Patent Publication Number: US-9839141-B2

Title: Method for manufacturing a component interconnect board

Description:
TECHNICAL FIELD 
     The present invention generally relates to the field of light emitting diode luminaires, and more particularly to a method for manufacturing a component interconnect board for light emitting diode luminaires. 
     BACKGROUND OF THE INVENTION 
     In the cost breakdown of light emitting diode (LED) luminaires, the component to circuitry interconnect solution, or when the component is part of the circuitry, herein under referred to as a level two (L2) interconnect, is becoming increasingly important because of two main reasons. Firstly, the LED costs are decreasing, and secondly, in many LED luminaire designs, there is little room left to have a cost down on for example the housing parts. Both reasons lead to a relative increase of importance of the L2 interconnect on the total system costs. 
       FIG. 1  schematically illustrates a typical L2 interconnect in which a component, here a packaged LED  10 , is interconnected with a LED board  50  being a printed circuit board, PCB, by means of soldering. A LED board is usually provided as a stack. The LED board  50  comprises a carrying substrate  51  for providing a required robustness, or flexibility, of the LED board  50 . One or more dielectric layers  55  for providing a basic insulation of the LED board  50  is typically laminated together with epoxy resin onto the substrate  51 . On top of the substrate  51 , a conductor layer is also laminated over the full surface area. The conductor layer is thereafter chemically etched to provide for the final conductor structure  52  and circuitry. This etching process is by nature a discontinuous batch process. The LED  10  is interconnected to the conductor structure  52  by means of soldering. Before applying solder  53  to the LED board it is typically coated with a solder mask, a patterned solder resist layer  54 , defining areas where solder is to be applied. The solder resist layer  54 , which is typically 20-30 micrometers thick, may be a polymer coating applied in an offset process of a dispensed and cured polymer material. The solder resist layer  54  prevents solder from bridging between conductors  52  thereby creating short circuits, and may further provide protection from the environment. 
     Whereas the whole stack of a PCB is typically produced by lamination, the actual final end result for the conductor circuitry  52  and solder resist  54  is created by a discontinuous process. These batch processes do not come down in cost significantly with larger volumes in production. 
     Further, with respect to material utilization, there is little flexibility in playing with the essential and valuable conductor layer properties. When providing the conductor structure  52 , firstly a copper layer is applied to the full L2 process plate surface, followed by patterning and removing of copper, which cost time and saturates the chemical etching dissolvent. Growing a thicker layer, for better heat management, also cost extra time and energy. 
     In addition to the standard PCB type of L2 interconnect described above, there are many other types of L2 interconnects, which are mostly not relevant for reasons of high cost and complexity. One solution that is of relative low cost and which may be used for less complex circuitries is using lead-frames. In general this means putting components on a rigid, possibly bended, conductor frame, which is processed in a final stage to provide for the required circuitry. The lead-frame can be produced in different ways depending mostly on size and complexity e.g. mechanical stamping or chemical etching. There are some typical drawbacks to this approach. Firstly with creating the final circuitry the initial lead-frame will lose its mechanical integrity literally falling apart. One can either design to have mechanical stresses going through the electrical components such as is typical in larger mechanical lead-frames, or one can introduce some feature, e.g. plastic overmoulding, to provide for the necessary rigidity while the final circuitry is created before electrical components can be placed. Furthermore in general these types of solutions do not provide for the necessary electronic insulation requirements whereas dielectrics are not or only applied in limited areas. Prescribed creepage and clearance distances are difficult to manage or incorporate into the L2 interconnect design and must mostly be managed on a luminaire/system level. Finally if one wants to optimize for thermal management and the heat spreader and/or heat sink is made out of conductive material one has to introduce a separate dielectric component on a luminaire/system level. 
     SUMMARY OF THE INVENTION 
     In view of the above, an object of the invention is to at least alleviate the problems discussed above. In particular, an object is to provide a method for manufacturing a component interconnect board in a more material efficient, faster and more economical manner. 
     This object is achieved by a method for manufacturing a component interconnect board according to the present invention as defined in claim  1 . The invention is based on the insight that by starting from sheet based solder resist and conductor materials, and utilizing the solder resist sheet to function as a carrier for the conductor sheet, mechanical processing can be used in order to make the final circuitry. 
     Thus, in accordance with an aspect of the present invention, there is provided a method for manufacturing a component interconnect board comprising a conductor structure for providing electrical circuitry to at least one component when mounted on the component board. The method comprises providing a conductor sheet with a first predetermined pattern, providing a solder resist sheet with a second predetermined pattern for defining solder areas of the component board, forming a subassembly by laminating the solder resist sheet on top of the conductor sheet, applying solder onto the subassembly, placing the at least one component onto the subassembly, performing soldering, and laminating the subassembly to a substrate. In the subassembly, the solder resist sheet is further arranged to act as a carrier for the patterned conductor sheet, thereby maintaining integrity of the subassembly, during steps of the manufacturing. The manufacturing and assembling of the circuitry are thus decoupled from the substrate, which is advantageous in that the substrate can be chosen freely e.g. to provide for proper thermal dissipation, provide low light leakage from LEDs mounted on the component interconnect board, or to provide controllable creepage and clearance distances. Decoupling further allows separate processing of the carrier substrate in order to optimize the substrate and use its specific mechanical, optical or thermal properties to the full extent. 
     Further, a high utilization factor of high value materials like copper and aluminium is obtainable when providing the conductor structure from a patterned conductor sheet. This is particularly true when one stretches a single circuitry sub assembly, or distributes multiple circuitry sub assemblies, over a larger substrate, as will be further described below. 
     Due to the use of sheet materials, the present method may be implemented in a roll-to-roll process which is advantageous. Contrary to batch processes, which as mentioned above do not come down in cost significantly with larger volumes in production, continuous processes, like roll-to-roll processes, are very sensitive to economies of scale and are therefore cost effective for high volume production. In a roll-to-roll process large dimensions of the circuitry are allowed, e.g. infinite length is possible. There is no need for expensive L2 interconnect to L2 interconnect connectors on a luminaire/system. 
     Further, the present inventive method can provide high capacity utilization of machinery, because the subassembly and intermediate products can be produced and stocked separately. Every step of the manufacturing method may correspond to its own flexible machine. 
     Generally the present invention provides for a high freedom in layout design compared to lead-frame type solutions. This is because in the design of a lead-frame type of L2 interconnect there is a trade off between freedom in circuitry layout and mechanical rigidity/integrity. In the present invention the two different functionalities are managed by two different layers. Furthermore in current PCB type of L2 interconnect solutions one can either chose to apply free shaping of contours by a relatively expensive milling process step, or one is limited to linear cutting resulting in typical rectangular shapes. In the present invention the circuitry assembly is decoupled from the substrate which means that designing for a larger or more complex final assembly does mostly affect the substrate design and material utilization whereas the more valuable circuitry assembly may remain unchanged. 
     According to an embodiment of the method, it further comprises cutting the subassembly to provide the conductor sheet with a final predetermined pattern corresponding to the conductor structure, which is advantageous if the first predetermined pattern is not corresponding to the desired conductor structure. 
     According to embodiments of the method, it further comprises providing mechanical deformation of the subassembly by means of one of splitting, trimming of the subassembly to a predetermined contour, and stretching. For instance, to create large dimensions of the component interconnect board, it may be advantageous to create the circuitry and do the pick and placing of components first, and subsequently stretch the subassembly to a desired size before finally transferring it to the substrate. Preferably, the conductor structure is provided with extractable conductor portions. The component pick and placing is then performed with an as high as possible density, which is advantageous. Preferably, the subassembly comprising the conductor structure with extractable conductor portions is then stretched to an optimized total surface area and thickness before putting it on a carrier substrate. The substrate may be a final product or carrier, like for instance a luminaire housing part, an internal or external reflector, glass window pane, acoustically absorbing foam etc. 
     The separation of the manufacturing of the circuitry from the substrate further allows integration of various further functionalities in the substrate, like mechanical fixation, optical reflector and electrical connector before applying the subassembly to the substrate. 
     According to embodiments of the method, it further comprises providing three dimensional deformation of the subassembly for providing one of: optical properties, like specular or diffuse reflector, mechanical fixation of the component interconnect board, e.g. by bending or protrusions, mechanical fixation of additional components, like near die optics or local heat sinks, thermal properties, and connector functionality. 
     According to embodiments of the invention, the substrate may be is flexible and/or three dimensional. 
     Further, the substrate may in an embodiment of the method be mechanically deformed. This may be done by one of splitting, and trimming of the substrate to a predetermined contour. 
     According to embodiments of the method, it further comprises providing three dimensional deformation of the substrate for providing one of optical properties, like specular or diffuse reflector, mechanical fixation of the component interconnect board, e.g. by bending or providing protrusions, mechanical fixation of additional components, like e.g. near die optics or local heatsinks, thermal properties, and connector functionality. Optionally, the subassembly can incorporate features for providing additional functionalities, such as mounting or positioning features, such as providing snap fit features, slots or holes for primary optics, increasing the stiffness, e.g. by profiling. 
     According to an embodiment of the method, the conductor structure is further arranged to function as a connector. 
     According to an embodiment of the method, at least one of the first predetermined pattern and the second predetermined pattern is done by means of cutting, punching, or slitting. 
     The described invention is broadly applicable in LED products. For LED modules, LED lamps which have a relatively high thermal load to volume ratio it is highly advantageous because the solution can be optimized for thermal management while still being low cost. For LED board platforms, and large area luminaries in general it is advantageous because the solution provides for a novel way to distribute LEDs over large surface areas while still being low cost and possibly even incorporate a luminaire housing functionality. 
     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 
       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: 
         FIG. 1  is a cross sectional side view schematically illustrating a prior art L2 interconnect; 
         FIG. 2  is a flow chart schematically illustrating an embodiment of a method according to the present invention; 
         FIG. 3  is a flow chart schematically illustrating an embodiment of a method according to the present invention; and 
         FIG. 4  is a schematic illustration of an embodiment of the method according to the present invention when implemented in a roll-to-roll manufacturing line. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Exemplifying embodiments of the method for manufacturing a component interconnect board according to the present invention is now described with reference to  FIGS. 2, 3 and 4 . The steps of the method are shown as a numerical sequence, however some of the steps may be performed in another order. 
     With reference now to  FIG. 2 , and starting at step  1100 , a conductor sheet  100  is initially provided. The conductor sheet  100  is preferably selected amongst a group of metal sheet materials comprising copper and silver. The conductor sheet  100  is in step  1101  pre-cut to apply a first predetermined pattern  115  corresponding to a specific electronic layout while still maintaining the necessary integrity. 
     In a parallel step  1102 , a solder resist sheet  112  is pre-cut to provide a second predetermined pattern, here defining openings  125  for defining solder areas, while still maintaining the necessary integrity. 
     The maintained integrity of the patterned conductor sheet  111  and the patterned solder resist sheet  112  is of particularly importance when at least steps of the method are implemented in a roll-to-roll process, which is described herein under with reference to  FIG. 4 . 
     To continue with reference to  FIG. 2 , the patterned conductor sheet  111  and the patterned solder resist sheet  112  is in step  1103  laminated to form a subassembly  120 . 
     In step  1104 , solder  113  is applied to cleared areas defined by the openings  125  in the solder resist sheet  112 . 
     Subsequently, in step  1105 , pick and placing of components  114 , being for instance LEDs, is performed followed by soldering, which may be a reflow soldering process, in step  1106 . Optionally, if necessary, the subassembly  120  is in step  1107  cut to provide for a final predetermined pattern  116  corresponding to the conductor structure, i.e. the electronic circuitry of the components  114 . In step  1108  the subassembly  120 , now containing both the final circuitry and components  114 , is split (cut) in multiple parts here forming two subassemblies,  120   a  and  120   b . In a final step  1109 , the parts  120   a  (not shown) and  120   b  of the subassembly  120  are laminated to an appropriate substrate  130 , taking into account creepage and clearances, resulting in a component interconnect board  150 . In embodiments of the method, the substrate or the component interconnect board may be further mechanically deformed to add functionalities, e.g. mounting features or positioning features for primary optics. 
     The mechanical deforming of the subassembly in step  1108  is optional, and can in embodiments of the method comprise trimming of the subassembly (and/or parts of the subassembly when performing splitting of the subassembly, as described in step  1108  above) to a predetermined contour. 
       FIG. 3  is a flow chart schematically illustrating an embodiment of a method according to the present invention. Starting at step  1200 , a conductor sheet  200  is initially provided. The conductor sheet  200  is in step  1201  pre-cut to apply a first predetermined pattern  215  corresponding to a specific electronic layout while still maintaining the necessary integrity. The first predetermined pattern  215  comprises of a matrix of m×n component areas, C n×m , arranged in n rows and m columns, here n=3 and m=3, see for instance connect areas  212   a  and  212   b , to which components are to be soldered, in the schematic close up of the patterned conductor sheet  211 , which connect areas  212   a  and  212   b  together constitute a component area C 3,1 . Further, substantially U-shaped conductor portions  216  are arranged to interconnect adjacent conductor areas C n,m . As illustrated in  FIG. 3 , the conductor portions  216  are cut out from the conductor sheet  200  with maintained integrity by keeping a bridge part  217   a.    
     In a parallel step  1202 , a solder resist sheet  212  is pre-cut to provide a second predetermined pattern, here comprising covering areas  226 , corresponding to each component area C n,m  of the patterned conductor sheet  211  in which openings  225  for defining solder areas are arranged. Further, each covering area  226  is interconnected with a bridge  217  arranged at positions corresponding to bridge parts  217   a  of the patterned conductor sheet  211 . 
     Maintained integrity of the patterned conductor sheet  211  and the patterned solder resist sheet  212  is of particularly importance when at least steps of the method are implemented in a roll-to-roll process, which is described herein under with reference to  FIG. 4 . 
     To continue with reference to  FIG. 3 , in step  1203  the patterned conductor sheet  211  is laminated to the patterned solder resist sheet  212  to form a subassembly  220 . In step  1204 , solder  213  is applied to cleared areas defined by the openings  225 . Subsequently, pick and placing of components  214 , being for instance LEDs, in step  1205  is performed followed by soldering, which may be a reflow soldering process, in step  1206 . 
     In step  1207 , the subassembly  220  is cut to provide for a final predetermined pattern corresponding to the conductor structure, i.e. the electronic circuitry to the components  214 . Here, the part of the bridges  217   a  and  217  are simultaneously trimmed, e.g. by punching, such that the conductor portions  216  are no longer bridged. 
     In step  1208  the subassembly  220 , now containing both the final circuitry and components  214 , is mechanically deformed. The matrix of component areas C n×m  is stretched, thereby straightening out the conductor portions  216  such that the distance between the components  214  and the L2 interconnect surface area (area of subassembly  220 ) increases. Here, the stretching is done in two dimensions. 
     In a final step  1209 , the stretched subassembly  220  is laminated to an appropriate substrate  230 , taking into account creepage and clearances, resulting in a component interconnect board  250 . In embodiments of the method, the substrate or the component interconnect board may be further mechanically deformed to add functionalities, e.g. mounting features or positioning features for primary optics. 
     According to an embodiment of the method according to the present invention, it is implemented in a roll-to-roll process to produce a large number of lighting devices (i.e. final product devices corresponding to component interconnect boards according to the present invention). In a roll-to-roll process, the production facilities are equipped to carry out major parts of the production with sheet materials feed by rolls instead of individual sheets. Referring now to  FIG. 4 , which schematically illustrates a roll-to-roll manufacturing line, including machinery used for patterning, slitting and laminating steps of the present method. The manufacturing line is here at least partly described with reference to reference numbers and method steps of the method as described with reference to  FIG. 2 . However, in the roll-to-roll process, the steps are described with reference to films instead of referring to individual sheets, e.g. conductor sheet  100  is here referred to as conductor film  100 . To continue with reference to  FIG. 4 , a conductor film  100  is supplied from a feed roll  400  [step  1100 ] into a pattering machine  401 , in which a continuous series of the first predetermined pattern  115  is punched out or cut out [step  1101 ]. In a parallel process, a patterned resistor film  112  is provided from a feed roll of resistor film  402  fed into a patterning machine  403 , in which a continuous series of the second predetermined pattern  125  is punched out or cut out [step  1102 ]. The patterned conductor film  111  and the patterned resistor film  112  are then fed into a laminating station  404  (e.g. by applying an adhesive and or mechanical pressure and high temperature) to form the subassembly film  120 . The subassembly film  120  is subsequently fed into machinery for applying solder [step  1104 ], pick and placing of components [step  1105 ], soldering of components [step  1106 ] and mechanical deformation of the subassembly film [steps  1107 ,  1108 ], e.g. to provide the final circuitry, and form separate subassemblies  120 ′ corresponding to respective lighting devices. Here, after the circuitry making is completed, desired substrates  130  are provided. The substrates may be separate substrates provided on a feeding line after steps of manufacturing including desired deformation etc. [step  1110 , not shown in  FIG. 2 ]. The substrates can also be provided directly from a feed roll. 
     Decoupling of the manufacturing of the subassembly from the substrate is advantageous since it allows specific processes to run in their natural speeds, and further increases the flexibility of the processing component interconnect boards of different designs in the same processing line. 
     Further, decoupling of the processing of the subassembly and the substrate facilitates for adding or removing optional processing steps from the process line, for instance a 3D shaping of the subassembly may be removed from the processing if not necessary for a specific component design. Also when changing from one design in factory to another design in factory, if processes are decoupled, it is not longer necessary to exchange tooling for all process steps before machines are made operable again. 
     To continue with reference to  FIG. 4 , finally the subassemblies  120 ′ and the substrate  130  are laminated to form final products  150  [step  1109 ]. Optionally, a final process step, e.g. cutting to separate products, applying extra environmental encapsulation etc. may be performed on the final products [step  1111 ]. 
     Machinery utilizing rolling tools are advantageous for their very high speed performance, and can be used one or more of the different method steps, which is indicated by dashed rolls in the steps  1101 ,  1102 ,  1107 ,  1108  and  1111  in  FIG. 4 . 
     In the steps of the roll-to-roll process as described above, there might exist a difference in speed for the respective step. In particular, the step of pick and placing of components [step  1105 ] is a process step that has different processing speed for different designs of the circuitry. This relatively valuable process step is associated with costly machines, and may therefore become a bottleneck for the whole manufacturing line. According to an embodiment of the method (not shown), the pick and placing of components is therefore moved out of line from the roll-to-roll process as described with reference to  FIG. 4 . This way with every new design the roll-to-roll manufacturing line can operate at an optimized speed providing for the highest machine utilization factor. The speed may further be variable within a specific design. Thereby, different designs of the respective lighting devices can be created using the same manufacturing line with flexible tooling. Ultimately this means that a manufactured subassembly  120  up to and including step  1103  is an intermediate result, which may be created with multiple designs  120   a ,  120   b  etc. These are put on stock to create a necessary buffer which is a result of the variable speed of pick and placing step  1105 . 
     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.