FORM - 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE
SPECIFICATION
(See section 10; rule 13)
METHOD FOR CONNECTING PRINTED CIRCUIT BOARDS CONTROL TECHNIQUES LTD
a British Company
of The Gro Pool Road Newtown
SY16 3BE
United Kingdom.
Inventor: WAIN Richard
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE rNVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

FIELD
The invention relates to printed circuit boards and to a method for connecting printed circuit boards.
BACKGROUND
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.
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.
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.
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
An invention is set out in the claims.
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.
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,
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 boating the through of the first PCB into holes of the second PCB includes deflecting the prongs of the first PCB substantially into the configuration of the holes of the second PCB.
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
Specific embodiments are described below by way 0f example only and with reference to the accompanying drawings, in which:
Figure 1 shows a connecting PCB;
Figure 2 shows a receiving PCB; and
Figure 3 shows the connecting PCB of Figure 1 connected to the receiving PCB of Figure
2.

OVERVIEW
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
Figure 1 shows a first PCB 10. The first PCB 10 is thin and substantially fiat. 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 Figure 3.
The connecting edge 12 comprises three prongs 16, 18, 20. In the PCB shown in Figure 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.
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 Figure 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.
According to an embodiment, each of the prongs 16, 18, 20 has a width of approximately lmm to 1.5mm, a thickness or depth of approximately 1mm to 1.5mm and projects outwardly from the main body 9 of the first PCB 10 by a distance of approximately lmm to 1.5mm.
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 Figure 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.
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 lmm.
Figure 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.

The second PCB 30 shown in Figure 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.
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.
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.
In Figure 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 0.5mm.
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.

It is possible to form an electrical connection and a mechanical connection between the first 10 and second 30 PCBs. Figure 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.
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.
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.
Additionally, because the centre of the thir