GB2602321A

GB2602321A - Thermally conductive polymer

Info

Publication number: GB2602321A
Application number: GB2020482.2A
Authority: GB
Inventors: King Simon; Gawel Przemyslaw; Fletcher Thomas
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 2020-12-24
Filing date: 2020-12-24
Publication date: 2022-06-29
Also published as: GB202020482D0

Abstract

A polymer comprising a repeating structure of formula (I): Wherein Ar is an arylene or heteroarylene; p is at least 2; one of Y1 and Y2 is CR1 wherein R1 is H or a substituent; and the other Y1 and Y2 is N. Also disclosed is a film comprising the polymer; a method of forming the film; an electronic device comprising the film; a heat sink comprising the film and a formulation comprising at least one or more monomers and a solvent wherein the monomers are dissolved for forming the polymer.

Description

THERMALLY CONDUCTIVE POLYMER

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.

Mary Liu and Wusheng Yin, -A novel high thermal conductive underfill for flip chip application" http://yincae.com/assets/wp-1000-03 2013.pdf discloses an underfill containing diamond powder.

Islam et al, "Enhanced Thermal Conductivity of Liquid Crystalline Epoxy Resin using Controlled Linear Polymerization", ACS Macro Lett 2018, 7, 10, 1180-1185 discloses liquid crystalline epoxy resin with a 2-D boron nitride filler.

WO 2019/143823 discloses thermally conductive quinoid-type conjugated polymer thin films fabricated by oxidative chemical vapour deposition.

Huang e al, "Thermal conductivity of polymers and polymer nanocomposites", Materials Science and Engineering: R: Reports, Vol. 132, October 2018, p. 1-22 describes thermal 15 transport mechanisms in polymers.

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: 175).-CH.-N

SUMMARY

The present disclosure provides a polymer comprising a repeating structure of formula (I): i(Ar)p-% (I) wherein Ar in each occurrence is an arylene or heteroarylene group; p is at least 2; one of Y1 and Y2 is CRI wherein RI is H or a substituent; and the other of Y1 and Y2 is N. Optionally, p is 2-5.

Optionally, each Ar of (Ar)p is independently selected from para-phenylene, thiophene, furan, and benzobisoxazole, each of which may independently be unsubstituted or substituted with Optionally, each Ar of (Ar)p is the same.

Optionally, one or more Ar groups of (Ar)p are substituted with one or more substituents selected from substituents R2 wherein R2 in each occurrence is independently selected from: F; CM; NO2; branched, linear or cyclic Cir20 alkyl wherein one or more non-adjacent C-atoms may be replaced with 0, S. NR5, SiR62, C=0 or COO wherein R5 in each occurrence is H or a substituent and R6 in each occurrence is independently a substituent; or an aryl or heteroaryl group Ars which is unsubstituted or substituted with one or more substituents.

Optionally. RI is H or a C1,20hydrocarbyl group Optionally, a divalent linker group L disposed in the polymer backbone, wherein L is selected 20 from 0, S. NR5 or a C1_12 alkylene group wherein one or more non-adjacent C atoms may be replaced with 0, S. NR5, SiR60, CO or COO wherein R5 in each occurrence is flora substituent and R6 in each occurrence is independently a substituent.

Optionally, the repeating structure of formula a) is comprised in a repeating group of formula (II), (III) or (IV). (H) +Ar)r

y2-(A0n-L-(Ar)"-\Y+ _((Ar)n L-(Ar)r Ykk (III) ft.^ y2 -kJ-kik' yl (IV) wherein: q is at least 1; n is 0 or a positive integer; m is 0 or a positive integer; and L is as described above.

The present disclosure provides a film comprising a polymer as described herein.

The present disclosure provides a method of forming a film as described herein comprising deposition of one or more monomers for forming the polymer onto a surface and polymerising the one or more monomers.

Optionally, the one or more monomers are deposited from a solution.

Optionally, the surface is a surface of a functional layer of an electronic device.

The present disclosure provides an electronic device comprising a film as described herein disposed on a functional layer thereof 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.

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 a heat sink comprising a first surface having fins extending therefrom and an opposing second surface having a film as described herein disposed thereon.

The present disclosure provides a method of forming a polymer as described herein comprising polymerisation of a monomer or monomers having reactive groups which react to form Y' =Y2.

Optionally, the method comprises polymerisation of a first monomer selected from formulae (M 1-A) and (M I -B) and a second monomer selected from formulae (M2-A) and (M2-B): X1-(Ar)p-X1 (M 1-A) X1-(Ar) p L-(Ar),-,-X1 (Ml-B) X2-(Ar) q X2 (M2-A) X2(Ai)n L-(Ar),,-X2 (M2-B) wherein one of XI and X2 is a group of formula -C=0)R1 and the other of XI and X= is NEti; 20 and q, n and m and L are as described above.

Optionally, the method comprises polymerisation of a monomer of formula (M3): X1-(Ar) p L-(Ar)n-X2 (M3) wherein one of XI and X2 is a group of formula -C(=0)R1 and the other of XI and X= is NI-11; n is 0 or a positive integer; and L is as described above.

The present disclosure provides a formulation comprising one or more monomers for forming the polymer as described herein and a solvent wherein the one or more monomers are dissolved in the solvent.

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; 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; Figure 3 schematically illustrates a 3D chip stack according to some embodiments; Figure 4 schematically illustrates a substrate for measurement of thermal conductivity of a film; 15 and Figures 5A and 5B schematically illustrate apparatus for measurement of thermal conductivity including the substrate of Figure 4.

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 are may be present. References to a layer "on" another layer when used in this application means that the layers are in direct contact.

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 Desciiption 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 he 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 high thermal conductivity may be provided by a film comprising or consisting of a polymer comprising a repeating structure of formula (I): i(A0p-Y1 % y2 (I) wherein Ar in each occurrence is an arylene or heteroarylene group; p is at least 2; one of Y1 and Y2 is CRI wherein RI is H or a substituent; and the other of YI and Y2 is N. The extended rigid-rod type structure of formula (I) may enhance thermal conductivity of the polymer as compared to the case where p = 1.

Optionally, thermal conductivity of polymers as described herein is at least 0.15 Win-'K' optionally at least 0.2 or 0.3 Wm-1K-1.

p is preferably 2-5.

Ar in each occurrence in (Ar)p may be the same or different, preferably the same.

Exemplary Ar groups include, without limitation, para-phenylene, thiophene, furan. and benzohisoxazole, each of which may independently be unsuhstituted or substituted with one or more substituents. Para-phenylene is preferred.

RI is preferably H or a CI -20 hydrocarbyl group, more preferably H. A C 1_20 hydrocarbyl group as described anywhere herein is preferably selected from Ci_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 0, 5, NR5, SiR62, C=0 or Coo; wherein R5 in each occurrence is H or a substituent, preferably H or a C1_20 hydrocarbyl group and R6 in each occurrence is independently a substituent, optionally a C1_20hydrocarbyl group; or an aryl or heteroaryl group Ars which 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 0, S, NR5, SiR62, C=0 or COO.

Preferably, at least one substituent R2, optionally each substituent R2, is C1-20 alkyl or C1-20 alkoxy, more preferably a Cup alkyl or C1-12 alkoxy.

The polymer may comprise a divalent linker group L disposed in the polymer backbone, wherein L is selected from 0, 5, NW or a C1_12 alkylene group wherein one or more nonadjacent C atoms may be replaced with 0, S. NR5, SiR62, CO or COO.

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 (1-C(121)=N-) group.

In some embodiments, the divalent linker group L is disposed between and linked directly to two imine (-C(RI)=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 (II)-(IV): yl y2 (Ar)q (1) -((Ar)p y2 (Ar)n-L-(Ar)rn _((Ar)n L-(Ar)p yl y2 (Ar),,, yl 2* (IV) 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.

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. 15 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 (II)-(IV). The polymer may contain one or more of: different repeat units of formula (II); different repeat units of formula (111); different repeat units of formula (IV); and a repeat unit selected from one of formulae S (11)-(IV) and at least one other repeat unit selected from another of formulae (11)-(IV). 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 (II) and a repeat unit of formula (III).

In the case where n and m are each 0, the repeat unit of formula (Hi) has formula (IIIa): tAr)p-Y.,1 y2 _L_yl The polymers may be substituted with groups for bonding together of polymer chains, e.g. hydrogen bonding or covalent bonding, to enhance lone-range ordering of the polymers.

Polymers as described herein are preferably at least partially crystalline.

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

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 1 x 103 to 1 x 108, and preferably 1x104 to 5x106. The polystyrene-equivalent weight-average molecular weight (Mw) of the polymers described herein may he 1x103 to 1x108, and preferably lx104 to 1x107.

Polymerisation The polymer may be formed by polymerising a monomer or monomers having reactive groups which react to form an imine.

In some embodiments, polymers as described herein are formed by polymerisation of a first 25 monomer comprising a group of formula (I) and two reactive groups XI with a second monomer comprising two reactive groups X2 wherein one of X1 and X2 is a group of formula -C(=0)R I and the other of XI and X2 is NH2.

Optionally according to these embodiments, the first monomer is selected from formulae (M1-A) -(MI-B) and and the second monomer is selected from formulae (M2-A) and (M2-B): X1-(Ar)p -X1 (M I-A) X1-(Ar) p L-(Ar),-X1 (Ml -B) X2 (AN X2 (M2-A) X2 -(Ar), L-(Ar),"-X2 (M2-B) In some embodiments, only one monomer substituted with XI groups and only one monomer substituted with X2 groups are reacted. It will be understood that the polymer formed from these monomers will contain only one repeat unit structure.

In some embodiments, two or more different monomers substituted with X1 groups and / or two or more different monomers substituted with X2 groups are reacted. It will be understood that the polymer formed from these monomers will contain two or more different repeat unit structures.

In some embodiments, a polymer comprising a repeating structure of formula a) may be formed by polymerisation of a monomer of formula (M3): X1-(Ar)p L-(Ar),-X2 (M3) In some embodiments, only one monomer of formula (M3) is reacted. It will be understood that the polymer formed from this monomer will contain only one repeat unit structure.

In some embodiments, two or more different monomers of formula (M3) are reacted. It will be understood that the polymer formed from these monomers will contain two or more different repeat unit structures The reaction between XI and X= may be catalysed by a Lewis acid. The Lewis acid may or may not be a Bronsted-Lowry acid.

The present inventors have surprisingly found that thermal conductivity of a polymer as described herein may be increased by polymerisation of monomers in the presence of a Lewis acid catalyst as compared to formation of the polymer without a catalyst Exemplary catalysts include, without limitation, sulfonic acids and salts thereof, for example p-toluene sulfonic acid; triflic acid; and salts thereof. An exemplary trillic acid salt is scandium triflate, Sc(Trf)3. The catalyst may be provided in an amount of 0.01-0.3 molar equivalents of the total number of moles of the monomer or monomers.

Film formation Formation of a film comprising a polymer as described herein may comprise formation of a precursor film comprising the monomer or monomers for forming the polymer followed by polymerisation of the monomers. The precursor film may consist of the monomer or monomers for forming the polymer or the precursor film may he a composition comprising one or more further materials, e.g. a Lewis acid catalyst as described herein.

Preferably, the precursor film formation comprises deposition of a formulation comprising the monomer or monomers dissolved in one or more solvents.

Solvents may be selected according to their ability to dissolve the, or each, monomer.

Exemplary solvents include, without limitation, benzene or naphthalene substituted with one or more substituents, optionally one or more substituents selected from C1_12 alkyl, C -12 alkoxy, F and Cl; ethers; esters; halogenated alkanes; ketones; sulfoxides; and mixtures thereof. Exemplary solvents include, without limitation, xylenes, 1,2,4-trimethylbenzene, mesitylene, I-meth yl naphthalene, 1-chl oro naphth al one, diiodo methane, ani sole, N-methylpyrrol idone, 1,2-dimethoxybenzene, dimethylsulfoxidc 1,3-dimethy1-2-imidazolidinone and cyclopentanone.

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

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 polymer precursor film may be heated before and / or after polymerisation. In some 10 embodiments, the polymer precursor film may be dried at a temperature of up to about 100°C, optionally 50-70°C. The dried film may be heated at a temperature above 100°C, optionally in the range of 100-200°C.

The 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; or one or more thermally conductive materials, for example boron nitride.

Optionally, a film comprising or consisting of a polymer as described herein has a thickness in the range of 1-100 microns, preferably 10-100 microns Applications A film comprising a polymer as described herein may be used in any known application of a thermally conductive film. Preferably, the film is a thermally conductive layer of an electronic device.

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 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.

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.

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 desciibed 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.

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.

lithe 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 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.

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

Polymer formation Polymers were formed by depositing a solution of a diamine monomer and a dialdehyde monomers as set out in Table 1 and reacting the monomers. Some monomers were reacted in the presence of a Lewis acid catalyst. Some polymers were annealed following polymerisation.

Monomers were dissolved in the solvent at the same desired concentration (w/v) e.g lOrng/ml, optionally applying heat up to 80°C to aid dissolution. The monomer inks were mixed by volume to produce an equimolar mixture of the monomers. A catalyst may be added by the same method of pre-dissolving and mixing by volume to achieve the desired molar ratio of catalyst to monomer. For illustration, a 1:1:0.15 molar mixture of monomers A2 (Mw 108.144) and B2 (Mw 454.65) illustrated below plus scandium triflate catalyst (Mw 492.16) at 10 mg/m1 is prepared by first preparing bulk solutions of each component at 10mg/ml, then mixing them in a 0.18:0.758:0.062 volume ratio.

After mixing the ink is promptly dropcast onto a substrate described below for thermal conductivity measurement. 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 (18x lOmm rectangle) for the dropcasting procedure. The wet film is dried by evaporation on a hotplate which may be at room temperature or any temperature below the solvent boiling point; for the examples in the table the drying temperature was consistently 50°C. After drying the gasket is removed and the film is optionally annealed at 170°C for 2 hours.

Table 1

Polymer Diamine monomer Dialdehyde monomer Solvent Catalyst Annealing temperature (°C) Thermal Conductivity (WmK-1) Comparative Polymer 1 Epoxy 0.14 Polymer Example lA Al B1 DMSO 0.11 Polymer Example 1B Al B1 DMSO ScTrf 0.17 Polymer Example 1C Al BI DMSO PTSA 0.17 Polymer Example 1D Al Bl DMSO PTSA 170 0.17 Polymer Example 2 Al B2 DMSO ScTrf - 0.14 Polymer Example 3A A2 B2 Cyclopentanone ScTrf 0.21 Polymer Example 3B A2 B2 Cyclopentanone ScTrf 170 0.32 Polymer Example 3C A2 B2 Cyclopentanone PTSA 0.18 Polymer Example 3D A2 B2 Cyclopentanone PTSA 170 0.31 Polymer Example 4 A3 B2 Anisole ScTrf 170 0.32 Sc(Trt)3 = Sc PTSA = para-toluenesulfonic acid H2N NH2 Al = terphenyl diamine NH2 A2 = phenyl diamine H 2N NH2 A3 = ethylene dianiline B1 = terphenyl dialdehyde C6H 3 B2 = hexyl terphenyl dialdehyde Comparative Polymer 1 was formed from a 1:1 v/v mixture of Epikote Resin 862 and Epikure 20 Curing Agent 866 (both Hexion) plus 0.1% tetrabutylphosphonium bromide (purchased from Sigma).

Thermal conductivity was measured as described below. It was found that monomers without alkyl substituents or an alkylene linker to enhance solubility formed poorer films, although the thermal conductivity of these films could be enhanced by annealing and / or use of a catalyst.

Thermal conductivity measurement A sensor substrate 600 (ca. 25 nun x 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, 251..tm) with a 200 nm thick heating structure consisting of a 20 micron wide heater line 610, 500 micron wide busbus 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 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 lmm width and -1mm 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 PM MA 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.

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, T. 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, lmm). 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 ic. 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,Fs) to adjust for these losses. The thermal conductivity (4) is then calculated by dividing the resulting film only conductance by the film thickness (cIF). The film thickness is determined using a digital micrometer by measuring the total thickness and subtracting the substrate thickness.

Qw KF = CF+S -CS C = 21,LIT -icd + 2hw2 dp

Claims

Claims 1. A polymer comprising a repeating structure of formula (I): i(Ar)r, -Y1 % y2 (I) wherein Ar in each occurrence is an arylene or heteroarylene group; p is at least 2; one of Y I and Y2 is CRI wherein RI is H or a substituent; and the other of Y' and Y2 is N.
2. The polymer according to claim 1 wherein p is 2-5.
3. The polymer according to claim 1 or 2 wherein each Ar of (Ar)p is independently selected from para-phenylene, thiophene, furan, and benzobisoxazole, each of which may independently be unsubstituted or substituted with one or more substituents
4. The polymer according to any one of the preceding claims wherein each Ar of (Ar)p is the same.
5. The polymer according to any one of the preceding claims wherein one or more Ar groups of (Ar)p are substituted with one or more substituents selected from substituents R2 wherein R2 in each occurrence is independently selected from: F; CN; branched, linear or cyclic Cl_in alkyl wherein one or more non-adjacent C-atoms may be replaced with 0, S. MR5. SiR62. C=0 or COO wherein R5 in each occurrence is H or a substituent and R6 in each occurrence is independently a substituent; or an aryl or heteroaryl group Ar5 which is unsubstituted or substituted with one or more substituents
6. The polymer according to any one of the preceding claims wherein RI is H or a Ci_io hydrocarbyl group
7. The polymer according to any one of the preceding claims wherein a divalent linker group L disposed in the polymer backbone, wherein L is selected from 0, S. NR5 or a C1-I2 alkylene group wherein one or more non-adjacent C atoms may be replaced with 0, 5, NR5, SiR62, CO or COO wherein R5 in each occurrence is H or a substituent and R6 in each occurrence is independently a substituent.
8. The polymer according to any one of the preceding claims wherein the repeating structure of formula (I) is comprised in a repeating group of formula (11), ( HI) or (IV). (Ar)py2 (Arlq (Ar)p y 1 2-(Ar)n-L-(Ar)\ ni-1 Y+ (Ar)n L -(Ai)p yl y2 -(Ar)q r x/1 (IV) wherein: q is at least 1; n is Dora positive integer; m is 0 or a positive integer; and L is as defined in claim 7.
9. A film comprising a polymer according to any one of the preceding claims.
10. A method of forming a film according to claim 9 comprising deposition of one or more monomers for forming the polymer onto a surface and polymerising the one or more monomers.
11. The method according to claim 10 wherein the one or more monomers are deposited from a solution.
12. The method according to claim 10 or 11 wherein the surface is a surface of a functional layer of an electronic device.
13. An electronic device comprising a film according to claim 9 disposed on a functional layer thereof.
14. The electronic device according to claim 13 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.
15. The electronic device according to claim 14 wherein the functional layer is a printed circuit board; an interposer; or a second chip.
16. The electronic device according to claim 13, 14 or 15 wherein the electronic device comprises a 3D chip stack.
17. A heat sink comprising a first surface having fins extending therefrom and an opposing second surface having a film according to claim 9 disposed thereon.
18. A method of forming a polymer according to any one of the claims 1-8 comprising polymerisation of a monomer or monomers having reactive groups which react to form yi=y2.
19. The method according to claim 18 comprising polymerisation of a first monomer selected from formulae (M1-A) and (M 1-B) and a second monomer selected from formulae (M2-A) and (M2-B): X1-(Ar)p-X1 (M1-A) X1-(Ar)p L-(Ar),-,-X1 (M1-B) X2-(Ar) q X2 (M2-A) X2-(Ar)n L-(Ar),,-X2 (M2-B) wherein one of XI and X= is a group of formula -C(=0)R1 and the other of XI and X= is NW; q, n and m are as defined in claim 8; and L is as defined in claim 7.
20. The method according to claim 18 comprising polymerisation of a monomer of formula (M3): X1-(Ar) p L-(Ar)n-X2 (M3) wherein one of XI and X= is a group of formula -C(=0)RI and the other of X1 and X= is NH2; n is 0 or a positive integer; and L is as defined in claim 7.
21. A formulation comprising one or more monomers for forming the polymer according to any one of claims 1-8 and a solvent wherein the one or more monomers are dissolved in the solvent.