Abstract:
There is provided a first, connecting printed circuit board (PCB) and a second, receiving PCB and a method for connecting the first and second PCBs. The first PCB has three projections or prongs extending from the main body of the PCB. The second PCB has three holes into which the prongs of the first PCB can be inserted to provide a secure mechanical connection whilst the PCBs are soldered together.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit and priority of Great Britain Application No. GB 1116522.2 filed Sep. 23, 2011. The entire disclosure of the above application is incorporated herein by reference. 
       FIELD 
       [0002]    The invention relates to printed circuit boards and to a method for connecting printed circuit boards. 
       BACKGROUND 
       [0003]    Printed circuit boards (PCBs) are widely used in electronics applications and are well known. In some applications it is necessary to join one PCB to another. The joining technique must provide a reliable electronic connection between the electronic contacts of the first PCB and the electronic contacts of the second PCB, as well a reliable mechanical connection between the two PCBs with accurate orientation. 
         [0004]    One technique for providing electronic connections on and between PCBs is known as wave soldering. In the wave soldering process, a quantity of molten solder is contained within a tank. A pattern of standing waves is induced on the surface of the molten solder and the PCB to be soldered is conveyed over the surface of the molten solder. The height of the standing waves is adjusted such that the peaks of the waves contact the surface of the PCB, thereby applying solder to the PCB. To join two PCBs together in this way, the use of a mechanical support or a jig is required to hold the PCBs in the desired orientation during application of the solder. Amongst the disadvantages of the wave soldering technique are the requirements for relatively large, expensive equipment and a large quantity of solder when joining two PCBs. Additionally, the wave soldering technique is not generally suitable for modern components having many small contact areas. 
         [0005]    Reflow soldering is a known technique for providing electronic connections between components via a PCB in which a solder paste is applied between one or more electrical components and their contact pads on a PCB. The solder paste temporarily secures the electrical components in place. The PCB and the electrical components are then heated, thereby melting the solder. As the assembly cools, the solder solidifies and permanently connects the electrical components to the PCB. 
         [0006]    Reflow soldering is advantageous over wave soldering in that it is cleaner, quicker and can be fully automated. Using reflow soldering, modern components which have many small legs or contact areas can be soldered. However, reflow soldering precludes the use of a jig or mechanical support because the jig or mechanical support will obstruct the heating process, thereby preventing the melting of the solder. Furthermore, the jig or mechanical support cannot be put in place automatically, hence it would negate one of the key advantages of the reflow soldering technique. 
       SUMMARY 
       [0007]    According to an aspect, a method for joining a first PCB and a second PCB is provided. The first PCB has a shape defined by one or more edges and includes a connecting edge. The connecting edge has one or more electrical contacts disposed thereon and comprises one or more prongs extending therefrom. The second PCB comprises one or more electrical contacts and one or more holes for receiving the prongs of the first PCB. The method comprises connecting the first PCB to the second PCB by locating the prongs of the first PCB into the holes of the second PCB, and soldering the first PCB to the second PCB. 
         [0008]    Because the prongs of the first PCB are located into the holes of the second PCB, a secure mechanical connection is provided between the first and second PCBs before the soldering process takes place. Thus, no external mechanical support or jig is required when soldering the first and second PCBs using any soldering process. Furthermore, because no external mechanical support or jig is required, a reflow soldering process can be employed to join the first and second PCBs. 
         [0009]    Optionally, the first PCB comprises a substantially planar body and has at least three prongs. The prongs of the first PCB can be deflected in a direction substantially perpendicular to the plane of the first PCB. The second PCB comprises at least three holes, wherein the centre of one or more of the at least three holes is displaced from an axis passing through the centres of another two of the at least three holes. The step of locating the prongs of the first PCB into the holes of the second PCB includes deflecting the prongs of the first PCB substantially into the configuration of the holes of the second PCB. 
         [0010]    Because the prongs of the first PCB are deflected substantially into the configuration of the holes of the second PCB, the prongs of the first PCB form a “tripod” shape, providing enhanced stability and further securing the mechanical connection between the first and second PCBs by providing leverage for resisting lateral forces on the PCBs. Thus the PCBs can be held in a desired orientation for soldering very effectively. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Specific embodiments are described below by way of example only and with reference to the accompanying drawings, in which: 
           [0012]      FIG. 1  shows a connecting PCB; 
           [0013]      FIG. 2  shows a receiving PCB; and 
           [0014]      FIG. 3  shows the connecting PCB of  FIG. 1  connected to the receiving PCB of  FIG. 2 . 
       
    
    
     OVERVIEW 
       [0015]    In overview, a first, connecting PCB is provided. The first PCB has three projections or prongs extending from the main body of the PCB. A second, receiving PCB is provided, the second PCB having three holes into which the prongs of the first PCB can be inserted, thereby providing a secure mechanical connection during the soldering process. 
       DETAILED DESCRIPTION 
       [0016]      FIG. 1  shows a first PCB  10 . The first PCB  10  is thin and substantially flat. Its shape is defined by a plurality of substantially straight edges and an edge having a varied profile across its width (W), referred to herein as a “connecting edge”  12 . One or more electrical contacts  14  are provided on the connecting edge  12 . The electrical contacts  14  can be electrically coupled to one or more electronic components  24  provided on the first PCB  10 , as shown in  FIG. 3 . 
         [0017]    The connecting edge  12  comprises three prongs  16 ,  18 ,  20 . In the PCB shown in  FIG. 1 , a first prong  16  is provided at a first end of the connecting edge  12 , a second prong  18  is provided at a second, distal end of the connecting edge  12  and a third prong  20  is provided generally at the centre of the connecting edge  12 , in between the first  16  and second  18  prongs. The first  16  and second  18  prongs have a first plating  26  and a second plating  28  respectively, for receiving solder, as discussed in more detail below. 
         [0018]    As mentioned above, the PCB  10  is thin and so the prongs  16 ,  18 ,  20  have a small depth (usually just a few millimetres). The prongs  16 ,  18 ,  20  each project outwardly from and substantially co-planar to a main body  9  (shown upward of the broken line in  FIG. 1 ) of the first PCB  10 , in a direction generally perpendicular to the width (W) of the connecting edge  12 . Each of the prongs  16 ,  18 ,  20  has a width that is equal to or greater than its thickness, such that the prongs  16 ,  18 ,  20  can be flexed in a direction perpendicular to the plane of the first PCB  10 . 
         [0019]    According to an embodiment, each of the prongs  16 ,  18 ,  20  has a width of approximately 1 mm to 1.5 mm, a thickness or depth of approximately 1 mm to 1.5 mm and projects outwardly from the main body  9  of the first PCB  10  by a distance of approximately 1 mm to 1.5 mm. 
         [0020]    The profile of the connecting edge  12  further defines first  21  and second  23  tabs. The first tab  21  is provided intermediate the first  16  and third  20  prongs, with recesses  22  separating the tab  21  from each of the prongs  16 ,  20  adjacent thereto. The second tab  23  is provided intermediate the third  20  and second  18  prongs, again with recesses separating the tab  23  from the adjacent prongs  18 ,  20 . In the PCB  10  shown in  FIG. 1 , the tabs  21 ,  23  are wider than each of the prongs  16 ,  18 ,  20 . The prongs  16 ,  18 ,  20  project outwardly from the main body  9  of the first PCB  10 , in a direction perpendicular to the width (W) of the connecting edge  12 , to a greater extent than the tabs  21 ,  23  do. The above-mentioned electrical contacts  14  are provided on the tabs  21 ,  23 . 
         [0021]    The recesses  22  between the tabs  21 ,  23  and the prongs  16 ,  18 ,  20  further enable the prongs  16 ,  18 ,  20  to be flexed in a direction perpendicular to the plane of the first PCB  10 . Each of the recesses  22  has a width of approximately 1 mm. 
         [0022]      FIG. 2  shows a section of a second, receiving PCB  30 . The second PCB  30  has one or more electrical contacts  32  provided on a surface thereof. The electrical contacts  32  can be electrically coupled to one or more electronic components  40  provided on the second PCB  30 . As will be understood further from the description below, the electrical contacts  32  of the second PCB  30  substantially correspond in size and layout to the electrical contacts  14  of the first PCB  10 . 
         [0023]    The second PCB  30  shown in  FIG. 2  has a first hole  34 , a second hole  36  and a third hole  38 . The third hole  38  is provided between the first  34  and second  36  holes on a surface of the second PCB  30 . The distances between the first  34  and third  38  holes and between the second  36  and third  38  holes substantially correspond to the distances between the first  16  and third  20  prongs and between the second  18  and third  20  prongs of the first PCB  10 , respectively. The sizes and shapes of the holes  34 ,  36 ,  38  substantially correspond to the sizes and shapes of the prongs  16 ,  18 ,  20 , respectively. 
         [0024]    The first hole  34  includes a first plating  42  and the second hole  36  includes a second plating  44 . Each plating  42 ,  44  comprises a metallic ring around the edge that is defined by the respective hole  34 ,  36  for receiving solder, as discussed in more detail below. 
         [0025]    The above mentioned electrical contacts  32  on the second PCB  30  are provided in first  46  and second  48  groups. The first group  46  of electrical contacts  32  is provided intermediate the first  34  and third  38  holes and the second group  48  of electrical contacts  32  is provided intermediate the third  38  and second  36  holes. 
         [0026]    In  FIG. 2 , the centre of the first hole  34  and the centre of the second hole  36  lie on a common axis (A). The axis (A) also passes across the third hole  38 , but the centre of the third hole  38  is offset from the axis (A). Therefore the holes  34 ,  36 ,  38  form a triangular configuration. According to an embodiment, the centre of the third hole  38  is offset from the axis (A) by a distance of approximately 0.5 mm. 
         [0027]    The first  10  and second  30  PCBs can be manufactured using any suitable material. According to an embodiment, they are manufactured from glass fill epoxy resin. The electrical contacts  14 ,  26 ,  28 ,  32 ,  34 ,  36  are preferably manufactured from copper. In order to manufacture the first PCB  10  with copper on an edge thereof, arced recesses  15  can be provided on the distal ends of the electrical contacts  14  on the connecting edge  12  of the first PCB  10 . 
         [0028]    It is possible to form an electrical connection and a mechanical connection between the first  10  and second  30  PCBs.  FIG. 3  shows the first PCB  10  and the second PCB  30  when connected. In the configuration shown, the prongs  16 ,  18 ,  20  of the first PCB  10  are located in the holes  34 ,  36 ,  38  of the second PCB  30 , respectively. The electrical contacts  14  of the first PCB  10  are in contact with the electrical contacts  32  of the second PCB  30 , thus enabling electrical connection between the electrical components  24  provided on the first PCB  10  and the electrical components  40  provided on the second PCB  30 . 
         [0029]    Because the prongs  16 ,  18 ,  20  project outwardly from the main body  9  of the first PCB  10  further than the tabs  21 ,  24  do, the prongs  16 ,  18 ,  20  can project into or even through the holes  34 ,  36 ,  38  of the second PCB  30  whereas the ends of the tabs  21 ,  23  rest substantially flush with the planar upper surface of the second PCB  30 . The prongs  16 ,  18 ,  20  may project through the holes  34 ,  36 ,  38  such that the prongs  16 ,  18 ,  20  extend out of an opposite face of the second PCB  30 , or the prongs  16 ,  18 ,  20  may project only part of the way through the holes  34 ,  36 ,  38 . Arranging the prongs  16 ,  18 ,  20  to project only part of the way through the holes  34 ,  36 ,  38 , improves the quality of the soldering which can be achieved between the PCBs  10  and  30 . 
         [0030]    By locating the prongs  16 ,  18 ,  20  in the holes  34 ,  36 ,  38 , a secure mechanical connection between the first PCB  10  and the second PCB  30  is provided. The substantial correspondence between the relative sizes and shapes of the prongs  16 ,  18 ,  20  and the holes  34 ,  36 ,  38  serves to further enhance the secure mechanical connection. 
         [0031]    Additionally, because the centre of the third hole  38  is displaced from the axis (A) defined between the centres of the first  34  and second  36  holes, when the first PCB  10  and the second PCB  30  are connected, the third prong  20  is bent or deflected in a direction substantially perpendicular to the plane of the first PCB  10  (shown by arrow D 1  in  FIG. 3 ). The first and second prongs  16 ,  18  are also bent or deflected in a direction opposite the direction in which the third prong  20  is bent or deflected (shown by arrow D 2  in  FIG. 3 ), perpendicular to the plane of the first PCB  10 . The prongs  16 ,  18 ,  20  thus form a “tripod”. Because of the nature of the tripod configuration, at least one of the three prongs  16 ,  18 ,  20  is deflected away from the plane defined by the face of the first PCB  10 . Leverage is therefore provided for resisting lateral forces, and so the first PCB  10  and second PCB  30  are held in the desired orientation very effectively. Additionally, when the first PCB  10  is positioned vertically and the second PCB  30  is positioned horizontally with respect to the ground, the centre of gravity of the first PCB  10  falls within the prongs  16 ,  18 ,  20 , thus providing additional stability to the assembly. 
         [0032]    Once a mechanical connection has been formed between the PCBs  10 ,  30 , they can be electrically connected using a soldering process. Advantageously, because of the secure mechanical connection between the first  10  and second  30  PCBs, the electrical contacts  14  can be soldered to the electrical contacts  32  using a wave soldering technique without the use of any additional mechanical support such as a jig to secure the first PCB  10  and the second PCB  30  together during the soldering process. 
         [0033]    Furthermore, because no jig is required, a reflow soldering process may advantageously be used to solder the first PCB  10  to the second PCB  30 . This is particularly useful for modern electrical components, as explained in the background section above. 
         [0034]    During the soldering process, the plating  26 ,  28  of the first and second prongs  16 ,  18  can be soldered to the plating  42 ,  44  of the first and second holes  34 ,  36  to further secure the mechanical connection between the first PCB  10  and the second PCB  30 . 
         [0035]    Although the PCBs  10 ,  30  are described above as having a plurality of substantially straight edges, the shape of one or both of the PCBs can be defined by any of one or more arced or irregular edges, a circular edge, or any combination or arced, circular, straight and irregularly-shaped edges. The connecting edge itself may also comprise any of (or any combination of) arced, circular and straight edges, or may have an irregular shape. 
         [0036]    Whilst the first PCB  10  has been described as having three prongs  16 ,  18 ,  20  and the second PCB  30  has been described as having three corresponding holes  34 ,  36 ,  38 , the first PCB  10  can have one, two, four or more prongs and the second PCB  30  can have one, two, four or more holes. The first PCB  10  can have a greater number of prongs than the number of holes in the second PCB  30 , and likewise the second PCB  30  can have a greater number of holes than the number of prongs of the first PCB  10 . For example, the second PCB  30  may have a first, elongated hole or slit into which the first  16  and second  18  prongs of the first PCB  10  can be located, and the second PCB  30  may have a second hole into which the third prong  20  of the first PCB  10  can be located. One or more of the prongs may be offset from the plane of the main body of the first PCB  10 , such that little or no bending or deflection is required to insert the prongs into the holes in the second PCB  30 . 
         [0037]    Two or more first PCBs  10  may be connected to a second PCB  30  by providing additional holes  34 ,  36 ,  38  and electrical contacts  32  on the second PCB  30 . Additionally or alternatively, a chain of two or more PCBs each having the features of both the first PCB  10  and the second PCB  30  disclosed above can be connected together. 
         [0038]    In the PCBs described above, the platings  26 ,  28 ,  42 ,  44  are included on the first  16  and second  18  prongs of the first PCB  10  and in the first  34  and second  36  holes of the second PCB  30 . These platings may be omitted. Additionally or alternatively, platings could be provided on the third prong  20  and in the third hole  38 , or platings could be provided on any combination of the first  16 , second  18  and third  20  prongs and first  34 , second  36  and third  38  holes. 
         [0039]    Whilst the above description relates to connecting together two PCBs, it should be readily appreciated that the techniques disclosed herein may be used in any application where a secure mechanical connection is required between two electronic components during a soldering process.