Abstract:
The inventions relates to a method of manufacturing a flexible surface mounted device, the method including bonding a main face of a conductive layer to an insulating layer; linking electrically and mechanically at least one electronic surface mounted component to the conductive layer; wherein the insulating layer is punched to produce through holes through which the electronic component is linked to said main face of the conductive layer.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method of manufacturing a surface mounted device and a surface mounted device manufactured by this method. 
       BACKGROUND OF THE INVENTION 
       [0002]    A Printed Circuit Board or PCB, is used to mechanically support and electrically connect electronic components. A surface mounted device is a PCB having electronic components mounted directly onto its surface. 
         [0003]    Originally, a method of manufacturing such surface mounted devices called through-hole construction was used. According to this method, the electronic components have wire leads that were inserted into holes (PTH-Plated Through Hole) drilled in a wafer either by manual assembly by hand placement or by the use of automated insertion mount machines. The wire leads of the electronic components were then soldered to pads located on the side of the wafer opposite to the side of the wafer carrying the electronic components. 
         [0004]    Nowadays, a method of manufacturing such surface mounted devices, called Surface Mount Technology (SMT), is used. According to this method, a conductive layer, usually made of copper, is bonded over its entire surface to an insulated layer. A temporary mask is applied on the conductive layer and unwanted copper is removed, for example, by etching to construct an interconnection pattern. This interconnection pattern comprises interconnection pathways and flat pads without holes, called solder pads. Solder paste is applied on the solder pads with a stainless steel or a nickel stencil using, for example, a screen printing process. After screen printing, the leads (which have no wire) of the electronic components are placed on the solder paste usually by pick-and-place machines. Afterwards, the wafers are conveyed into a reflow soldering oven to bond the electronic component leads to the solder pads. 
         [0005]    However, since the electronic components are small, around 0.4×0.2 mm, some electronic components may be lifted or shifted from the solder pads which leads to weak connexions. 
         [0006]    To remedy to this drawback, one technique consists in covering the interconnection pathways of the interconnection pattern with a solder resist before applying the solder paste. The solder resist is applied to build up retain walls around the solder pads wherein the solder paste is filled for example by the stencil. 
         [0007]    However, this method is quite expensive due to the use of a solder resist. Further, this method requires the application of a second mask on the conductive layer. 
         [0008]    In parallel, it has been known to manufacture flexible printed circuits, using continuous roll-to-roll processes, for example to produce flexible circuits such as circuits comprising contacts for smartcards. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention seeks to mitigate at least one these drawbacks by proposing a manufacturing method which is more efficient. 
         [0010]    The invention thus relates to a method of manufacturing a flexible surface mounted device. A main face of a conductive layer is bonded to an insulating flexible layer. 
         [0011]    One or more electronic surface-mounted component is/are electrically and mechanically linked to the conductive layer. 
         [0012]    Through holes are produced in the insulating layer. The electronic surface-mounted component(s) are linked to the main face of the conductive layer through these through holes. 
         [0013]    With this feature, surface mounted devices can be manufactured without using a solder resist and even without the use of a stencil for placing the solder resist. 
         [0014]    As a result, the manufacturing process is cheaper and more reliable. 
         [0015]    According to another aspect, the invention relates to a flexible surface mounted device. This device comprises a conductive layer and a flexible insulating layer stacked and bonded to the conductive layer. One or more electronic surface mounted component(s) is/are electrically and mechanically linked to the conductive layer. The electronic surface mounted component(s) is/are linked to the conductive layer via connexion passing through through-holes formed into the insulating layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    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 of the accompanied drawings. 
           [0017]    On the drawings : 
           [0018]      FIG. 1  is a flow chart illustrating the manufacturing steps of the method according to the invention; 
           [0019]      FIGS. 2 to 7  are cross-sectional schematic views of a part of a surface mounted device at different manufacturing steps; and 
           [0020]      FIG. 8  is a front schematic view of a band comprising surface mounted device to cut. 
       
    
    
       [0021]    On the different figures, the same references signs designate like or similar elements. 
       DETAILED DESCRIPTION 
       [0022]    In reference to  FIGS. 1 and 2 , the manufacturing method according to the invention begins with a step  12  of spreading glue  16  on a first main face  14  of an insulating layer  8 . 
         [0023]    The insulating layer  8  is made of a dielectric polymeric material, for example, glass epoxy material. This insulating layer  8  will form the substrate on which the electronic components will be electrically and mechanically connected. The insulating layer  8  has for example, a width of 8 to 10 mm and a thickness in the range of 50 to 250 μm, and more particularly in the range of 75 to 110 μm. 
         [0024]    Then, at step  18 , the insulating layer  8  is punched to produce through holes  20 , as shown on  FIG. 3 . The through holes are drilled on locations where leads of electronic components have to be fixed to produce the desired device. 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. For simplification reason,  FIGS. 2 to 7  show only a short part of a band on which one electronic component is mounted. The manufacturing method according to the invention is performed on a greater band surface on which several electronic components are electrically and mechanically connected to produce the device. 
         [0025]    At step  22 , a main face  24  of a conductive layer  10  is stacked on the glue-spreaded face  14  of the insulating layer and is bonded to it by adhesion and lamination to produce a flexible band  33 . 
         [0026]    The conductive layer  10  is, for example, a flexible layer of copper having a width of 8 to 10 mm and a thickness in the range of 10 to 30 μm. 
         [0027]    As a result, at least one of the openings  26  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 as shown on  FIG. 4 . Preliminary to its fixation, the main face  24  of the conductive layer might be treated by suitable treatments. 
         [0028]    At step  28 , the bottom regions  29  of the conductive layer are deoxidized, i.e., the regions of the main face  24  delimited by the through-holes are deoxidized. 
         [0029]    In variant, the entire main face  24  of the conductive layer is deoxidized. According to this variant, the deoxidization step  28  is performed before the bonding step  22 . 
         [0030]    At step  30 , 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 figure, as partially shown on  FIG. 5 . Afterwards, the main face  31  of the conductive layer opposite to the main face  24  is protected, for example, by applying a conformal coating by dipping or spraying. This coating prevents corrosion and leakage currents or shorting due to condensation. 
         [0031]    At step  32 , a solder paste  34  is brought in the blind holes  20 . To this end, the solder paste  34  is advantageously dispensed by scraping. The solder paste is, for example, deposited on the surface  44  of the insulating substrate and is pushed into the hole  20  by a fixed squeegee  36  as shown on  FIG. 6 . 
         [0032]    The squeegee scraps the excess solder paste away from the opening  20 . At step  38 , an electronic surface-mounted component  40  is placed on the wafer  31  with its leads  42  in contact to the solder paste  34 , for example by a pick-and-place machine. In particular, the electronic surface-mounted component  40  is placed in contact with the main face  44  of the insulating layer  8  which is opposite to the main face  24  bonded to the conductive layer  10 . 
         [0033]    The surface-mounted component has dimensions of at least 5 mm, with electrical contact sizes of about 0.1 to 1 mm. 
         [0034]    As shown on  FIG. 7 , the leads  42  of the electronic component are made of flat pads. These leads  42  do not comprise any wire. 
         [0035]    The electronic components comprise, for example, transistors, resistors printed circuit boards or light emitting diodes. 
         [0036]    At step  46 , the wafer  31  is submitted to reflow-soldering to melt the solder paste  34  for soldering the component leads  42 . After reflow soldering, the solder paste  34  forms an electrical and mechanical connexion  47  between the electronic component  40  and the main face  24  of the conductive layer  10 . 
         [0037]    Hence, the insulating layer itself is used as a solder mask. The surface-mounted components  40  are connected by way of the conductive layer  10  provided on the bottom face. 
         [0038]    Thus, the insulating layer  8 , the conductive layer  10  and the electronic components linked to them constitute a flat band  13 .. The flat band  13  comprises surface mounted devices  4  to be cut free from the band, as shown on  FIG. 8 . This band  13  is delivered to a client who cuts, at step  48  the required surface mounted devices  4 . Alternatively, the surface mounted devices  4  are cut before being delivered to clients. 
         [0039]    The steps described above are performed in a plurality of continuous apparatus disposed in series or in parallel. 
         [0040]    The surface mounted device  4  is, for example, a band of light emitting diodes. In this case, the electrical connexions to the LEDs are not visible, so that the lighting effect is better. For example, when the surface mounted device is a light emitting diode band, sections of several centimetres comprising several light emitting diodes are cut in band  13 . 
         [0041]    The invention is also related to a surface mounted device  4  manufactured by the above-mentioned method. This surface mounted flexible device  4  comprises a conductive layer  10 , an insulating layer  8  bonded to the conductive layer  10 , for example with glue and at least one electronic surface-mounted component  40  electrically and mechanically linked to the conductive layer  10  via through holes  20  punched into the insulating layer.