EP0473473B1 - Heated windscreen - Google Patents

Description

The invention relates to a screen-printed electrical network for heated glazing, in particular for a rear window of a motor vehicle. More specifically, the invention relates to a glazing unit fitted with all of its conductive strips, that is to say strips forming strictly speaking the heating network and collector strips intended to be connected by welding to the supplied current leads. by the vehicle's current source.

To produce a glazing with a heating network ensuring the functions of defrosting and demisting, it is known for example from patent FR-B-1 464 585 to deposit on the surface of a glass sheet a series of narrow resistance bands of an electroconductive composition. These resistance bands are screen printed on the glass sheet before its bending, so that the electrically conductive composition is baked during the heating preceding the bending and / or tempering operation of the glazing. The electroconductive composition is made of a pasty suspension, in an organic binder, of metallic silver and a frit, that is to say of a glass with low melting point. The resistance bands forming the network are relatively thin so as not to interfere with visibility through the glazing and lead to wider collecting strips, located near the edges of the glazing. These collector strips are made of a composition identical to that of the resistance bands and are deposited in the same way and preferably at the same time as these. Current leads are then welded to these collector strips.

The first heating glazings thus produced included very conductive resistance bands, of a particularly high metallic silver content, with a view to a high heating capacity; therefore, the electrical power dissipated by the heating network was also very high. However, such high dissipated powers are not well suited to small-displacement motor vehicles whose electrical reserve power is relatively low. This is all the more so since motor vehicles are today fitted with a whole series of electrical equipment which means that the maximum power capable of being dissipated at a given time is often less than the theoretical capacity required for simultaneous operation of all electrical equipment in the vehicle. Therefore, there is an increasing demand for heating glazing dissipating lower powers which - unless reducing the size of the heating circuit - requires the use of electrically conductive compositions having relatively lower silver contents. Concretely, while in recent years the heating networks were made from pasta with a silver content of around 75% or more, the market today is moving towards products made with pasta with a silver content is at least 71%.

In practice, this reduction in the silver content poses a problem of compatibility with the soldering of the solder, in other words the solder does not hold well. To remedy welding faults, the current industrial practice is to deposit at the places provided for welding a pellet made of a paste with a high silver content, this second deposition being carried out for example by screen printing or with a brush and covering a surface of in the range of 1-4 cm².

The authors of the present invention have however observed that this solution is far from always giving satisfaction, short-circuits occurring quite frequently. By examining the defective glazing, there is no problem with the weld. On the other hand, it appears that the pellet itself does not always adhere to correct way to the header strip. This phenomenon was completely unexpected given the great similarity between the different silver-based compositions which suggested a good bond between the two layers. (the only difference being ultimately the silver-sintered ratio). Furthermore, it should be noted that this phenomenon, in addition to its unpredictable nature, had the drawback of being hardly easy to be observed given the thin thickness of the layers in question.

The object of the present invention is to provide a more satisfactory solution to this technical problem of connection to the vehicle power source of screen-printed networks with low silver content.

This problem is solved according to the invention by a heating glazing comprising a glass sheet provided on one of its faces with resistance bands formed of an electrically conductive composition resulting from the baking of a suspension in an organic binder of metallic silver and a low-melting frit and collector strips formed of the same electroconductive composition, windows according to the invention being closed by pellets made of an electroconductive composition having a greater silver content than that of the forming composition the resistance bands and the collector bands and having a dimension slightly greater than that of the windows so as to form overlap zones, the current supply terminals being welded to said pellets.

The glazing according to the invention is thus presented with collecting strips with "holes" and pellets the silver content of which is most adequate for perfect welding of the current supply terminals. The window in the collecting strip allows the pellet to be directly in contact with the substrate and not superimposed as before. The substrate here is either glass or an enamelled layer previously deposited on the glass and used for example to mask the adhesive used during the installation of the glazing. In all cases, the connection between the substrate and the pellet is ensured by the frit so that there is no risk of the pellets peeling off.

Such glazing can for example be obtained simply by modifying the screen screen used for the deposition of the heating network so as to provide windows which are subsequently covered by the deposited patches, either by a new screen pass or simply with a brush.

In a typical case, the silver content of the composition forming the bands of the heating network is less than or equal to 71%, while for the pellets a silver content greater than 75% or even 80% is provided, which is very favorable. weld quality, the small size of the pellets allowing the use of rich pastes without resulting in a significant increase in the cost of glazing.

An important point of the invention is, as indicated above, the direct bonding on the substrate of the patch, so that the overlap zones between the pellets and the collector strips are fixed over time with respect to the surface of the sheet of film. glass. It can certainly happen as with glazing according to the art that the pellet made of a paste with a high silver content does not adhere correctly to the collecting strip at the level of the overlap zone, but the pellet being anchored to the substrate the overlap area is always well presented, so that generally conduction is ensured by the simple fact of the close proximity of the two layers.

In a more particularly preferred variant of the invention, the patch is placed under the collecting strip at the level of the overlap zone. Indeed, the collector strip has its low silver content offset by a higher sinter content. During baking, the collector strip will therefore diffuse very easily towards the glass of which it is very close due to its composition. On the other hand, this diffusion is less for the pastille depleted in frit. Tests have shown that a paste with a silver content of 75% thus diffuses half as much as a paste with a content of 71%. Under these conditions, if the collecting strip covers the pellet, its tendency to diffusing towards the glass will lead to the formation of an interpenetration layer having an average silver content, which guarantees perfect conduction, while leaving the pellet available to weld current leads.

In the latter case, it is preferably necessary to proceed in order to obtain the glazing according to the invention to a screen printing in two stages with in a first step the deposition exclusively of the pellets and in a second step the deposition of the conductive strips of the network heating and collector strips, the frame used has windows of adequate size in places already fitted with a pad.

Claims (4)

1. Heating pane comprising a glass sheet provided, on one of its faces, with resistance strips formed of an electrically conducting composition resulting from the baking of a suspension, in an organic binder, of metallic silver and a low melting point fritt, and with collector strips formed of the same electrically conducting composition, characterized by apertures formed in said collector strips, these apertures being obturated by pellets made of an electrically conducting composition having a higher silver content than that of the composition forming the resistance strips and collector strips and having a dimension such that overlap zones exist between the pellets and the collector strips, the electrical supply terminals being soldered to said pellets.
2. Heating pane according to Claim 1, characterized in that the silver content of the composition forming the resistance strips and the collector strips is less than or equal to 71% and in that the silver content of the composition forming the pellets is greater than or equal to 75%.
3. Heating pane according to Claim 1 or 2, characterized in that, at the position of the overlap zone, the pellet is disposed beneath the collector strip.
4. Method of production of a heating pane according to Claim 3, according to which there are carried out, in a first step, a deposition by screen printing of the pellets and, in a second step, the deposition of the resistance strips and of the collector strips, the latter comprising said apertures.