Glazing with electrical terminal

0001The present invention relates to a glazing provided with an electric circuit including an electrically conducting substrate and a terminal for making electrical connection thereto. In particular, the circuit may be a heating element for defrosting or demisting the glazing, or an antenna circuit. The glazing may be for a vehicle window, or a window for an appliance or a building; in the case of a vehicle it may be a windscreen or rear window.

0002Various techniques are known for making electrical connection to a terminal. For instance, it is known from <patcit id="pcit0001" dnum="GB2223385A"><text>GB-A-2 223 385</text></patcit> to solder a terminal to a busbar or current collector. Alternatively, <patcit id="pcit0002" dnum="EP410766A"><text>EP-A-410 766</text></patcit> discloses an electrical device comprising a substrate supporting a film-type heater track formed of conductive polymer ink. A connector member is bonded to the substrate by means of adhesive, and the heater track overlies at least an end portion of the connector member to provide an electrical contact between them.

0003It is also known from <patcit id="pcit0003" dnum="EP278611A1"><text>EP 278 611 A1</text></patcit> to employ an electrically conductive adhesive to bond a busbar onto a conductive layer formed on a substrate, which may be used as a window. Furthermore, in <patcit id="pcit0004" dnum="GB1393887A"><text>GB 1 393 887</text></patcit>, the use of steel filings to make electrical contact through an adhesive coating is disclosed in the context of a rear window heating circuit.

0004<patcit id="pcit0005" dnum="DE19536131C1"><text>DE 195 36 131 C1</text></patcit> discloses a terminal for use with a heating circuit and a diversity antenna provided on a glazing pane. The terminal comprises a flexible base having electrical conductors embedded therein.

0005However, <patcit id="pcit0006" dnum="FR2519477"><text>FR 2 519 477</text></patcit> teaches that electrically conductive adhesives have various disadvantages, for example, they are not sufficiently durable under the conditions to which a vehicle window is typically subjected, and they are expensive. Instead, <patcit id="pcit0007" dnum="FR2519477"><text>FR 2 519 477</text></patcit> suggests that it is preferable to employ a non-conductive adhesive to maintain a suitably configured terminal in intimate contact with an electrically conducting substrate so as to ensure electrical continuity.

0006Unfortunately, the invention of <patcit id="pcit0008" dnum="FR2519477"><text>FR 2 519 477</text></patcit> is also not without disadvantages; it has been found that the inevitable mismatch of the thermal expansion coefficients of glass, metal and adhesive results in a weakening of the adhesive bond to the extent that electrical contact is adversely affected. Consequently, soldering remains a widely used technique for attaching electrical terminals to glazings, although the strength of the bonds produced is not always satisfactory, and there are increasing environmental concerns about metals, such as lead, which are commonly used in solders. Furthermore, the actual soldering operation is inclined to produce thermal stresses in the glass, which sometimes lead to damage.

0007It would be desirable to improve the attachment of electrical terminals to glazings, avoiding the disadvantages described above.

0008According to the present invention there is provided a glazing in accordance with claim 1.

0009The provision of a secure and reliable electrical connection between the terminal and the substrate allows adhesives to replace solder without attendant problems of electrical continuity. Modern adhesives are able to offer many advantages over solder. They may be flexible in the cured or set condition, and thereby absorb stresses generated in, or exerted on, the terminal. Adhesives are available which do not require to be heated in order to form a bond, or only need to be heated to a modest temperature, and yet which still provide a strong bond. Furthermore, it is usually possible to arrange the manufacturing operation so that the adhesive is heated just before it contacts the glazing, thereby avoiding the need to heat the glazing directly. Durable electrically conducting adhesives are now available, thereby allowing the adhesive to fulfil both the mechanical and electrical functions of a conventional soldered joint. Generally, such adhesives contain finely dispersed metal powder to make them conductive. Providing that the electric current to be carried is not large, the adhesive constitutes a sufficient electrical connection. A further important advantage is that suitable adhesives are available which have fewer health and safety hazards and less adverse environmental impact than the conventionally used solders.

0010Advantageously, especially in the automotive industry, the base may be of a standardised design of general applicability, whereas the connector part may be tailored to the specific requirements of a particular glazing, vehicle or vehicle manufacturer. That is, the base is preferably usable with a variety of differing designs of connector part.

0011Means of electrical connection between the terminal and the substrate may, for example, include the use of an electrically conducting adhesive, and/or a soldered connection. In high current applications, a joint soldered directly between the terminal and the substrate is preferable as the additional means of electrical connection, in order to obtain a low resistance connection. In this situation the solder complements the adhesive, as the latter bears the mechanical load and stresses, leaving the solder to provide, or enhance, electrical continuity without being compromised. This makes it possible, for instance, to use solders which are less environmentally aggressive, even if there is some loss in bond strength.

0012The invention also provides a method in accordance with claim 6.

0013Improvements can also be made in the type and method of application of the adhesive.

0014Preferably the adhesive is provided in the form of a tack-free tablet which is placed in contact with the terminal. The use of a tack-free (i.e. non-sticking) form of adhesive is advantageous because handling of the adhesive is greatly facilitated.

0015It is also preferable for the adhesive to be pre-shaped to match the part of the terminal that the tablet is in contact with. This increases the area of the terminal that is bonded, and hence the strength of the bond, while reducing the likelihood of adhesive escaping from underneath the terminal and becoming visible, which is likely to be unsightly. It also results in less waste and hence a cost reduction.

0016Advantageously the adhesive is pre-applied to the terminal, e.g. the terminals may be supplied with adhesive already applied to the requisite part of the terminal.

0017Optionally the adhesive may be heated to activate or cure it. The term "activating" an adhesive is used to refer to any process which initiates the bonding process, e.g. one which makes a previously tack-free adhesive sticky, or one which starts the curing process. Activation may involve melting the adhesive (at least on its surface), initiating a heat-dependent chemical reaction, or removing or destroying a barrier which separates two reactants. The extent of heating an adhesive is frequently less than is required for solder, and with regard to heating for activation, the adhesive need not necessarily be in contact with the glazing at the time of heating.

0018In this specification, the 'term "solder" is used to denote a fusible alloy of metals. The term "adhesive" is used to denote a substance employed to bond other substances together, but does not include a solder as such. An adhesive may be wholly non-metallic or may contain a metallic component.

0019The invention will now be further described by way of the following specific embodiments, which are given by way of illustration and not of limitation, and with reference to the accompanying drawings in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a perspective view of part of a glazing which includes an electric circuit and has a terminal attached;</li><li><figref idref="f0002">Fig. 2</figref> is a perspective view of a terminal which comprises separate base and connector portions;</li><li><figref idref="f0003">Fig. 3 (a)-(c)</figref> are perspective views of pre-shaped tablets of adhesive.</li></ul>

0020<figref idref="f0001">Figure 1</figref> illustrates a glazing 1 including an electric circuit 2. In this case the electric circuit is a resistive heating circuit printed on a pane 3 of glazing material in a conductive ink. Such circuits are used for defrosting and demisting, e.g. in vehicles, and as they are well known they need not be described further. Other equally well-known heating circuits comprise fine wires or a thin electroconductive coating on the pane. Alternatively the circuit may be an antenna circuit, again as is well-known.

0021The pane 3 of glazing material may be a sheet of glass, which may be annealed or toughened, or a composite pane made up of two or more plies of glass (or other glazing material) laminated together. In the case of an automotive glazing, the glass would be safety glass, i.e. toughened or laminated glass. There are alternative glazing materials to glass including various plastics such as polycarbonate for example.

0022Attached to the glazing, or more precisely, to an electrically conducting substrate 4 which forms part of the electric circuit 2, is a terminal 5. The embodiment of terminal 5 shown in <figref idref="f0001">Figure 1</figref> comprises a base part 6 and an upper (as illustrated) or connector part 7, which are adapted to mutually engage, as will be explained in more detail below in connection with <figref idref="f0002">Figure 2</figref>. The base part 6 is attached to the glazing 1 by adhesive 8, again as will be explained in more detail below. The terminal may be in intimate physical contact with the substrate, but this is not necessary since an alternative means of electrical connection is provided, as described below.

0023In this embodiment, the electrically conducting substrate 4 constitutes a busbar supplying current to heating elements of the electric circuit 2. The substrate may be composed of thin sheet metal, such as a foil, or may be made from an ink which is applied to the pane (e. g. by screen printing), dried, and fired to provide a durable conducting path. For example, silver-containing inks are widely used in the manufacture of electrically heated rear windows for vehicles.

0024The terminal 5 includes a spade connector 9, to which an electrical lead (not shown) may be attached in conventional fashion by a corresponding female connector. Many variations on the type and embodiment of connector are possible; for example, it may be cranked, or otherwise bent, to facilitate access or to improve its aesthetic aspect. Alternative forms of connector, e.g. the press stud type, may also be used. The terminal is manufactured from thin sheet metal; in particular, copper sheet, preferably tinned to prevent oxidation, is a suitable material.

0025<figref idref="f0002">Figure 2</figref> shows more details of the terminal 5. As already mentioned, the terminal 5 comprises a base part 6 and a connector part 7, which are adapted to mutually engage each other. For example, base part 6 may be provided with tabs 20, which are bent around the connector part 7, e.g. around the spade 9.

0026It has already been mentioned that it is convenient for the terminals to be supplied with the adhesive pre-applied, especially in a tack-free form. The tablet of adhesive (generally comprising pre-mixed components) is pressed onto the surface of the terminal with just sufficient heat to cause it to adhere to the surface. When the terminal is in two (or more parts) as in <figref idref="f0001">Figures 1</figref> and <figref idref="f0002">2</figref>, it is especially convenient for the base part 6 to be of a standard universal design, with the adhesive pre-applied to it. This means that only one design of base part need be purchased, which is then used with a connector part which suits the particular vehicle glazing being manufactured. The base part may either be attached to the glazing first, or to the connector first, whichever is more convenient. In <figref idref="f0002">Figure 2</figref>, a tablet of adhesive 8 is applied to the terminal base part 6, as indicated by arrow A. The base part is then attached to the connector part 7 by means of tabs 20, as indicated by arrow B. Finally the terminal is attached to the glazing, as will be described below.

0027The adhesive may be electrically conducting, in which case no further electrical connection may be necessary. However, if the adhesive is non-conducting, or if the electric circuit has a high power requirement, a means of electrical connection will be required between the terminal and the electric circuit. This may conveniently be provided by a soldered joint between the terminal and the electrically conducting substrate. In <figref idref="f0002">Figure 2</figref>, the application of solder is diagrammatically represented by arrows C and solder 21. For an antenna, which of course only produces a very small current, the use of a conducting adhesive may often provide sufficient electrical connection.

0028<figref idref="f0003">Figure 3 (a)-(c)</figref> show shaped tablets 50, 51, 52 of adhesive, the tablet being pre-shaped to suit the shape of the terminal. Pre-shaping reduces waste, improves conformity to the terminal surface and improves flow control during application of adhesive. A stronger and neater bond is thereby obtained, and this technique may be used with either embodiment of terminal.

0029In <figref idref="f0003">Figure 3 (a)</figref> , a T-shaped block 50 of adhesive is shown, which is suitable for use in the correspondingly T-shaped part of the terminal (i.e. where the limbs of the "T" cross, one limb being the spade). <figref idref="f0003">Figure 3(b)</figref> shows a thin rectangular block 51 of adhesive and <figref idref="f0003">Figure 3(c)</figref> shows a thin cylindrical tablet 52 of adhesive.

0030There are many suitable adhesives for attaching terminals to glazing. Polyurethanes are a suitable class of adhesives, including moisture-cured polyurethane (e.g. Betaseal HV3 available from Gurit-Essex AG of Freienbach, Germany), moisture/heat cured polyurethane (e.g. Sika 360 HC available from Sika AG of Switzerland), reactive hot melt polyurethane (e. g, PUR-FECT 310 available from National Starch &amp; Chemical Company of Bridgewater, New Jersey, USA, a member of the ICI Group), two component polyurethane, and other polyurethane-based compositions (such as Techbond PUR available from A. Raybond SARL of 68300 Saint-Louis, France). A further suitable class of adhesives is that of epoxy based compositions, e.g. Raybond's Techbond EPO. Also suitable are structural adhesive tapes (e.g. 3M 9214 available from the Minnesota Mining and Manufacturing Company of St. Paul, Minnesota, USA). Suitable conducting adhesives contain finely dispersed metallic particles in sufficient quantity to pass the required current density.

0031The preferred tack-free adhesives in tablet form are available from A. Raybond SARL in both epoxy and polyurethane compositions (Techbond EPO and PUR), and terminal base parts can be supplied with the adhesive pre-applied. In a polyurethane-based composition such as Techbond PUR, the polyol and isocyanate reactants are pre-mixed in stoichiometric ratio, but the isocyanate is "end capped" or micro-encapsulated to prevent contact with the polyol..The adhesive is solid at room temperature. Application of heat activates the adhesive by destroying, e.g. melting, the capping or encapsulating membrane and allowing the isocyanate component into contact with the surrounding polyol so that the curing reaction starts. The method of use of these adhesives is as follows.

0032A pane of glass carrying an electric circuit is provided and cleaned with a cleaner such as Betaseal VP-04604 from Gurit-Essex; a terminal, or terminal base part, with tack-free adhesive pre-applied is also provided. The terminal is positioned over the pane, and rapidly heated to a temperature in the range 100°-140°C. Suitable rapid heating techniques include infra-red lamps, hot air jets, inductive heating or radio frequency dielectric heating. The heat activates the adhesive, and the terminal is pressed into position on the pane. During the initial cooling period the terminal should be held in place, until the temperature has reduced to a value in the region of 60°-80°C. The pane may then be moved, but the bond does not develop full strength until post polymerisation has been completed, which may require up to 20 minutes at room temperature. Note that the pane need not be directly heated, thereby reducing thermal stresses.

0033As mentioned above, in many cases it will be necessary to make a separate electrical connection between the terminal and the circuit, and a preferred method involves soldering. In this case, the heat of soldering may advantageously also be used to activate the adhesive and eliminate a separate operation. The locations on the terminal body to which solder is applied should be close to those to which adhesive is applied. A soldering tool (which may be automated, e.g. robot-controlled) is then used to solder the terminal in position, and simultaneously heat the adhesive to activate it. The soldering tool may be one which contacts the work, or a hot air soldering technique may be used. Generally, soldering requires a higher temperature than activation of the adhesive, so the temperature attained may be governed by soldering considerations. However, one of the advantages of the invention is that the mechanical bond is provided by the adhesive, so that the solder solely provides an electrical connection. Consequently, solders may be used which have a lower melting point than those used in the prior art, where both mechanical and electrical considerations apply.

0034In practice, certain types of adhesive are activated by melting them, and so the heat used to melt the solder preferably also melts the adhesive. It is therefore advantageous to select an adhesive and a solder which have similar melting points. Suitably, the adhesive and solder have melting points within 20°C, preferably 10°C, most preferably 5°C of each other.