Thermally conductive composition

Abstract

A method of forming a crosslinked film comprising crosslinking a precursor film disposed on a surface wherein the precursor film comprises a precursor polymer and a crosslinking agent; and: the prec

Description

BACKGROUND Thermally conductive materials are used in a wide variety of applications including in underfill for flip-chips to reduce thermally induced stresses following application of a flip chip. WO 2022 / 136584 discloses thermally conductive polymers formed by reaction of a dialdehyde and a diamine. CN116003871 discloses formation of a super-hydrophobic overall aerogel involving reaction of a diamine monomer with trifluoromethyl, terephthaldehyde and a cross-linking agent tris (2-aminoethyl) amine. SUMMARY The present disclosure provides a method of forming a crosslinked film comprising crosslinking a precursor film disposed on a surface wherein the precursor film comprises a precursor polymer and a crosslinking agent; and either: the precursor polymer is substituted with one or more carbonyl groups and the crosslinking agent comprises a plurality of amine groups, or the precursor polymer is substituted with one or more amine groups and the crosslinking agent comprises a plurality of carbonyl groups. Optionally, the precursor polymer is substituted with one or more carbonyl groups and the crosslinking agent comprises a plurality of amine groups. Optionally, formation of the precursor film comprises deposition of a formulation comprising the precursor polymer and the crosslinking agent dissolved in one or more solvents. Optionally, the polymer comprises a repeat unit comprising an Imine unit in the polymer backbone. Optionally, the polymer is a conjugated polymer. Optionally, the polymer comprises a repeating structure of formula (I): i,Ar) “4 < y2 / (I) wherein Ar in each occurrence is an arylene or heteroarylene group which is unsubstituted or substituted with one or more substituents; p is at least 1; one of Y1 and Y2 is CR2 wherein R2 is H or a substituent; and the other of Y1 and Y2 is N. Optionally, the one or more carbonyl groups are groups of formula -C(=O)R1 wherein R1 is H or a Ci 12 hydrocarbyl group. Optionally, R1 Is H. Optionally, the plurality of amine groups are -NH2 groups. Optionally, the crosslinking agent is a polymer substituted with amine groups. Optionally, the polymer substituted with amine groups is a branched polyethyleneimine. Optionally, the precursor polymer is substituted with one or more carbonyl groups and has a glass transition temperature in the range of 50-200°C. Optionally, the precursor film is crosslinked by heating. Optionally, the heating is at a temperature in the range of 100-400°C. The present disclosure provides a crosslinked film obtainable by either crosslinking a precursor film comprising a polymer substituted with one or more carbonyl groups and a crosslinking agent comprising a plurality of amine groups, or crosslinking a precursor film comprising a polymer substituted with one or more amine groups and a crosslinking agent comprising a plurality of carbonyl groups. The present disclosure provides an electronic device comprising a crosslinked film as described herein disposed on a surface of a functional layer of the electronic device. Optionally, the crosslinked film is disposed in a region between the surface of the functional layer and a first surface of a first chip electrically connected to the functional layer. Optionally, the functional layer is a printed circuit board; an interposer; or a second chip. Optionally, the electronic device comprises a 3D chip stack. The present disclosure provides apparatus comprising a heat-generating device, a heat transfer device configured to transfer heat away from the heat-generating device and a crosslinked film as described herein disposed between the heat-generating device and the heat transfer device. The present disclosure provides a heat sink comprising a first surface having fins extending therefrom and an opposing second surface having a crosslinked film as described herein disposed thereon. The present disclosure provides use of a crosslinked film as described herein as a thermally conductive film. The present disclosure provides a composition comprising a mixture of a polymer substituted with one or more carbonyl groups and a crosslinking agent comprising a plurality of amine groups. The present disclosure provides a composition comprising a mixture of a polymer substituted with one or more amine groups and a crosslinking agent comprising a plurality of carbonyl groups. The present disclosure provides a formulation comprising the composition as described herein and one or more solvents wherein the polymer and the crosslinking agent are dissolved in the one or more solvents. Optionally, the composition is a solid composition. DESCRIPTION OF DRAWINGS Figure 1 schematically illustrates an electronic device according to some embodiments comprising a flip-chip electrically connected to a substrate; Figure 2A schematically illustrates a method according to some embodiments of forming the electronic device of Figure 1 in which an underfill layer is formed between the substrate and the flip-chip; Figure 2B schematically illustrates a method according to some embodiments of forming the electronic device of Figure 1 in which a non-conducting film Is applied to the flip chip prior to connection to the substrate; Figure 3 schematically illustrates a 3D chip stack according to some embodiments; Figure 4 is the infrared spectra for annealed films of a polymer and a crosslinking agent at different crosslinking agent loadings; Figure 5 schematically illustrates a substrate for measurement of thermal conductivity of a film; Figures 6A and 6B schematically Illustrate apparatus for measurement of thermal conductivity including the substrate of Figure 5; and Figure 7 is a graph of thermal conductivity vs filler content for crosslinked films and precursor (uncrosslinked) films. The drawings are not drawn to scale and have various viewpoints and perspectives. The drawings are some implementations and examples. Additionally, some components and / or operations may be separated into different blocks or combined into a single block for the purposes of discussion of some of the embodiments of the disclosed technology. Moreover, while the technology is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the technology to the particular implementations described. On the contrary, the technology is intended to cover all modifications, equivalents, and alternatives falling within the scope of the technology as defined by the appended claims. DETAILED DESCRIPTION Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, In the sense of "including, but not limited to." Additionally, the words "herein," "above," "below," and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word "or," in reference to a list of two or more items, covers all of the following interpretations of the word: any of the Items in the list, all of the items in the list, and any combination of the items in the list. References to a layer "over" another layer when used in this application means that the layers may be in direct contact or one or more intervening layers may be present. References to a layer "on" another layer when used in this application means that the layers are in direct contact. References to a chemical element of the Periodic Table Include any isotopes of that element. The teachings of the technology provided herein can be applied to other systems, not necessarily the system described below. The elements and acts of the various examples described below can be combined to provide further implementations of the technology. Some alternative implementations of the technology may include not only additional elements to those implementations noted below, but also may include fewer elements. These and other changes can be made to the technology in light of the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the technology under the claims. To reduce the number of claims, certain aspects of the technology are presented below in certain claim forms, but the applicant contemplates the various aspects of the technology in any number of claim forms. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of implementations of the disclosed technology. It will be apparent, however, to one skilled in the art that embodiments of the disclosed technology may be practiced without some of these specific details. The present inventors have found that a robust film having high thermal conductivity may be provided either by crosslinking a precursor polymer having one or more carbonyl groups and a crosslinking agent containing a plurality of amine groups or by crosslinking a precursor polymer having one or more amine groups and a crosslinking agent containing a plurality of carbonyl groups. Optionally, thermal conductivity of a crosslinked film as described herein is at least 0.15 Wm^K'1, optionally at least 0.2 or 0.3 Wm^K'1. Polymer The polymer is preferably a conjugated polymer. The conjugated polymer as described herein has a backbone comprising arylene or heteroarylene groups which are conjugated together in the polymer backbone. The arylene or heteroarylene groups may be directly linked or may be linked by a conjugating group, for example a carbon-carbon double bond (alkene) group or a carbon-nitrogen double bond (imine) group. The conjugation may extend across the whole of the polymer backbone or the polymer backbone may comprise conjugated regions interrupted by nonconjugating repeat units. Polymers as described herein are preferably at least partially crystalline. Polymers as described herein may undergo pi-pi stacking when deposited as a film. Optionally, thermal conductivity of a film consisting of a polymer as described herein is at least 0.15 Wm^K1, optionally at least 0.2 or 0.3 Wm^K’1. The polymer preferably has a repeating structure comprising imine units. More preferably, the conjugated polymer comprises a repeating structure of formula (I): (I) wherein Ar in each occurrence is an arylene or heteroarylene group which is unsubstituted or substituted with one or more substituents; p is at least 1; one of Y1 and Y2 is CR1 wherein R1 is H or a substituent; and the other of Y1 and Y2 is N. p is preferably at least 2, optionally 2-5. The extended rigid-rod type structure of formula (I) may enhance thermal conductivity of the polymer as compared to the case where p = 1. Ar in each occurrence in (Ar)p may be the same or different, preferably the same. Exemplary (Ar)p groups include, without limitation, para-phenylene, pyridine pyrazine, biphenylene, terphenylene, thiophene, furan, and benzobisoxazole, each of which may independently be unsubstituted or substituted with one or more substituents. Paraphenylene is preferred. R1 is preferably H or a C1-20 hydrocarbyl group, more preferably H. A Ci-20 hydrocarbyl group as described anywhere herein is preferably selected from C1-20 alkyl; unsubstituted phenyl; and phenyl substituted with one or more C1-12 alkyl groups. Optionally, one or more Ar groups of (Ar)P are substituted with one or more substituents. Preferably, substituents are selected from substituents R2 wherein R2 in each occurrence is independently selected from: F; CN; NO2; branched, linear or cyclic Ci 20 alkyl wherein one or more non-adjacent C-atoms may be replaced with O, S, NR3, SiR42, C=0 or COO; wherein R3 in each occurrence is H or a substituent, preferably H or a C1-20 hydrocarbyl group and R4 in each occurrence is independently a substituent, optionally a C1-20 hydrocarbyl group; an aryl or heteroaryl group Ar3 which Is unsubstituted or substituted with one or more substituents. Optional substituents of Ar3, where present, are described above. Preferably, at least one substituent R2, optionally each substituent R2, is C1-20 alkyl or Ci-20 alkoxy, more preferably a C1-14alkyl or C1-14alkoxy. Exemplary groups -(Ar)P- include, without limitation, groups of formulae (IVa) and (IVb): (Ha) (lib) wherein R2 independently in each occurrence is a substituent and w in each occurrence is independently 0 or a positive integer. A preferred group -(Ar1)— has formula (IIb-1): R2 (IIb-1) The polymer may comprise a divalent linker group L disposed in the polymer backbone, wherein L is selected from 0, S, NR3 or a C1-12 alkylene group wherein one or more non-adjacent C atoms of a C212 alkylene group may be replaced with 0, S, NR3, SiR42, CO or COO. Preferably, L is a C2-12 alkylene group in which one or more non-adjacent C atoms are replaced with 0. In some embodiments, the divalent linker group L is disposed between and linked directly to two Ar groups. In some embodiments, the divalent linker group L is disposed between and linked directly to an Ar group and an imine (-C(R1) = N-) group. In some embodiments, the divalent linker group L is disposed between and linked directly to two imine (-C(R1) = N-) groups. The polymer may be formed by polymerising a monomer or monomers having reactive groups which react to form an imine. The repeating structure of formula (I) may be part of a larger repeat unit of the polymer formed by polymerising the monomer or monomers. Exemplary repeat units include, without limitation, formulae (111)-( V): 'p Y2—(Ar)q (HI) -^(Ar)p in------L-----(Ar)m (IV) -^(Ar)n---l Y2---(Ar)q (V) wherein Ar, p, Y1, Y2 and L are as described above; q is at least 1, preferably 1-5, more preferably 1-3; n is 0 or a positive integer, preferably 0 or 1-5, more preferably 0, 1, 2 or 3; and m is 0 or a positive integer, preferably 0 or 1-5, more preferably 0, 1, 2 or 3. For each Y^Y2, Y1 is one of CR1 and N and the other of Y^Y2 is the other of CR1 and N. In each of formulae (III)-(V), it will be understood that the two Y1 groups may both be the same one of CR1 and N; or one Y1 is CR1 and the other Y1 is N. Likewise, the two Y2 groups may both be the same one of CR1 and N; or one Y2 is CR1 and the other Y2 is N. In a preferred embodiment, one Y1 is CR1 and the other Y1 is N and, accordingly, one Y2 is CR1 and the other Y2 is N. If q is greater than 1 then each Ar of (Ar)q, may be the same or different, preferably the same. If n is greater than 1 then each Ar of (Ar)n, may be the same or different, preferably the same. If m is greater than 1 then each Ar of (Ar)m, may be the same or different, preferably the same. Preferred Ar groups of (Ar)q, (Ar)m and (Ar)m are as described with reference to (Ar)p. The repeat units of the polymer may be the same or different. In some embodiments, the polymer contains a mixture of different repeat units of formulae (III)-(V). The polymer may contain one or more of: different repeat units of formula (III); different repeat units of formula (IV); different repeat units of formula (V); and a repeat unit selected from one of formulae (III)-(V) and at least one other repeat unit selected from another of formulae (III)-(V). In a preferred embodiment, the polymer contains a repeat unit without a divalent linker group L and a repeat unit with a divalent linker group L, for example a repeat unit of formula (III) and a repeat unit of formula (IV). In the case where n and m are each 0, the repeat unit of formula (IV) has formula (IVa): (IVa) The polystyrene-equivalent number-average molecular weight (Mn) measured by gel permeation chromatography of the polymers described herein may be in the range of about lxlO3 to 1x10®, and preferably IxlO4 to 5xl05. The polystyrene-equivalent weightaverage molecular weight (Mw) of the polymers described herein may be lxlO3 to 1x10®, and preferably lxlO4 to lxlO6. Conjugated polymers comprising a repeating structure of formula (I) may be formed by polymerising a first monomer having two aldehyde groups with a second monomer having two diamine groups. In some embodiments, only one first monomer and only one second monomer is used in polymer formation. In some embodiments, two or more different first monomers and / or two or more different second monomers are used in polymer formation. In some embodiments, polymers comprising a repeating structure of formula (I) are formed by polymerisation of a first monomer comprising a group of formula (I) and two reactive groups X1 with a second monomer comprising two reactive groups X2 wherein one of X1 and X2 is a group of formula -C(=O)lV and the other of X1 and X2 is NHz. In a preferred embodiment, the precursor polymer comprises carbonyl end-groups, more preferably aldehyde end-groups. The carbonyl end-groups may be provided by reacting a molar excess of the first monomer or monomers comprising two carbonyl (preferably aldehyde) groups relative to the number of moles of the second monomer or monomers comprising two amine groups. In another embodiment, the precursor polymer comprises amine end-groups. The amine end-groups may be provided by reacting a molar excess of the second monomer or monomers comprising two amine groups relative to the number of moles of the first monomer or monomers comprising two carbonyl (preferably aldehyde) groups. By providing amine or carbonyl (preferably aldehyde) end groups arising from partially reacted monomers, it is not necessary to synthesise monomers with side groups that can be converted to amine or carbonyl groups after polymerisation or to carry out post polymerisation functionalisation (e.g., an activation or deprotection step) prior to crosslinking of the precursor polymer. The composition as described herein may contain a single polymer. The composition as described herein may contain two or more different polymers. Crosslinking agent The crosslinking agent comprises a plurality of amine or a plurality of carbonyl groups, preferably a plurality of amine groups. Preferably, the crosslinking agent contains at least two carbonyl or amine groups, more preferably more than two carbonyl or amine groups. Preferably, each amine group is a primary (-NH2) amine group. The crosslinking agent may be a non-polymeric or polymeric material substituted with a plurality of carbonyl or amine groups. Exemplary non-polymeric crosslinking agents comprise a core group substituted with at least two carbonyl or at least two amine groups, wherein the core group is selected from Ci-10 alkylenes, tris(Ci 10 alkylene)amines, C6-20 arylenes, and 5-20 membered heteroarylenes. Exemplary C6-20 arylenes and 5-20 membered heteroarylenes are phenylene; biphenylene; and triazine. An arylene or heteroarylene substituted with at least two carbonyl or amine groups may be substituted with one or more further substituents, for example one or more further substituents selected from C1-6 alkyl. Exemplary non-polymeric crosslinking agents are 1,2-ethylenediamine, 1,4-phenylenediamine, p-xylylenediamine, tris(2-aminoethyl)amine, 1,3,5-triaminobenzene, 1,2,4-triaminobenzene, 2,4,6-triamino-l,3,5-triazine(melamine), 1,2,4,5-benzenetetramine, 3,3'-diaminobenzidine, 2,6-diamino-9,10-anthraquinone, terephthaldicarboxaldehyde benzene-l,3,5-tricarbaldehyde, 4,4’- biphenyldicarboxaldehyde, glutaric dialdehyde and triformylmethane. Exemplary polymeric crosslinking agents are branched-polyethylenimine and polymers containing repeat units formed by polymerisation of 3-aminostyrene, 4-aminostyrene, N-(3-aminopropyl)methacrylamide, 2-aminoethyl methacrylate, 2-aminoethyl methacrylamide, methacrylamide poly(ethylene glycol) amine or 3-vinylbenzaldehyde monomers. In the case where the crosslinking agent comprises two or more carbonyl groups, each carbonyl is preferably an aldehyde group. The crosslinking agent is preferably provided in an amount of at least 0.5 weight % relative to the weight of the crosslinkable polymer, more preferably at least 2 weight %. Optionally, the crosslinking agent is provided in an amount of no more than 20 weight %, optionally no more than 10 weight %. Filler particles A crosslinkable composition as described herein may consist of a polymer and a crosslinking agent as described herein or may contain one or more further materials. Exemplary further materials are filler particles, for example one or both of boron nitride particles and alumina particles. Boron nitride particles as described herein preferably have a thermal conductivity of at least 10 Wnr1^1. In some embodiments, the surface of the boron nitride particles Is modified. The surface may be modified by attachment of a material comprising one or more groups selected from Ci-20 alkyl groups, aromatic groups, preferably an aromatic group, for example an oligo-(hetero)arylene comprising 1-10 arylene or heteroarylene groups, or a poly-(hetero)arylene. An exemplary surface group is an oligophenylene, for example biphenyl or terphenyl. A surface group may be bound to the particle surface by reaction of the particle with a compound comprising the surface group substituted with a reactive group. The reactive group may be an aldehyde, carboxylic acid, carboxylic anhydride, acid chloride, carboxylic ester, phosphonic acid or ester or a trialkoxysilane. In the case of boron nitride particles, hydroxyl groups are preferably present at the surface of the boron nitride and the reactive group is a reactive group capable of reacting with a hydroxyl group, e.g. in a nucleophilic substitution reaction. Boron nitride may be exfoliated with a metal hydroxide, e.g. sodium hydroxide, to provide hydroxyl groups at the boron nitride surface. The boron nitride particles are preferably in the form of flakes. Flake particles as described herein may have a largest dimension of up to 100 microns. Flake particles as described herein may have a mean average largest dimension of up to 100 microns. Flakes as described herein preferably have a mean average aspect ratio of at least 10 : 1. The aspect ratio of a particle having a length, width and thickness is a ratio of the length to thickness of the particle. A mean average dimension and a mean average aspect ratio as described herein may be determined from measurement of dimensions of a plurality of particles (e.g. at least 10 particles) In a scanning electron micrograph image of a sample of the particles. Flakes may be formed by exfoliation using methods known to the skilled person, e.g. ultrasonication and / or ball milling. Alumina particles are preferably spherical or spheroidal. A spheroidal particle as described herein preferably have a mean average aspect ratio of no more than 3 : 1, preferably no more than 2 : 1 and more preferably no more than 1.5 : 1. In some embodiments, the surface of the alumina is modified. The surface may be modified by attachment of a material comprising one or more groups selected from C1-20 alkyl groups, aromatic groups, preferably an aromatic group, for example an oligo-(hetero)arylene comprising 1-10 arylene or heteroarylene groups, or a poly-(hetero)arylene. An exemplary surface group is an oligophenylene, for example biphenyl or terphenyl, A surface group may be bound to the particle surface by reaction of the unmodified particle with compound comprising the surface group substituted with a reactive group. The reactive group may be an aldehyde, carboxylic acid, carboxylic anhydride, acid chloride, carboxylic ester, a phosphonic acid or a trialkoxysilane. Film formation Optionally, formation of a crosslinked film comprises formation of a precursor (uncrosslinked) film from a formulation comprising the polymer and crosslinkable agent dissolved in a solvent or solvent mixture; and crosslinking of the precursor film. Solvents may be selected according to their ability to dissolve or disperse the polymer and the crosslinking agent. Exemplary solvents include, without limitation, benzene or naphthalene substituted with one or more substituents, optionally one or more substituents selected from C1-12 alkyl, C1-12alkoxy, F and Cl; ethers; esters; halogenated alkanes; and mixtures thereof. Exemplary solvents Include, without limitation, tetrahydrofuran, toluene, 2-methyltetrahydrofuran, dichloromethane, chloroform, 1,2,4- trimethylbenzene, mesitylene, 1-methylnaphthalene, 1-chloronaphthalene, 1,2-xylene, chlorobenzene, 1,2-dichlorobenzene, anisole, and 1,2-dimethoxybenzene. The crosslinkable polymer of the composition, for example a polymer of Formula (I) described herein, preferably has a solubility of at least 0.1 mg / ml, optionally at least 0.5 mg / ml or at least 1 mg / ml, more preferably at least 10 mg / ml or 20 mg / ml in 1,2-dichlorobenzene at 50°C and at atmospheric pressure. The crosslinking agent preferably has a solubility of at least 0.1 mg / ml, optionally at least 0.5 mg I ml or at least 1 mg / ml in 1,2-dichlorobenzene at 50°C and at atmospheric pressure. The crosslinkable polymer and the crosslinking agent are preferably soluble in the same solvent. Filler particles, if present, are dispersed in the formulation. Formulations as described anywhere herein may be deposited by any suitable solution deposition technique including, without limitation, spin-coating, dip-coating, drop-casting, spray coating and blade coating. The formulation may be deposited onto an alignment layer, e.g. a rubbed polyimide. Following deposition of the formulation, the formulation may be processed during or after solvent evaporation to enhance ordering of polymer chains, e.g., by thermal annealing, hot-pressing, stretching or rubbing of the film. Preferably, the crosslinked film is not an aerogel. Preferably, the crosslinked film is substantially free of voids. By "substantially free of voids" is meant that less than 5 %, preferably less than 1 % of the film volume is air. Preferably, the crosslinked film has a density of at least 0.6 g / cm3, optionally at least 0.8 or 0.9 g / cm3. The crosslinking agent may be activated following deposition of the formulation to crosslink the polymer. Activation may be by thermal treatment. The thermal activation temperature is preferably at least 50°C, optionally at least 100°C, optionally at least 150°C, optionally no more than 400°C. Applications A film comprising a polymer as described herein may be used in any known application of a thermally conductive film. A product may comprise a first component, a second component and a thermal transfer film as described herein disposed between the first component and second component wherein, in use, a temperature gradient exists between the first component and the second component. Preferably, the product is an electronic device or apparatus, e.g. a semiconductor package. The film as described herein may be disposed between a surface of a heat-generating component and a heat transfer component configured to transfer heat away from the heatgenerating component, such as in any known thermal interface management application. It will be understood that in this arrangement the film is configured to transfer heat from the heat-generating component to the heat transfer component. The film preferably has a first surface in direct contact with a surface of the heat-generating component and / or a second surface opposing the first surface in direct contact with a surface of the heat transfer component. The thermally conductive film may be electrically insulating, i.e. in use the film does not provide an electrical conduction path between any electrically conductive surfaces that it may be in contact with. Optionally, the thermally conductive film has an electrical conductivity of no more than 1 x 10-8 S / m, optionally 1 x 10-9 S / m or 1 x 10 10 S / m. Any passive or active heat transfer component known to the skilled person may be used including, without limitation, a heat sink having a surface in contact with the film and an opposing surface comprising one or more heat-dissipating features, for example fins or a pipe or channel configured to transfer heat to a fluid flowing through the pipe or channel. The fluid may or may not undergo a phase change upon absorption of heat. In some embodiments, e.g. where the thermally conductive film is disposed on a surface of a heat sink, the thermally conductive film is electrically isolated. By "electrically isolated" is meant that the thermally conductive film is not electrically connected, directly or through any electrically conductive surface that it may be in contact with, to an electrical power source. In some embodiments, a film as described herein may be disposed on a surface of a heat sink opposing a surface of the heat sink having fins extending therefrom. In use, the film may be disposed between the heat sink and an electrical component. A film comprising a polymer as described herein may be used as an electrically non-conductive film, e.g. an underfill, for a flip chip including but not limited to 3D stacked multi-chips. Figure 1 illustrates an electronic device comprising a chip 105; a substrate 101, e.g. a printed circuit board; and electrically conductive interconnects 107 between electrically conductive pads 103 on the surface of the substrate 101 and the chip 105. Underfill 109 comprising a composition as described herein fills the region between the chip 105 and substrate 101 and surrounds the interconnects. Optionally, the polymer of the composition is crosslinked. With reference to Figure 2A, in some embodiments formation of an electronic device comprises bringing electrically conductive bumps 107', e.g. solder bumps, into contact with electrically conductive pads 103 disposed on a substrate 101, e.g. a printed circuit board to form interconnects 107 from electrically conductive bumps 107'. Formation of underfill 109 comprising a composition as described herein comprises application of a formulation into the overlap region between the chip 105 and the substrate 101. Optionally, the polymer is crosslinked following application of the formulation by heat treatment. With reference to Figure 2B, in some embodiments a film 109 comprising the composition is applied over a surface of the chip 105 carrying electrically conductive bumps 107. Figure 2B illustrates complete coverage of the conductive bumps 107 however it will be understood that the conductive bumps 107 may be partially covered such that a part of the conductive bumps 107 protrude from a surface of the film 109. The conductive bumps 107 are then brought into contact with conductive pads 103 disposed on a substrate 101, e.g. a printed circuit board, to form electrically conductive interconnects between the substrate and the chip. Formation of the electrically conductive interconnects may comprise application of heat and I or pressure. If the polymer of film 109 is crosslinked then crosslinking may take place before, during or after the conductive bumps 107 are brought into contact with the conductive pads 103. Two or more chips may be connected with a film comprising a polymer as described herein disposed between chips. Figure 3 illustrates a 3D stack of chips 105 according to some embodiments, wherein the chips 105 are interposed by an interposer 111 and a non-electrically conductive film 109 disposed between adjacent interposer and chip surfaces and between the substrate 101, e.g. a printed circuit board, and a first chip of the 3D stack. At least one non-electrically conductive film 109 comprises a composition as described herein. Through-vias 115 are formed through the chips 105 and the interposers. The 3D stack may comprise a heat sink 113 disposed on a surface thereof. In some embodiments, a film of a composition as described herein may be disposed between an electronic device and a heat sink. EXAMPLES Polymer 1 A polymer was formed by reacting 2',5'-dihexyloxy terphenyl 4,4"-dialdehyde (Dialdehyde Monomer 1) and a Diamine Monomer 1, shown below. OC6H13 / P H\= / ^= / \= / XH CbHisO Dialdehyde Monomer 1 XI X J Diamine Monomer 1 Dialdehyde Monomer 1 is disclosed in WO2022 / 136584, the contents of which are incorporated herein by reference. Diamine Monomer 1 (20.0 g, 69.8 mmol) and Dialdehyde Monomer 1 (37.3 g, 76.7 mmol) were combined with 600 ml of 2-methyltetrahydrofuran (2-MeTHF) and m-cresol (14.5 ml, 139 mmol) and dissolved at 50°C on the rotary evaporator. The solution was stirred for 30 minutes and reduced to dryness under reduced pressure. The polymer was redissolved in 600 ml of 2-MeTHF at 50°C on the rotary evaporator. The solution was cooled down to room temperature and transferred into a separating funnel. The polymer solution was washed three times with 300 ml of NaOAc (10wt% aqueous) and once with 300ml of water and with 30 ml of brine. The polymer solution was added dropwise to 4 L of methanol under vigorous stirring. The polymer was isolated by filtration. The polymer cake was triturated with 1.5 L of methanol for 30 minutes. The polymer was collected and sonicated for 1 hour with 500 ml of methanol, filtered, and dried in a vacuum oven at 50°C for 20 hours. The polymer was redissolved in 600 ml of 2-MeTHF at 50°C on the rotary evaporator for 1 hour. The solution was cooled down to room temperature and transferred into a separating funnel. The polymer solution was washed 2 x with 300 ml of NaOAc (10wt% aqueous) and once with 300ml of water + 30 ml of brine to help phase separation. The polymer solution was added dropwise to 4 L of methanol under vigorous stirring. The polymer was isolated by filtration. The polymer was washed with water then methanol and dried in a vacuum oven at 50°C for 20 hours. The polymer was redissolved in 600 ml of 2-MeTHF at 50°C on the rotary evaporator for 1 hour. The solution was cooled down to room temperature and transferred into a separating funnel. The polymer solution was washed once with 300ml of water. The polymer solution was added dropwise to 4 L of methanol under vigorous stirring. The polymer was isolated by filtration. The polymer was washed with methanol and dried in a vacuum oven at 50°C for 20 hours, yielding to 52.2 g of yellow solids, 95% yield. Functionalised boron nitride Boron nitride (BN) nanoflakes were prepared via a ball milling process. BN powder (2.76g, Goodfellow, nominal particle size 10 microns) was placed in a zirconia lined milling jar (50mL) with zirconia milling beads (114g, 2mm) and NaOH (2M aqueous) (15.1g). The mixture was milled in a planetary ball mill (Retsch PM 100) at 400 rpm for 6 hours. On completion the mixture was decanted from the milling jar and washed with deionised water. The milling beads were separated by sieving. The boron nitride nanoflakes were collected by filtration using a small pore filter, washing with water until the filtrate was neutral, then with methanol and dried in air at 130°C to yield 2.47g as white solid. The milled BN was functionalized by reaction with 2',5'-dihexyloxy terphenyl 4,4"-dialdehyde. 0.5 g of BN prepared above was dispersed in 100 ml DMSO with 0.593 g of 2',5'-dihexyloxy terphenyl 4,4"-dialdehyde. The reaction mixture was stirred for 20 hours at 100°C. The product was collected via filtration and washed with acetone and toluene to remove unreacted aldehyde. Solid was dried for 4 hours at 100°C to yield 0.44 g of light yellow solid. Crosslinkable solutions Solutions were formed by mixing 500 microlitres of a solution of Polymer 1 (40 mg / mL in THF) with solutions of branched polyethyleneimine (b-PEI) Mw~600 as set out in Table 1 to form Solutions 1-4 having, respectively, 1, 2, 5 and 10 % PEI by weight relative to Polymer 1. Solution b-PEI / Polymer 1 (wt / wt%) in mixed solution b-PEI solution (10 mg / ml in THF) Ib-PEI (100 mg / ml in hriF) 1 1% 20 pl ; 2 2% 40 pl :: - ; 3 5% - | 10 pl 4 10% - J 20 pl Film formation 200 microlitres of each of Solutions 1-4 were drop-casted onto stainless steel disks (diameter 16 mm) at room temperature. Solutions were allowed to dry at room temperature for 1 hour. 1 disk of each formulation was kept before annealing for analysis. The other disks were placed on an aluminium block and temperature was increased to 170°C. Temperature was then maintained for 1 hour at 170°C then left to cool down to room temperature. Crosslinking results Films with 5 and 10% b- PEI turned orange and films could not be peeled off the stainless steel disks. Films with 1 and 2% b-PEI could be peeled off. An attempt was made to dissolve the film formed using 2% b-PEI in THF but the film was no longer soluble indicating effective cross-linking. FT-IR. spectra of annealed films are given in Figure 4. The peak at 1700 cm 1 in the FTIR spectra corresponding to the C=O stretch of the aldehyde is still present after annealing of the blend with 1% b-PEI, however it has fully disappeared in the spectra of the blends containing 2wt% of b-PEI and above confirming the full cross-linking of Polymer 1. Table 2 shows differential scanning calorimetry (DSC) results of films before annealing. Table 2 Wt% of PEI Endotherm 1 peak temperature (°C) Endotherm 2 peak temperature (°C) Exotherm 1 peak temperature (°C) 0 73.8 212 245 2 72.4 Non-detected Broad 159.5 5 69.3 Non-detected Broad 183.8 10 69.3 Non-detected Broad 182.7 All samples have a first endothermic event at around 70°C which correspond to a softening of the films or Tg. The inclusion of b-PEI does not significantly impact this event indicating that this temperature is below a crosslinking temperature of the film. The second 5 endotherm is not seen in films containing b-PEI due to an exotherm corresponding to the reaction of Polymer 1 with the b-PEI and thus the cross-linking and setting of the film. Table 3 shows thermogravimetric analysis (TGA) of the films before annealing. Table 3 Wt% of PEI % Mass loss associated with water loss Onset degradation (°C) 0 Non-detected 416 1 0.84 417 2 2.86 411 5 4.58 399 10 5.57 353 10 TGA shows mass loss below 150°C for the blends containing b-PEI. This is associated with water loss contained in the b-PEI material used for the blends and seen in its TGA analysis. Onset degradation temperature of the blends decreased with increased b-PEI content, indicating that it is desirable to limit the amount of b-PEI to only that required for crosslinking. Thermal conductivity measurement A sensor substrate 600 (ca. 25 mm x 25 mm) illustrated in Figure 5 was used for measurement of thermal conductivity as described herein. The substrate has a polylmlde (PI) film (25 microns) with a 400 nm thick heating structure consisting of a 20 micron wide heater line 610, 500 micron wide busbars 620 for application of a current and contact pads 640. A sensing structure mirrors the heating structure except that the heater line is replaced with a 200 micron wide sensor line 630. With reference to Figures 6A and 6B, the sensor substrate 600 carrying the film to be measured is placed on a temperature-controlled aluminium block, regulated via a PID system such that the temperature may be controlled by software. The aluminium block has a long notch 720 of 1mm width and ~lmm depth cut into It. The sensor substrate 600 is placed over the notch such that the central heater line 610 Is aligned with the centre of the notch 720, and the sensor line 630 is aligned with the edge of the notch. A PMMA sheet 730 (3 mm thickness) with a notch cut-through matching that of the aluminium block 710 is placed over the top and an addition piece of plain PMMA sheet 740 (5 mm thickness) is placed on top to enclose the device. The entire assembly is clamped using bolts and nuts at positions 750. The heater line is connected to a sourcemeter unit (Keithley 2400) using a 4-wire measurement set up. The sensor line is connected to a multimeter unit (Keithley 2001) using a 4-wire set up. The temperature of the assembly is first stabilised at a predetermined temperature. The resistance of the heater line and the temperature sensor is then measured. To measure the resistance of the heater line without causing undue heating a low current is sourced and voltage measured in short pulses, with time allowed between pulses for heat to be dissipated. A constant DC current is then passed along the heater line to cause resistive heating. The arrangement of the substrate in the assembly causes heat to flow through the substrate and film to the aluminium block which acts as a heat sink, setting up an approximate one-dimensional steady state heat flux. The power dissipated in the heater line, and the resistance of the heater line and temperature sensor is additionally measured in this state. This process is repeated for increasing sourced current, and the complete process repeated at the next temperature setpoint. The resistances of the heater line and sensor lines under the condition of no heat flux at different temperature setpoints are used as calibration data in a straight-line fit of resistance and temperature, allowing the temperature of the resistive elements to be determined under the condition of steady state heat flux. As such the temperature gradient, AT, between the heater line and temperature sensor (aligned with the heatsink) can then be calculated. The power dissipated in the heater line is assumed to be completely converted to heat energy Q. A straight-line fit is then made between dT and Q with additional parameters for the length of the heater line over which power is measured (L, 14 mm), the distance between the voltage sense points) and the gap width (2w, 1mm). This provides a measure of the conductance C of the device under test and is affected by losses pertaining to conductive heat transfer in the substrate and convective and radiative heat transfer to the environment (h). To calculate a thermal conductivity k, the same measurement process is carried out on substrates without any test film (substrate only). We assume the losses will be approximately the same when measuring a coated vs uncoated substrate. We subtract the conductance of the substrate (Cs) from the device measurement (Cf+s) to adjust for these losses. The thermal conductivity (kp) is then calculated by dividing the resulting film only conductance by the film thickness (dp). The film thickness is determined using a digital micrometer by measuring the total thickness and subtracting the substrate thickness. C =----= Kd + 2hw2 2LAT Cp+s kf =-------- Up Thermal conductivity results Functionalised BN flakes were dried in an oven at 90°C for at least 4 hours before preparing the suspensions. Functionalised BN flakes were weighed after drying. A suspension of functionalised BN was prepared by mixing 1,2-dichlorobenzene and functionalised BN at a concentration of 20 mg / ml. The suspensions were homogenised by sonication (Fisherbrand 11207, 100% duty cycle, 37Hz, 50°C) for at least 1 hour. b-PEI was dissolved in o-dichlorobenzene (10 mg / mL) at room temperature and mixed with a solution of Polymer 1 in o-dichlorobenzene (20 mg / mL) to reach 98:2 (Polymer 1 : b-PEI wt:wt ratio). The solution was mixed at room temperature with the suspension of functionalised boron nitride. Weight ratios of BN and Solution of Polymer 1:b-PEI (98:2) are as shown in Table 4. For comparison, solutions and suspensions without b-PEI were prepared. The solutions were vortexed and immediately drop-casted on polyimide substrates at room temperature. After overnight drying at room temperature, the substrates were cured on a hotplate at 160°C for 1 hour. Thermal conductivities are shown in Figure 7 and Table 4. 5 Table 4 Example Matrix Wt:wt ratio of matrix: functionalised boron nitride Average thermal conductivity (—) 'm-K' Median thermal conductivity (—) 'm-K' Comparative example 1 Polymer 1 100:0 0.47 0.45 Example 1 Polymer 1: b-PEI (98:2) 100:0 0.33 0.31 Comparative example 2 Polymer 1 80:20 2.37 2.28 Example 2 Polymer 1: b-PEI (98:2) 80:20 2.08 2.30 Comparative example 3 Polymer 1 60:40 5.13 4.79 Example 3 Polymer 1: b-PEI (98:2) 60:40 5.26 5.00 As shown in these results, b-PEI does not have a significant effect on thermal conductivity. Consequently, stable thermoset films can be formed by crosslinking without significantly reducing the thermal conductivity of the films.

Claims

1. A method of forming a crosslinked film comprising crosslinking a precursor film disposed on a surface wherein the precursor film comprises a precursor polymer and a crosslinking agent; and either:the precursor polymer is substituted with one or more carbonyl groups and the crosslinking agent comprises a plurality of amine groups,orthe precursor polymer is substituted with one or more amine groups and the crosslinking agent comprises a plurality of carbonyl groups.

2. The method according to claim 1 wherein the precursor polymer is substituted with one or more carbonyl groups and the crosslinking agent comprises a plurality of amine groups.

3. The method according to claim 1 or 2 wherein formation of the precursor film comprises deposition of a formulation comprising the precursor polymer and the crosslinking agent dissolved in one or more solvents.

4. The method according to any one of the preceding claims wherein the polymer comprises a repeat unit comprising an imine unit in the polymer backbone.

5. The method according to any one of the preceding claims wherein the polymer Is a conjugated polymer.

6. The method according to any one of the preceding claims wherein the polymer comprises a repeating structure of formula (I):y27"(i)wherein Ar in each occurrence is an arylene or heteroarylene group which is unsubstituted or substituted with one or more substituents; p is at least 1; one of Y1 and Y2 is CR2 wherein R2 is H or a substituent; and the other of Y1 and Y2 Is N.

7. The method according to any one of claims 2-6 wherein the one or more carbonyl groups are groups of formula -0(=0^1 wherein R1 is H or a C1-12 hydrocarbyl group.

8. The method according to claim 7 wherein R1 is H.

9. The method according to any one of claims 2-8 wherein the plurality of aminegroups are -NH2 groups.

10. The method according to any one of claims 2-9 wherein the crosslinking agent is a polymer substituted with amine groups.

11. The method according to claim 10 wherein the polymer substituted with amine groups is a branched polyethyleneimine.

12. The method according to any one of the preceding claims wherein the precursor polymer is substituted with one or more carbonyl groups and has a glass transition temperature in the range of 50-200°C.

13. The method according to any one of the preceding claims wherein the precursor film is crosslinked by heating.

14. The method according to claim 13 wherein the heating is at a temperature in the range of 100-400°C.

15. A crosslinked film obtainable by either crosslinking a precursor film comprising a polymer substituted with one or more carbonyl groups and a crosslinking agent comprising a plurality of amine groups, or crosslinking a precursor film comprising a polymer substituted with one or more amine groups and a crosslinking agent comprising a plurality of carbonyl groups.

16. An electronic device comprising a crosslinked film according to claim 15 disposed on a surface of a functional layer of the electronic device.

17. The electronic device according to claim 16 wherein the crosslinked film is disposed in a region between the surface of the functional layer and a first surface of a first chip electrically connected to the functional layer.

18. The electronic device according to claim 16 or 17 wherein the functional layer Is a printed circuit board; an interposer; or a second chip.

19. The electronic device according to any one of claims 16-18 wherein the electronic device comprises a 3D chip stack.

20. Apparatus comprising a heat-generating device, a heat transfer device configured to transfer heat away from the heat-generating device and a crosslinked film according to claim 15 disposed between the heat-generating device and the heat transfer device.

21. A heat sink comprising a first surface having fins extending therefrom and an opposing second surface having a crosslinked film according to claim 15 disposed thereon.

22. Use of a crosslinked film according to claim 15 as a thermally conductive film.

23. A composition comprising a mixture of a polymer substituted with one or morecarbonyl groups and a crosslinking agent comprising a plurality of amine groups.

24. A composition comprising a mixture of a polymer substituted with one or more amine groups and a crosslinking agent comprising a plurality of carbonyl groups.

25. A formulation comprising the composition according to claim 23 or 24 and one or more solvents wherein the polymer and the crosslinking agent are dissolved In the one or more solvents.

26. The composition according to claim 23 or 24 wherein the composition is a solid composition.27

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