Abstract:
There is provided a method for manufacturing a light emitting diode, LED, matrix ( 100 ) comprising the steps of providing with a maintained integrity a conductor sheet ( 150 ) with a plurality of component areas ( 111 ) interconnected with meandering connection tracks ( 116 ), mounting a plurality of LEDs ( 120 ) to a respective component area thereby forming a subassembly ( 100″ ), trimming and stretching the subassembly thereby straightening the connection tracks such that an m×n LED conductor matrix is formed during the step of stretching.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of light emitting diode (LED) lighting devices, and more particularly to a method for manufacturing a light emitting diode, LED, matrix for large area LED lighting devices, and a corresponding lighting device. 
       BACKGROUND OF THE INVENTION 
       [0002]    Light emitting diodes (LEDs) in large area lighting devices have been used for some time, where a large number of LEDs are arranged as an array or matrix to evenly provide lighting over a large area. Typically, LEDs have been arranged on a printed circuit board (PCB) by means of traditional surface mounting techniques. However, recently there has been proposed other types of solutions to provide large area LED arrays. 
         [0003]    WO 2007/125566 A1 discloses a method for manufacturing a LED array in which N electrically conducting wires are arranged in parallel creating an array of wires having a width D perpendicular to the length direction of the wires. Further, LED components are arranged to the array of wires such that each LED component is electrically connected to at least two adjacent wires, and subsequently the array of wires is stretched such that the width of the array is increased. To connect to the wires, the LED components are each arranged with a printed circuit board or lead frame with snap locks, or by means of soldering, gluing, insulation displacement connection (IDC) or similar. The proposed method provides a low cost solution for applications where large lighting area is important. However, mounting of small components is difficult on wires, and the proposed method requires that the wires have to be placed on a temporary carrier which is removed after mounting of the components. 
       SUMMARY OF THE INVENTION 
       [0004]    In view of the above, an object of the invention is to provide an alternative and improved method of manufacturing a LED array, and to at least alleviate the problems discussed above. 
         [0005]    This object is achieved by a method for manufacturing a LED matrix according to the present invention as defined in claim  1 . Thus, in accordance with an aspect of the present invention, there is provided a method for manufacturing a light emitting diode, LED, matrix comprising providing a first conductor structure having a plurality of component areas interconnected with meandering connection tracks by providing a conductor sheet with a first predetermined pattern, mounting a plurality of LEDs to a respective component area of the first conductor structure thereby forming a subassembly, trimming the subassembly to provide a second conductor structure, and stretching the subassembly thereby straightening the connection tracks. The second conductor structure is arranged such that an m×n conductor matrix is formed during the step of stretching. Thereby, there is provided an effective method to manufacture a large area LED matrix in which, in a first step, a conductor structure is provided from a conducting sheet material. The sheet material may advantageously be fed from a roll and the first conductor structure may be mechanically punched out while maintaining integrity of the conductor sheet. This allows for feeding the patterned conductor sheet into a standard soldering (or optionally gluing) pick and place process for mounting of the LEDs onto a respective component area. After trimming, e.g. by means of laser cutting, or punching and/or cutting, of the conductor sheet to release any integrity governing bridges, the conductor structure is stretched such that the meandering connection tracks are straightened and the large area matrix is formed. Thus, the method is suitable for a mechanical reel-to-reel process using standard component mounting facilities which is highly advantageous for producing large volumes at low cost. Further, by performing e.g. the mounting of the components in a high density set up, efficient time and space usage of the mounting facility is provided, while still providing a large area of the final LED matrix. A further advantage with the present concept, as compared to the prior art wire solution, is that LED&#39;s (or other components) having more than two connection pads are mountable on the subassembly, when the first predetermined pattern is adapted for such component. 
         [0006]    In embodiments of the method, before stretching, additional coatings and/or optical elements are applied. Also bending the lead frame locally may be performed, which can be beneficial e.g. for changing the orientation of the LEDs. 
         [0007]    According to an embodiment, the method further comprises arranging the stretched subassembly onto a substrate. By decoupling the production of the LED matrix from a traditional substrate (contrary to when using a PCB based LED matrix), the substrate onto which the LED matrix is mounted may be more freely chosen. 
         [0008]    This substrate could act as reflector and in case of metal as additional heat sink According to an embodiment of the method, the first predetermined pattern is selected for providing at least one thermal pad area in the first or second conductor structure, which is advantageous for integrating predetermined areas with facilitated thermal dissipation into the LED conductor matrix. 
         [0009]    According to an embodiment, at least one thermal pad area is arranged at a component area, or at a connection track. The area of the thermal pad is preferably selected to provide sufficient thermal dissipation to cool a LED or component at which it is arranged. According to an embodiment of the method (and the correspondingly produced LED conductor matrix and a lighting device in which the LED conductor matrix is arranged), the areas of thermal pads arranged at a LED or component are arranged to have different size on different sides of the component, which is advantageous e.g. when the component is asymmetrically dissipating heat, or in case one of the component/LED interconnect pads dissipate more heat than the other pad (due to material choice, position in the electrical design, position on the substrate etc.) 
         [0010]    According to an embodiment, the method further comprises providing at least one recess in the component area for receiving LEDs or LED packages of a size less then 1×1×1 mm, which is advantageous. This is applicable for Wafer Level Chip Scale Package, WLCSP, which are expected to become the lowest cost package for LED&#39;s on the market. 
         [0011]    According to an embodiment, the step of mounting comprises defining solder areas on the first conductor structure, applying solder onto the solder areas, placing the LEDs on a respective component area, and performing soldering, which is advantageous. 
         [0012]    According to an embodiment, the first or second conductor structure is arranged with alignment features and/or stretch alignment features, like for instance holes. 
         [0013]    According to an embodiment, the method further comprises providing mechanical deformation of the subassembly by means of one of splitting, trimming of the subassembly to a predetermined contour. 
         [0014]    According to an embodiment, the method further comprises deforming at least one portion of the subassembly to provide a predetermined lighting direction of at least one LED. For instance, portions onto which specific LEDs are arranged may be bent to provide a predetermined lighting direction for the LEDs, thereby enabling the use of top emitting LEDs as side emitting LEDs. 
         [0015]    According to an embodiment, the method further comprises providing deformation of the subassembly for providing one of: optical properties, mechanical fixation of the component interconnect board, mechanical fixation of additional components, thermal properties, and connector functionality. 
         [0016]    According to an embodiment, at least one of providing the first predetermined pattern and the second conductor structure is done by means of etching, cutting, punching, slitting or laser cutting. 
         [0017]    According to a second and third aspects of the invention, there is provided a LED matrix produced with a method according to the invention, and a large area lighting device comprising such a LED matrix. These have the same advantages as mentioned above with respect to the method, e.g. the decoupled processing of the circuitry before mounting to the substrate allows for providing lighting devices with more optimized substrates. Further, the LED matrix produced according to the present method can be made very large without requiring interconnect between sub portions of a matrix (compare for instance with large area lighting with the standard PCB solution, which either requires use of very large PCB&#39;s, which as previously mentioned still are limited in size and expensive to produce, or use of an array of PCB&#39;s with interconnect liners between the individual PCBs). 
         [0018]    The described invention is broadly applicable in LED products but especially interesting for LED modules and lamps, which are really large measuring up to several m 2 . Other objectives, features and advantages will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein: 
           [0020]      FIGS. 1   a ) to  c ) are schematic top views, and  d ) to  e ) are perspective views of a LED matrix according to the present invention during different steps of manufacturing; 
           [0021]      FIGS. 2   a ) to  b ) are schematic illustrations of details of embodiments of a LED matrix according to the present invention; and 
           [0022]      FIGS. 3   a ) to  d ) are schematic illustrations of manufacturing steps of a detail of an embodiment of a LED matrix according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    An exemplifying embodiment of the method for manufacturing a LED matrix according to the present invention is now described with reference to  FIGS. 1   a )- e ). The steps of the method are described as a continuous sequence, however some of the steps may be performed in another order or some additional process step may be interleaved. The conductor structure in the LED matrix is also referred to as a lead frame herein under. 
         [0024]    With reference now to  FIG. 1   a ), and starting at a step [1100] of the method, a conductor sheet  150  is initially provided. The conductor sheet is preferably selected amongst a group of metal sheet materials comprising Cu, Cu/Ni, Ni/Fe, Cu plated Fe, Cu plated RVS (stainless steel), tin plated Fe, and metal plated polymers. The conductor sheet  150  is here a 200 μm thick copper sheet. 
         [0025]    A first predetermined pattern  115  is at a step [1101] applied to the conductor sheet  150 , thereby forming a first conductor structure  100 , which corresponds to a specific mechanical and electronic layout while still maintaining the necessary integrity. The step of providing the first predetermined pattern is here done by means of laser cutting. Optionally, the first predetermined pattern  115  in the conductor sheet  100  is provided by means of a punching, or a masking and etching process. The first predetermined pattern  115  is selected to provide a plurality of component areas  111  dedicated for electrical and mechanical connection of LEDs  120 . Each component area  111 , comprises a pair of connection pads  111   a,    111   b  separated by a gap A. The component areas  115  are further interconnected via meandering connection tracks  116  which are designed to, when stretched, form an electrical circuitry for a LED matrix. 
         [0026]    For a typical LED matrix, a series parallel configuration of the electrical circuitry is provided, which is advantageous for being forgiving for LED failures. That is, failures arising from an open circuit or short circuit in the electrical circuitry of the LED matrix will only affect one row in the array, and a lighting application in which the LED matrix is integrated will still be functional. The connection tracks are here based on leads with a cross section of 0.2*0.2 mm and the length of the connection tracks are selected such that the finished LED matrix has a LED pitch of 75 mm in each dimension. The LED pitch for the LED matrix layout is preferably selected within a range of 50 to 100 mm, and the cross section of the connection tracks are selected depending on the material used in the conductor sheet. 
         [0027]    As illustrated in  FIGS. 1   a ) and  1   b ), the first predetermined pattern  115  is selected such that integrity of the conductor sheet  150  is governed by bridging portions  117 . The first predetermined pattern  115  is here further selected for providing alignment holes  118  arranged in edge portions  125  of the conductor sheet  150  for alignment of the conductor sheet  150  in the mounting and trimming facilities during the mounting of LEDs, and stretch alignment holes  119  for applying a stretching tool, which will be described with reference to steps [1102] and [1104] respectively, herein under. 
         [0028]    With reference now to  FIG. 1   b ), in step [1102], the LEDs are mounted to the first conductor structure  100  forming a subassembly. The mounting may be done by means of a standard LED assembly process, i.e. solder is screen printed onto component areas connection pads  111   a,    111   b,  and LEDs  120  are placed onto the component areas  111  followed by soldering of the LEDs  120 , which soldering may be performed in a reflow soldering process. 
         [0029]    In a step [1103] a final predetermined pattern corresponding to a second conductor structure  100 ′, i.e. the electronic circuitry of the LEDs  120 , is provided to the subassembly by trimming of the bridging portions  117 . The second conductor structure  100 ′ with the bridging portions  117  removed is illustrated in  FIG. 1   c ). Further, the edge portions  125  of the conductor sheet  150  are trimmed off from the conductor circuitry. Trimming of the conductor sheet may be performed by e.g. laser cutting or punching. In a process of providing a number of subassemblies on a continuous conductor sheet, e.g. when the method is implemented in a reel-to-reel process, the processed conductor sheet is divided into a number of individual subassemblies (conductor structure with mounted LEDs). 
         [0030]    Referring now to FIG. id), in step [ 1104 ] the subassembly, i.e. the conductor structure  100 ′ containing the trimmed meandering connection tracks  116  which electrically connects the LEDs  120 , is stretched, thereby straightening the connection tracks  116 . The stretching is performed in two dimensions such that the lead frame is expanded in two dimensions (x, y) to a final LED matrix. The maximum stretching force is preferably selected to be a factor  10  lower than the break force of the connection tracks, which depends on the selected conductor sheet material and the selected cross section of the connection tracks. 
         [0031]    In the exemplifying embodiment, a 200 μm thick etched copper sheet provided with 13×13 LEDs interconnected with meandering (or folded) conductor portions is stretched and thereby expanded from an initial lead frame size of 30*200 mm to 600*600 mm. The area of the final LED matrix is thereby 60 times larger than the working size during manufacturing of the lead frame. Thus, the application of solder by screen printing, and mounting of components on the lead frame with meandering conductor portions, before stretching offers a very efficient, highly advantageous manufacturing method for large area LED matrices. 
         [0032]    In  FIG. 1   e ) the final LED array, i.e. the stretched subassembly  100 ″ is mounted on a substrate  200 , which here is a tile, thereby forming a light tile  300 . The final LED array may be arranged on any suitable substrate, for instance a reflector or an insulated heat sink. Light tiles according to the present inventions, as illustrated in  FIG. 1   e ), are preferably manufactured with dimensions ranging from 600*600 to 600*1200 mm. Further, other applications like for instance head light applications are applicable for the present inventive concept, e.g. for TL retrofit in which dimensions of 150*150 mm are preferred. 
         [0033]    According to an embodiment of the invention, the first predetermined pattern is selected such that thermal pads are integrated in the conductor structure. The thermal pads may be arranged at a component area, as illustrated in  FIGS. 2   a ) and  2   b ). In  FIG. 2   a ) a design for extra thermal pads  211  arranged at the component area  111  is shown, in which the thermal pad design is adapted for low power LEDs  120 . For high power LEDs  220 , larger thermal pads  212  are arranged at the component area  111  for facilitating thermal dissipation from the LEDs  220 , as visualized in  FIG. 2   b ). 
         [0034]    According to an embodiment of the invention, referring now to  FIG. 3 , for mounting of small package LEDs  320 , the component area  111  of the conductor structure is provided with a recess  311 . As illustrated in the sequence performed in  FIGS. 3   a )- 3   d ), here thermal pads  212  of the conductor structure (only a detail of the conductor structure is shown) are arranged at the component area  111 , which comprises two dedicated connection pads  111   a,    111   b  arranged having an initial gap A. The gap A is then reduced by means of forging tool  400 , as illustrated in  FIG. 3   b ). At the same time as the gap A is reduced, the forging tool  400  forms a recess  311  in the component area  111 . The size of the recess  311  is selected to fit a small package component  320 , and is here selected to fit a 0.5*0.5*0.3 mm Wafer Level Chip Scale Package, WLCSP. This step is advantageously integrated in a reel-to-reel process as described above. 
         [0035]    The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended claims.