Abstract:
The inventions relates to a method of manufacturing a circuit incorporating a solid state light emitting component, the method including providing an insulating layer, producing at least one through hole in the insulating layer, providing a conductive layer, bonding a main surface of the conductive layer to the insulating layer, and positioning at least one solid state light emitting component in the hole of the insulating layer and connecting this component to the conductive layer.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method of manufacturing a circuit having a light emitting component mounted thereon and to a circuit manufactured by way of this method. 
       BACKGROUND OF THE INVENTION 
       [0002]    Light emitting components, and in particular solid state light emitting components, are more and more used in electronic devices. In a context of electrical power saving solid state light emitting components have proven to be able to deliver high amounts of light with low electrical power consumption. Further, technologies of solid state manufacturing have developed in the recent years to a point that light emitting components can now be obtained with a size of a few hundreds of micro-meters in the case of Surface Mounted components—SMCs—and even with a size of a few tens of micro-meters in the case of bare chips or dies. This has allowed high amounts of light emitting elements to be placed in a same appliance such as for example a lighting bulb or a lighting tube for domestic use. 
         [0003]    Surface mount technology—SMT—is a method for constructing electronic circuits in which the components—usually called surface-mounted components or SMCs—are mounted directly onto the surface of a circuit such as a printed circuit board—PCB—. An electronic device so made is called a surface mounted device—SMD—. In the industry it has largely replaced the through hole technology construction method of attaching components with wire leads into holes in the circuit board. A surface mounted device is hence a type of circuit having electronic components mounted directly onto its surface. 
         [0004]    An SMT component is usually smaller than its through-hole-wired counterpart because it has either smaller leads or no leads at all. It may have short pins or leads of various styles, flat contacts, a matrix of solder balls, or terminations on the body of the component. 
         [0005]    Surface mount technology was developed in the 1960s and became widely used in the late 1980s. Part of those components were mechanically redesigned to have small metal tabs or end caps that could be directly soldered to the surface of a PCB. Components became much smaller and component placement on both sides of a board became far more common with surface mounting, allowing much higher circuit densities. Often only some solder joints hold the SMCs or a dot of adhesive may as well affix the SMC to the circuit. 
         [0006]    Surface mounted devices (SMDs) are usually made physically small and lightweight for these different reasons. Surface mounting lends itself well to a high degree of automation, reducing labor cost and greatly increasing production rates. SMDs can be one-quarter to one-tenth the size and weight, and one-half to one-quarter the cost of equivalent through-hole-wired parts. 
         [0007]    In a context of multiplying information devices such as smart phones, flat screen televisions, intelligent automobile conductor boards, and many other apparatuses that may visually display information, light emitting components are more and more adopted in everyday appliances thanks to their low size and low consumption. 
         [0008]    Despite the many progresses made in the field of light emitting components, these components remain however a source of heat and hence there remains a need for handling dissipation of heat in devices where such elements are used. This constraint is still more accurate when a large number of light emitting components are used in a same product. 
         [0009]    The invention aims at proposing a solution so as to ease thermal transfers away from a light emitting component in a circuit and hence enable an enhanced heat dissipation out of a circuit incorporating a light emitting component. The invention also aims at proposing such a solution that remains adapted to an industrial process and does not induce heavy costs when implemented in such process. 
         [0010]    This goal is achieved according to the invention thanks to a method of manufacturing a circuit incorporating a solid state light emitting component, the method comprising:
       providing an insulating layer,   producing at least one through hole in the insulating layer,   providing a conductive layer,   bonding a main surface of the conductive layer to the insulating layer,   positioning at least one solid state light emitting component in the hole of the insulating layer and connecting this component to the conductive layer.       
 
         [0016]    The invention also relates to a circuit incorporating a solid state light emitting component, the circuit comprising an insulating layer, said insulating layer having two opposite sides and at least one hole extending from one side to the other side of the insulating layer, the circuit also comprising a conductive layer, a main surface of the conductive layer being bonded to the insulating layer, characterized in that the solid state light emitting component is placed in the said at least one hole of the insulating layer and the solid state light emitting component is connected to the conductive layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Other characteristics and advantages of the invention will readily appear from the following description of one of its embodiments, provided as a non-limitative example, and from the accompanying drawings. 
           [0018]    On the drawings: 
           [0019]      FIG. 1  is a flow chart illustrating the manufacturing steps in an embodiment of the method according to the invention; 
           [0020]      FIGS. 2 to 8  are cross-sectional schematic views of a part of a circuit according to an embodiment of the invention at different manufacturing steps, 
           [0021]      FIGS. 9  is a top view of a portion of a flexible circuit according to an embodiment the invention; 
           [0022]      FIG. 10  is a schematic cross section of the structure of  FIG. 9 , 
           [0023]      FIG. 11  is a schematic cross section of the structure of  FIG. 10 , 
           [0024]      FIG. 12  is a bottom view of a component to be mounted onto the structure of  FIG. 9 , 
           [0025]      FIG. 13  depicts a bare chip used in an embodiment of the invention, 
           [0026]      FIG. 14  depicts a wired bare chip according to an embodiment of the invention, 
           [0027]      FIG. 15  is a partial perspective view of a surface-mounted-component used in an embodiment of the invention. 
           [0028]      FIG. 16  is a schematic cross section similar to  FIG. 11 , 
           [0029]      FIG. 17  shows schematically the electrical circuit of  FIG. 16 , 
           [0030]      FIG. 18  shows electrical and thermal connections of the flexible circuit of  FIG. 16 . 
       
    
    
       [0031]    On the different figures, the same reference signs designate like or similar elements. 
       DETAILED DESCRIPTION 
       [0032]    With reference to  FIG. 1 , the manufacturing method according to the invention begins with a step  12  of spreading glue  16  on a first main surface  14  of an insulating layer  8 . Possibly, this insulating layer  8  is a copper clad laminate with a dielectric layer  8   a  and a copper layer  15 . 
         [0033]    The dielectric layer  8   a  is made of a dielectric polymeric material, for example, glass epoxy material. The dielectric layer  8   a  has for example a thickness in the range of 50 to 250 μm and more particularly in the range of 75 to 110 μm. 
         [0034]    Then, at step  18 , the insulating layer  8  is punched to produce through holes  20 . Such holes have a size with the millimetre as an order of magnitude. For example, they are about 0.5 mm to 5 mm in size. 
         [0035]    The holes  20  may alternatively be realised by mechanical methods, such as punching, drilling or water jet or by chemical methods such as etching or dissolving. Holes  20  may be realised by any other type of method such as by laser engraving. 
         [0036]    At step  22 , a main surface  24  of a conductive layer  10  is stacked on the glue-spread face  14  of the insulating layer  8  and is bonded to it by adhesion and lamination to produce a flexible band  33 . 
         [0037]    The conductive layer  10  is, for example, a flexible layer of copper having a thickness in the range of 10 to 105 μm. 
         [0038]    As a result, at least one of the through-holes  20  is covered with the conductive layer  10 . The through holes  20  are now blind holes having bottom regions  29  made of conductive material. 
         [0039]    In the present example, the assembly comprising the conductive layer  20  and the insulating flexible layer  8  (for instance made of epoxy glass, polyimide, etc.) forms a flexible circuit, which is particularly adapted to be produced in a roll-to-roll process. 
         [0040]    Preliminary to its fixation, the main surface  24  of the conductive layer  20  might be treated by suitable treatments. For instance, a deoxidization is performed before the bonding step  22 . Bottom regions  29  of the conductive layer  20  are deoxidized, i.e., the regions of the main face  24  delimited by the through-holes are deoxidized. 
         [0041]    At step  28 , the conductive layer  10  is patterned, for example by screen printing, photoengraving or PCB milling to create an interconnection pattern, i.e. to create conductor pathways which will link the electronic components between them according to the desired electronic pattern. 
         [0042]    At step  32 , the flexible circuit and its copper conductive layer  10  are subjected to an electroplating process for producing a finishing treatment of conductive surfaces. 
         [0043]    An electroplating deposition is realised on the copper layer  15  making part of the copper laminate clad, which copper layer  15  is at this stage on an upper side of the dielectric layer  8   a  which is opposite to a lower side of dielectric layer which is bonded to the copper conductive layer  10 . Thanks to the electroplating of the upper copper layer  15  a high reflectivity is obtained in this area, which enhances the lighting ability of the assembly as a whole. 
         [0044]    The electroplating is also realised onto the copper conductive layer  10 , so that both copper layers  15  and  10  are protected against dirt and oxidation in particular during intermediary steps when the present assembly is stored waiting for further components to be placed onto the assembly as will be described here-under. The electroplating step  32  also provides a protection of the copper layers against aging due to exposure to light emitted by a light emitting component that will be described here-under and hence the electroplating contributes to make the assembly a long lasting device as required nowadays for lighting devices. 
         [0045]    Preferably, electroplating is performed onto both the main surface  24  of the conductive layer  10  and onto a surface  31  of the conductive layer which is on an opposite side of the conductive layer  10 . 
         [0046]    At step  34 , surface  31  of the conductive layer  10  which is opposite to the main surface  24  is further protected, for example, by applying a conformal coating  36 . Conformal coating  36  is realized for example by dipping or spraying. Conformal coating  36  prevents corrosion and leakage of currents or shortenings between conductive paths of the conductive layer  10  due to condensation. It also insulates the copper layer and consequently the conductive tracks between the components and/or other electrical circuits from one another. This coating  36  is here electrically insulating and thermally conductive. 
         [0047]    Hence by covering free spaces which are present between conductive pathways of the conductive layer  10 , the conformal layer  36  forms a barrier against dust and moisture which would otherwise penetrate into the free spaces and would electrically bridge the pathways together. 
         [0048]    Conformal coating  36  is here made of a glue which is electrically insulating. A glue thickness of 10-20 micro-meters is adequate so that thermal transfers take place easily through the glue coating layer. In another embodiment the conformal coating  36  is made of a composite material comprising a base made of a plastic material and electrically conductive particles embedded in such base so that the composite as a whole is electrically insulating but is of enhanced thermal conductivity. 
         [0049]    A heat sink is then affixed to the conformal coating  36 , the conformal coating  36  hence insulating the pathways of the conductive layer  10  from the heat sink which here has a conductive surface in contact with the flexible assembly comprising the isolative layer and the conductive layer. 
         [0050]    The heat sink may be a component available on the market, which is typically a metallic element, for example made of aluminium, either compact or made of a series of thin plates so as to provide a large area for thermal exchanges. 
         [0051]    The conductive layer  10  and the insulating layer  8  are here obtained by being cut free from respective flexible bands, the assembly thereof being flexible also. Due to the flexible nature of the circuit, such embodiment of the invention can be easily implemented using a continuous a roll-to-roll process. 
         [0052]    A portion of a flexible circuit according to the invention is shown on  FIG. 9 . The top copper layer  15  covers the flexible dielectric layer. The conductive layer  10  is seen through the punched holes  20 . The conductive layer  10  forms two contact pads  11 ,  12  and one thermal pad  13 . 
         [0053]      FIG. 10  shows a schematic cross section of the structure of  FIG. 9 . This structure corresponds to a flexible circuit which can be sold as such. A customer buying such a type of flexible circuit can choose the type of components he will place in the holes  20 . However the flexible circuit described here is particularly suitable and adapted for receiving components  50  such as LEDs with three pads comprising two small electrical pads  51 ,  22  and one large thermal pad  54  such as represented on  FIG. 12 . 
         [0054]      FIG. 11  shows a schematic cross section of the structure of  FIG. 10  with one component  50  already in place in a hole  20  and one which is going to be placed in another hole. The component  50  is soldered to the copper pads  11 ,  12  and  13  by brazing with SnAgCu solder for instance. 
         [0055]    At least one of the pads of the component is connected to one of the two main surfaces of the conductive layer whilst the other of the two main surfaces of the conductive layer is here designed to be placed in thermal conduction relationship with a heat sink. Consequently, the thermal energy can be very efficiently evacuated from the component to the heat sink through the conductive layer. The conductive layer is a made of a thermally and/or electrically conductive material. For instance, the conductive layer is made of a copper alloy. The same conductive layer is advantageously used for evacuating the thermal energy of a set of several such components as component  50 . 
         [0056]    In the present embodiment, the solid state light emitting component is a chip or bare chip. 
         [0057]    The bare chip may also be called a die, due to the usual industrial process used for obtaining such a bare chip. Such usual process consists in producing large batches of a same circuit made of patterned diffusion of trace elements onto the surface of a thin wafer. The wafer is then cut (“diced”) into many pieces, each containing one copy of the circuit. Each of these pieces hence constitutes a “die”. 
         [0058]    The bare chip  50  of  FIG. 12  is a flip chip whose pads  51 ,  52 ,  54  are adapted for direct connection of the chip to conductive pathways without intermediary wire-bonding. 
         [0059]    Such a bare chip or die is represented on  FIG. 13 . The light emitting bare chip  50  is made of a substrate  55  onto which a stack  56  of semi-conductor layers are deposited, which stack of layers  56  has the ability to emit light when a voltage is applied onto different layers of the stack. Such a bare chip is typically a few tens of micro-meters large, and the layers of the stack are typically a few nano-meters thick. For being able to contact the different layers of the stack separately and thereby apply a differential voltage in the stack  56 , pads  57  and  58  are realised on a side of the bare chip  50  which is opposite to the side constituted by the substrate  55 . These pads  57  and  58  are connected to two different layers of stack of layers  56  by means of electrical connections which are themselves realised by deposition. Due to the size of such a bare chip, the pads  57  and  58  have a size of a few tens of micro-meters, typically between 50 and 100 micro-meters. 
         [0060]    In the alternate embodiment of  FIG. 14 , the bare chip is connected to pathways of the conductive layer  10  by means of wires  61 ,  62  which are soldered to pads  57 ,  58  and connected by their opposite end to the conductive layer  10  through additional through hole  70  of the insulating layer  8 . 
         [0061]    In such case, the chip  50  may be placed in the hole  20  so that contact pads  51 ,  52 ,  54  of the chip are on the side of chip which is facing away from the conductive layer  10 . The bonding wires  91 ,  62  then extend from theses pads  57 ,  58  and back to the conductive layer  10 . 
         [0062]    In an alternate embodiment, the solid state light emitting component is Surface Mount Component—SMC. 
         [0063]    Such an SMC is represented on  FIG. 15  under the general reference  100 . 
         [0064]    SMC  100  comprises a wafer element  110  and a bare chip  120  which is similar to the light emitting bare chip described above. The bare chip  120  is affixed to a first side  115  of the wafer element  110 . The wafer element  110  carries conductive pads  116 ,  117  on a second and opposite side of the wafer element. Pads  126 ,  127  of the bare chip  120  are here directed so as to face away from the wafer element  110 . Pads  126 ,  127  are connected to the pads  116 ,  117  of the wafer element  110  by means of connecting wires  160 ,  170 . A non-represented encapsulating body is over-molded over the bare chip  120  and the bonding wires  160 ,  170  so that the surface mounted component  100  constitutes a protected and robust component able to be easily and directly mounted onto a support having conductive pathways. 
         [0065]    The surface mounted component  100  has typically a size around half a millimeter. The pads  116 ,  117  of such a surface mounted component  100  are typically a few hundreds of micro-meters large. Connection of the surface mounted component  100  can be made by means of bonding wires, in particular when the surface mounted component is placed so that the pads of the surface mounted component face away from the wafer element  110 . Connection of the surface mounted component  100  can also be made by placing the surface mounted component so that its pads  116 ,  117  come directly into contact with the conductive layer. The SMC may hence be connected to the conductive layer by being oriented so that the pads  116  and  117  are placed in the bottom of the cavity and come into contact with corresponding pathways of the conducting layer in the same way as described previously for a flip chip. 
         [0066]    Although described as being entirely received inside a hole in the insulating layer, only part of the bare chip or of the SMC may be received in the hole, a lower part of the bare chip or SMC being inside an overall thickness of the hole while an upper part of the SMC emerges from the hole out of the insulating layer on the side of the insulating layer which is opposite to the side which is bonded to the conductive layer. 
         [0067]    In both cases of a bare chip or a SMC, the solid state light emitting element is preferably electrically, thermally and mechanically connected to the conductive layer  10 . In such case the product comprises a conductive layer and an insulating layer which are stacked and bonded together with one or more solid state light emitting elements which is/are electrically, thermally and mechanically linked to the conductive layer. When the conductive layer is electrically conductive, it may hence be also used for electrically connecting different components of the circuit between them or for electrically connecting components of the circuit with another electronic circuit. 
         [0068]      FIG. 16  shows a schematic cross section of a circuit according to the invention which is here a flexible circuit with three LEDs  50 —Light Emitting Diodes—respectively constituted of such surface mounted components. The three LEDs are here mounted between two nods referenced as A and B as indicated also on the schematic representation of  FIG. 17 . 
         [0069]      FIG. 18  shows electrical and thermal connections of the same flexible circuit, between the electrical pads  51 ,  52  of the LEDs and portions or tracks  11 ,  12  of the conductive layer  10  and thermal connections of the thermal pads  53  of the LEDs with a same track  13  of the same conductive layer  10 . Track  13  forms an extended element which covers the thermal pads  53  of the different LEDs  50  so as to collect the heat of the set of different LEDs of the circuit. The LEDs  50  being connected in series, a track  11  connected with a pad  51  of a LED constitutes also a track  12  which is connected with a pad  52  of an adjacent LED. However, a track  11 ,  12  is insulated from a same neighbour track  11 ,  12  and from the thermal track  13 .