-containing polymers can comprise a dopant, such as boron, to control sintering and crystallization behavior.

[0032] The method of crosslinking the silicon-containing polymer is not particularly limited and depends on the type of polymer. Some polymers can be crosslinked using heat only, or with an optional crosslinking or curing agent. The temperature of the crosslinking step varies depending upon the type of silicon-containing polymer and the crosslinking agent that are used. For example, the crosslinking temperature can be about 50 to about 200⁰C.

[0033] A polymer such as poly(methyl-vinyl)silazane can be crosslinked using dicumyl peroxide as the crosslinking agent. Other crosslinking agents include organic peroxides such as dibenzoyl peroxide, bis-p-chlorobenzoyl peroxide, bis-2,2-dichlorobenzoyl peroxide, di-t-butyl peroxide, dicumyl peroxide, t-butyl perbenzoate, 2,5-bis(t-butylperoxy)- 2,3-dimethylhexane and t-butyl peracetate. Vinyl substituted siloxane polymers can be crosslinked using a peroxide catalyst or a transition metal catalyst, in particular a platinum catalyst, for example STARFIRE® SP020 sold by Starfire Systems, Inc.

[0034] The crosslinking agent is present in an amount sufficient to effect crosslinking of the silicon-containing polymer. Therefore, the actual amount of the crosslinking agent will depend on the activity of the crosslinking agent used and the amount of the silicon-containing polymer present. An exemplary amount of crosslinking agent is about 0.05-5 wt.%, based on the weight of the silicon-containing polymer to be cured, more particularly about 0.1-1 wt.%.

[0035] The amount of precursor compound applied to the surface of the untreated BN powder can vary depending on the type of precursor, the BN particle size and surface area, and other considerations. When used, the precursor compound can be present in amounts of about 1 to 20 wt.%, more specifically about 1 to 10 wt.%, and even more specifically about 1 to 6 wt.%, each based on total weight of the untreated BN particulate.

[0036] The surface treatment can comprise depositing a coating material onto a surface of an untreated BN particle in a single step. Alternatively, the surface treatment can comprise multiple steps. The surface treatment typically involves contacting the untreated BN particles with a precursor compound of the coating material to form a BN intermediate filler, and thermally or chemically treating the BN intermediate filler to form the coated BN filler comprising the coating material disposed on a surface thereof. The thermal treatment can be performed at a temperature of 500 to 1500 ⁰C for an effective time, e.g., about 4 to about 18 hours. The thermal treatment can be performed in the presence of air, an inert atmosphere, or a vacuum. Alternatively, the coating material can be deposited directly to the surface of the non-treated BN particles to form the coated BN filler, followed by an optional thermal treatment to fixedly bond the coating material to the surface of the BN particles.

[0037] Exemplary commercially available coated BN fillers include BORONID™ VSN 1215, an hexagonal BN (hBN) filler in the form of agglomerates wherein the hBN particles have an average particle size of 9 micrometers and comprise 6 wt. % alumina/bo ehmite on the surface based on total weight of the hBN filler; BORONID™ VSN 1216, an hBN filler in the form of agglomerates wherein the hBN particles have an average particle size 15 micrometers and comprise 6 wt.% alumina/boehmite on the surface; and BORONID™ VSN 1217, an agglomerated hBN filler wherein the hBN particles have an average particle size of 3 micrometers and comprise 1 wt.% silica on the surface, each manufactured by ESK Ceramics.

[0038] The coated BN particles can be further surface treated with a coupling agent. Coupling agents promote the formation of or participate in covalent bonds that improve adhesion between the filler and the thermosetting or thermoplastic polymer matrix. Other properties can also be improved, such as reduction in water absorption. Exemplary coupling agents include silanes, zirconates, titanates, and the like, such as vinyltrichlorosilane, vinyltrimethoxysilane, trivinylmethoxysilane, vinyltriethoxysilane, vinyltris(β- methoxyethoxy)silane, β-(3 ,4-epoxycyclohexyl)ethyltrimethoxysilane, γ- glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ- glycidoxypropyltriethoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ- methacryloxypropyltrimethoxysilane, γ-methacryloxypropylmethyldiethoxysilane, γ- methacryloxypropyltriethoxysilane, N-β(aminoethyl)γ-aminopropylmethyldimethoxysilane, N-β(aminoethyl)γ-aminopropyltrimethoxysilane, bis(trimethoxysilylethyl)benzene, bis(triethoxysilyl)ethylene, triethoxysilyl-modified butadiene, styrylethyltrimethyloxysilane, N-β(aminoethyl)γ-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane, γ- aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane and γ- mercaptopropyltrimethoxysilane. Combinations comprising one or more of the foregoing can also be used. In one embodiment, the silane is trivinylmethoxysilane, bis(trimethoxysilylethyl)benzene, bis(triethoxysilyl)ethylene, triethoxysilyl-modified butadiene, styrylethyltrimethyloxysilane, 3 -mercaptopropylmethyldimethoxy silane, 3- mercaptopropyltrimethoxy silane, or a combination comprising one or more of the foregoing. The BN surface is notoriously difficult to surface treat. Use of the inorganic coating as described above allows for surface treatments such as these to be more effective in improving the properties of the composite.

[0039] The coated BN filler can be surface treated with a coupling agent before combining with the other components of the composition used to make the dielectric layer, or while blending the components of the composition used to make the dielectric layer. When used, coupling agents can be present in amounts of about 0.5 to about 5.0 wt.% based on the total weight of BN.

[0040] The coated BN particles are combined with a polymer composition used to manufacture the dielectric layer. For convenience, the term "polymer composition" as used herein refers to all components of the compositions used to form the dielectric layer except for the coated BN filler and any additional optional fillers. The coated BN filler can be present in the polymer composition in an amount of 1 to 50 wt.%, more particularly 5 to 35 wt.%, and even more particularly 10 to 25 wt.% based on total weight of the polymer composition. [0041] The polymer composition can be a thermoplastic or thermosetting polymer. A variety of thermoplastic polymers are known for their use in dielectric layers including, for example, fluoropolymers such as polytetrafluroethylene (PTFE), liquid crystal polymers (LCP), thermoplastic polyimides, polyarylene ether ketones including polyether ether ketone (PEEK), polyetherimides (PEI), polyether sulfones, and combinations comprising at least one of the foregoing.

[0042] A variety of thermosetting polymers are also known for use in dielectric layers including, for example, polymers and copolymers derived from ethylenically unsaturated monomers having one or more carbon-carbon double bonds, such as butadiene and/or isoprene; epoxy modified polymers; siloxane modified polymers; allylated polyethers, and combinations thereof. More particularly, the thermosetting composition comprises low dielectric constant and low loss polymers, including those based on thermosetting polymers such as 1,2-polybutadiene, polyisoprene, polybutadiene-polyisoprene copolymers, poly(phenylene ether) polymers, and those based on allylated poly(phenylene ether) polymers. These materials, while exhibiting the desirable features of low dielectric constant and low loss, also exhibit low copper peel strength. The copper peel strength of such materials containing BN filler can be significantly improved by the use of the coated filler as described herein. It is also important that the peel strength remain relatively high at elevated temperatures to allow for "rework," i.e., the removal and replacement of soldered components on the circuit board. Combinations of low polarity polymers with higher polarity polymers can also be used, non-limiting examples including epoxy and poly(phenylene ether), epoxy and poly( ether imide), and cyanate ester and poly(phenylene ether). Compositions containing polybutadiene, polyisoprene, and/or butadiene- and isoprene- containing copolymers are especially useful.

[0043] Particularly useful thermosetting polymer compositions comprise a thermosetting polybutadiene and/or polyisoprene polymer. As used herein, the term "thermosetting polybutadiene and/or polyisoprene polymer" includes homopolymers and copolymers comprising units derived from butadiene, isoprene, or mixtures thereof. Units derived from other copolymerizable monomers can also be present in the polymer, for example in the form of grafts. Exemplary copolymerizable monomers include, but are not limited to, vinylaromatic monomers, for example substituted and unsubstituted monovinylaromatic monomers such as styrene, 3-methylstyrene, 3,5-diethylstyrene, 4-n- propylstyrene, alpha-methylstyrene, alpha-methyl vinyltoluene, para-hydroxystyrene, para- methoxystyrene, alpha-chlorostyrene, alpha-bromostyrene, dichloro styrene, dibromostyrene, tetra-chlorostyrene; and substituted and unsubstituted divinylaromatic monomers such as divinylbenzene, divinyltoluene. Combinations comprising at least one of the foregoing copolymerizable monomers can also be used. Exemplary thermosetting polybutadiene and/or polyisoprene polymers include, but are not limited to, butadiene homopolymers, isoprene homopolymers, butadiene-vinylaromatic copolymers such as butadiene-styrene, isoprene- vinylaromatic copolymers such as isoprene-styrene copolymers.

[0044] The thermosetting polybutadiene and/or polyisoprene polymers can also be modified, for example the polymers can be hydroxyl-terminated, methacrylate-terminated, carboxylate-terminated polymers. Post-reacted polymers can be used, such as epoxy-, maleic anhydride-, or urethane-modified butadiene or isoprene polymers. The polymers can also be crosslinked, for example by divinylaromatic compounds such as divinyl benzene, e.g., a polybutadiene-styrene crosslinked with divinyl benzene. Suitable polymers are broadly classified as "polybutadienes" by their manufacturers, for example Nippon Soda Co., Tokyo, Japan, and Sartomer Company Inc., Exton, PA. Mixtures of polymers can also be used, for example, a mixture of a polybutadiene homopolymer and a poly(butadiene-isoprene) copolymer. Combinations comprising a syndiotactic polybutadiene can also be useful.

[0045] The thermosetting polybutadiene and/or polyisoprene polymer can be liquid or solid at room temperature. Suitable liquid polymers can have a number average molecular weight greater than about 5,000 but generally have a number average molecular weight of less than about 5,000 (most preferably about 1,000 to about 3,000). Thermosetting polybutadiene and/or polyisoprene polymers having at least 90 wt.% 1,2 addition are preferred because they exhibit the greatest crosslink density upon cure, due to the large number of pendent vinyl groups available for crosslinking. Exemplary thermosetting polybutadiene and/or polyisoprene polymer compositions are disclosed, for example, in U.S. Pat. No. 6,2913,74.

[0046] The polybutadiene and/or polyisoprene polymer is present in the polymer system in an amount of up to 100 wt.%, specifically about 60 wt.% with respect to the total polymer system, more specifically about 10 to about 55 wt.%, even more specifically about 15 to about 45 wt.%, based on the total weight of the polymer components.

[0047] Other polymers that can co-cure with the thermosetting polybutadiene and/or polyisoprene polymers can be added for specific property or processing modifications. For example, in order to improve the stability of the dielectric strength and mechanical properties of the electrical substrate material over time, a lower molecular weight ethylene propylene elastomer can be used in the polymer systems. An ethylene propylene elastomer as used herein is a copolymer, terpolymer, or other polymer comprising primarily ethylene and propylene. Ethylene propylene elastomers can be further classified as EPM copolymers (i.e., copolymers of ethylene and propylene monomers) or EPDM terpolymers (i.e., terpolymers of ethylene, propylene, and diene monomers). Ethylene propylene diene terpolymer rubbers, in particular, have saturated main chains, with unsaturation available off the main chain for facile cross-linking. Liquid ethylene propylene diene terpolymer rubbers, in which the diene is dicyclopentadiene, are preferred.

[0048] Useful molecular weights of the ethylene propylene rubbers are less than 10,000 viscosity average molecular weight. Suitable ethylene propylene rubbers include an ethylene propylene rubber having a viscosity average molecular weight (MV) of about 7,200, which is available from Uniroyal Chemical Co., Middlebury, CT, under the trade name TRILENE CP80; a liquid ethylene propylene dicyclopentadiene terpolymer rubbers having a molecular weight of about 7,000, which is available from Uniroyal Chemical Co. under the trade name of TRILENE 65; and a liquid ethylene propylene ethylidene norbornene terpolymer, having a molecular weight of about 7,500, which is available from Uniroyal Chemical Co. under the name TRILENE 67.

[0049] The ethylene propylene rubber is preferably present in an amount effective to maintain the stability of the properties of the substrate material over time, in particular the dielectric strength and mechanical properties. Typically, such amounts are up to about 20 wt. % with respect to the total weight of the polymer components, more specifically about 4 to about 20 wt.%, even more specifically about 6 to about 12 wt.%.

[0050] Another type of co-curable polymer is an unsaturated polybutadiene- or polyisoprene-containing elastomer. This component can be a random or block copolymer of primarily 1,3 -addition butadiene or isoprene with an ethyl enically unsaturated monomer, for example a vinylaromatic compound such as styrene or alpha-methyl styrene, an acrylate or methacrylate such a methyl methacrylate, or acrylonitrile. The elastomer is preferably a solid, thermoplastic elastomer comprising a linear or graft-type block copolymer having a polybutadiene or polyisoprene block, and a thermoplastic block that preferably is derived from a monovinylaromatic monomer such as styrene or alpha-methyl styrene. Suitable block copolymers of this type include styrene-butadiene- styrene triblock copolymers, for example those available from Dexco Polymers, Houston, TX, under the trade name VECTOR 8508M, from Enichem Elastomers America, Houston, TX, under the trade name Sol-T-6302, and those from Fina Oil and Chemical Company, Dallas, TX, under the trade name FINAPRENE 401; styrene-butadiene diblock copolymers; and mixed triblock and diblock copolymers containing styrene and butadiene, for example those available from Shell Chemical Corporation, Houston, TX, under the trade name KRATON Dl 118. KRATON Dl 118 is a mixed diblock / triblock styrene and butadiene containing copolymer, containing 30 volume% styrene.

[0051] The optional polybutadiene- or polyisoprene-containing elastomer can further comprise a second block copolymer similar to that described above, except that the polybutadiene or polyisoprene block is hydrogenated, thereby forming a polyethylene block (in the case of polybutadiene) or an ethylene-propylene copolymer block (in the case of polyisoprene). When used in conjunction with the above-described copolymer, materials with greater toughness can be produced. An exemplary second block copolymer of this type is KRATON GXl 855 (commercially available from Shell Chemical Corp.), which is believed to be a mixture of a styrene-high 1 ,2-butadiene-styrene block copolymer and a styrene-(ethylene-propylene)-styrene block copolymer. Another exemplary block copolymer is KRATON G (commercially available also from Shell Chemical Corp.), a maleic anhydride modified block copolymer of styrene-ethylene-butylene-styrene.

[0052] Typically, the unsaturated polybutadiene- or polyisoprene-containing elastomer component is present in the polymer system in an amount of about 10 to about 60 wt. % with respect to the total polymer system, more specifically about 20 to about 50 wt.%, or even more specifically about 25 to about 40 wt.%, based on the total weight of the polymer components.

[0053] Still other co-curable polymers that can be added for specific property or processing modifications include, but are not limited to, homopolymers or copolymers of ethylene such as polyethylene and ethylene oxide copolymers; natural rubber; norbornene polymers such as polydicyclopentadiene; hydrogenated styrene-isoprene-styrene copolymers and butadiene-acrylonitrile copolymers; unsaturated polyesters; and the like. Levels of these copolymers are generally less than 50 vol.% of the total volume of the polymer components.

[0054] In addition to the one or more of the polymers described above, the thermosetting or thermoplastic polymer composition can further optionally comprise additives such as crosslinking agents, fluoropolymers, curing agents, secondary inorganic fillers, coupling agents, antioxidants, woven or nonwoven fibers, flame retardants (including particulate magnesium hydroxide fillers), viscosity modifiers, and wetting agents. The particular choice of additives depends upon the nature of the conductive layer and the thermosetting or thermoplastic composition, and are selected so as to enhance or not substantially adversely affect adhesion between a conductive layer and the dielectric layer, dielectric constant, thermal conductivity, dissipation factor, water absorbance, flame retardance, and/or other desired properties of the dielectric layer.

[0055] Crosslinking agents can optionally be added to increase the crosslink density of polymer(s). Examples of cross-linking agents include, without limitation, triallylisocyanurate, triallylcyanurate, diallyl phthalate, divinyl benzene, and multifunctional acrylate monomers (e.g., the Sartomer polymers available from Arco Specialty Chemicals Co.), and combinations thereof, all of which are commercially available, with triallylisocyanurate being particularly exemplary. The cross-linking agent content of the polymer composition can be readily determined by one of ordinary skill in the art, depending upon the desired flame retardancy of the composition, the amount of the other constituent components, and the other properties desired in the final product. UL-94, an Underwriters Laboratories flammability test, provides four possible ratings, HB, V-2, V-I, and V-O. V-O is the most difficult rating to obtain, requiring that five bars of material self extinguish with an average flame out time of five seconds or less without dripping. More particularly, the amount of cross-linking agent depends upon the loading of fillers and amount(s) of the other components in the polymer composition, and attaining excellent flame retardancy, electrical and moisture properties. When used to increase the crosslink density, effective quantities are about 0.5 to about 10 wt.%, specifically about 1 to about 7 wt.%, more specifically about 5 to about 7 wt.%, based on the total weight of the polymer components.

[0056] Particulate fluoropolymers can optionally be included in the thermosetting or thermoplastic polymer composition. Exemplary particulate fluoropolymers include those known in the art for use in circuit subassemblies, and include but are not limited to fluorinated homopolymers, for example polytetrafluoroethylene (PTFE), and fluorinated copolymers, e.g., copolymers oftetrafluoroethylene with hexafluoropropylene or perfluoroalkylvinylethers such as perfluorooctylvinyl ether, or copolymers of tetrafluoroethylene with ethylene. Combinations of fluorinated polymers, copolymers, and terpolymers can also be used. The fluoropolymers can be in fine powder, dispersion, or granular form, including fine powder (or "coagulated dispersion") PTFE, made by coagulation and drying of dispersion-manufactured PTFE, generally manufactured to exhibit a particle size of about 400 to about 500 micrometers; granular PTFE made by suspension polymerization, generally having two different particle size ranges (median particle size of about 30 to about 40 micrometers for the standard product, and about 400 to about 500 micrometers for the high bulk density product); and/or granular PTFE, FEP, or PFA. The granular fluoropolymers can be cryogenically ground to exhibit a median particle size of less than about 100 micrometers.

[0057] When present, the effective particulate fluoropolymer content of the thermosetting or thermoplastic polymer composition can be readily determined by one of ordinary skill in the art, depending upon the desired flame retardancy of the composition, the amount of the other components, and the other properties desired in the final product. More particularly, the amount of the fluoropolymer depends upon the amounts of BN, and other fillers and particulate flame retardants in the polymer composition. In general, effective quantities are greater than or equal to about 1 to about 90 parts by weight per hundred parts by weight of the other polymer components (phr), specifically about 5 to about 75 phr, and most specifically about 10 to about 50 phr (parts per hundred parts by weight) of the total polymer components.

[0058] A curing agent can be added to accelerate the curing reaction of the polyenes having olefinic reactive sites. Examples include, but are not limited to, azides, peroxides, sulfur, and sulfur derivatives. Free radical initiators can be desirable as cure initiators. Examples of free radical initiators include peroxides, hydroperoxides, and non-peroxide initiators such as 2,3-dimethyl-2, 3-diphenyl butane. Specifically useful curing agents are organic peroxides such as, dicumyl peroxide, t-butyl perbenzoate, 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane, a,a-di-bis(t-butyl peroxy)diisopropylbenzene, and 2,5-dimethyl-2,5-di(t-butyl peroxy) hexyne-3, all of which are commercially available. They can be used alone or in combination. Typical amounts of curing agent are from about 1.5 to about 10 wt.% of the total polymer components.

[0059] The thermosetting or thermoplastic polymer composition can further optionally comprise a secondary inorganic filler, in particular a dielectric and/or flame retardant filler, in combination with the coated BN filler. Secondary inorganic dielectric fillers include titanium dioxide (rutile and anatase), barium titanate, strontium titanate, silica (particles and hollow spheres) including fused amorphous silica; corundum, wollastonite, fiberglass, Ba₂Ti₉O₂O, glass spheres, quartz, boron nitride, aluminum nitride, phosphorous compounds (e.g., flame retardant phosphorus-containing compounds), brominated compounds, (e.g., flame retardant bromine-containing compounds), silicon carbide, beryllia, alumina, magnesia, mica, talcs, nanoclays, aluminosilicates (natural and synthetic), fumed silicon dioxide (e.g., Cab-O-Sil, available from Cabot Corporation). Other secondary fillers can be used, for example magnesium hydroxide and other uncoated BN fillers. Combinations of the secondary dielectric fillers can also be used. More specifically, rutile titanium dioxide and amorphous silica are especially desirable because these fillers have a high and low dielectric constant, respectively, thereby permitting a broad range of dielectric constants combined with a low dissipation factor to be achieved in the final cured product by adjusting the respective amounts of the two secondary dielectric fillers in the composition. Specific dielectric fillers include rutile titanium dioxide and amorphous silica. The secondary fillers can be in the form of solid, porous, or hollow particles. Secondary fillers, when used, are typically present in an amount of about 15 to about 60 weight %, specifically about 20 to about 50 weight %, based on the total weight of the polymer composition.

[0060] To improve adhesion between the secondary fillers, in particular the dielectric fillers, and the thermosetting or thermoplastic polymer system, the dielectric filler can be treated with one or more coupling agents, such as silanes, zirconates, or titanate coupling agents. The dielectric filler can be surface treated with a coupling agent before combining with the other components of the composition used to form the dielectric layer, or while blending the components of the components used to make the dielectric layer. When used, coupling agents can be present in amounts of about 0.5 to about 5.0 wt.%, based on the total weight of BN.

[0061] The dielectric layer can optionally further include woven, thermally stable webs of a suitable fiber, specifically glass (E, S, and D glass) (such as 1080 glass from Clark- Schwebel), including flat glass or close-weaved fiber glass, or high temperature polyester fibers (e.g., KODEL from Eastman Kodak). Such thermally stable fiber reinforcement provides a circuit laminate with a means of controlling shrinkage upon cure within the plane of the laminate. In addition, the use of the woven web reinforcement renders a circuit substrate with a relatively high mechanical strength.

[0062] The above-described thermosetting and thermoplastic polymer compositions, together with the coated BN and any optional additional fillers, can be used to manufacture a dielectric layer, for use, for example, as a bond ply, cover film, or build-up material, and which, when combined with a conductive layer, provides circuit laminates, resin-coated cap layers, and circuit subassemblies. Any one or more of the foregoing can be used can be used to form circuits and multilayer circuits. Suitable conductive layers include a thin layer of a conductive metal such as a copper foil presently used in the formation of circuits, for example, electrodeposited copper foils. Useful copper foils typically have thicknesses of about 9 to about 180 micrometers.

[0063] The copper foil can be made either by the electrodeposition (ED) on a rotating stainless steel drum from a copper sulfate bath, or by the rolling of solid copper bars. Where ED copper foil is used, the initial roughness of the base foil is created in the foil plating process on the "bath side" (or matte side) of the foil. Additional roughness is created in a secondary plating step. Where rolled foil used, roughness is imparted to the initially smooth and shiny foil by a secondary plating step. Conventional ED copper foil made for the circuit industry has had treated side Rz values of 7 to 20 micrometers (um) (corresponding to Rq values of about 1.2 to 4 um) when measured by the WYCO Optical Profiler. Contact profilometers tend to yield lower values, due to the stylus deforming the copper treatment as the measurement is made. The treated side of rolled copper foil exhibits Rz values of 3.5 - 5.5 um (corresponding to Rq values of 0.45 - 0.9 um). "Reverse treated" ED foils, such as Oak-Mitsui MLS-TOC-500 can also exhibit Rq values similar to those of rolled foils. The lower profile ED foils currently exhibit Rz values of 2 to 3 um. By WYCO measurement, the shiny side of rolled foil exhibits an Rz value of about 0.7 um and a corresponding Rq of about 0.1 um. More recently, other types of low profile electrodeposited foils have been commercially available. These include Oak Mitsui products SQ-VLP, with an Rq value measured by the WYCO of 0.7 um and MQ-VLP with a WYCO Rq value of 0.47 um.

[0064] Both rolled and ED foils specially treated for the circuit industry are available from a number of commercial manufacturers. For example, low profile copper foils are commercially available from Oak Mitsui under the trade name "TOC-500," 'TOC-500- MLS," and "TOC-500-LZ," from Nippon Denkai under the trade name "USLP," and from Furukawa under the trade name "FlWS." High profile copper foils are commercially available from Circuit Foil under the trade name "TWS."

[0065] In general, a composition for the formation of the dielectric layer is processed as follows. First, all the components (polymers, coated BN filler, other optional fillers such as dielectric fillers, optional coupling agents (e.g., such as silanes, zirconates and titanates), and other desired additives such as a curing agent) are thoroughly mixed in conventional mixing equipment. Mixing continues to form a slurry or dispersion wherein the ingredients are uniformly dispersed throughout the composition. For those applications where the composition comprises a melt-processable thermoplastic, the mixing can be carried out in the molten phase of the polymer using an extruder or other suitable thermoplastic compound mixing equipment. For those applications where the composition is to impregnate a woven glass web to form a prepreg, conventional prepreg manufacturing methods can be employed. Typically the woven web is impregnated with the slurry, metered to the correct thickness, and then any solvent removed to form a prepreg. If the prepreg contains certain thermosetting polymers, it can then optionally be B-staged to form a partially cured dielectric layer before use in forming a circuit subassembly.

[0066] A solvent or dispersing medium can optionally be present to adjust the viscosity of the slurry. The solvent is selected so as to dissolve or disperse the polymer components of the thermosetting or thermoplastic composition and to have a convenient evaporation rate for applying and drying the coating. A non-exclusive list of possible solvents and dispersing media is water, xylene, toluene, methyl ethyl ketone, methyl isobutyl ketone, hexane, and higher liquid linear alkanes, such as heptane, octane, nonane, cyclohexane, isophorone, and various terpene-based solvents. Specific solvents and dispersing media for thermosetting compositions include xylene, toluene, methyl ethyl ketone, methyl isobutyl ketone, and hexane, and more specifically xylene and toluene. The concentration of the thermosetting composition in solution is not critical and will depend on the solubility of the components, the method of application, and other factors. A specific dispersing media for thermoplastic compositions especially PTFE and other fluoropolymers is water.

[0067] In accordance with various specific embodiments, FIG. 1 is a schematic cross- sectional diagram of an exemplary circuit subassembly 10 comprising a dielectric layer 14 disposed on a conductive layer 12, e.g., a copper foil. As used herein, "disposed" means at least partial intimate contact between conductive layer 12 and the dielectric layer 14. It is to be understood that in all of the embodiments described herein, the various layers can fully or partially cover each other, and additional copper foil layers, patterned circuit layers, and dielectric substrate layers can also be present. The dielectric layer 14 comprises the coated BN, and can be uncured, partially cured, or fully cured. The dielectric layer 14 can be a dielectric substrate layer, a resin in a resin-coated copper layer, or a dielectric substrate in a core layer.

[0068] Figure 2 is a schematic cross-sectional diagram of an exemplary circuit subassembly 20 comprising a dielectric layer 24 disposed between a first conductive layer 22, e.g., a copper foil, and a second conductive layer 26, e.g., a copper foil. The thermosetting dielectric layer 24 comprises the coated BN, and can be uncured, partially cured, or fully cured. The dielectric layer 24 can be a double clad dielectric substrate layer for a conventional multilayer circuit, or a dielectric in a core layer.

[0069] The following examples are illustrative, but do not limit the claims. EXAMPLES

[0070] The materials shown in Table 1 were used in the Examples. Table 1.

[0071] Copper peel strength was tested in accordance with the "Peel strength of metallic clad laminates" test method (IPC-TM-650 2.4.8).

[0072] The laminates were tested for solder float by floating them on a pot of molten solder at a temperature of 288°C for 10 seconds. This procedure is repeated five times on each sample. A failure in the solder float test is noted if there is blistering or delamination of the copper foil from the laminate surface. [0073] Thermal conductivity was measured in accordance with ASTM C-518, Unitherm 2021).

Preparation of P120-PhSi.

[0074] PT120 boron nitride (1000 g) was coated with phenyl silane (50 g) using a V- blender. The treated boron nitride was subsequently dried at 13O⁰C for 12 hours to form BN particles comprising phenyl silane on the surface.

Preparation of PTlOO-SiOC.

[0075] PTlOO boron nitride (1000 g) was treated with POLYRAMIC RD212A from Starfire Systems (25 g dry polycarbosiloxane) and 0.15 g of STARFIRE SP020 (platinum catalyst) using a V-blender. The treated boron nitride was first dried at 175⁰C for 12 hours, and then subsequently sintered at 900⁰C for 6 hours to form BN particles comprising amorphous silicon oxycarbide (about 85 wt% conversion as tested by TGA) on the surface of the particles.

Examples 1-5 and Comparative Example A.

[0076] Thermosetting compositions were prepared based on a formulation of the type described in described in U.S. Pat. No. 6,2913,74, as shown in Table 2, and formed into prepregs. Amounts shown are in grams. The compositions further contained 0.6 g of an antioxidant and 1.3 g of a peroxide.

[0077] Example formulations (Ex. 1 to Ex. 5) were made with coated BN fillers. The comparative formulation (CEx. A) was made with uncoated BN filler (hexagonal PT120 BN filler). Table 2.

[0078] Circuit laminates were formed from the prepregs by forming a layered structure comprising a prepreg between two sheets of adhesive-coated 0.5 oz. TWS copper foil, and laminating the layered structure in a press using a rapid ramp to 345°F (174°C), a 15 minute hold at 345°F (174°C), followed by a ramp to 475°F (246°C), and an additional hour hold at 475°F (246°C). A pressure of 1200 psi (70.3 kilogram/square centimeter) was maintained throughout the ramp cycles. Each laminate was tested for copper peel strength (IPC TM-650-2.4.8), thermal conductivity (ASTM C-518, Unitherm 2021), interlamination (interplies) adhesion (qualitatively), and solder float. The results are shown in Table 3.

Table 3.

[0079] As can be seen from the above data, copper peel strength and interlaminar adhesion of Exs. 3, 4, and 5 (alumina-boehmite-coated BN and silica-coated BN) are better than in CEx. A (uncoated BN). In addition, the laminates made with alumina/boehmite or silane-coated boron nitride (Ex. 2) are improved in these properties when compared with laminates made with uncoated boron nitride (CEx. A) or silicon oxycarbide-coated boron nitride (Ex. 1). Also, the laminates of Ex. 3, 4 and 5 have good thermal conductivity (at least 1.0 W/mK). Moreover, Ex. 3, 4, and 5 pass the solder float test. The combination of improved adhesion and good thermal conductivity is highly desirable in circuit laminates.

[0080] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. The endpoints of all ranges directed to the same characteristic or component are independently combinable and inclusive of the recited endpoint. All references are incorporated herein by reference. As used herein and throughout, "disposed," "contacted," and variants thereof refers to the complete or partial physical contact between the respective materials, substrates, layers, films. Further, the terms "first," "second," and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another.

[0081] While specific embodiments have been shown and described, various modifications and substitutions can be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation. [0082] What is claimed is:

Claims

1. A circuit subassembly, comprising a dielectric layer formed from a composition comprising a thermosetting or thermoplastic polymer composition, and a surface coated boron nitride particulate filler, wherein the surface coating comprises a ceramic, a metal oxide, or a metal hydroxide.

2. The circuit subassembly of claim 1, further comprising a conductive layer.

3. The circuit subassembly of claim 2, wherein the conductive layer is patterned to form a circuit.

4. The circuit subassembly of any of claims 2-3 , wherein the conductive layer comprises copper.

5. The circuit subassembly of any of claims 1-4, wherein the surface coating comprises silica, alumina, boehmite, magnesium hydroxide, titania, silicon carbide, silicon oxycarbide, or a combination thereof.

6. The circuit subassembly of any of claims 1-4, wherein the surface coating is derived from a polysilazane, polycarbosilane, siloxane, polysiloxane, polycarbosiloxane, silsesquioxane, polysilsesquioxane, or a polycarbosilazane.

7. The circuit subassembly of any of claims 1 -4, wherein the boron nitride filler comprises an alumina/boehmite surface coating in an amount of about 1 to about 20 wt.%, based on the total weight of the boron nitride filler.

8. The circuit subassembly of any of claims 1 -4, wherein the boron nitride filler comprises a silica coating in an amount of about 1 to about 10 wt.%, based on the total weight of the BN filler.

9. The circuit subassembly of any of claims 1 -8, wherein the boron nitride filler is further surface treated with a coupling agent.

10. The circuit subassembly of claims 1-9, wherein the coated filler comprises an average agglomerated particle size of 2 to 200 micrometers.

11. The circuit subassembly of any of claims 1-11, wherein the polymer composition is a thermosetting composition comprising a polybutadiene polymer, a polyisoprene polymer, or a combination comprising at least one of the foregoing polymers.

12. The circuit subassembly of any of claims 1-10, wherein the polymer composition is a thermoplastic composition

13. The circuit subassembly of any of claims 1-10, wherein the polymer composition comprises a fluoropolymer.

14. The circuit subassembly of any of claims 1-13, further comprising a coupling agent.

15. The circuit subassembly of claim 14, wherein the coupling agent is a silane.

16. The circuit subassembly of claim 15, wherein the silane is vinyltrichlorosilane, vinyltrimethoxysilane, trivinylmethoxysilane, vinyltriethoxysilane, vinyltris(β- methoxyethoxy)silane, β-(3 ,4-epoxycyclohexyl)ethyltrimethoxysilane, γ- glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ- glycidoxypropyltriethoxysilane, γ-methacryloxypropylmethyldimethoxysilane, γ- methacryloxypropyltrimethoxysilane, γ-methacryloxypropylmethyldiethoxysilane, γ- methacryloxypropyltriethoxysilane, N-β(aminoethyl)γ-aminopropylmethyldimethoxysilane, N-β(aminoethyl)γ-aminopropyltrimethoxysilane, bis(trimethoxysilylethyl)benzene, bis(triethoxysilyl)ethylene, triethoxysilyl-modified butadiene, styrylethyltrimethyloxysilane, N-β(aminoethyl)γ-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane, γ- aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane γ- mercaptopropyltrimethoxysilane, 3 -mercaptopropylmethyldimethoxysilane, 3 - mercaptopropyltrimethoxysilane, or a combination comprising at least one of the foregoing silanes.

17. The circuit subassembly of claim 16, wherein the silane is trivinylmethoxysilane, vinyltriethoxysilane, bis(trimethoxysilylethyl)benzene, bis(triethoxysilyl)ethylene, triethoxysilyl-modified butadiene, styrylethyltrimethyloxysilane, 3-mercaptopropylmethyldimethoxysilane, 3 -mercaptopropyltrimethoxysilane, or a combination comprising at least one of the foregoing silanes.

18. The circuit subassembly of any of claims 1-17, wherein the circuit subassembly has greater interlaminar adhesive strength compared to an otherwise identical circuit subassembly comprising the boron nitride filler without the surface coating.

19. The circuit subassembly of any of claims 1-18, wherein the circuit subassembly has a Z-direction thermal conductivity, as measured by ASTM C-518, Unitherm 2021, of 90% or more compared to the otherwise identical circuit subassembly comprising the boron nitride filler without the surface coating.

20. The circuit subassembly of any of claims 1-19, in the form of a circuit laminate, a resin-coated conductive layer, a bond ply, or a buildup material.

21. A circuit comprising the circuit subassembly of any of claims 1-20.

22. An multilayer circuit comprising the circuit subassembly of any of claims 1 - 20.

23. A method for the manufacture of a dielectric layer for a circuit subassembly, the method comprising forming a dielectric layer comprising a thermosetting polymer composition, and a surface coated boron nitride particulate filler, wherein the surface coating comprises a ceramic, a metal oxide, or a metal hydroxide; and at least partially curing the dielectric layer.

24. A method for the manufacture of a dielectric layer for a circuit subassembly, the method comprising melt-blending a composition comprising a thermoplastic polymer composition, and a surface coated boron nitride particulate filler, wherein the surface coating comprises a ceramic, a metal oxide, or a metal hydroxide; and forming a dielectric layer from the melt-blended composition.

25. The method of claims 23-24, wherein the dielectric layer is formed on a conductive layer.

26. The method of claim 25, wherein the conductive layer is copper.

27. A method of preparing a circuit subassembly, comprising forming a layered structure comprising a dielectric layer and a conductive layer, wherein the dielectric layer is formed from a composition comprising a thermosetting or thermoplastic polymer composition and a surface coated boron nitride particulate filler, wherein the surface coating comprises a ceramic, a metal oxide, or a metal hydroxide; and laminating the layered structure at a temperature and pressure effective to adhere the dielectric layer to the conductive layer to form the circuit subassembly.

28. The method of claim 27, wherein the conductive layer is copper.