WO2009043849A1

WO2009043849A1 - Backlight module for liquid crystal display

Info

Publication number: WO2009043849A1
Application number: PCT/EP2008/063087
Authority: WO
Inventors: Abel Frans Pot; Van Hans Klaas Dijk; Robert Hendrik Catharina Janssen
Original assignee: Dsm Ip Assets B.V.
Priority date: 2007-10-01
Filing date: 2008-09-30
Publication date: 2009-04-09
Also published as: TW200924627A

Abstract

The invention relates to a backlight module for liquid crystal displays (LCDs), the backlight module comprising a light source and a heat sink with cooling fins, wherein the heat sink including the cooling fins is made from a thermoplastic material having a through-plane conductivity in the range of 1 to 10 W/mK and the cooling fins have height (H) and thickness (T) dimensions wherein the H/T ratio i\ s at least 3:1. The invention also relates to an LCD comprising the backlight module.

Description

BACKLIGHT MODULE FOR LIQUID CRYSTAL DISPLAY

The invention relates to a backlight module for liquid crystal displays (LCDs), the backlight module comprising a light source and a heat sink with cooling fins. The invention also relates to an LCD comprising the backlight module.

In general, liquid crystal displays can be classified into reflective LCD, transmissive LCD and transflective LCD according to their light source. A transmissive or transflective LCD mainly comprises a liquid crystal panel and a backlight module. The liquid crystal panel has a structure comprising a liquid crystal layer sandwiched between a pair of transparent substrates. The back light module illuminates the liquid crystal panel with a planar light source so that data can be clearly seen on the liquid crystal display screen.

However, apart from producing light, the light source inside the backlight module also produces heat. When the heat dissipates into the liquid crystal panel the display quality is affected. Furthermore because the transfer of heat to the liquid crystal panel from the light source is unlikely to be uniform, the liquid crystal layer within the liquid crystal panel is subjected to different degrees of heating. Aside from affecting the liquid crystal molecules inside the display, the non-uniform distribution of heat may also affect the switching of thin film transistors inside the liquid crystal display. Ultimately, the overall display quality of the liquid crystal panel deteriorates.

A backlight module for an LCD capable of limiting the amount of heat passing into the liquid crystal panel is known from US-2005/004141 1-A1. This known backlight module comprises a frame, a reflective plate, a light source, a caved transparent plate, a diffusion plate, and an optical film. The light source in the backlight module of US-2005/004141 1-A1 is a lamp tube, a light bulb, a light-emitting diode array or a fluorescent lamp and is positioned inside the frame. To limit the amount of heat generated by the light source passing into the liquid crystal panel and resulting in a non-uniform display, US-2005/0041411-A1 provides the backlight module with a caved transparent plate, positioned between the light source and the diffusion plate. The caved transparent plate is fabricated using a transparent material such as acrylic. The diffusion plate is set up over the caved transparent plate such that a gap is formed between the diffusion plate and the caved transparent plate. Through the gap thermal conduction is said to be retarded so that the amount of heat transferring to the liquid crystal panel is immensely reduced. In US2006/0227554A1 a light emitting assembly and backlight device employing the same are disclosed. The light emitting assembly comprisies a metal circuit board and a cooling module with cooling fins, both made of metal. For the cooling fins copper or aluminium or a combination alloy are mentioned.

Another backlight module is known from US-2006/0138951-A1. This known backlight module comprises a light emitting diode (LED) lighting system as the light source and can be used in LCDs to illuminate the information to be displayed. This backlight module is said to have a high LED packaging density to achieve a high light intensity. The LEDs are comprised in optical plates positioned approximately perpendicular to an optical panel. The LEDs emit light travelling approximately parallel to the optical panel, and being reflected by optical protrusions comprised by the optical panel. To further enhance the performance and increase the lifetime of the LED light source it is used together with a heat sink. The optical plate comprises a heat dissipation core plate and one electrical circuit layer connected to the heat dissipation core plate. The heat dissipation core plate can be made of a material chosen from dielectric material, electrical material and thermal conductor. No details are given about the heat sink apart from one of the figures showing schematically a heat sink, positioned with one side to the backside of an optical panel and with ribs supposedly being cooling protruding from the other side of the heat sink.

Recent developments include high power LEDs adopted in illumination systems and backlight units for large sized LCDs for monitors. Such high power LEDs give off a lot of heat and require superior thermal dissipation performance and good heat management capabilities of the entire display.

Another recent development is the use of LEDs in LCD systems in passenger cars. These LCDs used to be equipped with CCFL (cold cathode fluorescent lamp) backlighting systems which also produce heat but more delocalized and less concentrated, thus posing less heat problems than the LEDs and allowing the frames of the modules to be made of plastic. However, LEDs as light source for LCDs have some advantages over conventional CCFLs such as increasing the colour spectrum, no smearing effect, improvement of contrast, longer operating life, and less environmental hazard (free of mercury). Another important advantage of LEDs is a low power consumption per unit of light output. Unfortunately, the backlight modules for LCD systems in cars are confined in very small spaces with limited to no possibility for using forced cooling to avoid unwanted heating. This further enhances the problems with regard to heat management. In an early approach the plastic frame used in the former CFFT backlighting system was replaced by aluminium to improve the performance of the LED backlighting system. This resulted in the desired performance for the heat dissipation but also resulted in a strong increase in weight. Copper as another metal also shows a good performance, but it is even heavier and more expensive. Substituting a highly thermally conductive plastic material for the metal did not result in a proper heat dissipation performance. This corresponds with observations of others that in many situations thermally conductive plastic heat sinks cannot provide sufficient thermal conductivity to achieve the desired heat transfer capabilities.

The increasing trend of using LEDs as light source in backlight modules for LCD systems is ongoing. This also requires for a cost effective solution for the heat management problem.

The aim of the present invention is to provide a cost effective solution for the heat management problem in LCD systems resulting in a satisfying heat dissipation performance without substantially increasing the weight of the LCD system. This aim has been achieved with a backlight module for liquid crystal displays (LCDs), the backlight module comprising at least one light source and a heat sink including cooling fins as an integral part, characterized in that the heat sink including the cooling fins is made from a thermoplastic material having a through-plane conductivity in the range of 1 to 10 VWmK and the cooling fins have height (H) and thickness (T) dimensions wherein the H/T ratio is at least 3:1. The present invention also relates to a liquid crystal display (LCD) comprising a liquid crystal panel and said backlight module.

The present invention is further directed to different uses of said LCD. The backlight module of the present invention uniquely combines high heat dissipating effect, low weight, and good mechanical strength. Surprisingly, a sufficiently high heat dissipating effect can be achieved without the need to use materials having a higher heat conductivity. The backlight module according to the present invention is superior to backlight modules comprising a heat sink with such slender cooling fins but made of a thermoplastic material having a higher heat conductivity as those backlight modules suffer from higher weight values and defectively moulded and/or mechanically poor cooling fins. Enhancing the thickness of the cooling fins thereby reducing the H/T ratio leads to a further increase in weight whereas the heat dissipation does not increase to the same extent. Thus making the heat sinks made of the materials having a higher heat conductivity still performing less good than the heat sinks in the backlight module according to the invention made of the thermoplastic material having only a moderate heat conductivity. - A -

The essential components of the backlight module according to the present invention are the at least one light source and the heat sink comprising the cooling fins. The light source(s) is (are) in direct or indirect thermal contact with the heat sink to allow transfer of the heat from the light source(s) to the heat sink. Typically, the backlight module comprises a housing to accommodate the light source(s) and further optional components. Preferably, the heat sink constitutes an integral part of the housing. More preferably, the heat sink constitutes the housing i.e. in addition to its heat dissipating effect, the heat sink is also effective as a housing.

In a preferred embodiment of the present invention the backlight module further comprises a reflective plate. Typically, the reflective plate is set up at a bottom section of the housing. It mainly serves as a reflective surface for deflecting light heading away from the liquid crystal panel so that utilization efficiency of the light source(s) is increased.

The light source may be a lamp tube, a light bulb, a light-emitting diode array or a fluorescent lamp, for example a straight tube, a U-shaped tube or a flat fluorescent light source. Due to several advantages of LEDs mentioned above, the light source of the backlight module according to the present invention preferably comprises or is constituted of a plurality of LEDs, typically mounted on a mother board or printed circuit board. The board suitably comprises a metal body or metal core. The metal core in the metal core printed circuit board (MC-PCB) advantageously acts as heat conductive member transferring heat from the LEDs to the heat sink.

Depending on the location of the light source(s) the backlight module of the present invention may further comprise a light guide for spreading the emitted light in order to evenly illuminate the liquid crystal panel. Optionally, the backlight module may comprise a diffusion plate, also known as diffuser sheet, which is typically located at an upper section of the housing and oriented towards the liquid crystal panel. The diffusion plate is usually a thin layer of a transparent polymer such as an acrylic resin or polycarbonate. It may be coated by one or more optical films. When light directly emitted from the light source and/or light guide and light after being reflected pass through the diffusion plate, the diffusion plate diffuses the light uniformly to form a planar light.

The cooling fins are an integral part of the heat sink and promote the heat dissipation. Other than cooling fins, the heat sink comprises a main body which constitutes a base from which the cooling fins protrude. The height H of the fins is measured from the main body of the heat sink. The heat sink may also comprise further elements. Such elements may be structural elements, such as ribs, which might be used, for example, to increase the mechanical strength or to prevent warpage of the heat sink when heated. The dimensions of the cooling fins are an essential feature of the present invention. The height to thickness ratio (H/T ratio) must be at least 3:1 to ensure a sufficient heat dissipating effect. The term "thickness" means the average thickness of a single fin in case the thickness is non-uniform, e.g. for tapered shapes. It is understood that the H/T ratio need not be the same for all single fins, i.e. the H/T ratio may differ from fin to fin. It is not detrimental to the present invention if, in addition to the fins satisfying the H/T requirement, the heat sink comprises additional fins having a H/T ratio outside the specified range. Of course, those additional fins have a minor contribution to the heat dissipating effect. However, the presence of those additional fins is not excluded by the wording of the claims.

The "thickness" of the fin denotes the smallest dimension of the fin. The "height" of the fin denotes the dimension of the fin measured from the distal end of the fin to the surface of the main body of the heat sink. The "length" of the fin denotes the third dimension of the fin.

Although it is mostly true that a higher H/T ratio of the cooling fins improve the heat dissipating effect, practical considerations may limit the H/T ratio. Fins having excessive heights do not provide a further improvement of the heat dissipation effect; the present material is not sufficiently thermally conductive for very high fins to suck up the heat all the way to the top. From the viewpoint of mechanical stability and ease of manufacturing the preferred H/T ratio of the cooling fins is in the range of 3:1 to 10:1 ; more preferably the H/T ratio is in the range of 5:1 to 8:1.

For reasons concerning production technology and mechanical stability the minimum thickness of the cooling fins is preferably 0.2 mm, more preferably 0.3 mm, and even more preferably 0.5 mm. Most preferably, the thickness of the cooling fins is about 1 mm.

Depending on the intended use of the backlight module and the complete LCD, respectively, the maximum height of the cooling fins is preferably 20 mm. Typically, the minimum length of the cooling fins is 10 mm.

Limiting the absolute dimensions allows a high number of cooling fins per unit surface area (dense packing of the fins) and accordingly, the cooling surface area per unit weight of the heat sink can be high, while the overall dimensions of the heat sink can be kept limited. This is in particular favourable for applications where there is limited space for positioning the backlight module, such as for backlight modules for build-in LCD navigation systems and car infotainment systems in dashboards of passenger cars. In order to maximize the heat dissipating effect it is preferred to pack the fins as close as possible; however, a certain minimum distance between the fins is determined by a value below which the heat convection may be disturbed and the surface area of the fins may be no longer accessible to flowing cooling air.

The cooling fins are typically protruding from the outer surface of the main body of the heat sink. They need not be uniformly distributed over the whole heat sink. It rather may be advantageous to design the heat sink of the present backlight module in a way that the cooling fins are primarily located where they are most effective, i.e. near to the light source(s) emitting the heat to be dissipated.

The position, number and actual dimensions (including thickness, height, and length) of the cooling fins of the heat sink can be determined by the person skilled in the art of making heat dissipating parts based on experience and by routine testing. Fine tuning of these variables is amongst others governed by the desired heat dissipating effect and the production method and material of the heat sink.

The heat sink of the backlight module according to the present invention may be prepared by any known method suitable for processing thermoplastic materials. Preferably, the heat sink is prepared by injection moulding as this production method is especially suited to form the slender cooling fins.

In order to improve the injection moldability of the thermoplastic material used to prepare the heat sink, the thermoplastic material should have good flow properties. Preferably, the thermoplastic material has a spiral flow length of at least 60 mm, more preferably at least 80 mm, and most preferably at least 100 mm. The spiral flow length is determined by injecting the molten thermoplastic material into a long spiral-channel cavity having dimensions 280 x 15 x 1 mm and the length of the resulting flow for that material is its spiral flow length. The material is injected by using a 22 mm Engel 45B L/d = 19 injection moulding machine having a theoretical shot volume of 38 cm³; under the conditions of a cylinder temperature being 10⁰C above the melting point of the main polymer component, a mould temperature of 120⁰C and an injection pressure of 100 MPa.

Depending on the intended use of the backlight module a certain mechanical performance of the backlight module, especially of the heat sink comprising the slender cooling fins, may be desired. Typically, the heat sink is made from a thermoplastic material having a tensile strength of at least 40 MPa, preferably at least 60 MPa and more preferably at least 70 MPa. Typically, the heat sink is made from a thermoplastic material having an elongation at break of at least 0.5 %, preferably at least 1.0 %, more preferably at least 1.5 %, and most preferably at least 1.8 %. Typically, the heat sink is made from a thermoplastic material having a Young's modulus of at least 6,000 MPa, more preferably at least 9,000 MPa. Tensile modulus, tensile strength and elongation at break are determined at 23°C and 5 mm/min according to ISO 527; the dried granulate of the thermoplastic material to be tested was injection moulded to form the test bars for the tensile tests having a thickness of 4 mm conforming to ISO 527 type 1A. The thermoplastic material of the heat sink has a through-plane thermal conductivity in the range of 1 to 10 W/mk, preferably 1 to 5 VWmK, more preferably 1 to 4 W/mK, and most preferably 2 to 3 VWmK. Herein, the thermal conductivity is derived from the thermal diffusivity (D) measured by laser flash technology according to ASTM E1461-01 on injection moulded samples of 80x80x2 mm in through plane direction, the bulk density (p) and the specific heat (Cp), at 20⁰C, using the method described in Polymer Testing (2005, 628-634).

The thermal conductivity of a thermoplastic material is herein understood to be a material property, which can be orientation dependent and which also depends on the history of the composition. For determining the thermal conductivity of a thermoplastic material, that material has to be shaped into a shape suitable for performing thermal conductivity measurements. Depending on the composition of the thermoplastic material, the type of shape used for the measurements, the shaping process as well as the conditions applied in the shaping process, the plastic composition may show an isotropic thermal conductivity or an anisotropic, i.e. orientation dependent thermal conductivity. In case the thermoplastic material is shaped into a flat rectangular shape, the orientation dependent thermal conductivity can generally be described with three parameters: A₁, Au and Λ₊. A₁ is the through-plane thermal conductivity, A,, is the in-plane thermal conductivity in the direction of maximum in-plane thermal conductivity and Λ+ is the in-plane thermal conductivity in the direction of minimum in-plane thermal conductivity. It is noted that the through-plane thermal conductivity is indicated elsewhere also as "transversal" thermal conductivity.

The number of parameters can be reduced to two or even to one depending on whether the thermal conductivity is anisotropic in only one of the three directions or even isotropic. In case of a thermoplastic material with a dominant unidirectional orientation of thermal conductive fibres in one orientation, A// can be much higher than A₁, whereas A₁ might be very close or even equal to A₁. In case of a thermoplastic material with a dominant parallel orientation of plate-like particles in plane with the planar orientation of the plate, the plastic composition may show an isotropic in-plane thermal conductivity, i.e. A,, is equal to A₁. In case of a thermoplastic material with an overall isotropic thermal conductivity, A₁, A// and A₁ are all equal and identical to the isotropic thermal conductivity Λ.

For measurement Of A₁ samples with dimensions of 80 x 80 x 2 mm were prepared from the material to be tested by injection moulding using an injection moulding machine equipped with a square mould with the proper dimensions and a film gate of 80 mm wide and 1 mm high positioned at one side of the square. Of the 1 mm thick injection molded plaques the thermal diffusivity D, the density (p) and the heat capacity (Cp) were determined. The thermal diffusivity was determined through plane (D₁) according to ASTM E1461-01 with Netzsch LFA 447 laserflash equipment.

The heat capacity (Cp) of the plates was determined by comparison to a reference sample with a known heat capacity (Pyroceram 9606), using the same Netzsch LFA 447 laserflash equipment and employing the procedure described by W. Nunes dos Santos, P. Mummery and A. Wallwork, Polymer Testing 14 (2005), 628- 634. From the thermal diffusivity (D₁), the density (p) and the heat capacity (Cp), the thermal conductivity of the moulded plaques was determined in a direction perpendicular to the plane of the plaques (A₁) according to following formula:

A₁ = D₁ ^* p ^* Cp.

Typically, the thermoplastic material of the heat sink comprises a thermoplastic polymer and a thermally conductive filler.

The thermoplastic polymer suitably is an amorphous, a semi- crystalline or a liquid crystalline polymer, an elastomer, or a combination thereof. Liquid crystal polymers are preferred due to their highly crystalline nature and ability to provide a good matrix for the filler material. Examples of liquid crystalline polymers include thermoplastic aromatic polyesters.

Suitable thermoplastic polymers that can be used in the matrix are, for example, polyethylene, polypropylene, acrylics, acrylonitriles, vinyls, polycarbonates, polyesters, polyamides, polyphenylene sulphides, polyphenylene oxides, polysulfones, polyarylates, polyimides, polyetheretherketones, and polyetherimides, and mixtures and copolymers thereof.

Suitable thermoplastic elastomers include, for example, styrene- butadiene copolymer, polychloroprene, nitrite rubber, butyl rubber, polysulfide rubber, ethylene-propylene terpolymers, polysiloxanes (silicones), and polyurethanes.

Preferably, the thermoplastic polymer is a chosen from the group consisting of polyesters, polyamides, polyphenylene sulphides, polyphenylene oxides, polysulfones, polyarylates, polyimides, polyetheretherketones, and polyetherimides, and mixtures and copolymers thereof.

Suitable polyamides include both amorphous and semi-crystalline polyamides. Suitable polyamides are all the polyamides known to a person skilled in the art, comprising semi-crystalline and amorphous polyamides that are melt- processable. Examples of suitable polyamides according to the invention are aliphatic polyamides, for example PA-6, PA-11 , PA-12, PA-4,6, PA-4,8, PA-4,10, PA-4,12, PA- 6,6, PA-6,9, PA-6, 10, PA-6, 12, PA-10,10, PA-12, 12, PA-6/6,6-copolyamide, PA-6/12- copolyamide, PA-6/11-copolyamide, PA-6,6/11-copolyamide, PA-6, 6/12-copolyamide, PA-6/6,10-copolyamide, PA-6,6/6,10-copolyamide, PA-4,6/6-copolyamide, PA- 6/6,6/6,10-terpolyamide, and copolyamides obtained from 1 ,4-cyclohexanedicarboxylic acid and 2,2,4- and 2,4,4-trimethylhexamethylenediamine, aromatic polyamides, for example PA-6,1, PA-6, 1/6, 6-copolyamide, PA-6, T, PA-6,T/6-copolyamide, PA-6,T/6,6- copolyamide, PA-6,l/6,T-copolyamide, PA-6,6/6,T/6,l-copolyamide, PA-6,T/2-MPMDT- copolyamide (2-MPMDT = 2-methylpentamethylene diamine), PA-9,T, copolyamides obtained from terephthalic acid, 2,2,4- and 2,4,4-trimethylhexamethylenediamine, copolyamide obtained from isophthalic acid, laurinlactam and 3,5-dimethyl-4,4-diamino- dicyclohexylmethane, copolyamides obtained from isophthalic acid, azelaic acid and/or sebacic acid and 4,4-diaminodicyclohexylmethane, copolyamides obtained from caprolactam, isophthalic acid and/or terephthalic acid and 4,4-diaminodicyclohexylmethane, copolyamides obtained from caprolactam, isophthalic acid and/or terephthalic acid and isophoronediamine, copolyamides obtained from isophthalic acid and/or terephthalic acid and/or other aromatic or aliphatic dicarboxylic acids, optionally alkyl- substituted hexamethylenediamine and alkyl-substituted 4,4- diaminodicyclohexylamine, and also copolyamides and mixtures of the aforementioned polyamides. More preferably, the thermoplastic polymer comprises a semi-crystalline polyamide. Semi-crystalline polyamides have the advantage of having good thermal properties and mould filling characteristics. Also still more preferably, the thermoplastic polymer comprises a semi-crystalline polyamide with a melting point of at least 200⁰C, more preferably at least 220⁰C, 240°C, or even 260 ⁰C and most preferably at least 280⁰C. Semi-crystalline polyamides with a higher melting point have the advantage that the thermal properties are further improved. With the term melting point is herein understood the temperature measured by DSC with a heating rate of 5°C falling in the melting range and showing the highest melting rate. Preferably a semi-crystalline polyamide is chosen from the group comprising PA-6, PA-6,6, PA-6,10, PA- 4,6, PA-11 , PA-12, PA-12,12, PA-6, 1, PA-6,T, PA-6,T/6,6-copolyamide, PA-6,T/6-copolyamide, PA-6/6,6-copolyamide, PA-6,6/6,T/6,l-copolyamide, PA-6,T/2-MPMDT- copolyamide, PA-9,T, PA-4,6/6-copolyamide and mixtures and copolyamides of the aforementioned polyamides. More preferably PA-6,1, PA-6,T, PA-6,6, PA-6,6/6T, PA-6,6/6,T/6,l-copolyamide, PA-6,T/2-MPMDT- copolyamide, PA-9,T or PA-4,6, or a mixture or copolyamide thereof, is chosen as the polyamide. Still more preferably, the semi-crystalline polyamide comprises PA-4,6.

A "thermally conductive filler" within the meaning of the present invention includes any material that can be dispersed in the thermoplastic polymer and that improves the heat conductivity of the thermoplastic composition. The thermally conductive filler material has an intrinsic thermal conductivity of at least 5 VWmK, preferably at least 10 VWmK. Non-limiting examples of thermally conductive filler include boron nitride, silicon nitride, aluminium nitride, alumina, calcium oxide, titanium oxide, zinc oxide, carbon materials such as graphite, expanded graphite, carbon black, carbon fibres e.g. pitch and pan based carbon fibres; ceramic fibres and metals such as aluminium, copper, magnesium, brass. The filler particles may have any shape, for example they may be in the form of spheres, ellipsoids, platelets, strands, polyhedra, and fibres including whiskers. Mixtures of different filler materials and shapes may also be used. The preferred thermally conductive filler is boron nitride and/or graphite.

Depending amongst others on the heat conductivity of the selected thermally conductive filler material itself and the desired physical properties of the final thermoplastic material, the thermoplastic material typically comprises 5 to 60 % by weight, preferably 15 to 45 % by weight of the thermally conductive filler, based on the total weight of the thermoplastic material including filler and further optional components.

The thermoplastic material from which the heat sink of present backlight module is made, may also comprise, next to the thermoplastic polymer and the thermally conductive material, also other components, denoted herein as additives. As additives, the thermoplastic material may comprise any auxiliary additives, known to a person skilled in the art that are customarily used in polymer compositions. Preferably, these other additives should not detract, or not in a significant extent, from the invention. Whether an additive is suitable for use in a heat sink according to the invention can be determined by the person skilled in the art of making polymer compositions for heat sinks by routine experiments and simple tests. Such other additives include, in particular, other fillers not considered thermally conductive such as non-conductive reinforcing fillers; pigments; dispersing aids; processing aids, for example lubricants and mould release agents; impact modifiers; plasticizers; crystallization accelerating agents; nucleating agents; UV stabilizers; antioxidants; and heat stabilizers. In particular, the non-conductive fillers include non-conductive inorganic fillers. Suitable for use as a non-conductive inorganic fillers are all the fillers, such as reinforcing and extending fillers, known to a person skilled in the art, for example asbestos, mica, clay, calcinated clay, talcum, silicates such as wollastonite, and silicon dioxide, especially glass fibres. In this context, "not thermally conductive" or "non-conductive" is used to describe filler materials having an intrinsic thermal conductivity of less than 5 VWmK in order to differentiate these materials from those thermally conductive filler materials described in the paragraph above. It is understood that various of those thermally conductive filler materials also have some reinforcing effect. However, their thermal conductivity classifies them as thermally conductive fillers within the meaning of the present invention.

In one preferred embodiment the thermoplastic material of the heat sink comprises glass fibres, typically 5 to 20 % by weight of glass fibres, based on the total weight of the thermoplastic material including filler and further optional components.

The present invention is further directed to a liquid crystal display (LCD) comprising a liquid crystal panel and a backlight module as described above.

The liquid crystal panel can be any known liquid crystal display panel, e.g. a common active matrix liquid crystal display typically comprising a thin film transistor (TFT) array, a colour filtering substrate and a liquid crystal layer. The liquid crystal panel may optionally be covered by or comprise one or more optical films. The design and type of the liquid crystal panel is not critical for the present invention.

Of course, both the backlight module and the LCD may comprise further optional elements not explicitly mentioned, such as support elements and mounting means. The LCD according to the present invention may be used in, for example, flat-panel TVs, computer monitors, notebooks, portable information terminals, personal digital assistants (PDAs), mobile phones, clocks and watches, digital video cameras, digital photographic cameras, and LCD systems in dashboards of passenger cars such as navigation systems and infotainment systems.

The present inventors especially succeeded in solving the heat dissipation performance of a backlighting system wherein the CCFL backlighting system was replaced by an the LED based backlighting system. The former approach to replace the plastic material of the original heat sink by aluminium solved the heating problem but also led to an enormous increase in weight. The original heat sink without any cooling fins could properly be made using a highly thermally conductive plastic material, however, the heat dissipation performance was insufficient to prevent unacceptable heating of the liquid crystal panel. Modifying the frame with cooling fins with height and thickness dimensions according to the invention and using a moderately thermally conductive material according to the invention solved the heating problem. Using a highly thermally conductive plastic material in the modified heat sink would result in defectively moulded and/or mechanically poor cooling fins.

The design of the backlight module according to the present invention is illustrated by a non-limiting embodiment depicted in Fig. 1. Fig. 1 is schematic cross-sectional view of a LCD according to the present invention comprising a liquid crystal panel 6 and a backlight module comprised of a heat sink (housing) 1 , cooling fins 2, a light guide 3, an LED array 4 mounted on a printed circuit board (PCB) 8, and a reflective plate 5. The liquid crystal panel 6 is covered by a stack of optical films 7.

Claims

1. A backlight module for liquid crystal displays (LCDs), the backlight module comprising - at least one light source

- and a heat sink including cooling fins as an integral part, characterized in that the heat sink including the cooling fins is made from a thermoplastic material having a through-plane conductivity in the range of 1 to 10 W/mK and the cooling fins have height (H) and thickness (T) dimensions wherein the H/T ratio is at least 3:1 , and wherein the thermal conductivity is derived from the thermal diffusivity (D) measured by laser flash technology according to ASTM E1461-01 on injection moulded samples of 80x80x2 mm in through plane direction, the bulk density (p) and the specific heat (Cp), at 20⁰C, using the method described in Polymer Testing (2005, 628-634).

2. The backlight module according to claim 1 , wherein the light source comprises a plurality of light emitting diodes (LEDs) mounted on a printed circuit board, preferably a metal core printed circuit board (MC-PCB).

3. The backlight module according to claim 1 or 2, further comprising a light guide.

4. The backlight module according to any of claims 1-3, wherein the H/T ratio of the cooling fins is in the range of 3:1 to 10:1 , preferably 5:1 to 8:1.

5. The backlight module according to any of claims 1-4, wherein the thickness of the cooling fins is at least 0.2 mm and/or wherein the height of the cooling fins is at most 20 mm.

6. The backlight module according to any of claims 1-5, wherein the heat sink is prepared by injection moulding.

7. The backlight module according to any of claims 1-6, wherein the thermoplastic material has a spiral flow length of at least 60 mm and/or wherein the thermoplastic material has a tensile strength of at least 40 MPa, an elongation at break of at least 0.5 %, and a Young's modulus of at least

6,000 MPa.

8. The backlight module according to any of claims 1-7, wherein the through- plane conductivity of the thermoplastic material is in the range of 1 to 5 VWmK, preferably 2 to 3 W/mK.

9. The backlight module according to any of claims 1-8, wherein the thermoplastic material comprises a thermoplastic polymer and a thermally conductive filler, and optionally glass fibres.

10. The backlight module according to claim 9, wherein thermoplastic material comprises 5 to 20 % by weight of glass fibres, based on the total weight of the thermoplastic material.

11. A liquid crystal display (LCD) comprising a liquid crystal panel and a backlight module according to any of claims 1-10.

12. Use of the liquid crystal display (LCD) according to claim 1 1 in flat-panel TVs, computer monitors, notebooks, portable information terminals, personal digital assistants (PDAs), mobile phones, clocks and watches, digital video cameras, digital photographic cameras, and LCD systems in dashboards of passenger cars.