The invention relates to a polymer matrix composite, articles having at least one surface coated with such a polymer matrix composite, and methods of manufacturing such articles.

Background

Materials that are both electrically insulating and thermally conductive are sought after in the field of electronics. Many electronic devices and electronic chips generate heat. In order to function properly, this heat needs to be removed from the device or chip. Devices and chips often need to be mounted in electrical insulation. Materials that insulate electrically tend to perform poorly as thermal conductors.

Some materials, such as certain ceramics, may be capable of both thermal conduction sufficient to remove heat from an electronic device and sufficient electrical insulation. Such materials are brittle and limited in their application. Electronic chips, for example, are often connected to electrical contacts and mounted by encapsulation in a polymer. Polymers provide suitable insulation properties and allow for convenient encapsulation, for example using an injection moulding process. Polymers, however, have poor thermal conductivity. Ceramics are not suitable for encapsulating most electronic chips.

Summary of the Invention

In its various aspects, the invention provides a polymer matrix composite, an article of manufacture, a method of manufacturing an electronic device, and a packaged electronic device substantially as defined in the appended independent claims. Preferred and/or advantageous features of the invention are set out in dependent subclaims.

Thus, in a first aspect the invention may provide a polymer matrix composite comprising a first filler phase, a second filler phase, and a matrix phase.

The first filler phase consists of a plurality of substantially spherical first phase particles of an electrically insulating first phase material. This plurality of substantially spherical first phase particles can be defined by a first phase average particle diameter, and a first phase particle size distribution. The first phase particle size distribution is defined by a first phase DIO diameter and a first phase D90 diameter.

The second filler phase consists of a plurality of second phase particles of an electrically insulating second phase material. This plurality of second phase particles can be defined by a second phase average particle diameter, and a second phase particle size distribution. The second phase particle size distribution is defined by a second phase DIO diameter and a second phase D90 diameter.

The matrix phase comprises a polymeric material surrounding particles of the first filler phase and the second filler phase.

The first phase average particle diameter is between 25 micrometres to 500 micrometres, and the first phase DIO diameter is preferably within 25%, more preferably within 15%, and most preferably within 10% of the first phase D90 diameter. Thus, the particles of the first filler phase are extremely uniform and have a narrow size distribution.

The first phase DIO diameter is greater than the second phase D90 diameter.

In general, when referring to a particle size distribution of a number of similar and homogenous particles, the term DIO diameter refers to a diameter that is equal to or greater than 10% of the particles in the distribution. In other words 90% of the number of particles in the distribution have a diameter greater than the DIO diameter. The term D90 diameter refers to a diameter that is equal to or greater than 90% of the particles in the distribution. In other words 10% of the number of particles in the distribution have a diameter greater than the D90 value.

The proportion of the polymer matrix composite that is made up of filler particles, or filler, can be referred to as a total filler loading. The total filler loading within the polymer matrix composite is equal to, or exceeds, a percolation threshold such that there is long-range connectivity between filler particles. Furthermore, the first phase material and/or the second phase material is a ceramic or glass having a thermal conductivity greater than 10 W/mK. The ceramic or glass may have a dielectric strength of greater than 10 kV/mm, for example greater than 15 kV/mm.

In general, percolation theory is frequently applied to understand how properties in a composite alter with variation in filler loading. Once the filler loading of a polymer matrix composite reaches a percolation threshold for that composite, there is a long-range connectivity between the filler particles within the composite. This may result in a sharp increase in thermal conductivity and/or electrical conductivity as the percolation threshold is reached.

The precise filler loading required to achieve the percolation threshold will vary depending on the size, shape, and distribution of the filler particles used. The percolation threshold is simple to determine, however. In practice, a user can prepare a number of samples of a polymer matrix composite, each sample having a slightly higher filler loading than the preceding sample. The thermal conductivity of each sample is measured, starting with the lowest filler loading and testing the samples in order of increasing filler loading. The sample for which the thermal conductivity increases dramatically (generally at least doubling and possibly increasing by a factor of four or five times) is the sample that has a filler loading that is equal to, or exceeds, the percolation threshold. The long-range connectivity between the filler particles for this sample enables thermal percolation through the sample. Once the filler loading required to reach the percolation threshold is known for the particular filler and resin matrix combination, then it is straightforward to use the derived weight percentage of the specific filler (or volume percentage if desired) to produce the desired polymer matrix composite.

Where a particle is not perfectly spherical, but a diameter of the particle is referred to, the term "diameter" may refer to a largest dimension of the particle. Alternatively, the term "diameter" may refer to the diameter of a perfectly spherical particle having the same volume as the not perfectly spherical particle.

The term "average particle diameter", as used herein, may refer to a number average particle diameter. Other methods of determining average particle diameter are known. Thus, the average particle diameter may be, for example, a volume average particle diameter.

Unless otherwise mentioned, values given for average particle diameters in this specification refer to a "number average particle diameter". Specifically, a "number average particle diameter" is calculated as a sum of the diameters of the particles in a group divided by the number of particles in the group. Mathematically, this can be expressed as:

In the above equation, N is the total number of particles, and D _n is the diameter of the n ^th particle. As an alternative, "volume average particle diameter" may be used. Specifically, the term "volume average particle diameter" may refer to a sum of [the diameter of the particle multiplied by the volume of the particle] for each particle, divided by the sum of the volumes of the particles. Mathematically, this can be expressed as:

In the above equation, N is the total number of particles, D _n is the diameter of the n ^th particle, and V _n is the volume of the n ^th particle.

The first phase particles of the first phase material are substantially spherical and have a narrow size distribution. Where the polymer matrix composite is used as a coating, the diameter of the first phase particles may help control the thickness of the coating. For example, the polymer matrix composite may flow when applied as a coating to a planar surface and it may be possible to control the thickness to be approximately equal to or not substantially greater than the average diameter of the first phase particles. Advantageously, the thickness of a coating may be precisely controlled by selecting the diameter of the first phase particle. The first phase particles may be referred to as spacer particles because they can control the thickness of a coating formed from the polymer matrix composite and, thus, the spacing between two components separated by the composite.

The second phase particles of the second filler phase may be spherical or non-spherical. There may be more than one different material forming the second filler phase. The long-range connectivity between filler particles means that there are many paths of thermal conductivity through the polymer matrix composite. The total filler loading expressed in volume percentage may be between 60 and 90 volume percent of the total volume of the composite. The total filler loading expressed in weight percentage will vary depending on the densities of the materials forming the composite. Preferably, at least the second phase material has a thermal conductivity greater than 10 W/mK, which allows heat to be conducted through the polymer matrix composite via the many paths of thermal conductivity provided by contacts between neighbouring particles.

Advantageously, the first phase material and/or the second phase material may be any material selected from the list consisting of aluminium nitride, alumina, silicon, glass, and boron nitride.

The polymeric material may be a thermoplastic, for example: polyamide including nylon 6, nylon 6,6, and nylon 12, polyolefin including all grades of polyethylene such as linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), medium-density polyethylene (MDPE), high-density polyethylene (HDPE), and ultra-high-molecular-weight polyethylene (UHMWPE), polypropylene (PP), polybutylene, thermoplastic polyesters including polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), thermoplastic fluoropolymers including polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and ethylene chlorotrifluoroethylene (ECTFE), polyvinylchloride (PVC) acrylics including poly methyl methacrylate (PMMA), acrylonitrile butadiene styrene (ABS), polyether ether ketone (PEEK), polyetherketone (PEK), and polyetherketoneketone (PEKK), polystyrene, polycarbonates, polyimides including polyetherimide (PEI), polyoxymethylene (POM), polyphenylene sulfide (PPS), polyphenylene pxide (PPO), polysulfones such as polyether sulfone, or polylactic acid (PLA).

The Polymeric material may be a thermoplastic elastomer, for example styrene- butadiene-styrene (SBS) and styrene-ethyl-butyl-styrene (SEBS). The polymeric material may be a co-polymer of two or more of any of the above polymeric materials.

The polymeric material may be a thermoset, for example thermosetting polyesthers, thermosetting polyurethanes, polyuria / polyurethane, vulcanised rubber, phenols, phenol formaldehyde, melamine, epoxy resins, thermosetting polyamides, bismaleimides, cyanate esters, furans, silicones, vinyl esters, thermosetting acrylics, cyanoacrylates, a sheet moulding compound, thermosetting polyimides.

Preferably, the first filler phase forms less than 10 volume percent, for example between 0.5 and 10 volume percent, of the total volume of the composite with the remaining volume formed by the second filler phase and the matrix phase. Thus, it is preferable that a significantly higher proportion of the filler loading derives from the second filler phase than the first filler phase. The second filler phase, for example, preferably forms between 50 volume percent and 89.5 volume percent of the total volume of the composite.

In a second aspect the invention may provide an article of manufacture comprising a first element having a substantially planar first surface coated with a layer of a polymer matrix composite. The polymer matrix composite comprises a first filler phase consisting of a plurality of substantially spherical first phase particles of an electrically insulating first phase material, a second filler phase consisting of a plurality of second phase particles of an electrically insulating second phase material, and a matrix phase comprising a polymeric material. The average particle diameter, for example the number average particle diameter or the volume average particle diameter, of the first filler phase is greater than the average particle diameter, for example the number average particle diameter or the volume average particle diameter, of the second filler phase. The average thickness of the layer of polymer matrix composite is determined by the average particle diameter of the first filler phase, the average thickness being between zero percent and fifteen percent greater than the average particle diameter of the first filler phase.

Preferably, the average thickness of the layer of polymer matrix composite is substantially the same as the average particle diameter of the first filler phase.

Preferably, total filler loading within the polymer matrix composite is equal to, or exceeds, a percolation threshold such that there is long-range connectivity between filler particles.

Advantageously, the polymer matrix composite may be any polymer matrix composite defined above.

The layer of polymer matrix composite may have a first composite surface disposed in contact with the planar first surface of the article of manufacture and a second composite surface, in which a second element is disposed in contact with the second composite surface such that the layer of polymer matrix composite is sandwiched between the first element and the second element.

The first element may be an electrically conducting material and/or a thermally conducting material. For example the first element may be a sheet of electrically conducting material and/or thermally conducting material, or a tape formed from electrically conducting material and/or thermally conducting material. Thus, the polymer matrix composite may provide an electrically resistive but thermally conductive barrier to a first surface of the electrically and/or thermally conducting material of the first element. The first element may comprise a metallic sheet or tape with a layer of the polymer matrix composite on at least one surface. In some embodiments, the article of manufacture may be suitable for use as a thermal backplane and/or Electromagnetic Interference (EMI) shield of an electronic device.

In some embodiments, the first element may be a heat sink made from a solid metal or other thermally conducting material.

In some embodiments, the first element may be an electronic chip or an electronic device.

In some embodiments, the first element may be an electronic chip and the second element may be a thermal and/or EMI shield backplane.

The thickness of the layer of polymer matrix composite may be between 25 micrometres and 500 micrometres, for example between 40 micrometres and 250 micrometres, for example between 50 micrometres and 150 micrometres.

Preferably, the thermal conductivity of the layer of polymer matrix composite is greater than 3 W/mK. For example the thermal conductivity of the layer of polymer matrix composite may be greater than 5 W/mK, or greater than 10 W/mK. The thermal conductivity of the layer of polymer matrix composite may be between 10 and 150 W/mK, or between 15 and 100 W/mK.

Preferably, the dielectric strength of the layer of polymer matrix composite is greater than 10 kV/mm, for example between 10 and 20 kV/mm, preferably greater than 15 kV/mm.

In a further aspect the invention may provide a method of manufacturing an electronic device comprising steps of: providing an electronic chip having a body comprising electronic contacts and a substantially planar first surface; encapsulating the body of the electronic chip apart from the first surface with a first encapsulant material; and attaching a thermal backplane to the first surface with a layer of a second encapsulant disposed between the first surface and the thermal backplane, the second encapsulant being a polymer matrix composite comprising a first filler phase consisting of a plurality of substantially spherical first phase particles of an electrically insulating first phase material, a second filler phase consisting of a plurality of second phase particles of an electrically insulating second phase material, and a matrix phase comprising a polymeric material, in which the volume average particle diameter of the first filler phase is greater than the volume average particle diameter of the second filler phase, and in which the thickness of the layer of the second encapsulant is determined by the volume average particle diameter of the first filler phase, the thickness being between zero percent and fifteen percent greater than the volume average particle diameter of the first filler phase.

Advantageously, the second encapsulant may be a polymer matrix composite as defined above.

Preferably, the polymeric material forming the matrix phase of the composite is formed from a thermoplastic.

In one embodiment, the step of attaching the thermal backplane may involve steps of: applying the polymer matrix composite to the first surface at a temperature of between 100 °C and 450°C such that the composite is flowable; applying a thermal backplane to the polymer matrix composite; applying pressure to the thermal backplane to urge the thermal backplane towards the first surface, the distance between the thermal backplane and the first surface being determined by the volume average particle diameter of the first filler phase to be between zero percent and fifteen percent greater than the volume average particle diameter of the first filler phase; and allowing the polymer matrix composite to cool.

In another embodiment, the thermal backplane may be a metallic element having a substantially planar first surface coated with a layer of the second encapsulant, the thickness of the layer of the second encapsulant being determined by the volume average particle diameter of the first filler phase, the thickness being between zero percent and fifteen percent greater than the volume average particle diameter of the first filler phase, and the step of attaching the thermal backplane may involve steps of: heating the planar first surface of the electronic chip to a temperature of between 100 °C and 450°C; bringing the encapsulant layer of the thermal backplane into contact with the first surface of the electronic chip; applying pressure to the thermal backplane to urge the thermal backplane towards the first surface; and allowing the polymer matrix composite to cool.

Advantageously, the thermal backplane may have a substantially planar first surface in contact with or coated with a layer of the second encapsulant, and a substantially planar second surface parallel to the substantially planar first surface, the temperature of the second surface of the backplane being monitored during manufacture to ensure desired thermal contact between the backplane and the electronic chip.

Preferably, the first encapsulant may be a second polymer matrix composite comprising a filler and a polymer matrix.

The viscosity of the first encapsulant at a temperature of 370 degrees Centigrade may be less than 1000 Pa s, or less than 800 Pa s.

In a further aspect, the invention may provide a packaged electronic device comprising an electronic chip, the electronic chip having a body comprising electronic contacts and a substantially planar first surface. A portion of the electronic chip including the electronic contacts is encapsulated in a first encapsulant. A portion of the electronic chip including the first surface being encapsulated in a second encapsulant, the second encapsulant being a polymer matrix composite comprising a first filler phase consisting of a plurality of substantially spherical first phase particles of an electrically insulating first phase material, a second filler phase consisting of a plurality of second phase particles of an electrically insulating second phase material, and a matrix phase comprising a polymeric material, in which the volume average particle diameter of the first filler phase is greater than the volume average particle diameter of the second filler phase.

The viscosity of the first encapsulant may be less than the viscosity of the second encapsulant. For example, at a temperature of 370 degrees Centigrade, the viscosity of the first encapsulant may be less than the viscosity of the second encapsulant.

Advantageously, the second encapsulant may be a polymer matrix composite as described above. The packaged electronic device may further comprise a thermal backplane separated from the first surface of the electronic chip by a layer of the second encapsulant.

Preferably, the first encapsulant is miscible with the second encapsulant and/or the first encapsulant is weldable to the second encapsulant.

In a further aspect, the invention may also be used to create a thermal path between at least two components, which have a gap after assembly. For example, an aspect of the invention may provide a method of forming a thermal path between a first component and a second component, for example between an electronic substrate and a heatsink.

In a preferred embodiment, during assembly, a thermally conductive polymer matrix composite may be provided between the first component and the second component. The polymer matrix composite may comprise a polymeric matrix phase and a filler phase, in which the filler phase comprises filler particles having high thermal conductivity such as alumina, silicon, glass, boron nitride or any other suitable filler material. The polymer matrix composite may comprise a first filler phase consisting of a plurality of substantially spherical first phase particles of an electrically insulating first phase material, a second filler phase consisting of a plurality of second phase particles of an electrically insulating second phase material, and a matrix phase comprising a polymeric material. The average particle diameter, for example the number average particle diameter or the volume average particle diameter, of the first filler phase is greater than the average particle diameter, for example the number average particle diameter or the volume average particle diameter, of the second filler phase. The average thickness of the layer of polymer matrix composite is determined by the average particle diameter of the first filler phase, the average thickness being between zero percent and fifteen percent greater than the average particle diameter of the first filler phase.

Preferably, the average thickness of the layer of polymer matrix composite is substantially the same as the average particle diameter of the first filler phase.

Preferably, total filler loading within the polymer matrix composite is equal to, or exceeds, a percolation threshold such that there is long-range connectivity between filler particles.

Advantageously, the polymer matrix composite may be any polymer matrix composite defined above.

The polymer matrix composite may be applied to fill a gap between the first component and the second component and then heated and/or cured to form the thermal path.

The polymer matrix composite may be heated and then applied to fill a gap between the first component and the second component. It may be preferable for example, if the polymer matrix composite comprises a rigid thermoplastic or filed thermoplastic matrix phase, that the polymer matrix composite is heated first, for example in an extruder feeding an injection moulding or extrusion welding machine, and then injected or pressed into the gap between the first component and the second component. Other possible methods of application of the polymer matrix composite to fill the gap in the melted state may comprise steps of melting using hot gas, a heated tool, a laser, friction, infra-red, induction heating, or ultrasonic heating.

The polymer matrix composite may be applied to the gap in the form of a solvent cement, comprising a thermoplastic or thermosetting polymer matrix, a solvent, and optionally filler particles having high thermal conductivity such as alumina, silicon, glass, boron nitride or any other suitable filler material. In a further aspect, the invention may provide a method of assembling a device comprising a first component and a second component to optimise thermal and/or electrical paths between the first component and the second component. The invention may provide a method of assembling an electronic device or package to optimise thermal and/or electrical paths between components of the electrical device or package.

In an example, it may be desirable that, in an assembled device, a first portion of a first component is spaced from a first portion of a second component by means of a thermally conductive but electrically insulating layer or interface, for example spaced by a layer of a thermally conductive polymer or a thermally conductive polymer matrix composite. It may also be desirable that a second portion of the first component is spaced from a second portion of the second component by an electrically conductive layer or interface, for example spaced by a layer of conductive material such as a solder layer.

In an example, it may be desirable that, in an assembled device, a first component is spaced from a second component by means of a thermally conductive but electrically insulating layer or interface, for example spaced by a layer of a thermally conductive polymer or a thermally conductive polymer matrix composite. It may also be desirable that the second component is spaced from a third component by an electrically conductive layer or interface, for example spaced by a layer of conductive material such as a solder layer.

The method of assembling the device may comprise steps of, providing a sinterable material or solder between two components of the device, providing a thermally conductive polymer, for example a polymer matrix composite, between two components of the device, heating the device to sinter the sinterable material, or reflow the solder, and simultaneously melt the thermally conductive polymer, and cooling the device to form the device.

The method of assembling the device may comprise steps of, providing a sinterable material or solder between two components of the device, introducing a melted thermally conductive polymer, for example a polymer matrix composite, between two components of the device, heat from the melted thermally conductive polymer causing the sinterable material to sinter, or causing the solder to reflow, and cooling the device to form the device.

The method of assembling the device may comprise steps of, providing a sinterable material or solder between two components of the device, introducing a melted polymer, for example a polymer matrix composite, to encapsulate the two components of the device, heat from the melted thermally conductive polymer causing the sinterable material to sinter, or causing the solder to reflow, and cooling the device to form the device.

Advantageously, reflow of solder or sintering of a material within the structure of an assembled device, for example an electronic module, may create a thermal and electrical path to aid in improving the overall efficiency of the device. As an example, it may be desirable to assemble or manufacture an electronic package or module where heat needs to be transported from the device in operation to an externally facing heat spreading plane from which heat can be dissipated externally during operation of the electronic device or module. Heat may be dissipated, for example, by means of conduction (for example to an external heat sink), convection (for example by exposure to a fluid which can remove heat) and/or by radiation into free space. The module may be assembled using an injection moulding or compression moulding process to apply a polymeric encapsulant, and/or to provide a polymeric material between components of the device. An injection moulding or pressure moulding process as described above may be used. Prior to the injection of any polymeric material, components of the device may be connected electrically to the other electronic components of the device. Other components of the device performing the functions of providing structural strength and providing a pathway of preferentially high thermal conductivity may be assembled using soldering, sintering or bonding for example. A final interface, for example an interface providing an optimal structural and thermally conducting pathway between the device and a heat spreader on one external surface of the device (by sintering or soldering) is assembled but not effected. That is, the solder or sinterable material for forming the final interface is not heated to form the final interface. The assembly in this form is then placed into the mould cavity of a compression moulder or injection moulder to receive an encapsulating layer of polymeric material. The heat and pressure generated by the injection or compression moulding machine retains the assembly in its required geometry. The polymeric material then effects the joint between the heat spreader plate and the components with high thermal conductivity connected to the hot electronic device. Heat from the molten polymeric material causes the sinterable material to sinter, or the solder to reflow, thereby effecting the final interface in situ within the injection or compression moulding machine.

The polymeric material may be a polymer matrix composite. The polymer matrix composite may comprise a polymeric matrix phase and a filler phase, in which the filler phase comprises filler particles having high thermal conductivity such as alumina, silicon, glass, boron nitride or any other suitable filler material. The polymer matrix composite may comprise a first filler phase consisting of a plurality of substantially spherical first phase particles of an electrically insulating first phase material, a second filler phase consisting of a plurality of second phase particles of an electrically insulating second phase material, and a matrix phase comprising a polymeric material. The average particle diameter, for example the number average particle diameter or the volume average particle diameter, of the first filler phase is greater than the average particle diameter, for example the number average particle diameter or the volume average particle diameter, of the second filler phase. The average thickness of the layer of polymer matrix composite is determined by the average particle diameter of the first filler phase, the average thickness being between zero percent and fifteen percent greater than the average particle diameter of the first filler phase.

Preferably, the average thickness of the layer of polymer matrix composite is substantially the same as the average particle diameter of the first filler phase. Preferably, total filler loading within the polymer matrix composite is equal to, or exceeds, a percolation threshold such that there is long- range connectivity between filler particles. Advantageously, the polymer matrix composite may be any polymer matrix composite defined above.

Features described above in relation to one aspect may be applicable to one or more of the other aspects.

List of figures

Figure 1 is a schematic illustration of a polymer matrix composite according to an embodiment of the invention;

Figure 2 is a graph illustrating how the measured thermal conductivities of a polymer matrix composite may vary with the total filler loading of the polymer matrix composite;

Figure 3 is a schematic illustration of an article of manufacture according to an embodiment of the invention; Figure 4 is a schematic illustration of an electronic device according to an embodiment of the invention;

Figure 5 is a schematic illustration of a packaged electronic device according to an embodiment of the invention, and

Figure 6 is a schematic illustration of a packaged electronic device according to an further embodiment of the invention.

Specific embodiments of the invention

Various embodiments of the invention will now be described by way of example.

Figure 1 is a schematic illustration of a polymer matrix composite. The polymer matrix composite 100 comprises a first filler phase, a second filler phase, and a matrix phase 102.

The first filler phase consists of a plurality of substantially spherical first phase particles 104, 106,

108 of aluminium nitride (AIN). Other suitable materials for the first phase particles of the first filler phase include alumina, silicon, glass, and boron nitride. The plurality of substantially spherical first phase particles 104, 106, 108 have a first phase average particle diameter of 45 microns. The first phase particle size distribution is defined by a first phase D10 diameter of 40 microns and a first phase D90 diameter of 50 microns. Thus, the particle size distribution is narrow and the particles forming the first filler phase are substantially the same size.

The first filler phase in this embodiment was obtained by sieving a commercially available aluminium nitride powder.

The first filler phase in this embodiment makes up approximately 1% of the total weight of the polymer matrix composite. This corresponds to about 0.7% of the total volume of the polymer matrix composite.

The second filler phase consists of a plurality of second phase particles 110, 112, 114 of AIN. Other suitable materials for the second phase particles of the second filler phase include alumina, silicon, glass, boron nitride or any other suitable material. The plurality of second phase particles 110, 112, 114 have a second phase average particle diameter of 2 microns. The second phase particle size distribution is defined by a second phase D10 diameter of 1.5 microns and a second phase D90 diameter of 2.5 microns.

The second filler phase in this embodiment is a commercially available aluminium nitride powder.

The second filler phase in this embodiment makes up approximately 74% of the total weight of the polymer matrix composite. This corresponds to about 53.2% of the total volume of the polymer matrix composite.

The matrix phase 102 is formed by polyetherimide (PEI). The matrix phase 102 surrounds particles of the first filler phase and the second filler phase.

The matrix phase 102 in this embodiment makes up approximately 25% of the total weight of the polymer matrix composite. This corresponds to about 46.1% of the total volume of the polymer matrix composite.

In this embodiment, the polymer matrix composite has a melt viscosity of around 1627 Pa s at 370 degrees Centigrade. The total filler loading within the polymer matrix composite exceeds the percolation threshold. This means that there is long-range connectivity between filler particles. Long-range connectivity allows for thermal percolation through the polymer matrix composite, resulting in higher thermal conductivity for the polymer matrix composite than would be the case if the total filler loading was below the percolation threshold.

The filler loading necessary to achieve the percolation threshold will depend on the size and shape of the particles in the composite. The percolation threshold may be easily determined for any specific filler (i.e. the total of all first phase particles, second phase particles, and any other filler particles). One method is set out in the general description above. The skilled person would understand that there are numerous ways to determine the percolation threshold.

Once the percolation threshold has been determined, one can select a suitable total filler loading for the polymer matrix composite that equals or exceeds the percolation threshold. For the polymer matrix composite 100 illustrated in figure 1, the total filler loading is approximately 53.9% by volume. In other examples, similar aluminium nitride particles have been loaded to form composites with total filler loading of up to 75% by volume.

In order to create the polymer matrix composite, the polymer and the first and second filler phases are thoroughly mixed at a temperature greater than the melting temperature of the polymer. For PEI, it is preferred that the mixing occurs at a temperature of about 350 °C. Suitable means for mixing the heated polymer and the filler particles are known. A preferential mixing system may be a screw mixing system, for example a screw mixing system comprising two counter-rotating screws which cause shear mixing of the polymer melt and the first and second filler phases. The feed chamber of a polymer screw extruder may provide suitable mixing.

Once mixed, the polymer matrix composite may be pressed, or extruded, or otherwise formed, in order produce a suitable shape and size. The shaped polymer matrix composite is then cooled to solidify.

In a second embodiment of the polymer matrix composite, the first filler phase consists of a plurality of substantially spherical first phase particles of glass. The plurality of substantially spherical first phase particles have a first phase average particle diameter of 475 microns, a first phase D90 particle diameter of 500 microns, and a first phase D10 particle diameter of 450 microns. This first filler phase was obtained by sieving SiLi ^® glass balls from Sigmund Lindner GmbH (product code 4502).

The first filler phase in this embodiment makes up approximately 1% of the total weight of the polymer matrix composite. This corresponds to about 0.9% of the total volume of the polymer matrix composite. The second filler phase consists of a plurality of second phase particles of aluminium nitride. The plurality of second phase particles have a second phase average particle diameter of around 10 microns. The second phase particle size distribution is defined by a second phase D10 diameter of 7.5 microns and a second phase D90 diameter of 12.5 microns. The second filler phase in this embodiment makes up approximately 74% of the total weight of the polymer matrix composite. This corresponds to about 53.4% of the total volume of the polymer matrix composite. The matrix phase is formed by polyetherimide. The matrix phase in this embodiment makes up approximately 25% of the total weight of the polymer matrix composite. This corresponds to about 45.7% of the total volume of the polymer matrix composite. In this embodiment, the polymer matrix composite has a melt viscosity of around 693 Pa s at 371 degrees Centigrade.

Figure 3 is a schematic illustration of an article of manufacture comprising the polymer matrix composite described above in relation to figure 1. The article of manufacture 300 comprises a layer of the polymer matrix composite 100, coated onto a substantially planar first surface of a first element 320.

The first element 320 is formed from a copper sheet or foil. Particularly suitable materials for forming at least the first surface of the first element include copper, aluminium, or silver. Materials that provide high levels of thermal conductivity or high levels of electrical conductivity may be suitable materials to form at least the first surface of the first element.

To manufacture the article, the polymer matrix composite 100 is coated onto the first surface of the first element 320, and then a force is applied to the polymer matrix composite 100. This occurs at a temperature higher than the melting temperature of the polymer of the polymer matrix composite 100. The first element 320 may also be heated, for example to a temperature greater than the melting temperature of the polymer of the polymer matrix composite 100. The force, acting from above the polymer matrix composite 100 when looking at figure 3, may act to 'spread' the composite. In other words, the force may create a coating of composite 100 of substantially uniform thickness, the thickness being equal to, or slightly greater than, for example less than 15% greater than, the average particle diameter of the first phase particles. Since the particle size distribution of the first phase particles is narrow, the article of manufacture may therefore have a well-defined thickness roughly equal to the thickness of the first element added to the average particle diameter of the first phase particles. The first phase particles may act as spacer particles and may allow a coating of substantially uniform thickness to be applied to a planar surface. Thickness of such a coating may be controlled by selecting the dimensions of the first filler phase particles. The polymer matrix composite 100 and first element 320 are then allowed to cool.

Figure 4 is a schematic illustration of an electronic device. The device 400 comprises the polymer matrix composite 100 described in relation to figure 1, an electronic chip 402, and a thermal backplane 404. The thermal backplane is preferably formed from a conductive material such as copper or aluminium. The thermal backplane may be a foil or a tape of thermally conductive material, or may be a solid component such as a heat sink.

The electronic chip 402 will generate heat when in use. The polymer matrix composite 100 electrically insulates the backplane 404 from the electronic chip 402. The polymer matrix composite 100 also has a relatively high thermal conductivity, due to the presence of filler particles that have a relatively high thermal conductivity which are present in the composite at a loading level that is equal to or greater than the percolation threshold, and allows heat generated by the electronic chip 402 to be dissipated through the thermal backplane 404. Further still, the narrow particle distribution of the first phase particles of the first filler phase of the polymer matrix composite 100 ensures that the spacing between the backplane 404 and the electronic chip 402 is substantially constant. The polymer matrix composite layer is, thus, electrically constant which may minimise parasitic capacitance, also known as stray capacitance.

The electronic device 400 is manufactured by providing an electronic chip having a body comprising electronic contacts and a substantially planar first surface, and encapsulating the body of the electronic chip with the polymer matrix composite. This may occur, for example, by an injection moulding process as is known in electronic package manufacture.

It is important to ensure that good thermal contact is made between the chip 402 and the backplane 404. Good thermal contact between the chip 402 and the backplane 404 is achieved by applying the polymer matrix composite 100 to the first surface of the electronic chip 402 at a temperature of between 300°C and 450°C such that the composite is relatively flowable, applying the thermal backplane 404 to the polymer matrix composite 100, and applying pressure to the thermal backplane 404 to urge the thermal backplane 404 towards the first surface of the electronic chip 402. Thus, the distance or space between the thermal backplane 404 and the first surface of the electronic chip 402 is determined by the average particle diameter of the first filler phase, and is between zero percent and fifteen percent greater than the volume average particle diameter of the first filler phase. The polymer matrix composite 100 is then allowed to cool.

Good thermal contact between the chip 402 and the backplane 404 can be verified practically by arranging for the electronic chip 402 to have a temperature well above room temperature during attachment of the backplane 404 to the electronic chip 402, and monitoring the temperature of the upper surface of the backplane (the surface of the backplane 404 furthest from the electronic chip 402). A higher temperature measured at this surface of the backplane, or a high temperature measured in a shorter period of time, may indicate good thermal contact between the chip 402 and the backplane 404. In this way, a good thermal contact between backplane 404 and electronic chip 402 can be verified.

Figure 5 is a schematic illustration of a packaged electronic device. The packaged electronic device 500 comprises an electronic chip 502. The electronic chip 502 has a body 504 comprising electronic contacts 506, 508 and a substantially planar first surface 510. At least a portion of the electronic chip 502, including the first surface 510, is encapsulated by an encapsulant. The encapsulant is the polymer matrix composite 100 described in relation to figure 1. The packaged electronic device 500 further comprises a thermal backplane 512. The thermal backplane 512 is separated from the first surface 510 of the electronic chip 502 by a layer of the second encapsulant. It is noted that the figures are schematic and, for example, the relative thickness of the encapsulant 100 is exaggerated compared to the dimensions of the electronic chip 502.

Figure 6 is a schematic illustration of a further embodiment of a packaged electronic device 600. The polymer matrix composite described above is loaded with a high proportion of filler (i.e at or above the percolation threshold). Thus, the flowability of the polymer matrix composite may make full encapsulation of an electronic chip difficult to achieve without damaging contacts. An alternative method of manufacture may utilise two different encapsulants for the electronic chip.

The packaged electronic device 600 comprises an electronic chip 602. The electronic chip 602 comprises electronic contacts 606, 608 and a substantially planar first surface 610. A portion of the electronic chip 602 including the electronic contacts 606, 608 is encapsulated in a first encapsulant 650. A portion of the electronic chip 602 including the first surface 610 is encapsulated in a second encapsulant 660. The second encapsulant 660 is the same as the polymer matrix composite 100 described in relation to figure 1. The packaged electronic device 600 further comprises a thermal backplane 612. The thermal backplane 612 is separated from the first surface 610 of the electronic chip 602 by a layer of the second encapsulant 660.

Thus, a method of producing a packaged electronic device 600 as described in relation to figure 6 may comprise the step of encapsulating the body of the electronic chip 602, other than the first surface 610, with a first encapsulant material 650. As a subsequent step, the thermal backplane 612 is attached to the first surface 610 with a layer of a second encapsulant 660 disposed between the first surface 610 and the thermal backplane 612. The second encapsulant is, in this case, the polymer matrix composite as described in relation to figure 1. The first encapsulant is preferably a polymer or a polymer matrix composite that has a relatively low viscosity, for example less than 1000 Pa s, or less than 800 Pa s, at a temperature of 370 degrees Centigrade.

The polymer phase of the second encapsulant should be miscible with, or weldable to, the polymer phase of the first encapsulant. Advantageously, the polymer phase of the second encapsulant may be the same as the polymer phase of the first encapsulant.

A packaged electronic device having two different encapsulating materials may be manufactured using an injection moulding process and two different injection ports. Alternatively, the first encapsulant may be applied to the underside of the electronic chip using an injection moulding process leaving a partially encapsulated electronic chip having an exposed first surface. The second encapsulant and the thermal plane may then be added, separately or together, to the exposed first surface of the electronic chip, for example in the form of a tape.