Abstract:
A display tile having a frame whose compliance is increased in a well determined region to accommodate the different thermal expansions of the frame and one or more substrates to which the frame is fastened. The display tile has also regions of increased compliance that will deform to allow different parts of the frame to be brought into contact and glued to the lateral sides of two superimposed substrates when at least one lateral side of one of the substrates is not coplanar with the corresponding lateral side of the other substrate.

Description:
[0001]    The present invention relates to a frame for a display or display tile, a display or display tile itself, or a method of constructing such a frame for a display or display tile. 
       BACKGROUND 
       [0002]      FIG. 1  shows a schematic representation of a tiled display. 
         [0003]    The display tiles T can be an emissive display apparatus like e.g. a Liquid Crystal Display apparatus or it can be a rear projection display apparatus. 
         [0004]      FIG. 1  also shows schematically how the picture elements or pixels P are arranged on the display tile T. Three sub-pixels green, red and blue lie close to one another and produce in their totality a picture element of the image displayed. The picture elements follow one another with a spacing A. The broken line indicates the inner area of the display tile T on which an image can be formed and viewed by a viewer. The space between the broken line and the edges of a display tile T is a non-display area. The non-display area often corresponds to a space occupied by a bezel B whose primary function is to hold the LCD panel of the LCD in position. In the prior art the spacing between two adjacent picture elements P displayed onto adjacent display tiles T is largely determined by the distance separating two adjacent display tiles T and the width of the bezel B. The space between the display areas of two adjacent display tiles is often referred to as the seam (SEAM). The minimum distance between two adjacent display tiles T depends on the clearance needed to allow thermal and humidity expansion of the display tiles T without misalignment, bowing or damaging of the Liquid Crystal panel (or projection screen) of the display tile T. 
         [0005]    The problem of thermal and humidity expansion and the relative movement of tiles with respect to each other and its impact on the inter-tile seams is well known in the art too. 
         [0006]    The width of the bezel can be decreased or the bezel can be eliminated as e.g. proposed in U.S. Pat. No. 8,446,540 “Display device having an enlarged display area”. 
         [0007]    The problem is that the solution may not be reliable for large liquid crystal panels (e.g. with more than one meter of diagonal) as the weight of the panel could overcome the forces exerted by the “fixing member” and “panel guide portion”. 
         [0008]    Another problem is that the panel guide portion may not be in metal to avoid scratching the liquid crystal panel. Therefore, it needs to be thick enough to withstand the forces applied on it. The seam will thus be at least twice as wide as the panel guide portion will be thick. 
         [0009]    The author of U.S. Pat. No. 8,446,540 also excludes the use of adhesive tape to fasten the liquid crystal panel to a frame because it is incompatible with reliability and durability of the assembly. 
         [0010]    The reliability and durability could be improved if the adhesive tape were thick enough. 
         [0011]    For instance, with values of α-aluminum=23.8·10 −6  1/K and α-glass=8.1·10 −6  1/K (α being the coefficient of thermal expansion) and a total bond length at room temperature of L0=1500 mm for a change in temperature of ΔT=40 K a difference in length at both ends of ΔL=ΔT·L0/2·(α-Al−α-Glass)=0,471 mm will occur in a stress-free assembly (i.e. when aluminum and metal are not bonded together). A pressure sensitive mounting tape with a thickness of 0.8 mm should be suitable to compensate Δα-induced stress in this configuration. 
         [0012]    But such a thickness of pressure sensitive mounting tape is not compatible with a seam of less than 1 mm. 
         [0013]    The art also does not provide solutions for reliably fastening a liquid crystal panel affected by tolerances (e.g. a mismatch between the lateral dimensions of the glass panel delimiting the liquid crystal layer) while at the same time avoiding the use of a bezel or rim. 
         [0014]    There is a need to improve the art. 
       SUMMARY OF THE INVENTION 
       [0015]    It is an object of the present invention to provide a frame for a display or display tile, a display or display tile itself, or a method of constructing such a frame for a display or display tile. 
         [0016]    A frame for a display or display tile, a display or display tile itself, or a method of constructing such a frame for a display or display tile according to embodiments of the present invention may overcome one or more of the disadvantages mentioned above. 
         [0017]    According to an aspect of the present invention, a display tile comprises a frame made of a first material with a first coefficient of thermal expansion fastened to a substrate made of a second material with a second coefficient of thermal expansion by means of a heat activated adhesive tape; the frame being characterized in that it is divided into three parts: a first part to be glued to a lateral side of the substrate, a third part to serve as fastening interface to another structure like e.g. a backlight element or a support structure and a second part positioned between the first and third part to isolate the third part from strain in the first part e.g. as strain relief. 
         [0018]    Isolating the first part of the frame that is heated when the adhesive tape is activated from the third part of the frame that remains at a lower temperature than that of the first part prevents excessive deformation of the substrate once fastening of the frame to the substrate is complete. 
         [0019]    In particular, the first coefficient of thermal expansion is preferably higher than the second coefficient of thermal expansion. 
         [0020]    In particular, the first material is preferably a metal with a coefficient of thermal expansion larger or equal to 23.8 10-6 1/K. The metal can be aluminum, for example. 
         [0021]    In particular the second material is preferably glass with a coefficient of thermal expansion. The glass can be, for example, a borosilicate glass with a coefficient of thermal expansion lower or equal to 9.5 10 −6  1/K. 
         [0022]    In a further aspect of the invention the frame is divided into two or more frame elements or frame strips. Each frame strip has a first part to be glued to a lateral side of the substrate, a third part to serve as fastening interface to another structure like e.g. a backlight element or a support structure and a second part positioned between the first and third part to isolate the third part from strain in the first part, e.g. to operate as strain relief. 
         [0023]    Isolating the third part of the frame strip from strain in the first part of the frame strip can be assured by one or more openings in the frame strip. The opening extends across the first and second part of the frame strip away from the substrate. 
         [0024]    The openings can for instance be rectangular. The corners of the rectangle can be rounded. 
         [0025]    In another aspect of the invention, the third part of the (frame) strip is preferably isolated from strain in the first part of the frame strip by grooves formed, for example by etching or stamping in the frame part or by any equivalent method. 
         [0026]    The grooves can be formed on both sides of the frame strip. The depth of the grooves is preferably more than half the thickness of the frame strip in the first and second part of the frame strip. 
         [0027]    In a further aspect of the invention, a display tile comprises a frame fastened to a first substrate and a second substrate by means of an adhesive tape; the frame being characterized in that it is divided into 3 parts: a first part to be glued by means of the adhesive tape (e.g. a heat activated adhesive tape) to a lateral side of the first substrate, a third part to be glued by means of the adhesive tape (e.g. a heat activated adhesive tape) to a lateral side of the second substrate and a second part positioned between the first and third part, the second part having higher compliance than the first and third part thereby allowing the first and third part to be positioned in different planes. 
         [0028]    In another aspect a method of fabricating a frame for a display tile is provided, the frame being made of a first material with a first coefficient of thermal expansion, the method comprising: 
         [0000]    fastening the frame to a substrate made of a second material with a second coefficient of thermal expansion by means of a heat activated adhesive tape;
 
fabricating the frame in three parts:
 
gluing a first part by means of the heat activated adhesive tape to a lateral side of the substrate, fastening a third part to another structure and positioning a second part between the first and third part to isolate the third part from strain in the first part.
 
         [0029]    In another aspect a method of fabricating a frame for a display tile is provided, the method comprising:
       fastening the frame to a first substrate and a second substrate by means of an adhesive tape;
 
fabricating the frame in 3 parts:
 
gluing a first part by means of a heat activated adhesive tape to a lateral side of the first substrate,
 
gluing a third part by means of the heat activated adhesive tape to a lateral side of the second substrate, and
 
positioning a second part between the first and third part, the second part having a higher compliance than the first and third part thereby allowing the first and third part to be positioned in different planes.
       
 
         [0031]    It is an advantage of that aspect of the invention that it allows a frame to be glued to a first substrate and a second substrate, the first substrate being parallel to the second substrates even when at least one lateral side of the first substrate is not coplanar with the corresponding lateral side of the second substrate. 
         [0032]    In a further aspect of the invention grooves are formed in the second part of the frame; the grooves being substantially parallel to the first and second substrates. 
         [0033]    The grooves will increase the compliance of the frame in the desired direction and will allow different parts of the frame to be in different planes. 
         [0034]    In a further aspect of the invention the grooves are formed on both sides of the frame. 
         [0035]    In a further aspect of the invention the depth of the grooves is at least 50% of the thickness of the frame. 
         [0036]    In another aspect the invention proposes a frame for a display tile, the frame being made of a first material with a first coefficient of thermal expansion and being fastened to a substrate made of a second material with a second coefficient of thermal expansion by means of a heat activated adhesive tape; the frame being divided into three parts: a first part to be glued by means of the heat activated adhesive tape to a lateral side of the substrate, a third part to serve as fastening interface to another structure and a second part positioned between the first and third part to isolate the third part from strain in the first part. In a further aspect at least one opening extends from an edge of the frame and extends across the first part and second part of the frame. In a further aspect grooves are formed in the first part and/or the second part of the frame. The grooves may be formed on both sides of the frame. The depth of the grooves may be at least 50% of the thickness (x) of the frame. In a further aspect the frame is split into two or more frame elements or strips. Any or every feature relating to the frame can relate to one or more of the frame elements or strips. According to a further aspect said another structure is a backlight element or a support structure. 
         [0037]    In another aspect the invention proposes a frame for a display tile, the frame being fastened to a first substrate and a second substrate by means of an adhesive tape; the frame being divided into parts: a first part to be glued by means of the adhesive tape to a lateral side of the first substrate, a third part to be glued by means of the adhesive tape to a lateral side of the second substrate and a second part positioned between the first and third part, the second part having a higher compliance than the first and third part thereby allowing the first and third part to be positioned in different planes. Grooves may be formed in the second part of the frame, the grooves being substantially parallel to the first and second substrates. The grooves may be formed on both sides of the frame. The depth of the grooves s at least 50% of the thickness (x) of the frame. 
         [0038]    In another aspect the invention proposes a display tile comprising a frame as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0039]      FIG. 1  shows a schematic representation of a tiled display. 
           [0040]      FIG. 2A  shows an exploded view of a display tile, its frame and a backlight structure. 
           [0041]      FIG. 2B  shows an example of profile for a non-planar frame element. 
           [0042]      FIGS. 3A, 3B and 3C  show how the substrate and the frame are fastened together by means of a heat activated adhesive tape in accordance with an embodiment of the present invention. 
           [0043]      FIGS. 4A and 4B  show the deformation of the frame and the substrate during and after heat activation of the adhesive tape of an embodiment of the present invention. 
           [0044]      FIG. 5  shows a first adaptation of the frame to prevent deformation of the substrate caused by strain in the region of the frame that is glued to the substrate in accordance with an embodiment of the present invention. 
           [0045]      FIG. 6  shows a second adaptation of the frame in accordance with an embodiment of the present invention. 
           [0046]      FIGS. 7A and 7B  illustrate the deformations of the second part of the frame. 
           [0047]      FIG. 8  illustrates how first and second adaptation of the frame can be combined in accordance with an embodiment of the present invention. 
           [0048]      FIGS. 9A  to D illustrate a first and second substrate fastened to a frame in accordance with an embodiment of the present invention. 
           [0049]      FIG. 10  illustrates the problem to fasten first and second substrate to a frame when the first and second substrates have different lateral dimensions and/or are misaligned. 
           [0050]      FIGS. 11 and 11A  show an adaptation of the frame to allow deformation (i.e. compliance) of the frames and to allow the frame to be glued to first and second substrate even when at least one lateral side of one of the substrate is not coplanar with the corresponding lateral side of the other substrate in accordance with an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0051]    The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. 
         [0052]    Embodiments of the present relate to a design of a display tile T that has one, or some or all of the following advantages:
       keeps its form and flatness,   has only negligible thermal expansion regardless of its environment (high humidity, temperature fluctuations . . . ) and/or   will stay aligned with adjacent display tiles when used in a tiled display.       
 
         [0056]    The invention is also related to tiled displays where the width of the seam will be as small or smaller than the spacing between adjacent pixels on a display tile. 
         [0057]    To that end, with reference to  FIGS. 2, 3, 5, 9 and 11  a stable transparent substrate  1  like for example glass is used. The substrate  1  is either part of a rear projection screen or a glass plate in a liquid crystal panel. The glass is for instance soda lime glass. Any suitable transparent material or other types of glass are possible. The coefficient of thermal expansion of the material used for the carrier substrate is advantageously lower than 9.5·10 −6  K −1 . The substrate  1  has a first side or entry face that is substantially planar. The entry face is the face or side of the substrate  1  through which the light projected by a projector or a backlight enters. The substrate  1  has a second side or exit face that is substantially planar. The exit face is the face or side of the substrate  1  through which the light exits towards a viewer. The first side and second side of the substrate  1  are substantially parallel. When a line or surface, in particular a plane, is said to be perpendicular to the substrate, it means that the line or surface is perpendicular to the first and second sides of the substrate  1 . 
         [0058]    The substrate  1  is usually a parallelepiped with a first surface, a second surface parallel to the first surface and lateral sides  1 A,  1 B,  1 C and  1 D as shown in  FIG. 2A . 
         [0059]    A display (whether it is a rear projection display or an emissive display like a liquid crystal display or LCD), can be a fixed format display. The substrate  1  is associated with other layers. For instance the substrate  1  can support an optical element such as a Fresnel lens and/or a diffuser layer. The substrate  1  can also be part of an LCD panel. In that case, the substrate  1  is but one component of a liquid crystal panel: thin film transistors can be formed on the substrate  1 ; a the first glass substrate  1  and second glass substrate parallel to the first delimit a space to enclose a layer of liquid crystals, additional layers like e.g. colour filters can be formed on the second surface etc. . . . . In the description of the invention below, only the first substrate  1  will be explicitly mentioned. The display could also be a plasma display. 
         [0060]    To avoid a non-display area along the perimeter of the substrate  1 , the substrate is fastened to a frame  2  by mean of an adhesive film  3  distributed along the lateral sides  1 A,  1 B,  1 C and  1 D of the substrate as shown in  FIGS. 3B, 3C, 9C, 9D, 10, 11 . Avoiding a non-display area is useful both in e.g. a single panel LCD and a tiled display panel. 
         [0061]    The frame  2  is advantageously made of metal. This allows reducing the thickness of the frame without compromising on the solidity of the assembly. The frame can be e.g. an aluminium ribbon cut in a sheet with a thickness between 0.1 mm and 1 mm. For instance, the thickness of the frame is 0.2 mm. 
         [0062]    An example of adhesive film  3  is given in the utility model DE202009015262U1 “Latent reaktive, hitzeaktivierbare Klebmasse and damit hergestellte Klebemittel” which is incorporated herein by reference. Heat activation of the heat activated adhesive film can be done e.g. by near infrared heating or hot air (as provided by e.g. a heat gun). 
         [0063]    Heat activated adhesive films not being fluid like glue; it is easier to control the thickness of the adhesive film  3  that will contribute to the seam between adjacent tiles. 
         [0064]    Heat activated adhesive films are also easy to apply and cause no risk of glue penetration between layers associated with the substrate  1  or glue spilling onto one the first and/or second side of the substrate  1 . This is very important because glue penetration would cause visual artefacts. 
         [0065]    The frame  2  is not necessarily of one piece. It can for instance be split into e.g. 2 or more frame strips ( 2 A,  2 B,  2 C and  2 D) as illustrated on  FIG. 2A . As illustrated on  FIG. 2A , several frame strips can be glued to a lateral side of the substrate  1  (e.g.  1 B). The frame strips need not be planar as illustrated by  FIG. 2B  that shows an example of profile for a non-planar frame strip  2 B. The frame strips ( 2 A,  2 B,  2 C and  2 D) are used as mechanical interface between the substrate  1  and a backlight structure B (in the case of e.g. a liquid crystal display) or a support structure B (in the case of e.g. a rear projection display). Fastening of the frame  2  to the structure B can be done e.g. by means of screws like screw S and nuts like nut N. 
         [0066]    A strip, e.g.  2 A, is pressed against the heat activated adhesive tape  3  applied to one of the lateral sides, e.g.  1 A, of the substrate  1  as illustrated on  FIGS. 3A and 3B . Heat and pressure are applied to the frame strip with help of e.g. a bonding head  4  as illustrated on  FIG. 3C . The use of a bonding head allows application of heat and pressure in a well delimited region of the frame strip (the shaded area on  FIG. 3A ). 
         [0067]    When heat is applied to activate the adhesive tape  3 , the frame strip heats and expands. Heating and expansion will be highest right under the bonding head. Even if the frame strip is made of a metal like e.g. aluminum, the temperature will vary across the width W of the frame strip from a higher temperature T+ to a lower temperature T−. As a result, the frame strip will not expand uniformly as illustrated on  FIG. 4A . At the same time, the dimensions of the glass substrate  1 , and in particular the length of the lateral side ( 1 A,  1 B,  1 C or  1 D) being glued to the frame strip ( 2 A,  2 B,  2 C or  2 D), will change as well but to a lesser extent than the frame strip. 
         [0068]    As it cools down, the frame strip  2 A will return to its initial length, inducing deformations of the substrate as illustrated on  FIG. 4B  where the substrate  1  buckles under compressive stress by one or more of the frame strips. The impact of such deformations on the seam between adjacent tiles causes variations of the seam width along a seam and from seam to seam that will make the seam between tiles more perceptible to a viewer than would otherwise be the case. In some cases, the contraction can cause failure of the adhesive tape. 
         [0069]    To solve this problem, the shape of the metallic frame strip is modified as illustrated on  FIG. 5  in accordance with one embodiment of the present invention. 
         [0070]    The frame strip  2  is divided in three parts: a first part  21 , a second part  22  and a third part  23 . A first part  21  will be glued to a lateral side of the substrate  1 . 
         [0071]    A second part  22  acts as a strain relief structure between the first part  21  and the third part  23 . The second part  22  is preferably more compliant than the other two parts,  21  and  23 . 
         [0072]    At least some strain relief is assured by one or more openings or slits  5  in the frame strip. An opening  5  extends from the edge of the frame strip and extends across the first and second part of the frame strip. 
         [0073]    The opening  5  can be shaped as a rectangle but other geometries are possible. For instance, the corners of the opening can be rounded as illustrated on FIG.  5 . 
         [0074]    Strain relief can also be assured by grooves  6  etched or stamped in the first and/or second part of the frame strip as illustrated on  FIG. 6 . 
         [0075]    The grooves  6  are formed, e.g. by etching on both sides of the frame strip  2  giving a corrugated aspect to the region where they are formed. 
         [0076]    The depth of the grooves is ideally more than 50% of the thickness (x) of the frame strip. 
         [0077]    The grooves  6  are preferably substantially perpendicular to the substrate  1 . In other words, the angle made by a groove  6  and the substrate  1  is preferably 90.0 degrees of angle +/−0.1 degree of angle. Alternatively, the angle made by a groove  6  and the substrate  1  is preferably 90.0 degrees of angle +/−0.5 degree of angle. Alternatively, the angle made by a groove  6  and the substrate  1  is preferably 90.0 degrees of angle +/−1.0 degree of angle. Alternatively, the angle made by a groove  6  and the substrate  1  is preferably 90.0 degrees of angle +/−5.0 degree of angle. Alternatively, the angle made by a groove  6  and the substrate  1  is less than 90.0 degrees of angle but more than 45.0 degrees of angle. 
         [0078]    It is also possible to combine openings  5  and grooves  6  to further improve the isolation of the third part  23  of the frame strip from strain in the first part  21  of the frame strip. Openings  5  and grooves or corrugations  6  can be combined in different manners. An example of how openings  5  and grooves or corrugation  6  can be combined is illustrated on  FIG. 8 . In this example, the grooves  6  are etched right under the openings  5 . 
         [0079]    As mentioned earlier, the substrate  1  may be associated to other layers and/or substrates as is e.g. the case in a liquid crystal panel. 
         [0080]    In that case, the different substrates and layer may have slightly different dimensions as illustrated on  FIGS. 9A , B, C, e.g.  FIG. 9D  where substrate  1  is slightly smaller than substrate  7  resulting in a mismatch M between the lateral side of substrates  1  and  7 . Alternatively, both substrates can have close to identical dimensions but they can be misaligned, resulting in their lateral sides not being coplanar. 
         [0081]    When the dimensions of substrates  1  and  7  differ too much and/or when both substrates are misaligned, it can be difficult to glue them together to the frame  2  (the bond may be weaker than expected or even inexistent with one or both of the substrates). This is illustrated on  FIG. 10  where a cross section of a frame strip  2 A the heat activated layer  3  and substrates  1  and  7  are shown. As can be seen on  FIG. 10 , the mismatch M between the substrates lateral sides  1 A and  7 A makes bonding of the lateral sides  1 A to the frame strip very difficult or impossible. Two adhesive tapes  3 A and  3 B can be used on the lateral sides  7 A and  1 A of substrates  7  and  1  respectively but this will not prevent that there will be no contact between the adhesive strip  3 B and the frame element  2 A. 
         [0082]    A solution to that problem in accordance with an embodiment of the present invention is to increase the compliance (i.e. flexibility) of the frame element  2 A in the region of the frame element that will correspond to the border between substrates  1  and  7 . The compliance can, for instance, be increased by etching grooves  8  in the frame element as illustrated on  FIG. 11 , especially  FIG. 11A . The depth of the grooves  8  is preferably more than 50% of the thickness of the frame element  2 . 
         [0083]      FIG. 11  shows frame strip  2 , the heat activated adhesive layer  3 A and substrates  1  and  7 . A heat activated adhesive tape  3 A and optionally  3 B can be used on the lateral sides  7 A and optionally sides  1 A of substrates  7  and  1  respectively. 
         [0084]    As shown in  FIG. 11 , frame  2  can be fastened to the first substrate  1  and the second substrate  7  by means of an adhesive tape  3 A  3 B, respectively. The frame  2  is divided into 3 parts: a first part  31  which can be optionally glued by means of a heat activated adhesive tape  3 B to a lateral side of the first substrate  1 , a third part  33  to be glued by means of the heat activated adhesive tape  3 A to a lateral side of the second substrate  7  and a second part  32  positioned between the first  31  and third  33  part, the second part  32  having a higher compliance (i.e. better able to comply or adjust) than the first  31  and third  33  part thereby allowing the first  31  and third  33  part to be positioned in different planes. 
         [0085]    The grooves  8  can be formed in the second part  32  of the frame  2 , the grooves  8  being substantially parallel to the first  1  and second  7  substrates. The depth of the grooves  8  is preferably more than 50% of the thickness of the frame  2 . The grooves  8  can be formed on both sides of the frame  2 . 
         [0086]    The frame  2  can be split into two or more frame elements or strips  31 ,  32 ,  33 . A frame  2 , e.g.  2 A,  2 B,  2 C . . . that will accommodate both the thermal expansion during heat activation and the mismatch between the position of the lateral sides (e.g.  1 A and  7 A) of substrates  1  and  7  can be obtained by positioning the grooves  8  in the first part  31  of a frame  2 , e.g.  2 A,  2 B,  2 C . . . . The desired deformation of the frame  2  will be obtained with grooves  8  substantially parallel to the first and second substrate  1 ,  7  to which substrates the frame  2  is glued. 
         [0087]    In other words, the angle made by a groove  8  and the first and second substrates  1  and  7  is preferably 0.0 degrees of angle +/−0.1 degree of angle. Alternatively, the angle made by a groove  8  and the first and second substrates  1  and  7  is preferably 0.0 degrees of angle +/−0.5 degree of angle. Alternatively, the angle made by a groove  8  and the first and second substrates  1  and  7  is preferably 0.0 degrees of angle +/−1.0 degree of angle. Alternatively, the angle made by a groove  8  and the first and second substrates  1  and  7  is preferably 0.0 degrees of angle +/−5.0 degree of angle. Alternatively, the angle made by a groove  8  and the first and second substrates  1  and  7  is larger than 0.0 degrees of angle but less than 45.0 degree of angle.