Abstract:
An improved electrical connector for use with a glazing is disclosed. The glazing preferably comprises a ply of glazing material having a first electrically conductive component mounted thereon, and a second electrically conductive component, joined to the first by a solder. The second component comprises first and second connector feet linked by a bridge portion, the bridge portion being at a height h above each of the connector feet, and each of the feet comprises at least one protrusion having a height d. At least one of h or d is chosen to maximise the adhesion between the second electrically conductive component and the first electrically conductive component. Preferably, the glazing is an automotive glazing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electrical connector for an automotive glazing. 
     BACKGROUND DISCUSSION 
     A variety of electrical connectors are used throughout the automotive glazing industry. For example, T-piece connectors may be used to connect an electrically conductive circuit, such as a circuit printed on the surface of a ply of glass, or an array of wires fixed within a laminated glazing, to the wiring harness of a vehicle. Such circuits generally find use as heating circuits, to promote de-misting or de-icing, or antenna circuits. The T-piece connector is soldered to an electrically conductive substrate known as a bus bar, which may be provided directly on the surface of a piece of glass, or fully or partly on a fired, printed band on the glass, known as an obscuration band. The bus bar is typically printed using a silver-containing ink. Traditionally, the solder used to join the bus bar and the connector contains lead. However, lead is known to be harmful, and there is increasing pressure to use lead-free solders in the automotive industry, for example, such as that described in WO2004/068643. WO2004/068643 discloses tin-based solders (up to 90% by weight tin), comprising a mechanical stress modifier selected from bismuth or indium. The solder may also contain silver and/or copper. 
     However, one disadvantage to using a lead-free solder is that the adhesion between the connector and the bus bar may not be as high as that given by a lead containing solder. One solution to this is to additionally use an adhesive to bear the mechanical load on the connector, as in EP 1 256 261. 
     Even if additional adhesive means are used, the adhesion between the connector and the bus bar due to the solder will degrade over time due to environmental conditions, such as extremes of temperature and humidity. Once the joint between the connector and the bus bar starts to degrade, the adhesion decreases and the connector may become loose or detach from the bus bar altogether. In addition, when the joint deteriorates, the quality of the electrical connection achieved may become poor. This degradation may be quicker if there are regions of poor adhesion due to air bubbles or debris at the surface where the connector and bus bar are joined. To improve the reliability of the joint therefore, great care must be taken not only in adhesive and solder selection, but with processing techniques and conditions. 
     SUMMARY 
     It is therefore desirable to be able to find an alternative manner in which to improve the reliability of the joint between the connector and the bus bar when exposed to various environmental conditions, which does not rely on materials selection or special processing techniques, but which aids in preventing degradation of both adhesion and electrical connectivity. 
     The present invention aims to address these problems by providing a glazing comprising a ply of glazing material having a first electrically conductive component mounted thereon, and a second electrically conductive component, joined to the first by a solder, the second component comprising first and second connector feet linked by a bridge portion, the bridge portion being at a height h above each of the connector feet, and each of the feet comprising at least one protrusion having a height d, wherein at least one of h or d is chosen to maximise the adhesion between the second electrically conductive component and the first electrically conductive component. 
     By optimising the design of the connector, the reliability of the joint between the first and second components is improved, without the need to provide additional adhesive or implement complex processing steps. 
     Preferably, h is in the range 1.0 mm to less than or equal to 5.0 mm. Preferably d is in the range 0.00 mm to 1.0 mm. More preferably, d is in the range 0.3 mm to 0.75 mm. 
     When the first electrically conductive component is placed on flat float glass, and the second electrically conductive component soldered thereto, at least one of h or d is chosen such that the mean load required to remove the second electrically conductive component is preferably greater than 20 kg. 
     More preferably, both h and d are chosen to improve the adhesion between the second electrically conductive component and the first electrically conductive component. Both h and d may be chosen to improve the adhesion between the second electrically conductive component and the first electrically conductive component, after 14 days at 50° C. and 95% relative humidity. 
     The second electrically conductive component may be made of copper. The first electrically conductive component may be a busbar. The surface of the ply of glazing material may be printed around its periphery with a fired ink band. In this case, at least part of the first electrical component may be provided on the fired-ink band. 
     Preferably, the second electrically conductive component is a T-piece connector. Preferably, the solder is a lead-free solder. Preferably, the glazing is an automotive glazing. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING FIGURES 
       The invention will now be described by way of example only, and with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view (not to scale) of a glazing having an electrical connector mounted thereon; 
         FIG. 2  is a perspective view (not to scale) of the electrical connector used in  FIG. 1 ; and 
         FIG. 3  is a schematic cross-section of part of the connector of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     A variety of electrical connectors are used within the automotive glazing industry to connect circuitry provided on or within glazing panels with the wiring harness of a vehicle. A typical connector is known as a T-piece, and is generally in the shape of a “T”, although other connectors, for example, J-shaped and strip-shaped connectors are known. Each type of connector has three features in common: two connector feet joined by a raised bridge portion; protrusions on the bottom of each connector foot; and each is formed from a thin strip or sheet of a metallic material, such as copper. However, it is also common to use connectors which do not have protrusions on the connector feet. Although the following examples are described in terms of T-pieces, the principles of the invention may be applied to all types of electrical connectors sharing these features. 
       FIG. 1  is a perspective view (not to scale) of an automotive glazing  10  having an electrical connector  14  soldered to an electrically conductive layer  13 . The automotive glazing  10  comprises a sheet of glazing material  11 , such as a ply of toughened glass, having a fired, black ceramic obscuration band  12  on one surface. An electrically conductive layer  13 , known as a bus bar, is provided on the obscuration band  12 , typically by printing the surface of the obscuration band  12  with an electrically conductive silver-containing ink. The obscuration band  12  is provided on glazings forming windscreens backlights and some rooflights, but may not be provided on glazings forming sidelights. In this case, the bus bar  13  is provided directly onto the surface of the glazing material  11 . 
     An electrical connector  14  is mounted on the bus bar  13 . The electrical connector  14  comprises a pair of connector feet  15 , for attaching to the bus bar  13 , and a connector arm  16 , for attaching to the wiring harness of a vehicle in which the glazing is fitted. The connector feet  15  are linked together and joined to the connector arm  16  by a raised portion known as a bridge  17 . Each of the connector feet  15  is attached to the glazing by means of a layer of solder  18 , between each foot  15  and the bus bar  13 . 
       FIG. 2  is a perspective view (not to scale) showing the electrical connector  14  in more detail. The electrical connector  14  is formed from a thin strip or sheet of a metallic material, such as copper. The bridge  17  is in a fixed position with respect to the connector feet  15  and connector arm  16 , so that when the electrical connector  14  is soldered in place, the bridge  17  is at a height h above the base of the connector feet  15 . 
       FIG. 3  is a schematic cross-section of a single connector foot  15  mounted on the glazing  10 . A pair of protrusions or dimples  19  are provided on the base of the connector foot  15 , each having a height d. The protrusions  19  ensure that the main body of the foot  15  is kept at a set height above the bus bar  13 . In use, the layer of solder  18  will completely surround the protrusions  19  and contact the base of the connector foot  15 , but this is omitted from  FIG. 3  for clarity. 
     The electrical connector  14  may be soldered to the bus bar using various techniques, including hot air, hot iron and resistive heating. The typical soldering time is between 1 and 3 seconds, with 2 to 4 seconds cooling time. During soldering of the connector feet  15  to the bus bar  13 , the region of the glazing material  11  between the feet remains relatively cool compared with the connector bridge  17 , which is heated almost to the soldering temperature. This leads to a differential in expansion between the connector bridge  17  and the glazing material  11 . As the solder layer  18  sets, the temperature change on cooling for the connector bridge  17  is much greater than for the glazing material  11 , leading to a differential in the contraction between the connector bridge  17 , and the glazing material  11 . This expansion and contraction mismatch leads to the generation of stress in the surface of the glazing material in the region where the solder  18  has been applied. In the region of the glazing material  11  directly underneath the connector foot  15 , a tensile stress is produced, whereas in the region of the glazing material  11  underneath the connector bridge  17 , a compressive stress is produced. Any flaw at the glazing material surface produces a region of high tensile stress. 
     In the present invention it has been appreciated that optimising the design of the connector offers a practical alternative to the approaches of the prior art for improving the reliability of the solder joint between an electrical connector and a bus bar on an automotive glazing. In particular, two features of the design of the connector may be optimised to improve the adhesion between the connector and the bus bar: 
     a) the height of the connector bridge h; and 
     b) the height of the protrusions on the connector feet d. 
     In each of the tests below, the metallic connectors used are of the T-piece type. 
     Connector Bridge Height 
     Tests were carried out to determine the effect of connector bridge height on adhesion immediately after soldering. Samples were made from 4 mm thick flat float glass, having a fired-ink obscuration band (printed using IT57M202 black ceramic ink, available from Johnson Matthey, Fregatweg 38, 6222 NZ Maastricht, The Netherlands), and bus bars printed using SP1876 silver-containing ink (available from Ferro AG, Gutleutstrasses 215, PO Box 110403, D60039, Frankfurt-am-Main, Germany). The solder used was a lead-containing solder, including, by weight, 25% tin, 62% lead, 3% silver and 10% bismuth (available from Anglo Production Processes, Saxon Business Park, Hanbury Road, Stoke Prior, Bromsgrove, Worcestershire B60 4AD). The glass was toughened between printing and soldering. A vertical pull test, where the connector was pulled perpendicular to the surface of the glass, was used to measure the load required to pull the connector from the bus bar. Connectors with bridge heights of 0.5 mm, 1.0 mm, 1.5 mm and 2.0 mm were used. The mean loads required to remove the connectors are shown in Table 1. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 mean applied load for bridge height test samples 
               
             
          
           
               
                 Bridge Height 
                 Bridge Height 
                 Bridge Height 
                 Bridge Height 
               
               
                 0.5 mm 
                 1.0 mm 
                 1.5 mm 
                 2.0 mm 
               
               
                 Load 
                 Load 
                 Load 
                 Load 
               
               
                 (kg) 
                 (kg) 
                 (kg) 
                 (kg) 
               
               
                   
               
             
          
           
               
                 29.0 
                 33.7 
                 35.1 
                 35.4 
               
               
                   
               
             
          
         
       
     
     Three failure mechanisms were observed: the connector became detached through failure of the soldered joint; failure occurred at both the soldered joint and the glass surface; and the glass in the region of the soldered joint shattered before the soldered joint failed. 
     As can be seen from Table 1, the mean load applied to remove the connector increases with increasing bridge height. Therefore, the adhesion of the connector increases with bridge height. A desirable range for the bridge height is 1.0 mm to 5.0 mm, with the upper range only limited by factors including connector feeding and space constraints. 
     Connector Foot Protrusion Heights 
     In order to determine how the height of protrusions on each foot of the connector affected the adhesion of the connector to the bus bar, both immediately after soldering and after accelerated aging, four sets of samples, each having different protrusion heights, were pull tested. Each connector foot has two protrusions, at heights of 0 mm (i.e. no protrusions) 0.3 mm, 0.5 mm and 0.75 mm. The connectors were soldered to a bus bar printed using a mix of 80% silver and 50% silver (to give an overall silver content of 77%) pastes, (available as 1749 and 1752 from Chimet Thick Film Division, Via di Pescaiola 74, 52040 Viciomaggio (Arezzo), Italy) on 5.0 mm thick flat clear float glass (sample size 300 mm×300 mm). The glazing was toughened after printing and before soldering. 
     Table 2 shows the load required to pull the connector from the bus bar for each protrusion height, immediately after soldering. Three different solders were used: a lead containing solder (including, by weight, 62% lead, 25% tin, 3% silver and 10% bismuth (available from Litton, 6 First Avenue, Globe Park, Marlow, SL7 1YA)); lead-free solder (1) (including, by weight, 42% tin, 57% bismuth and 1% silver (available from Indium (UK), 7 Newmarket Court, Kingston, Milton Keynes, MK10 0AG)) and lead-free solder (2) (including, by weight, 95.5% tin, 3.8% silver and 0.7% copper (available from Multicore Solders Ltd., Kelsey House, Wood Lane End, Hemel Hempstead, HP2 4RQ)). 
     
       
         
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Mean Load necessary to remove connector 
               
               
                 immediately after soldering 
               
             
          
           
               
                   
                 Protrusion 
                 Protrusion 
                 Protrusion 
                 Protrusion 
               
               
                   
                 Height 
                 Height 
                 Height 
                 Height 
               
               
                   
                 0.00 mm 
                 0.30 mm 
                 0.50 mm 
                 0.75 mm 
               
               
                   
                 Mean 
                 Mean 
                 Mean 
                 Mean 
               
               
                   
                 Load (kg) 
                 Load (kg) 
                 Load (kg) 
                 Load (kg) 
               
               
                   
                   
               
             
          
           
               
                 Lead-containing Solder 
               
             
          
           
               
                   
                 42 
                 38 
                 39 
                 36 
               
             
          
           
               
                 Lead-free Solder (1) 
               
             
          
           
               
                   
                 21 
                 28 
                 31 
                 33 
               
             
          
           
               
                 Lead-free Solder (2) 
               
             
          
           
               
                   
                 26 
                 29 
                 30 
                 36 
               
               
                   
                   
               
             
          
         
       
     
     Table 3 shows the load required to pull the connector from the bus bar for each protrusion height, after accelerated aging. The samples were aged for 14 days in a weathering cabinet at 50° C. and 95% relative humidity. Again, three different solders were used: a lead containing solder (including, by weight, 62% lead, 25% tin, 3% silver and 10% bismuth); lead-free solder (1) (including, by weight, 42% tin, 57% bismuth and 1% silver) and lead-free solder (2) (including, by weight, 95.5% tin, 3.8% silver and 0.7% copper), all available as before. 
     
       
         
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Mean Load necessary to remove connector 
               
               
                 after 14 days at 50° C., 95% RH 
               
             
          
           
               
                   
                 Protrusion 
                 Protrusion 
                 Protrusion 
                 Protrusion 
               
               
                   
                 Height 
                 Height 
                 Height 
                 Height 
               
               
                   
                 0.00 mm 
                 0.30 mm 
                 0.50 mm 
                 0.75 mm 
               
               
                   
                 Mean 
                 Mean 
                 Mean 
                 Mean 
               
               
                   
                 Load (kg) 
                 Load (kg) 
                 Load (kg) 
                 Load (kg) 
               
               
                   
                   
               
             
          
           
               
                 Lead-containing Solder 
               
             
          
           
               
                   
                 38 
                 33 
                 36 
                 32 
               
             
          
           
               
                 Lead-free Solder (1) 
               
             
          
           
               
                   
                 19 
                 22 
                 30 
                 26 
               
             
          
           
               
                 Lead-free Solder (2) 
               
             
          
           
               
                   
                 13 
                 14 
                 21 
                 18 
               
               
                   
                   
               
             
          
         
       
     
     For the lead containing solder, the results in Tables 2 and 3 indicate that an increase in protrusion height has little effect on the adhesion between the connector and the bus bar, both immediately after soldering, and after accelerated aging. However, for both lead-free solder compositions, increasing the protrusion height causes an increase in adhesion immediately after soldering. After aging, an optimum protrusion height of 0.50 mm is seen for both lead-free solder compositions. From the trends in Tables 2 and 3, an upper limit of 1.0 mm for the protrusion height is desirable, with the height preferably in the range 0.3 mm to 0.75 mm. 
     In order to optimise the design of the electrical connector, at least one of these factors may be used to increase the adhesion between the connector and the bus bar. Preferably, both may be combined. In order to provide sufficient adhesion, it is preferred that d and h are in the ranges 1.0 mm≦h≦5.0 mm and 0.00 mm≦d≦1.00 mm, and that the mean load required to remove the connector from the bus bar is greater than or equal to 20 kg. By altering and optimising each of the bridge height and protrusion height, it is possible to produce a T-piece design, which improves the adhesion between the connector and bus bar to which it is applied. The optimised design is particularly effective when used with a lead-free solder.