Abstract:
A method of forming an electrical circuit for an electrical component comprises the steps of; producing an electrical circuit from a planar conductive material, the circuit including tracks and tie bars between tracks; over-moulding the electrical circuit ith plastics material leaving tie bars in apertures; and removing the tie bars after the over-moulding process.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention concerns improvements in and relating to the manufacture of electrical circuits for electrical components. 
       BACKGROUND OF THE INVENTION 
       [0002]    The shape and functionality of electrical devices such as, for example, circuit breakers, residual current devices, ground fault interrupters and arc fault interrupters, has remained unchanged for years. Within the electrical switchgear and circuit protection and monitoring industries there is an increasing drive and desire to reduce the size of electrical devices and to integrate more functionality into the devices. 
         [0003]    Increasing the functionality of such electrical devices typically involves integration of electronic components. However, for known electrical devices, this leads to the problem of insufficient internal surface area to accommodate additional printed circuit boards, or other similar substrates, required to enable the increased functionality, while maintaining the external size of the device. 
         [0004]    In our co-pending application we propose over-moulding stamped or etched electrical circuits in plastics material. There is, however, a need to provide support for such circuits during the moulding process. 
       SUMMARY OF THE INVENTION 
       [0005]    An object of the present invention is to provide a method of manufacturing electrical circuits for electrical components in which the electrical circuits are over-moulded with plastics material. 
         [0006]    According to the invention there is provided a method of forming an electrical circuit for an electrical component comprising the steps of:
       a) producing an electrical circuit from a planar conductive material, the circuit including tracks and tie bars between tracks;   b) over-moulding the electrical circuit with plastics material leaving tie bars in apertures; and   c) removing the tie bars after the over-moulding process.       
 
         [0010]    The present invention further provides an electrical circuit for an electrical component manufactured by the steps of:
       a) producing an electrical circuit from a planar conductive material, the circuit including tracks and tie bars between tracks;   b) over-moulding the electrical circuit with plastics material leaving tie bars in apertures; and   c) removing the tie bars after the over-moulding process.       
 
         [0014]    The electrical circuit may be formed in any suitable way but typically will be formed by stamping or etching the planar conductive material. Planar sheet copper alloy is preferably used for the electrical circuit. 
         [0015]    Over-moulding is preferably carried out in a mould having formations that mate with tie bar locations to form apertures around the tie bars in the over-moulded electrical circuit. Preferably such a mould formation will comprise a platform and raised portions that locate either side of a tie bar. Preferably the raised portions of the mould formation a tight fit between the tie bar and connected tracks, in order to prevent flashing or leakage of plastics material. 
         [0016]    The tie bars are preferably removed by laser cutting, although a fluid jet or mechanical punching could be used. Laser cutting of the tie bars may be carried out as a single shot process or may be by cutting the tie bars at opposite track edges to form a slug that drops away. Preferably tie bar material is removed during or after laser cutting by blown gas or by suction. The laser cutting operation is preferably microprocessor controlled. 
         [0017]    After removal of the tie bars a second overmoulding process is preferably carried out and this overmoulding preferably fills gaps created by removal of tie bars. 
         [0018]    Where the electrical circuit has multiple tracks alongside each other it is preferred that tie bars be staggered, so that inaccurate removal of tie bars is less likely to cause loss of integrity of tracks and or impair track to track isolation characteristics, especially for high voltage tracks. 
         [0019]    The spacing between adjacent tracks is advantageously greater adjacent the tie bars. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    This invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0021]      FIG. 1  shows a typical lead frame for an electrical component; 
           [0022]      FIG. 2  shows the lead frame of  FIG. 1  over-moulded with tie bar apertures; 
           [0023]      FIG. 3  shows tie bar detail: 
           [0024]      FIG. 4  shows mould tool detail for forming tie bars; 
           [0025]      FIG. 5  shows a preferred tie bar configuration; 
           [0026]      FIG. 6  shows worst case cutting of tie bars of a  FIG. 5  type tie bar configuration; 
           [0027]      FIG. 7  shows worst case cutting of tie bars of an alternative tie bar configuration; and 
           [0028]      FIGS. 8 and 9  show a tie bar geometry wherein the spacing between adjacent tracks is greater adjacent the tie bars. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Referring to the accompanying drawings, in order to create an electrical circuit for an electrical component a lead frame  10  is formed from a planar sheet of copper alloy by stamping or chemical etching. The lead frame includes various tracks  12  and tie bars  14  holding tracks together for over-moulding with plastics material  16  to create an over-moulded lead frame as shown in  FIG. 2  of the drawings. 
         [0030]    The tie bars  14  are provided to support the tracks and hold the lead frame together during the over-moulding process. Thereafter the tie bars are removed by laser cutting and the over-moulded component can be further over-moulded to provide desirable surface features, such as location formations for other over-moulded lead—frames. The additional overmould also forms an effective electrical isolation barrier allowing high voltage circuits to be placed in close proximity to one another whilst maintaining a high breakdown voltage, typically in the order of 4 Kv @ 1 mm separation. 
         [0031]    In order to form the tie bars  14  the over-moulding has to leave space around the tie bars, so that the laser cutting does not damage the plastics material in over-mould burning or melting Therefore, as shown in  FIG. 3  the plastics material is moulded so as to leave apertures  18  around the tie bars  14 . To achieve this the mould ( FIG. 4 ) for the over-moulding process incorporates platforms  22  with castellations  24  that locate between tracks  12  either side of a tie bar  14 . Such a mould creates the apertures  18  around the tie bars as shown in  FIG. 3 . A close fit between castellation  24 , tie bar  14  and track  12  is needed in order to prevent flashing or leakage of plastics material. 
         [0032]    After over-moulding the tie bars need to be cut and laser cutting is preferred to burn away the tie bars. However, cutting of the tie bars may alternatively be carried out using a fluid jet—either a gas or liquid. When using a laser the aperture around a tie bar is needed to reduce or eliminate burning of plastics material surrounding the tie bar. Mechanical punching to remove tie bars is a possible alternative to laser or fluid jet cutting but mechanical punches are relatively expensive and difficult to manufacture at the sizes required, would be prone to damage from use and would transmit undesirable mechanical stress to the surrounding over-mould and track. Additionally, circuit density, track and gap width would have to be less than for a circuit prepared using laser or fluid jet cutting and tie bar apertures would need to be larger to accommodate a suitably sized punch and die block. 
         [0033]    Advantageously using laser or fluid jet cutting allows a non-dedicated tool to be used for different track layouts and no mechanical force is imparted to the lead frame. Mechanical punching may deform or delaminate tracks from the overmoulding. The laser cutting or fluid jet process is controlled by a microprocessor, which can be reprogrammed according to the electrical circuit design. 
         [0034]    Laser or fluid jet cutting may be carried out as a single shot, wherein the tie bar is removed in its entirety. Alternatively, a cut line method may be used, wherein two lines are cut at the track edges allowing the remaining slug of the tie bar to fall away. The aperture around the tie bar means that the molten tie bar material or the tie bar slug are not captured in the surrounding plastics material. 
         [0035]    The tie bar size is important for providing sufficient support to its adjacent tracks as well as allowing effective removal thereof. The preferred track width to track gap ratio is between about 1.68:1 and 1:1, especially between 1.68:1 and 1.5:1. A typical circuit example has a tie bar width of 0.30 mm and track spacing of 0.42 mm. In order to remove the tie bar effectively by laser, the laser has to have a spot size of 0.40 mm in diameter. The minimum aperture gap either side of the tie bar area to be cut is 0.40 mm, whilst the minimum aperture width is the track spacing plus 2½ times the half track width. This gives an effective minimum aperture size of 1.22 mm×0.80 mm. The aperture formed by the moulding itself is undrafted, due to the relative thinness of the over-mould. 
         [0036]    After removal of the tie bars a second overmoulding typically of an engineering thermoplastic, resin or encapsulent, is carried out in order to fill the gaps created by removal of the tie bars. 
         [0037]      FIGS. 5 to 7  illustrate other factors to be taken into account when siting tie bars between multiple track arrangements. In these circumstances desirably tie bars between adjacent pairs of tracks sited close together are staggered relative to each other as shown in  FIG. 5 .  FIG. 6  shows the effect of laser removal of tie bars when the laser cuts are misplaced so that they cut up to 50% into the tracks. As can be seen the integrity of each track is maintained. However, as shown in  FIG. 7  with non-staggered tie bars misplaced laser cutting up to 50% into the tracks can lead to a very thin section of track remaining or complete loss of track integrity rendering the circuit useless. 
         [0038]      FIG. 8  shows a preferred embodiment wherein the tracks,  112   a,    112   b  and  112   c  are joined by tie bars,  114   a  and  114   b,  and separated by a space  126  extending parallel with adjacent tracks between tie bars. In this region the dimension of the space  126  between adjacent tracks is less than approximately 0.4 mm. In the region of the tie bars  114  the dimension of the space  126  between adjacent tracks is greater and the track width is reduced to a minimum of about 0.25 mm ( 127 ) thereby forming an enlarged space  128 . 
         [0039]      FIG. 9  shows the preferred embodiment, of  FIG. 8 , wherein the track spacing is increased and the track width decreased adjacent the tie bar. 
         [0040]      FIG. 9   a  shows tracks  112  and tie bars  114  having geometry as shown in  FIG. 8  compared to the track  12  and tie bar  14  on the left as previously described, with reference to  FIGS. 4 to 7 . 
         [0041]      FIG. 9   b  shows how an accurately executed cut removes the tie bars  14  and  114 , of both the previously described track and tie bar configuration, of  FIGS. 4 to 7 , and the profiled track and tie bar configuration of  FIG. 8 . 
         [0042]      FIG. 9   c  shows how an inaccurately executed cut fails to successfully remove the tie bar  14  of the previously described track  12  and tie bar  14  configuration, of  FIGS. 4 to 7 , but successfully removes the tie bar  114  of the profiled track  114  and tie bar  112  configuration of  FIG. 8 . 
         [0043]    The preferred embodiment of  FIGS. 8 and 9  therefore provides a significant advantage in that it is more tolerant of the manufacturing process.