EP3576930A1

EP3576930A1 - Planar heat sink made of high thermal conductivity and high mechanical strength composite material

Info

Publication number: EP3576930A1
Application number: EP18708475.1A
Authority: EP
Inventors: Alessandro DEODATI; Giuseppe Vendramin; Andrea Tinti; Antonella Tarzia; Alessandra PASSARO
Original assignee: Niteko Srl
Current assignee: Niteko Srl
Priority date: 2017-02-02
Filing date: 2018-01-31
Publication date: 2019-12-11
Also published as: WO2018142298A1; IT201700011367A1

Abstract

Heat sink (1) made of composite material in the form of planar element characterized by a combination of unidirectional and thermally conductive reinforcing carbon fibers embedded in a polymer matrix composed of a thermosetting epoxy resin. The invention also relates to a process for manufacturing a heat sink made of a composite material and a combination of heat sink made of composite material with an electronic circuit specifically comprising a light source of LED type.

Description

Planar heat sink made of high thermal conductivity and high mechanical strength composite material

DESCRIPTION

FIELD OF THE ART TO WHICH THE INVENTION RELATES

The present invention relates to a planar heat sink made of composite material characterized by high thermal conductivity, high mechanical strength and low specific weight, used in the field of LED technology for dissipating the heat produced during the operation by one or more LED devices applied thereto .

STATE OF THE PRE-EXISTING ART

The light source that in the last years is emerging the most compared to conventional sources (filament sources, halogen lamps, fluorescent, gas- discharge sources) is the HB (High Brightness) LED (Light Emitting Diode) source. Specifically, in outdoor applications, the last generation of high luminance light emitting diode provides substantially better performances, both energetically and qualitatively (photometrically and colorimetrically) , compared to conventional high pressure mercury vapor sources, metal halide vapor sources, high and low pressure sodium vapor sources.

Other advantages of this new technology are: high reliability, long lifetime, high light efficiency, lower heat generation, reduced weight and bulk, robustness, drastic reduction of maintenance costs . The attention to the environment protection and the continuous research for energy saving make the LED source the best light source presently in the aforementioned fields. Furthermore, still being a very new technology compared to the others, it has huge margins for growth and development.

In fact LEDs do not contain toxic or harmful substances for the human health, animal and environment health, differently from some kind of conventional lamps containing mercury and other hazardous substances.

A LED lighting device is composed of different parts: A MCPCB, that is a Metal Core Printed Circuit Board, substrate i.e. characterized by a main core composed of aluminium or copper, which are metals with good thermal conductivity to which the LEDs are welded, a passive heat sink - generally made for example of the frame of the same lighting apparatus and conventionally made of aluminium alloy, subjected to anti-corrosion processing and/or varnishing, which varnishing is generally made with polyester powders - comprising a plurality of fins, a possible active cooling technology such as for example cooling fans directing a given air flow directly among the heat sink fins, a solution for transferring heat between MCPCB and heat sink, (such as for example a double- sided tape adapted to the thermal transfer, possible system for directing the light, such as for example lenses and/or reflectors, a mechanical system for protecting from water and dust, for example generally made of a methacrylate or tempered glass screen, which is assembled on the heat sink by screws, possibly interposing a proper gasket between the latter, a driver, such as for example an AC/DC converter, in order to power supply the lighting apparatus, possible remote or stand alone controlling electronics, and power supplying devices to power supply the possible active cooling elements.

The LED diodes turn nearly all the absorbed power into visible light; nevertheless the use of a quite bulky cooler is required in order to prevent the P-N junction from heating and the following degradation of optical properties from occurring. Being able to reduce the mass of such a component of the power LED apparatus, it is key for saving material and application easiness.

Despite a higher efficiency compared to other light sources, LEDs can have reliability issues if any one of the parts of the lighting system, including the driver, is not properly protected from overheating .

Since the LEDs do not emit in the infrared by radiation, the produced heat has to be removed by conduction or convection. If the heat loss is insufficient, the LED may be subject to untimely color change: specifically, the supply voltage starts to decrease, resulting in an increase of the load on the LED driver components and causing their temperature to increase; at even higher temperatures the emission wavelength can be shifted, thereby causing the orange lights to turn to red and the white lights to a blued color. In addition, a thermally stressed LED becomes less efficient and the luminous flux decreases: if the heat is not dissipated, the LED junction can be damaged, thereby leading the entire element to breakage; other consequences can include internal delamination of the soldering point, damage to the epoxy resin and yellowing of the lenses.

The operating parameter that characterizes the efficiency of a heat sink, summarizing the number of factors affecting the same - composing material, sizes, shape, color, surface finishing, ventilation conditions, assembling position, etc.- is its thermal resistance Rthda, that is the thermal resistance at the interface between the heat sink and the surrounding environment. This is defined as the temperature increase to which the heat sink is subjected due to 1 watt electrical power being applied, in accordance with what can be deduced from the equation (1) :

Td - T_a = Pd ^* Rthda (1)

wherein Td (°C) is the heat sink temperature, T_a (°C) is the room temperature and Pd (W) is the power dissipated by the electronic device of interest, such as for example the LED lighting system. The heat sink thermal resistance Rthda (°C/W) is a measure of the difficulty of dissipating the heat generated by the LEDs and transmitted to the heat sink to which they are integral, to the environment.

As it can be deduced from equation (1) , a low thermal resistance is desirable since: the heat sink, and thus also the LED, operating temperature being the same, it would be possible to dissipate higher power or, the dissipated power being the same, it would be possible to reduce the operating temperature of the heat sink, and thus the LEDs, with evident positive effects in terms of efficiency, lifetime and spectral emission of the junction devices. Advances in technology are making the thermal management of LEDs if at all possible even more complex, due to continuous increase of power and current needed to create diodes with greater and greater luminous flux.

In order to address such a problem, the solutions are oriented to the implementation of new configurations and geometries that optimize the heat dissipation, and to the use of new materials that may offer better performances. Presently, many LED extruded or printed heat sinks use finned arrangements, which increase the thermal exchange surface and produce a more effective convective exchange. The presence of such fins results in issues of impurities nesting among the fins and this leads to a degradation of the thermal performances of the same heat sinks .

In the state of the art, the materials currently used in the implementation of the heat sinks are aluminium, which has good thermal conductivity and relatively low cost, and copper, which has twice the conductivity but a much higher cost.

Both the materials are however characterized by high specific weight and thus, due to the increase of involved powers , they lead to a remarkable increase of the whole weight of the entire lightning apparatus with subsequent effects in terms of cost and bulks of both the lightning system and the structures intended for fastening and supporting the same.

Furthermore in order to implement the aluminium heat sinks by extrusion techniques or die-casting, expensive equipment, steel molds and manufacturing processes are needed all requiring an enormous amount of energy and expensive facilities.

Another implication of the use of heat sinks made by conventional technologies is the corrosion due to the atmospheric agents when varnishes or surface passivations would be degraded by aging due to the exposition to sun rays and atmospheric agents.

Purpose of the present invention is therefore to implement a heat sink for LED lightning apparatuses, able to overcome the drawbacks of the known art as afore highlighted, through the development of new solutions .

Within this purpose, an object of the invention is to implement a system for thermally managing the heat generated by the LED source, which system allows better performances of the LED lightning apparatus to be obtained; the same is in fact characterized by a heat sink made of thermally conductive composite material on which LED modules are applied preferably by double-sided thermally conductive tape. The consequence of what afore allows an important object of the invention to be reached, which is to obtain a heat sink and thus a lightning apparatus with a substantially lower weight compared to conventional aluminium heat sinks made by extrusion process or by aluminium die-casting. Such a characteristic allows supporting brackets much less robust, and thus much cheaper, compared to those associated with lamps using conventional type heat sinks, to be used.

Further object of the invention is to implement a heat sink with preferably planar geometry and thus with a large surface adapted to the thermal exchange by thermal convection, still not using the conventional cooling fins. This leads to another advantage, which is to prevent impurities and dust of any kind from nesting in the gap obtained among the cooling fins thus impairing the thermal performances.

Further object of the invention is thus to obtain heat sinks and thus LED lightning apparatuses, substantially less cumbersome compared to those using heat sinks made with conventional techniques.

Additional object of the invention is to allow the implementation of lightning apparatuses with supporting structure composed of the heat sink made of high mechanical strength composite material, whereas the remaining mechanical parts, including the compartment housing the drivers and the controlling electronics, free from excessive operating loads, can be preferably made of technopolymer by the plastic injection process thereby obtaining lower raw material costs, low tolerances on the product, lower processing costs, electrical insulation, higher finishing possibilities, longer life of the mould, less processing swarf and more limited weight of the produced pieces compared to the metal alloy die- casting process.

The use of the technopolymer for the realization of the aesthetic and functional parts of the invention allows pursuing another object of the invention, which is to eliminate the need of protecting the body of the apparatus by varnishing, thus leading to significant reduction of the manufacturing costs.

The consequence of what afore allows another object of the invention to be obtained, which is to reduce the pollution and, generally, the ecological footprint on the environment, thanks to the use of fully recyclable plastics, the lower energy used in the process of plastic injection compared to the metal alloy die-casting process and to the absence of varnishes.

Furthermore the object of the following invention is not subjected to degradation due to corrosion phenomena due to the exposure to atmospheric agents and solar radiations for the whole lifetime of the LED sourced applied thereto.

Additional object of the invention is to use the moulds for the implementation of the present invention, which have a substantially lower cost compared to those used in the aluminium die-casting technology and in the aluminium extrusion technology, to implement a component having equal thermo- mechanical performances.

The specific technical terms used in the description and claims have the following meaning: with the term prepreg composite fibers pre- impregnated with a thermosetting resin are meant. The fibers can be arranged in any desired way and direction and can also be interwoven and not, for example in the form of a fabric, non-woven fabric, three-dimensional fabric and other presently known shapes and the resin is used to bind the fibers to each other. The resin hardening process is only partially carried out, i.e. the curing is not completed whereby the pre-impregnated material is easily manipulable and can be subjected to processing in a subsequent treatment process, such as for example forming, lamination and other processes. With the term carbon fibers of pitch type a typology of carbon fibers based on the process for obtaining the same is denoted, i.e. which are obtained from pitch fibers subjected to stabilization and carbonization.

With the term mesophase pitch or MPP the process for obtaining carbon fibers from mesogenic pitch turned into mesophase pitch during the spinning is meant; such a mesophase pitch is then subjected to stabilization, carbonization and high temperature thermal treatment .

With the term PAN the carbon fibers of the type obtained through stabilization, carbonization and possible high temperature thermal treatment of polyacrylonitrile are denoted.

The present invention is, therefore, now described, for illustration but not limitative purposes, according to preferred but not exclusive embodiments thereof, specifically referring to the figures of the accompanying drawings, wherein:

figure 1 shows a perspective exploded view of a possible composition and stratification of the object of the present invention;

figure 2 depicts a schematic view of the heat flow within a planar body object of the present invention ;

figure 3 depicts a cross-sectional exploded view of a possible configuration of use of the present invention ;

figure 4 shows a flow diagram relating to a preferable sequence of lamination instructions related to the present invention; figure 5 sequentially shows the orientation of the individual fabrics in the various layers related to the lamination process of the heat sink object of the present invention.

DESCRIPTION OF THE FIGURES

Figure 1 shows a perspective exploded view of a possible embodiment of the lamination of four layers of balanced unidirectional prepreg superimposed sequence 0/+45/-45/90 described in the present invention.

Figure 2 depicts a schematic view of the heat flow flowing by conduction in a laminate with balanced stratification object of the present invention .

Figure 3 depicts a cross-sectional exploded view of a possible configuration of use of the present invention, wherein the heat sink (1) of thermally conductive composite material, the double-sided thermally conductive tape (2) for the coupling and dissipating and the MCPCB circuits (3) with assembled LEDs are recognizable.

Figure 4 shows a flowchart describing the operations for molding a laminate object of the present invention.

Figure 5 depicts a flowchart describing the lamination sequence of the prepreg fabric layers on the mould.

In a preferred embodiment, the heat sink (1) object of the present invention is made of a composite material, and as such it is composed of a matrix and a reinforcement. The reinforcement is obtained by the use of continuous carbon fibers. Specifically, high thermal conductivity carbon fibers so called of "pitch" type, that differ from the conventional carbon fibers so called of "PAN" type by the typology of chemical precursor used in the pyrolytic synthesis process with whom they are manufactured, are used: in the first case mesophase pitch and in the second case polyacrylonitrile (PAN) . The use of a different chemical precursor for the synthesis of the PAN and pitch carbon fibers causes significant differences in the microstructure and thus in the properties of the same fibers. The pitch carbon fibers have higher degree of crystalline order, the carbon layers being observable - in section - aligned to each other and to the fiber axis, as in the graphite structure. This results in carbon fibers of the pitch type having higher elastic modulus and thermal conductivity compared to carbon fibers of the PAN type, which conversely have higher mechanical strength. The thermal conductivity of the fiber is meant as longitudinal, i.e. along the fiber axis. The longitudinal thermal conductivity of specific degrees of carbon fiber of pitch type can by far outweigh the longitudinal thermal conductivity of the carbon fibers of PAN type and also the aluminium thermal conductivity. Typical examples of commercial products amount to values much higher than 100 W/ (m -K) and also up to 1000 W/ (m -K) . A problem shared by both the carbon fiber typologies, both pitch and PAN, is conversely the low transverse thermal conductivity, i.e. in a direction normal to the fiber axis. This amounts in both cases to values on the order of 10 W/ (m -K) . For this reason planar geometries of the heat sinks implemented according to the present invention are preferred. The matrix of the composite material composing the heat sink object of the present invention is a thermosetting, specifically epoxy, resin. Such resins, as it is typical of the organic based materials, generally have very low thermal conductivity values, usually lower than 1 W/ (m -K) . Therefore, a composite laminate composed of reinforcing pitch carbon fibers and epoxy resin matrix will have good thermal conductivity in the laminate plane (in-plane) - as long as it is a balanced laminate - but poor thermal conductivity in the direction perpendicular to the laminate plane (out-of-plane) . In order to even partially compensate for this problem, in the present invention the addition of a thermally conductive filler such as for example metal particles (copper, aluminium, etc.), ceramic particles (alumina, etc.) or carbonaceous particles (carbon black, graphite, etc.) to the resin, in proper contents, is provided.

For the manufacturing of the heat sink made of composite material object of the present invention, a so called "prepreg" fabric is used, i.e. a pitch carbon fiber fabric already pre-impregnated with resin in the proper proportions .

According to a first embodiment, the prepreg fiber layer has preferably unidirectional textile architecture, i.e. with the fibers all oriented in the same direction.

An embodiment variation provides alternatively also the use of bi-dimensional fabrics of varying weave - for example plain, twill or satin, this specifically if a so called "carbon look" finishing is desired to be provided to the final component. Vice versa, by using a unidirectional prepreg, the component surface finishing will be matt black.

The whole grammage of the prepreg, expressed in grams per square meter of planar surface, is variable upon the thermal and mechanical performances required for the final component.

The prepreg composing resin is of epoxy type or different in nature, however thermosetting. The resin shows high glass transition temperature, higher than the maximum operating temperature expected for the component, taking into account that the heat sink is subjected to heating by both conductive transfer by the LEDs and radiation transfer if exposed to direct sunlight (outdoor application) .

As afore mentioned, according to an embodiment, the resin embeds a thermally conductive filler increasing the thermal conductivity thereof. Furthermore, the resin embeds a UV absorbing filler filtering the ultraviolet radiation hitting the component, if it is exposed to direct sunlight, thus reducing the photo-chemical degradation phenomena otherwise caused in the resin itself and the subsequent reduction of the mechanical properties thereof .

The resin occurs in the prepreg in the minimum amount useful to ensure a proper impregnation of the carbon fibers of the pitch type and thus proper mechanical properties for the component. For the purposes of thermal dissipation, in fact, using as low as possible resin amount is desirable, since this is a thermal insulator.

According to an implementation, as an indication, the resin occurs in the prepreg at 40% by weight or lower value, and the resin content is uniform and constant throughout the prepreg.

The prepreg of carbon fibers of pitch type is trimmed to proper sizes, as provided by the cut plane specifically prearranged for this purpose.

The trimmings of carbon fibers pre-impregnated with thermosetting resin, i.e. the trimmings of prepreg material are laminated on a mould, made of aluminium, reproducing the heat sink geometry. The "mould side" finish, perfectly smooth, is reserved for the component surface expected to be exposed in the final application.

According to a preferred lamination process for the making of the component according to the present invention, the pre-impregnated carbon fibers, i.e. the prepreg material , are laminated in balanced stratification as shown in fig. 1, i.e. with at least one layer of carbon fiber in each of the four main in-plane directions (0°, ±45°, 90°) , so as to maximize the in-plane thermal conductivity of the heat sink.

According to the afore said process, in case of pre-impregnated carbon fibers (i.e. prepreg material) wherein the fibers have unidirectional orientation, at least four superimposed layers of said pre- impregnated carbon fibers are required in order to obtain a balanced stratification: the first layer with the carbon fibers all or at least mainly oriented in 0° direction (arbitrarily defined, fig. 1-a) ; the second layer with the carbon fibers all or at least mainly oriented in +45° direction (fig. 1- b) ; the third layer with the carbon fibers all or at least mainly oriented in -45° direction (fig. 1-c) ; the fourth layer with the carbon fibers all or at least mainly oriented in 90° direction (fig. 1-d) . Such a stratification can be briefly denoted as 0°/+45°/-45°/90° (fig. 1).

The lamination sequence of the four layers is described in detail by fig. 5 and can be varied and specifically reversed.

At the step 500 the first layer of pre- impregnated carbon fibers with unidirectional orientation is placed on the mould. The fiber orientation is arbitrary and is taken as geometric reference orientation corresponding to 0° direction. At the step 510 a second layer of pre-impregnated carbon fibers with unidirectional orientation is laid on the first layer. The orientation of these fibers to those of the first layer is corresponding to a rotation by a 45° clockwise angle.

At the step 520 a third layer of pre-impregnated carbon fibers with unidirectional orientation is laid on the second layer. The orientation of these fibers to those of the first layer is corresponding to a rotation by a 45° anti-clockwise angle, i.e. opposite to the direction of the second layer of carbon fibers .

At the step 530 a fourth layer of pre- impregnated carbon fibers with unidirectional orientation is laid on the third layer. The orientation of these fibers to those of the first layer is corresponding to a rotation by a 90° angle, i.e. perpendicular to the orientation of the fibers in the first layer of carbon fibers.

As shown in figure 4, these steps are summarized as balanced lamination at the step 400, while the heat sink finishing process takes place thanks to the steps 410 of consolidation in oven and/or autoclave by which the resin matrix curing process is completed and by the next step 420 of the component cutting and/or trimming to size.

According to an additional variation of the present manufacturing process, a number of layers larger than the afore mentioned four can be provided as long as the balancing condition applies. By way of example the following stratifications can be carried out :

six layers with 0° /90° /+45° /-45° /90° /0° sequence or

eight layers with 0°/90^ο/+45^ο/-45^ο/-45^ο/+45^ο/90^ο/0° sequence.

The minimum stratification, four layers with 0°/+45°/-45°/90° sequence, can be used after verifying that the relevant mechanical requirements are met.

According to an additional embodiment variation two layers of pre-impregnated carbon fibers fabric can also be used, wherein each fabric layer has a weave of fibers oriented in two not parallel directions, such as for example two layers of carbon fibers in the form of a plain, twist or satin, or like fabric.

Still according to an alternative two layers of unidirectional fibers, between which a layer of carbon fibers is interposed, can be combined to each other in the form of fabric with fibers oriented in two directions different from each other, such as for example better detailed afore and wherein the two layers of unidirectional fibers have orientations rotated by 90° to each other, whereas the intermediate layer formed by a fabric with interwoven carbon fibers and oriented in two directions transverse to each other, preferably perpendicular, is rotated to the direction of the fibers of the two layers with unidirectional fibers by such an amount that the directions of the carbon fibers of the fabric with bidirectional fibers are rotated by 45° clockwise and anti -clockwise respectively to the carbon fibers with unidirectional orientation.

Referring to the balanced lamination conditions of several layers of carbon fibers according to any one of the afore described variations, laboratory experiments carried out by infrared thermography showed how a balanced laminate, as afore defined, behaves as good thermal conductor, comparable to a metal. The heat generated by the operating LEDs is effectively dissipated in the laminate plane to which they are integral, due to the high longitudinal thermal conductivity of the carbon fibers of the pitch type and to the balanced stratification of the same laminate. Thanks to the fiber orientation in the four main in-plan directions (0°, ±45°, 90°) , in fact, a steady thermal flow is established in the laminate in the eight corresponding directions (0, ±45°, ±90°, ±135°, 180°) , which flow disperses heat by conduction towards the side corners, wherein the convection exchange occurs with the surrounding air as depicted in fig. 2. This effect, as it has been experimentally verified, results in the surface distribution of the laminate temperature being fairly uniform and the maximum detected temperature being relatively low, even some degrees lower than an aluminium plate of equal sizes used as control. This corresponds to low thermal resistance of the heat sink, such as defined in the equation (1) . A low thermal resistance of the heat sink ensures that its temperature remains everywhere lower than the glass transition temperature of the resin, thus preventing the structural weakening of the thermally activated composite from occurring. Furthermore, a low thermal resistance of the heat sink ensures that the regime operating temperature of the junction devices remains within the limits recommended by the manufacturer, therefore maximizing the lifetime thereof. The afore described thermal dissipation performances can not be equally obtained by a not-balanced laminate stratification, for example by providing that the carbon fibers of the pitch type are oriented in only two in-plane directions (0° and 90°) . Such results have been confirmed by the thermo-structural FEM analysis of the system under study. The numerical simulations also demonstrated how a balanced laminate has better mechanical characteristics compared to a not-balanced laminate, allowing higher safety coefficients to be obtained compared to the operative stresses the component can experience (self weight and electronic component weight, wind strength, etc . ) .

On the laminate surface opposite to the mould surface the vacuum bag is arranged: the depression created inside the bag, thanks to a compressor suctioning the air, allows the fiber impregnation to be improved and the voids in the component to be reduced. The "bag side" finish, not perfectly smooth, is reserved for the component surface expected to not be exposed in the final application. Alternatively, a counter-die can be used, ensuring smooth finish on both the component surfaces, if required by the final application .

Thus, the component - inside the vacuum bag - is consolidated in oven or autoclave depending on the resin curing cycle.

Once the curing process has been completed, the raw component, as demoulded from the mould, is subjected to edge trimming according to a proper cutting mask. The carbon heat sink in its ultimate geometry is thus obtained. Such a processing flow is specified in fig. 4.

Alternatively to the addition of an UV absorbing filler to the resin, as afore mentioned, a post- treatment of the finished component can be provided, aiming to reduce the phenomena of the thermal resin degradation caused by the extended and uninterrupted exposure to atmospheric agents usually active in a possible outdoor application: ultraviolet radiation, humidity and temperature, also combined with each other. In this way the phenomena of erosion of the matrix constituting the composite, the subsequent exposure of the carbon fiber on the component surface and thus the possible release of the same fiber to the environment are avoided. For such a purpose the application on the finished component of a protective, for example polyurethane-based, coat providing the resin with durability over time, comparable to the lifetime of the LED lamp, against the effects of the environmental exposure, can be provided. Hereinafter the realization details related to a particular configuration of the present invention are depicted.

For the production of the heat sink made of composite material, a pure unidirectional prepreg is used, i.e. a layer of carbon fibers with unidirectional orientation and which are pre- impregnated with a thermosetting resin. The set of carbon fibers is free from the limiting warp made of glass or carbon fiber and is directly obtained by roving .

The reinforcement of the afore described prepreg material is composed of continuous carbon fibers of the pitch type having high thermal longitudinal conductivity, on the order of 200 W/ (m -K) or higher value. The prepreg material matrix is a two-component epoxy resin having high glass transition temperature (110÷130 °C) , purposely formulated for processes of low pressure curing (out-of-autoclave vacuum bag) . The resin is loaded, in convenient content, with a particle filler increasing the thermal conductivity thereof .

According to an embodiment, by way of example, the conductive filler consists of ceramic particles of tri-hydrated alumina, added to the resin at 50% by weight or higher value. In this case, the thermal matrix conductivity increases from values on the order of 0.2 W/ (m -K) or lower (pure resin) to values on the order of 1 W/ (m -K) or more (loaded resin) .

According to an additional characteristic, the resin further embeds a UV filter able to absorb the ultraviolet component of the solar radiation possibly hitting the component in normal operating conditions (outdoor applications) .

According to an embodiment, the unidirectional prepreg material based on carbon fiber of pitch type has matrix content on the order of 35÷40% by weight and a whole grammage on the order of 500÷600 g/mq or lower value.

As already described above, the prepreg material is laminated on an aluminium mould in the number of four layers according to a balanced superimposing sequence, such as to maximize the in-plane thermal conductivity of the heat sink.

Specifically, the first layer is arranged with the fibers oriented in 0° direction (arbitrarily defined) , the second layer is arranged with the fibers oriented in +45° direction; the third layer is arranged with the fibers oriented in -45° direction; the fourth layer is arranged with the fibers oriented in 90° direction. The lamination sequence of the four layers can be reversed.

The mould, together with the prepreg laminated thereon, is closed by a vacuum bag and the component is consolidated in oven according to the epoxy resin curing cycle, for example: 60 minutes at 120 °C, 90 minutes at 110°C or 120 minutes at 100 °C.

Figure 2 shows a schematic example wherein a heat sink 1 according to what above described is combined, for example by double-sided thermally conductive adhesive tape to an electronic circuit 3 comprising at least one light source of the LED type 301 mounted on a printed circuit board 302.

In figure 3 a side exploded view is shown wherein the planar heat sink 1, in the form of plate or the like, the layer of thermally conductive adhesive denoted with 2 and the electronic circuit 3 comprising the printed circuit board or header or the like 302 on which the LED 301 are mounted, can be seen. Note that such electronic circuits are known and therefore are not described in detail since the same are not object of the present invention. The boards can comprise the individual LEDs or also other electronic circuit components for a LED power supply and/or control circuit.

It has thus been shown that the planar heat sink made of high thermal conductivity and high strength composite material, described according to the present invention, obtains the predetermined objects and purposes, solving the problems of the known art.

A number of changes can be made by the person skilled in the art without departing from the protection scope of the present invention. The protection scope of the claims, therefore, should not be limited by the illustrations or the exemplary preferred embodiments shown in the description, but rather the claims must comprise any feature of patentable novelty inferable from the present invention, including all the features the field technician would consider as equivalents.

Claims

1) Heat sink made of composite material in the form of planar element characterized by a combination of unidirectional and thermally conductive reinforcing carbon fibers embedded in a polymer matrix composed of a thermosetting epoxy resin.

2) Heat sink according to claim 1, wherein the carbon fibers are of pitch type.

3) Heat sink according to claim 1 or 2, wherein the polymer matrix is composed of a thermosetting epoxy resin embedding a conductive filler that increases the thermal conductivity thereof.

4) Heat sink according to one or more of the preceding claims , wherein the polymer matrix contains a UV absorbing filler filtering the ultraviolet radiation hitting the component.

5) Heat sink according to one or more of the preceding claims, characterized in that it is composed of two or more, preferably at least four layers of pre-impregnated carbon fibers and laminated on each other, which four layers comprise each one a layer of unidirectionally oriented fibers within the same layer, said layers being arranged on each other according to a balanced superimposing sequence, that is according to which the orientation of the carbon fibers of the individual layers relative to the direction of a first layer of carbon fibers, whose orientation is defined as reference at 0°, is +45°/- 45°/90° .

6) Heat sink according to claim 5, characterized in that it is composed of :

a first layer with the carbon fibers all or at least mainly oriented in direction 0° ; a second layer with the carbon fibers all or at least mainly oriented in direction +45° ;

a third layer with the carbon fibers all or at least mainly oriented in direction -45° ; a fourth layer with the carbon fibers all or at least mainly oriented in direction 90° ;

said layers being superimposed on each other in a condition of combinations of carbon fibers unidirectionally oriented within a layer and pre- impregnated with a thermosetting resin matrix in partial hardening condition, that is curing condition, and said layers being attached to each other by being pressed one against the other and being consolidated by heating according to the curing cycle namely hardening or curing of the epoxy resin.

7) Heat sink according to one or more of preceding claims 1 to 5 characterized in that it has a sequence arranging the layers according to one of the following alternatives:

- six layers with 0° /90° /+45° /-45° /90° /0° sequence or

eight layers with

0°/90°/+45°/-45°/-45°/+45°/90°/0° sequence.

8) Process for manufacturing a heat sink made of composite material comprising the following steps: providing at least one layer of carbon fibers, preferably of pitch type, which fibers have all a single substantially equal direction within said layer, which fibers are pre-impregnated with a thermosetting resin which is in a partially hardened condition, that is partially cured condition, namely a so called prepreg layer; subjecting said layer to compression and completing the hardening process, that is the curing process of said layer of pre-impregnated carbon fibers, namely of said prepreg.

9) Process according to claim 8 wherein the following steps are provided:

providing two or more, preferably at least four layers of carbon fibers, preferably of pitch type, which fibers have all a single substantially equal direction within the corresponding layer, which fibers are pre-impregnated with a thermosetting resin which is in a partially hardened condition, that is partially cured condition, namely two or more, preferably four layers of the so called prepreg;

superimposing said two or more, preferably said four layers one on each other by turning them with respect to each other such that the fibers of each layer are oriented in a direction different than the fibers of the other layers;

preferably and optionally arranging the individual layers on each other with a relative orientation between the directions of the fibers of the different layers with respect to each other according to the following angles: 0° for the first layer, 45° clockwise to the first layer, for the second layer, 45° anti-clockwise to the first layer, for the third layer, 90° to the first layer, for the fourth layer;

subjecting said combination of superimposed layers to compression and consolidation by completing the hardening process, that is the curing process of the epoxy resin matrix. 10) Method according to claims 8 or 9, wherein the number of layers to be superimposed can be more than four and optionally according to one of the two alternatives :

- six layers with 0° /90° /+45° /-45° /90° /0° sequence or

eight layers with

0°/90^ο/+45^ο/-45^ο/-45^ο/+45^ο/90^ο/0° sequence.

11) Method according to one of claims 8 to 10, wherein at least one of the two outermost layers has an exposed face with smooth finishing by being compressed against the forming surface of a mould.

12) Combination of a heat sink made of composite material according to one or more of the preceding claims with an electronic circuit in the form of a board, which electronic circuit comprises at least one thermal energy generating element, particularly a light source of the LED type and wherein said electronic circuit, that is said board, is coupled by double-sided thermally conductive tape (2) to an exposed face of said planar heat sink.

13) LED lamp comprising a heat sink of the type according to one or more of claims 1 to 7 or a combination of heat sink and electronic circuit according to claim 12.