WO2026008802A1

WO2026008802A1 - Thermally conductive composition

Info

Publication number: WO2026008802A1
Application number: PCT/EP2025/069054
Authority: WO
Inventors: Florence BOURCET; Yi-Chun Chin; Farzaneh HASSANI; Nir YAACOBI-GROSS
Original assignee: Cambridge Display Technology Ltd; Sumitomo Chemical Co Ltd
Current assignee: Cambridge Display Technology Ltd; Sumitomo Chemical Co Ltd
Priority date: 2024-07-05
Filing date: 2025-07-03
Publication date: 2026-01-08
Anticipated expiration: 2027-01-05
Also published as: GB202409834D0

Abstract

A composition comprising a polymer, boron nitride particles and alumina particles wherein, as a percentage of the total composition weight: the boron nitride particles weight percentage is greater than 10 wt%; the alumina particles weight percentage is at least 10 wt%; and the polymer weight percentage is at least 20 wt %. A thermally conductive film comprising the composition is also disclosed.

Description

THERMALLY CONDUCTIVE COMPOSITION

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.

WO2022/207695 discloses a composition containing a conjugated polymer and thermally conductive flakes.

Zonglin Li et al, "Formation of more efficient thermally conductive pathways due to the synergistic effect of boron nitride and alumina in poly(3-hydroxylbutyrate)", Thermochimica Acta 652 (2017) 9-16 discloses poly(3-hydroxylbutyrate)/boron nitride/A Os composites with high thermal conductivity fabricated by a melt blending process.

Zhendong Wang et al, "Thermal Conductivity and Electric Breakdown Strength properties of Epoxy/ Alumina /Boron Nitride Nanosheets Composites", 2016 IEEE International Conference on Dielectrics (ICD) discloses a composite of boron nitride nanosheets, alumina particles and an epoxy resin.

SUMMARY

The present disclosure provides a composition comprising a polymer, boron nitride particles and alumina particles wherein, as a percentage of the total composition weight: the boron nitride particles weight percentage is greater than 10 wt%; the alumina particles weight percentage is at least 10 wt%; and the polymer weight percentage is at least 20 wt %.

Optionally, the boron nitride particles weight percentage is at least 20wt %.

Optionally, the alumina particles weight percentage is at least 15wt %;

Optionally, the alumina particles weight percentage is no more than 50wt %. Optionally, the polymer weight percentage is at least 40wt %.

Optionally, the boron nitride : alumina weight ratio is in the range of 1 : 3 - 4 : 1.

Optionally, the boron nitride weight is the same as or higher than the alumina weight.

Optionally, an organic group is bound to the surface of the boron nitride particles.

Optionally, an organic group is bound to the surface of the alumina particles.

Optionally, the polymer is a conjugated polymer.

Optionally, the conjugated polymer comprises imine repeat units in the conjugated polymer backbone

Optionally, the conjugated polymer comprises a repeat unit of formula (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.

The present disclosure provides a dispersion comprising a composition as described herein and one or more solvents wherein the polymer is dissolved in the one or more solvents and the boron nitride and alumina particles are dispersed in the one or more solvents.

The present disclosure provides a film of a composition as described herein.

The present disclosure provides a method of forming a film as described herein wherein formation of the film comprises deposition of a dispersion as described herein and evaporation of the one or more solvents.

An electronic device comprising a film as described herein disposed on a surface of a functional layer of the electronic device.

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 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.

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.

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 schematically illustrates a substrate for measurement of thermal conductivity of a film;

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

Figure 6 shows thermal conductivity of a film containing Polymer 1 and functionalised boron nitride 1 only; and

Figure 7 shows thermal conductivity of a film containing Polymer 1, functionalised boron nitride 1 and Alumina 1.

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 high thermal conductivity may be provided by a film comprising or consisting of a polymer, boron nitride particles and alumina particles.

Preferably, the weight of the boron nitride and alumina particles together as a proportion of the thermally conductive particles + polymer weight is less than or equal to 70 wt%, preferably 25-70 wt%. Optionally, the amount of thermally conductive particles is below a percolation threshold of a film of the composition.

Optionally, thermal conductivity of compositions as described herein is at least 0.5 WITT ^XK ¹, optionally at least 1 Wm ^-1K ^-1.

Polymer

The polymer may be a non-conjugated polymer or a conjugated polymer.

A non-conjugated polymer as described herein suitably has no sp2-hybridised atoms in the polymer backbone. An exemplary non-conjugated polymer is poly(vinylpyrrolidone) (PVP).

The polymer is preferably a conjugated polymer.

A 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 Wnr'K'¹, optionally at least 0.2 or 0.3 Wirt'¹.

In some embodiments, the conjugated polymer comprises a repeating structure of formula (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 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 C1-20 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 C1-20 alkyl wherein one or more non-adjacent C-atoms may be replaced with O, S, NR³, SiR⁴2, C=O 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 C1-14 alkyl or C1-14 alkoxy.

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 -(Ar^m- has formula (IIb-1):

(IIb-1)

The polymer may comprise a divalent linker group L disposed in the polymer backbone, wherein L is selected from O, S, NR³ or a C1-12 alkylene group wherein one or more non- adjacent C atoms of a C2-12 alkylene group may be replaced with O, 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 O. 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 (III)-( 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. 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) :

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 lxlO⁸, and preferably lxlO⁴ to 5xl0⁵. The polystyrene-equivalent weightaverage molecular weight (Mw) of the polymers described herein may be lxlO³ to lxlO⁸, and preferably lxlO⁴ to lxlO⁶. Conjugated polymers comprising a repeating structure of formula (I) may be formed by polymerising a monomer or monomers having reactive groups which react to form an imine.

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)R¹ and the other of X¹ and X² is NH2.

The composition as described herein may contain a single polymer. The composition as described herein may contain two or more different polymers.

Preferably, the polymer weight as a percentage of the total composition weight (which is the polymer + boron nitride particles + alumina particles weight in the case where the composition consists of the polymer + boron nitride particles + alumina particles) is at least 20%, preferably at least 30%, more preferably at least 40%. Preferably, the polymer weight is no more than 90% optionally no more than 75%, optionally no more than 60% of the total composition weight.

Boron nitride particles

Boron nitride particles as described herein preferably have a thermal conductivity of at least 10 Wm ^-1K ^-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 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 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 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, preferably up to 10 microns or up to 1 micron. Flake particles as described herein may have a mean average largest dimension of up to 100 microns, preferably up to 10 microns or up to 1 micron. Optionally, the mean average largest dimension is at least 0.1 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 I or ball milling.

Alumina particles

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 or heteroaromatic groups, preferably an aromatic group, more preferably phenyl, for example an oligo-(hetero)arylene comprising 1-10 arylene or heteroarylene groups, or a poly-(hetero)arylene; a Ci-2o-alkylene-(hetero)aryl, preferably a Ci-20-alkylene-phenyl group.

An aromatic or heteroaromatic unit of a surface group may be unsubstituted or substituted with one or more substituents, optionally one or more substituents selected from F, Cl, NO2, CN and C1-12 alkyl in which one or more non-adjacent C atoms may be replaced with O, S, CO or COO.

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. The surface group may comprise a group capable of forming a hydrogen bond with the H atom of OH groups at the alumina surface.

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. Alumina particles preferably have an average particle diameter as measured by scanning electron microscopy (SEM) in the range of 0.1-10 microns, preferably 0.1-7 microns, yet more preferably 0.1-5 microns.

Alumina particles preferably have a Dso diameter as determined by dynamic light scattering (DLS) in the range of 0.1-10 microns, optionally 0.1-1 microns.

Film formation

The polymer of the composition, for example a polymer of Formula (I) described herein, is preferably soluble. A polymer of Formula (I) preferably has a solubility of at least 0.1 mg I ml, optionally at least 0.5 mg I ml or at least 1 mg I ml in xylene at 50°C and at atmospheric pressure.

In some embodiments, formation of a film may comprise deposition of a formulation comprising the polymer dissolved in a solvent or solvent mixture, the boron nitride and alumina being dispersed in the solvent or solvent mixture.

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 stretching, rubbing or thermal annealing of the film.

A crosslinker, if present, may be activated following deposition of the formulation to crosslink the polymers. Activation may be by thermal treatment and I or irradiation.

Solvents may be selected according to their ability to dissolve or disperse the polymer and (if present) any other soluble components of the formulation. 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; and mixtures thereof. Exemplary solvents include, without limitation, 1,2,4-trimethylbenzene, mesitylene, 1-methylnaphthalene, 1- chloronaphthalene, 1,2-xylene, 1,2-dichlorobenzene, diiodomethane, anisole, and 1,2- dimethoxy benzene.

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 I 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 IO^-8 S/m, optionally 1 x IO^-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, e.g. by heat and I or UV 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.

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 1

Boron nitride (BN) nanoflakes were prepared via a ball milling process. BN powder (2.76 g, Goodfellow, nominal particle size 10 microns) was placed in a zirconia lined milling jar (50mL) with zirconia milling beads (114 g, 2mm) and 2M aqueous NaOH (15.1g). The mixture was milled in a planetary ball mill (Retsch PM100) 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.

Alumina 1

Alumina having a Dso of 0.4 microns was used as "Alumina 1".

Alumina 2

Alumina (0.6 g, having a Dso of 4 pm), 4-phenylbutanoic acid (0.24 g, 1.46mmol) and anhydrous toluene (100 ml) were combined and heated to reflux overnight. The mixture was then cooled to room temperature and the particles were collected by suction filtration (membrane filter - 0.45 um pores - hydrophilic PVDF). The solid was washed with MeOH and dried in air to yield 0.46 g of white solid.

Film formation

Functionalised BN 1 flakes were dried in an oven at 90°C for at least 4 hours before preparing the suspensions. BN flakes were weighed after drying. Suspensions of Alumina 1 (Dso of 0.4 pm), Alumina 2 (functionalised, Dso of 4 pm) and pre-dried functionalised boron nitride 1 were prepared by mixing 1,2-diclorobenzene and each filler at a concentration of 20 mg/ml. Suspensions were homogenised by sonication (Fisherbrand 11207, 100% duty cycle, 37Hz, 50°C) for 1 hour. Suspensions were mixed with Polymer 1 or polyvinylpyrrolidone (PVP, average molecular weight ~8000Da) dissolved in 1,2- dichlorobenzene (20 mg/ml) at 80°C. Weight ratios of Alumina , BN, and polymer are as shown in Table 1, 2 and 3. The mixtures were heated back up 80°C. They were then vortexed and drop casted on substrates at 80°C, followed by room temperature drying in air overnight, before thermal conductivity measurement.

The films containing PVP as the matrix polymer were hot pressed (MTI YLJ-PE with HP- 100; ATC KT1 chiller) using following procedures:

For temperature: a hot press was warmed up for 30 minutes from the room temperature to 170°C. Then the 170°C temperature was maintained for 60 minutes and finally the hot press was cooled down back to room temperature at a constant rate of 2°C per minute.

For pressure: top and bottom hot press plates were closed on the substrates, which rested in this state for 100 minutes. Then pressure was ramped up over 1 minute to 0.3MPa and was maintained at this level for the next 60 minutes.

Both procedures were started at the same time and were run simultaneously. Films were protected by 0.4mm thick Teflon sheet (to prevent sticking to the hot press upper plate).

For comparison, films were also made using the same approach containing Polymer 1 only, Polymer 1 and functionalised boron nitride 1 only, Polymer 1 and Alumina 1 only, or Polymer 1 and Alumina 2 only (suspension preparation of each filler in 1,2-dichlorobenze, followed by sonication at 100% power, 37 Hz, at 50°C, before preparing solution).

Thermal conductivity measurement A sensor substrate 600 (ca. 25 mm x 25 mm) illustrated in Figure 4 was used for measurement of thermal conductivity as described herein. The substrate has a polyimide (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 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 (5mm 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 (k ) is then calculated by dividing the resulting film only conductance by the film thickness (d ). The film thickness is determined using a digital micrometer by measuring the total thickness and subtracting the substrate thickness.

Results

With reference to Figure 6, adding functionalised boron nitride only to Polymer 1 increases thermal conductivity up to a boron nitride loading of about 40 weight %.

With reference to Figure 7 and Table 1, films containing Polymer 1 (50 wt%) and both functionalised boron nitride 1 and Alumina 1 (Dso: 0.4 pm) show a sharp increase in thermal conductivity at a boron nitride loading of 30 wt% and an alumina loading of 20 wt% .

Table 1 :

As shown in Figure 7 and table 1, adding up to 50 wt% of Alumina 1 to a film containing Polymer 1 has only limited effect in increasing the thermal conductivity of the material (from 0.6 W/(m.K) to 1.0 W/(m.K)). Surprisingly the thermal conductivity of a comparative film containing Polymer 1 (70 wt%) and functionalised boron nitride 1 (30 wt%) only is much lower than the film of Polymer 1 (50 wt%), functionalised boron nitride 1 (30 wt%) and Alumina 1 (20 wt%), pointing to a synergistic effect in the combination of boron nitride and alumina.

Results with Alumina 2 are set out in Table 2. Table 2

As shown in Table 2, changing the alumina impacts the thermal conductivity of the composite formulation. However the trend is the same as in Table 1 where there is an advantage of using a combination of filler over using only one filler, with the maximum thermal conductivity being reached for the composition containing the same component ratios: Polymer 1 (50 wt%), functionalised boron nitride 1 (30 wt%) and Alumina 2 (20 wt%).

Results for compositions containing PVP are set out in Table 3.

Table 3

As shown in Table 3, changing the conjugated polymer 1 to a non-conjugated polymer (PVP) impacts the thermal conductivity of the composite formulation. However the trend is the same as in Table 1 and 2, where the maximum thermal conductivity is reached for the composition containing the same component ratios: PVP (50 wt%), functionalised boron nitride 1 (30 wt%) and Alumina 1 (20 wt%).

Claims

1. A composition comprising a polymer, boron nitride particles and alumina particles wherein, as a percentage of the total composition weight: the boron nitride particles weight percentage is greater than 10 wt%; the alumina particles weight percentage is at least 10 wt%; and the polymer weight percentage is at least 20 wt %.

2. The composition according to claim 1 wherein the boron nitride particles weight percentage is at least 20wt %.

3. The composition according to claim 1 or 2 wherein the alumina particles weight percentage is at least 15wt %.

4. The composition according to any one of the preceding claims wherein the alumina particles weight percentage is no more than 50wt %.

5. The composition according to any one of the preceding claims wherein the polymer weight percentage is at least 40wt %.

6. The composition according to any one of the preceding claims wherein the boron nitride : alumina weight ratio is in the range of 1 : 3 - 4 : 1.

7. The composition according to any one of the preceding claims wherein the boron nitride weight is the same as or higher than the alumina weight.

8. The composition according to any one of the preceding claims wherein an organic group is bound to the surface of the boron nitride particles.

9. The composition according to any one of the preceding claims wherein an organic group is bound to the surface of the alumina particles.

10. The composition according to any one of the preceding claims wherein the polymer is a conjugated polymer.

11. The composition according to claim 10 wherein the conjugated polymer comprises imine repeat units in the conjugated polymer backbone

12. The composition according to claim 11 wherein the conjugated polymer comprises a repeat unit of formula (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.

13. A dispersion comprising a composition according to any one of the preceding claims and one or more solvents wherein the polymer is dissolved in the one or more solvents and the boron nitride and alumina particles are dispersed in the one or more solvents.

14. A film of a composition according to any one of claims 1-12.

15. A method of forming a film according to claim 14 wherein formation of the film comprises deposition of a dispersion according to claim 13 and evaporation of the one or more solvents.

16. An electronic device comprising a film according to claim 14 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 16 wherein the functional layer is a printed circuit board; an interposer; or a second chip.

19. The electronic device according to claim 16 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 14 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 14 disposed thereon.