Thermally conductive polymer

Abstract

A polymer comprising a repeat unit of formula (I) X1 and X2 are each independently selected from formulae (II) and (III) with the proviso that at least one of X1 and X2 is a group of formula (III): (II) - (III) Ar1, Ar2 and Ar3 independently in each occurrence is an arylene or heteroarylene group; m is at least 1; p is at least 1; q is at least one; and L is an optionally substituted chain of methylene groups and O atoms; one of Y1 and Y2 is CR1 wherein R1 is H or a substituent; and the other of Y1 and Y2 is N; and one of Y3 and Y4 is CR1; and the other of Y3 and Y4 is N

Description

[0001] THERMALLY CONDUCTIVE POLYMER

[0002] BACKGROUND

[0003] 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.

[0004] Suematsu et al, “Polyimine, a C=N Double Bond Containing Polymers: Synthesis and Properties” Polymer Journal, Vol. 15, No. I, pp 71-79 (1983) discloses a polyimine of formula:

[0005] WO 2022 / 136584 discloses thermally conductive polymers formed by reaction of a dialdehyde and a diamine.

[0006] SUMMARY

[0007] The present disclosure provides a polymer comprising a repeat unit of formula (I): wherein:

[0008] X1and X2are each independently selected from formulae (II) and (III) with the proviso that at least one of X1and X2is a group of formula (III):

[0009] -(Ar^m- Formula (II)

[0010] -(Ar2)p-L-(Ar3)q- Formula (III) wherein Ar1, Ar2and Ar3independently in each occurrence is an arylene or heteroarylene group; m is at least 1; p is at least 1; q is at least one; and L is an optionally substituted chain of methylene groups and O atoms; one of Y1and Y2is CR1wherein R1is H or a substituent; and the other of Y1and Y2is N; and one of Y3and Y4is CR1; and the other of Y3and Y4is N.

[0011] Optionally, each R1is H or a Ci-2ohydrocarbyl group.

[0012] Optionally, L contains at least 3 atoms separating Ar2and Ar3.

[0013] Optionally, the atoms of L which are bound to Ar2and Ar3, are each O.

[0014] Optionally, one of X1and X2is a group of formula (II) and the other of X1and X2is a group of formula (III).

[0015] Optionally, m is at least 2.

[0016] Optionally, Y1and Y4are both the same one of CR1and N; and Y2and Y3are both the same and are the other one of CR1and N.

[0017] Optionally, Ar1, Ar2and Ar3are each, independently in each occurrence, selected from a 6-12 membered arylene or an optionally fused 5- or 6-membered heteroarylene.

[0018] Optionally, Ar1, Ar2and Ar3independently in each occurrence is an unsubstituted or substituted para-phenylene.

[0019] Optionally, p and q are each 1.

[0020] The present disclosure provides a method of forming a polymer comprising a repeat unit of formula (I), the method comprising reacting a first monomer of formula Ml and a second monomer of formula M2:

[0021] RG^XhRG1Ml

[0022] RG2-X2-RG2M2 wherein each RG1is a first reactive group selected from C(=O)R1and Nth and each RG2 is a second reactive group which is the other of C(=O)R1and Nth. The present disclosure provides a film comprising a polymer comprising a repeat unit of formula (I).

[0023] In some embodiments, the film is formed by a thermal processing method. Optionally, the thermal processing method is selected from extrusion, injection moulding, thermocompressive bonding and hot press or melt press moulding.

[0024] Optionally, formation of the film comprises deposition of the polymer on a surface, heating the deposited polymer to above a phase transition temperature of the polymer and cooling the polymer to below the phase transition temperature at a rate of no more than 2°C per minute, optionally at a rate of no more than 1°C per minute. Optionally, the polymer is cooled to at least 25°C or at least 50°C below the phase transition temperature at a rate of no more than 2°C per minute, optionally at a rate of no more than 1°C per minute.

[0025] The present disclosure provides an electronic device comprising a film as described herein disposed on a surface of a functional layer of the electronic device.

[0026] Optionally, the 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.

[0027] Optionally, the functional layer is a printed circuit board; an interposer; or a second chip.

[0028] Optionally, the electronic device comprises a 3D chip stack.

[0029] 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 film as described herein disposed between the heat-generating device and the heat transfer device.

[0030] The present disclosure provides a heat sink comprising a first surface having fins extending therefrom and an opposing second surface having a film as described herein disposed thereon.

[0031] The present disclosure provides a formulation comprising a first monomer of formula Ml and a second monomer of formula M2 dissolved or dispersed in a solvent or solvent mixture:

[0032] RG1- i -RG1Ml

[0033] RG2-X2-RG2M2 wherein X1and X2are as described above; each RG1is a first reactive group selected from C(=O)R1and Nth and each RG2is a second reactive group which is the other of C(=O)R1and NH2.

[0034] The present disclosure provides a method of forming a polymer comprising reacting a polymerisation mixture comprising a first monomer of formula Ml and a second monomer of formula M2:

[0035] RG1-X3-RG1Ml

[0036] RG2-X4-RG2M2 wherein each RG1is a first reactive group selected from C(=O)R1and Nth and each RG2is a second reactive group which is the other of C(=O)R1and Nth; R1is H or a substituent; and X3and X4are each independently selected from groups of formula (II) and formula (III):

[0037] -(Ar^m- Formula (II)

[0038] -(Ar2)p-L’-(Ar3)q- Formula (III) wherein Ar1, Ar2and Ar3independently in each occurrence is an arylene or heteroarylene group; m is at least 1; p is at least 1; q is at least one; and L’ is an optionally substituted Ci-io alkylene wherein one or more non-adjacent C-atoms may be replaced with O, S, NR5, SiR62, C=O or COO wherein R5in each occurrence is H or a substituent and R6in each occurrence is independently a substituent, and wherein the polymerisation mixture comprises a solvent or solvent mixture in which is dissolved a first monomer, a second monomer and an aromatic alcohol.

[0039] Optionally, the aromatic alcohol is a benzene substituted with at least one hydroxyl group.

[0040] Optionally, the solvent or solvent mixture comprises an alkylated benzene.

[0041] Optionally, the solvent or solvent mixture comprises tetrahydrofuran.

[0042] DESCRIPTION OF DRAWINGS

[0043] 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;

[0044] Figure2B 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;

[0045] Figure 3 schematically illustrates a 3D chip stack according to some embodiments;

[0046] Figure 4 schematically illustrates a substrate for measurement of thermal conductivity of a film;

[0047] Figures 5A and 5B schematically illustrate apparatus for measurement of thermal conductivity including the substrate of Figure 4;

[0048] Figures 6A and 6B are photographs of a thermally treated film of a polymer according to some embodiments; and

[0049] Figures 7 A and 7B are photographs of a film of a polymer according to some embodiments which did not undergo thermal treatment.

[0050] 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.

[0051] DETAILED DESCRIPTION

[0052] 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 include isotopes of that element.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] The present inventors have found that a high thermal conductivity may be provided by a film comprising or consisting of a polymer comprising a repeat unit of formula (I): wherein:

[0057] X1and X2are each independently selected from formulae (II) and (III) with the proviso that at least one of X1and X2is a group of formula (III):

[0058] -(Ar^m- Formula (II)

[0059] -(Ar2)p-L-(Ar3)q- Formula (III) wherein Ar1, Ar2and Ar3in each occurrence is an arylene or heteroarylene group; m is at least 1; p is at least 1; q is at least one; and L is an optionally substituted chain of methylene (-CH2-) groups and O atoms; one of Y1and Y2is CR1wherein R1is H or a substituent; and the other of Y1and Y2is N; and one of Y3and Y4is CR1; and the other of Y3and Y4is N.

[0060] R1is preferably H or a Ci-2ohydrocarbyl group, more preferably H or a C1-6 alkyl group, most preferably H.

[0061] The C and O atoms of L may be arranged in any order. Suitably, no two O atoms of the chain are directly adjacent. Preferably the terminal atoms of L, i.e., the atoms of L which are bound to Ar2and Ar3, are each O. One or more H atoms of the methylene groups of L may be replaced with one or more substituents. Optionally, one or more H atoms are replaced with F, Ci-6 alkyl or Ci-6 fluoroalkyl.

[0062] Preferably, L contains at least 3 or at least 4 atoms separating Ar2and Ar3.

[0063] Exemplary groups L include, without limitation, -OCH2CH2O- and -OCH2O-.

[0064] Optionally, each of X1and X2is a group of formula (III).

[0065] Preferably, one of X1and X2is a group of formula (II) and the other of X1and X2is a group of formula (III).

[0066] Preferably, Y1and Y4are both the same one of CR1and N; and Y2and Y3are both the same and are the other one of CR1and N. Preferably, R1of each CR1is the same.

[0067] In a particularly preferred embodiment, X1is a group of formula (II); X2is a group of formula (III); Y2and Y3are each N; and Y1and Y4are each CR1, wherein each R1is preferably the same.

[0068] Preferably, Ar1, Ar2and Ar3are each, independently in each occurrence, selected from a 6-12 membered arylene or an optionally fused 5- or 6-membered heteroarylene, e.g., an unfused 5- or 6-membered heteroarylene or a benzene-fused 5- or 6-membered hetero arylene. Exemplary groups Ar1, Ar2and Ar3include, without limitation, para-phenylene, thiophene, furan, and benzobisoxazole, each of which may independently be unsubstituted or substituted with one or more substituents. Para-phenylene is preferred. m is preferably more than 1, preferably 2-5, more preferably 2 or 3. p and q are preferably each independently 1 or 2, more preferably 1.

[0069] Each of Ar1, Ar2and Ar3is independently unsubstituted or substituted with one or more substituents R2.

[0070] Preferably, where present R2in each occurrence is independently selected from:

[0071] F;

[0072] CN; N02; branched, linear or cyclic C1-40 alkyl, preferably C1-20 alkyl, wherein one or more non-adjacent C-atoms may be replaced with O, S, NR5, SiR62, C=O or COO; wherein R5in each occurrence is H or a substituent, preferably H or a C1-20 hydrocarbyl group and R6in each occurrence is independently a substituent, optionally a C1-20 hydrocarbyl group; or an aryl or heteroaryl group Ar5which is unsubstituted or substituted with one or more substituents, optionally phenyl which is unsubstituted or substituted with one or more substituents selected from F, CN, NO2 and branched, linear or cyclic C1-20 alkyl wherein one or more non-adjacent C-atoms may be replaced with O, S, NR5, SiR62, C=O or COO.

[0073] Preferably, at least one substituent R2, optionally each substituent R2, is C1-20 alkyl, C1-20 alkoxy, or a group of formula -(Ak1)y-(OCH2CH2)z-Ak2wherein Ak1is a C1-4 alkylene group; y is 0 or 1; z is 1-15; and Ak2is a C1-4 alkyl group. More preferably R2is a C1-12 alkyl or C1-12 alkoxy. C1-12 alkoxy is particularly preferred.

[0074] Exemplary groups -(Ar^m- include, without limitation, groups of formulae (IVa) and (IVb):

[0075] (IVa) (IVb) wherein R2independently in each occurrence is a substituent and w in each occurrence is independently 0 or a positive integer.

[0076] A preferred group -(Ar^m- has formula (IVb-1):

[0077] (IVb-1) Ar2and Ar3are each preferably optionally substituted 1,4-phenylene, more preferably unsubstituted 1,4-phenylene.

[0078] The polymer may be formed by polymerisation of a first monomer containing X1and two reactive groups and a second monomer containing X2substituted with two reactive groups, wherein the reactive groups of the first and second monomers react to form imine (-CR^N-) linkages between X1and X2. Suitable reactive groups for reaction to form imine linkages are aldehyde or ketone (-C(=O)R1) reactive groups, preferably aldehyde reactive groups, and amine (NH2) reactive groups.

[0079] In some embodiments, polymerisation is between a first monomer of formula Ml and a second monomer of formula M2:

[0080] RG^X^RG1Ml

[0081] RG2-X2-RG2M2 wherein each RG1is a first reactive group selected from C(=O)R1and NH2 and each RG2 is a second reactive group which is the other of C(=O)R1and NH2.

[0082] Preferably, each RG1is C(=O)R1and each RG2is NH2.

[0083] The reaction between RG1and RG2may be catalysed by a Lewis acid. The Lewis acid may be a Bronsted-Lowry acid. Exemplary catalysts include, without limitation, sulfonic acids and salts thereof, for example p-toluene sulfonic acid; triflic acid; and salts thereof; and aromatic alcohols, more preferably benzene which is substituted with at least one hydroxyl group and, optionally, one or more further substituents for example one or more C1-6 alkyl groups.

[0084] An exemplary triflic acid salt is scandium triflate, Sc(Trf)3.

[0085] Exemplary aromatic alcohols are phenol which is unsubstituted or substituted with one more C1-6 alkyl groups, more preferably one or more methyl groups, for example m-cresol.

[0086] The catalyst may be provided in an amount of 0.01-0.3 molar equivalents of the total number of moles of the monomers. The catalyst is suitably soluble in the solvent or solvent mixture in which the monomer or monomers are dissolved. Optionally, the solvent or solvents are selected from benzene or naphthalene substituted with one or more substituents, optionally one or more substituents selected from C1-12 alkyl, C1-12 alkoxy, F and Cl; ethers; esters; halogenated alkanes; ketones; sulfoxides; and mixtures thereof. Exemplary solvents include, without limitation, toluene, xylenes, 1,2,4-trimethylbenzene, mesitylene, tetrahydrofuran, 1 -methylnaphthalene, 1 -chloronaphthalene, diiodomethane, anisole, N-methylpyrrolidone, 1,2-dimethoxybenzene, dimethylsulfoxide l,3-dimethyl-2- imidazolidinone and cyclopentanone.

[0087] Polymers as described herein are preferably at least partially crystalline.

[0088] Polymers as described herein are preferably linear (non-branched) polymers. Linear polymers may be formed by reaction between a monomer having two reactive groups RG1and a monomer having two reactive groups RG2as described herein.

[0089] Polymers as described herein may undergo pi-pi stacking when deposited as a film.

[0090] Optionally, thermal conductivity of polymers as described herein is at least 0.5 Win 'K’1, optionally at least 0.6 or 0.7 Wm^K’1.

[0091] 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 IxlO3to IxlO8, and preferably IxlO4to 5xl06. The polystyrene-equivalent weight-average molecular weight (Mw) of the polymers described herein may be IxlO3to IxlO8, and preferably IxlO4to IxlO7.

[0092] Polymer processing

[0093] Formation of a film comprising a polymer as described herein may comprise formation of a precursor film comprising the monomers for forming the polymer followed by polymerisation of the monomers, referred to hereinafter as in situ polymerisation. The precursor film may consist of the monomers for forming the polymer or the precursor film may be a composition comprising one or more further materials, e.g., a catalyst as described herein and / or thermally conductive particles such as boron nitride particles.

[0094] Preferably, formation of a film comprising a polymer as described herein comprises deposition of a polymer formulation comprising the polymer dissolved or dispersed in one or more solvents. The formulation may consist of the polymer and one or more solvents or it may comprise one or more further materials, e.g., a catalyst as described herein and / or thermally conductive particles such as boron nitride particles. Inorganic thermally conductive particles such as boron nitride as described herein may be substituted with an organic group, optionally 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. Surface groups of a thermally conductive particle are disclosed in WO2022 / 207695, the contents of which are incorporated herein by reference.

[0095] In some preferred embodiments of in situ polymerisation, the precursor film formation comprises deposition of a monomer formulation comprising the monomers dissolved in one or more solvents. According to these embodiments, in-situ polymerisation preferably takes place in solution. The monomer formulation may or may not comprise a catalyst.

[0096] Solvents for a monomer or polymer formulation may be selected according to their ability to dissolve the monomers or the polymer. Exemplary solvents include, without limitation, benzene or naphthalene substituted with one or more substituents, optionally one or more substituents selected from C1-12 alkyl, C1-12 alkoxy, F and Cl; ethers; esters; halogenated alkanes; ketones; sulfoxides; and mixtures thereof. Exemplary solvents include, without limitation, toluene, xylenes, 1,2,4-trimethylbenzene, mesitylene, 1 -methylnaphthalene, 1- chloronaphthalene, diiodomethane, anisole, tetrahydrofuran, N-methylpyrrolidone, 1,2- dimethoxybenzene, dimethylsulfoxide l,3-dimethyl-2-imidazolidinone and cyclopentanone.

[0097] The concentration of each monomer, or the polymer, dissolved in the monomer formulation is preferably in the range of about 1-50 mg / ml, more preferably about 10-40 mg / ml. The monomer or polymer formulation may be heated to achieve dissolution of the monomer or monomers or the polymer.

[0098] The polymer precursor film may be heated before and / or after polymerisation. In some embodiments, the polymer precursor film may be dried at a temperature of up to about 100°C, optionally 50-90°C. The dried film may be heated at a temperature above 100°C, optionally in the range of 100-200°C. The temperature applied before, during or after drying may be below a melting point of the monomer having the lowest melting point. The temperature applied before, during or after drying may be at or above a melting point of the monomer having the lowest melting point. In some preferred embodiments of in-situ polymerisation, a monomer formulation comprising monomer particles mixed with a liquid is deposited on a surface to form a polymer precursor film and the film is heated to at least the melting point of the monomer or, if more than one monomer is present, to at least the melting point of the monomer having the lowest melting point. Optionally, the polymer precursor film is heated at below the lowest monomer melting point to drive off the liquid before the heating temperature is increased to at least this melting point. It will be appreciated that the amount and / or nature of the liquid is such that the monomer particles are not dissolved in the liquid. Preferably, each monomer is sparingly soluble or insoluble in the liquid. The liquid may be a single liquid material or a mixture of two or more liquid materials, for example one or more liquids selected from water and Ci-6 alcohols. The monomer formulation according to these embodiments may be, for example, a suspension or a paste and a suitable deposition method may be selected accordingly.

[0099] Monomer or polymer formulations as described anywhere herein may be deposited by any suitable solution deposition technique including, without limitation, spin-coating, dip-coating, jet dispensing, drop-casting, spray coating and blade coating.

[0100] In some embodiments, a film formed by in-situ polymerisation or by deposition of a polymer formulation comprising a preformed polymer may be formed directly on a surface of a device or apparatus may be used as a thermal transfer film without further manipulation of the film.

[0101] In other embodiments, following formation of a polymer film, either by in-situ polymerisation, deposition of a pre-formed polymer or any other method, the polymer film may be processed using a thermal processing technique to form a thermal transfer film on a surface of a device or apparatus. Exemplary thermal processing techniques include, without limitation, extrusion, injection moulding, thermocompressive bonding and hot press or melt press moulding.

[0102] The thermal transfer film may consist of the polymer or may contain one or more further materials, optionally one or more amorphous polymers, e.g. polystyrene, polyethylene or polypropylene; and / or one or more thermally conductive materials, for example boron nitride.

[0103] In some embodiments, the film comprises thermally conductive particles, such as boron nitride, dispersed therein. In some embodiments, the film does not comprise any thermally conductive particles. Optionally, a thermal transfer film comprising or consisting of a polymer as described herein has a thickness in the range of 1-100 microns, preferably 10-100 microns.

[0104] Applications

[0105] A film comprising a polymer as described herein may be used in any known application of a thermally conductive film. The film as described herein may be disposed between a surface of a heat-generating device and a heat transfer device configured to transfer heat away from the heat-generating device, such as in any known thermal interface management application.

[0106] It will be understood that in this arrangement the film is configured to transfer heat from the heat-generating device to the heat transfer device. The film preferably has a first surface in direct contact with a surface of the heat-generating device and / or a second surface opposing the first surface in direct contact with a surface of the heat transfer device.

[0107] The heat-generating device may be an electronic device.

[0108] Any passive or active heat transfer device 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.

[0109] Preferably, the film is a thermally conductive layer of an electronic device.

[0110] Heat may be transferred from a surface by bringing a layer comprising a thermally conductive film as described herein adjacent to the surface. The thermally conductive film may be in direct contact with the surface or it may be spaced apart from the surface by one or more thermally conductive layers.

[0111] 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.

[0112] A film as described herein may be a heat spreader layer disposed on a surface of a printed circuit board, for example a PCB for use in LED arrays. A film 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.

[0113] 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 or consisting of a polymer as described herein fills the region between the chip 105 and substrate 101. Optionally, the polymer is crosslinked.

[0114] 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 polymer as described herein comprises application of a formulation comprising the monomer or monomers into the overlap region between the chip 105 and the substrate 101. Optionally, the polymer is crosslinked following application of the formulation and reaction of the monomer or monomers, e.g., by heat and / or UV treatment.

[0115] With reference to Figure 2B, in some embodiments a polymer precursor film is formed over a surface of the chip 105 carrying electrically conductive bumps 107’. Figure 4B 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 / or pressure.

[0116] 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.

[0117] 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 polymer 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.

[0118] In some embodiments, a film comprising or consisting of a polymer as described herein may be disposed between an electronic device and a heat sink. EXAMPLES

[0119] Polymer formation

[0120] Polymers were formed by reacting 2’, 5 ’-dihexyloxy terphenyl 4,4”-dialdehyde (Aldehyde

[0121] Monomer 1) and a diamine monomer, shown below.

[0122] Diamine monomer

[0123]

[0124] Synthesis of Example polymer 1 m-cresol (2 eqv) was added to a stirred solution of monomer Bl (0.288 mg, 1 eqv) and monomer Al (0.5 g, 1.1 eqv) in a mixture of toluene (5 ml) and THF (50 ml). The mixture was left to stir at room temperature for 30 minutes, before it was concentrated in vacuo to dryness, upon which a yellow oil formed, which began to solidify on standing. This was resuspended in toluene / THF (~50 ml) and stirred for 5 minutes, before being concentrated to dryness again. The process was repeated a second time, after which the solid was suspended in warm dichloromethane (DCM) and was precipitated into methanol. The product was collected as a powdery yellow solid. Yield = 66%.

[0125] Synthesis of Example polymer 2 m-cresol (2 eqv) was added to a stirred solution of monomer B2 (430 mg, 1 eqv) and monomer Al (1.0 g, 1.1 eqv) in a mixture of toluene (5 ml) and THF (50 ml). The mixture was left to stir at room temperature for 30 minutes, before it was concentrated in vacuo to dryness, upon which a yellow oil formed, which began to solidify on standing. This was resuspended in toluene / THF (~50 ml) and stirred for 5 minutes, before being concentrated to dryness again. The process was repeated a second time, after which the solid was dissolved in warm DCM and was precipitated into methanol. The product was collected as a powdery yellow solid. Yield = 86%.

[0126] Synthesis of Example polymer 3 m-cresol (2 eqv) was added to a stirred solution of monomer B3 (424 mg, 1 eqv) and monomer Al (1.0 g, 1.1 eqv) in a mixture of toluene (5 ml) and THF (50 ml). The mixture was left to stir at room temperature for 30 minutes, before it was concentrated in vacuo to dryness, upon which a yellow gel formed. This was resuspended in toluene (50 ml) and stirred for 5 minutes, before being concentrated to dryness again. The process was repeated a second time, after which the solid was dissolved in DCM and was precipitated into methanol. The product was collected as a powdery yellow solid. Yield = 71%.

[0127] Synthesis of Example polymer 4 m-cresol (2 eqv) was added to a stirred solution of monomer B4 (532 mg, 1 eqv) and monomer Al (1.0 g, 1.1 eqv) in a mixture of toluene (5 ml) and THF (50 ml). The mixture was left to stir at room temperature for 1 hr, before it was concentrated in vacuo to dryness, upon which a yellow oil formed, which began to solidify on standing. This was resuspended in toluene (~50 ml) and stirred for 5 minutes, before being concentrated to dryness again. The process was repeated a second time, after which the solid was suspended in a DCM / toluene mixture and was precipitated into methanol. The product was collected as a powdery yellow solid. Yield = 42%.

[0128] Synthesis of Example polymer 5 m-cresol (2 eqv) was added to a stirred solution of monomer B5 (536 mg, 1 eqv) and monomer Al (1.0 g, 1.1 eqv) in a mixture of toluene (5 ml) and THF (50 ml). The mixture was left to stir at room temperature for 30 minutes, before it was concentrated in vacuo to dryness, upon which a yellow oil formed, which began to solidify on standing. This was resuspended in toluene (~50 ml) and stirred for 5 minutes, before being concentrated to dryness again. The process was repeated a second time, after which the solid was suspended in warm DCM and was precipitated into methanol. The product was collected as a powdery yellow solid. Yield = 70%.

[0129] Film deposition

[0130] Inks of polymer were prepared by dissolving the polymer in o-dichlorobenzene at 20 mg / ml, optionally applying heat up to 80°C to aid dissolution.

[0131] Film formation After dissolution of polymer in the solvent, the resultant ink is promptly drop-cast onto a substrate, described above for thermal conductivity measurement, at a temperature of 80°C. A gasket prepared from 0.5mm thick fluorosilicone rubber sheet (Silex Silicones Ltd) and applied to the substrate is used to contain the ink within a prescribed area (18x10mm rectangle) for the drop-casting procedure. The drop-cast solution was maintained at 80°C for 30 minutes and then heating was turned off and the solution was allowed to cool to room temperature. No further annealing step was taken after solvent evaporation of the drop-cast film.

[0132] Thermal conductivity measurement

[0133] A sensor substrate 600 (ca. 25 mmx 25 mm) illustrated in Figure 4 was used for measurement of thermal conductivity as described herein. The substrate has a polyethylene naphthalate (PEN) film (Dupont Teonex Q83, 25pm) with a 200 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.

[0134] With reference to Figures 5A and 5B, 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 (2mm 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 (4mm 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 2000) using a 4 wire set up.

[0135] 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.

[0136] 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, 4T, 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, 14.4mm), 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).

[0137] 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 ( ki ) 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.

[0138] C =

[0139] Results are set out in Table 1.

[0140] Table 1

[0141] Phase transition temperatures were determined using melting point apparatus configured to heat a sample in a tube and detect changes in transmission of the sample. The range given in Table 1 is the range as observed by a video of the sample starting from the material changing from a powder to a liquid or glossy substance to the temperature at which it is a transparent liquid.

[0142] As set out in Table 1, although Comparative Polymer 1 has the highest thermal conductivity of the polymers in this table, its relatively high melting point makes it less suitable for processing using thermal processing techniques as described herein. Polymer Examples 1 and 2 possess a good combination of relatively low onset phase transition and good thermal conductivity. The data in Table 1 suggests that a relatively low melting point may be achievable by providing a chain L having a 3 or 4 atom chain length. Surprisingly, for a given chain length of chain L, inclusion of oxygen in the chain was found to increase thermal conductivity.

[0143] Further, without wishing to be bound by any theory, polymers in which chain L contains an odd number of chain atoms may pack differently from polymers in which chain L contains an even number of chain atoms.

[0144] Film formation conditions

[0145] After drop casting polymer solutions on substrates with fluorosilicone rubber sheet gaskets, substrates were kept on the hot stage at 80°C for 30 minutes before heating was turned off. As this temperature is close to onset phase transition point point of the polymer, it is believed that chain movements are facilitated at this temperature, which results in more uniform film formation.

[0146] A film of Polymer Example 2 was prepared as described above except that the film was cooled more rapidly to room temperature by removing it from contact with the heated surface after 30 minutes at 80°C. With reference to Figures 6A and 6B, this rapid cooling resulted in films with large domains within the polymer film. In contrast, slow cooling as described above resulted in much more uniform films, as shown in Figures 7A and 7B. Without wishing to be bound by any theory, slow cooling at a phase transition onset temperature allows for optimal orientation of polymer films.

Claims

ClaimsA polymer comprising a repeat unit of formula (I):wherein:X1and X2are each independently selected from formulae (II) and (III) with the proviso that at least one of X1and X2is a group of formula (III):-(Ar1),,,- Formula (II)-(Ar2)p-L-(Ar3)q- Formula (III) wherein Ar1, Ar2and Ar3independently in each occurrence is an arylene or heteroarylene group; m is at least 1 ; p is at least 1 ; q is at least one; and L is an optionally substituted chain of methylene groups and O atoms; one of Y1and Y2is CR1wherein R1is H or a substituent; and the other of Y1and Y2is N; and one of Y3and Y4is CR1; and the other of Y3and Y4is N.

2. The polymer according to claim 1 wherein each R1is H or a C1-20 hydrocarbyl group.

3. The polymer according to claim 1 or 2 wherein L contains at least 3 atoms separating Ar2and Ar3.

4. The polymer according to any one of the preceding claims wherein the atoms of L which are bound to Ar2and Ar3, are each O.

5. The polymer according to any one of the preceding claims wherein one of X1and X2is a group of formula (II) and the other of X1and X2is a group of formula (III).

6. The polymer according to claim 5 wherein m is at least 2.

7. The polymer according to any one of the preceding claims wherein Y1and Y4are both the same one of CR1and N; and Y2and Y3are both the same and are the other one of CR1and N.

8. The polymer according to any one of the preceding claims wherein Ar1, Ar2and Ar3are each, independently in each occurrence, selected from a 6-12 membered arylene or an optionally fused 5- or 6-membered hetero arylene.

9. The polymer according to any one of the preceding claims wherein Ar1, Ar2and Ar3independently in each occurrence is an unsubstituted or substituted para-phenylene.

10. The polymer according to any one of the preceding claims wherein p and q are preferably each 1.

11. A method of forming a polymer according to any one of the preceding claims comprising reacting a first monomer of formula Ml and a second monomer of formula M2:RG^-RG1MlRG2-X2-RG2M2 wherein each RG1is a first reactive group selected from C(=O)R1and NH2 and each RG2 is a second reactive group which is the other of C(=O)R1and NH2.

12. A film comprising a polymer according to any one of claims 1-10.

13. A method of forming a film according to claim 12 wherein the film is formed by a thermal processing method.

14. The method according to claim 13 wherein the thermal processing method is selected from extrusion, injection moulding, thermocompressive bonding and hot press or melt press moulding.

15. The method according to claim 13 or 14 comprising deposition of the polymer on a surface, heating the deposited polymer to above a phase transition temperature of the polymer and cooling the polymer to below the phase transition temperature at a rate of no more than 2°C per minute16. An electronic device comprising a film according to claim 12 disposed on a surface of a functional layer of the electronic device.

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

19. The electronic device according to claim 16, 17 or 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 film according to claim 12 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 film according to claim 12 disposed thereon.

22. A formulation comprising a first monomer of formula Ml and a second monomer of formula M2 dissolved or dispersed in a solvent or solvent mixture:RG^-RG1MlRG2-X2-RG2M2wherein X1and X2are as defined in claim 1 or claim 5; each RG1is a first reactive group selected from C(=O)R1and Nth and each RG2is a second reactive group which is the other of C(=O)R1and Nth.

23. A method of forming a polymer comprising reacting a polymerisation mixture comprising a first monomer of formula Ml and a second monomer of formula M2:RG1-X3-RG1MlRG2-X4-RG2M2 wherein each RG1is a first reactive group selected from C(=O)R1and Nth and each RG2is a second reactive group which is the other of C(=O)R1and Nth; R1is H or a substituent; and X3and X4are each independently selected from groups of formula (II) and formula (III):-(Ar^m- Formula (II)-(Ar2)p-L’-(Ar3)q- Formula (III) wherein Ar1, Ar2and Ar3independently in each occurrence is an arylene or heteroarylene group; m is at least 1; p is at least 1; q is at least one; and L’ is an optionally substituted Ci-io alkylene wherein one or more non-adjacent C-atoms may be replaced with O, S, NR5, SiR62, C=O or COO wherein R5in each occurrence is H or a substituent and R6in each occurrence is independently a substituent, and wherein the polymerisation mixture comprises a solvent or solvent mixture in which is dissolved a first monomer, a second monomer and an aromatic alcohol.

24. The method according to claim 23 wherein the aromatic alcohol is a benzene substituted with at least one hydroxyl group.

25. The method according to claim 23 or 24 wherein the solvent or solvent mixture comprises an alkylated benzene.

26. The method according to any one of claims 23-25 wherein the solvent or solvent mixture comprises tetrahydrofuran.

Images