FR2823454A1 - Electroconducting track production on transparent substrate comprises silk screen printing using electroconducting paste under conditions to produce thin tracks - Google Patents

Abstract

The production of electroconducting tracks on a transparent substrate comprises applying, by silk screen printing, an electroconducting paste on the surface of the substrate to form a predetermined motif of tracks and subjecting it to firing. The process uses a thixotropic paste with a ratio of the viscosity without cutting pressure to the viscosity under cutting pressure in silk screen printing conditions of at least 50 and having a silver content greater than 35% and an impression screen of which the coating is provided, at least partially, with slots of which the narrowest width is at most equal to 0.25 mm \! 0.05 mm so that the width of the smallest electroconducting tracks formed by impression may be less than or equal to 0.3 mm. Independent claims are also included for the following: (a) the utilization of an impression screen incorporating at least 90 wires/cm for forming the electroconducting tracks; (b) a transparent substrate incorporating electroconducting tracks applied by silk screen printing; and (c) the utilization of the substrate as a heated window for a motor vehicle or as an antenna or as an alarm.

Description

with a higher pressure than that of the sheet.

  METHOD FOR MANUFACTURING ELECTROCONDUCTIVE TRACKS

  ON A TRANSPARENT SUBSTRATE AND SUBSTRATE OBTAINED

  The invention relates to a process for the manufacture of electrically conductive tracks on a transparent substrate, by serigraphy of a paste.

  electrically conductive, and to the transparent substrate provided with said tracks.

  For several years now, it has been known to provide trans substrates

  parents, in particular glazing, of conductive tracks which can serve as ele

  heating elements or antenna or alarm elements.

  These tracks are generally obtained by the serigraphy method used.

  a paste containing metallic silver particles. It is known from EP-

  A-0 712 814 that the dough has a high silver content, that is to say from 60 to 90% by weight relative to the solids. Furthermore, EP-A-0 079 854 describes a paste, capable of being deposited by screen printing on glass, which comprises from 45 to 90

  % by weight of metallic silver particles of size less than 1 m.

  The electroconductive tracks can also be obtained by other methods than screen printing, for example by extruding a thermosetting conductive paste directly on the glass to form narrow wires (see

DE-A-1 796310).

  The electrically conductive tracks which are obtained after baking (baking which is generally carried out at the same time as the treatment of the glazing for the purpose of forming and tempering) have sufficient mechanical strength. This avoids the additional galvanizing step, which is difficult to implement

  due to the pollution risks associated with it.

  Glazing comprising electroconductive tracks are widely used in the automotive field. These tracks are most often used as heating tracks, in particular on rear glasses, but they can also be placed on the glazing to provide an alarm and / or antenna function. The aforementioned documents give no indication as regards the width of the electrically conductive tracks thus produced. In

  practical, the electroconductive tracks are formed industrially by serigra-

  phie ciassique and they have, after firing, a width of between 0.4 and 1.2 mm and a thickness which varies according to the nominal heating power and

  of the ohmic resistance per unit of surface considered.

  Because of the interesting functions provided by these tracks, their number on the same glazing tends to increase over the years which can cause problems of space and visibility. Thus, when the tracks are located in the field of vision of the glazing, they are clearly visible from the inside, which can annoy the driver, and incidentally from the outside, which

harms the aesthetics of the vehicle.

  Furthermore, it is already known to use screen printing stencils for

  form various patterns on glass (see DE-A 32 31 382 and DE-A-35 06 891).

  It is thus possible to apply the paste in a thicker and / or wider layer in certain places of the glass in a single step, (without multiple printing), for example to form the busbars (or "bus bars") of the electric current of the glazing heating. In this way, it is possible to adjust the temperature as well as possible over the entire glass surface, this temperature not to exceed 50 ° C. in the area of the busbars under normal ambient temperature conditions for heating power. up to 450 Watts. The examples appearing in the abovementioned patent applications are produced under a DC voltage commonly used for automobiles,

from approximately 1 1 to 14 Volts.

  Heated glazing and glazing with antenna are also known, the conductive tracks of which consist of fine tungsten wires a few micrometers in diameter. These wires are only present on laminated glazing and they are embedded in the adhesive material constituting the intermediate sheet because otherwise they cannot be fixed directly on the glass in a secure manner. Being thinner, these wires are therefore less visible than the

  conductive tracks obtained by screen printing.

  There is a need on the part of automobile manufacturers, in particular for high-end vehicles, to have glass panes of

  safety hardened or laminated with conductive tracks barely visible to the naked eye.

  The object of the present invention is to form conductive tracks on a transparent substrate, which, while being narrower than the known tracks, are

  able to fulfill the electrical conduction function assigned to them.

  This object is achieved by the method of the invention which consists in forming electrically conductive tracks on the surface of a transparent substrate, by the application by screen printing of an electrically conductive paste forming a predetermined pattern, and in subjecting said tracks to a baking, said method being characterized in that a thixotropic dough having a silver content greater than 35% and of which at least 98% of the particles which constitute it is of size less than, μm, and a sieve having at least one is used 90 wires per cm, the width of the narrowest of the individual electroconductive tracks per print being less

or equal to 0.3 mm.

  It is important to obtain the electrically conductive tracks having the required width to carefully check all the elements of the method according to the invention. In this respect, special attention should be paid to the characteristics of the dough, in particular the thixotropy and the size of the particles entering into its constitution, and to the parameters of the sieve, in particular the size of the meshes, the thickness of its coating and the width of the openings (in this case slots) to be provided in said coating which corresponds directly to the width of the tracks to be printed by screen printing. Thanks to the invention, glazing can be produced industrially in series with a redeterminate pattern of tracks.

  particularly fine and barely visible to the naked eye.

  The opening of the meshes but also the size of the openings or slots provided in the coating of the screen for printing said pattern on the surface of a glazing have a direct influence on the width of said tracks. Since the dimension of the slits corresponds substantially to the width of the tracks obtained, it is necessary to form extremely narrow slits (as a rule of the order of 0.25 mm + 0.05) in said coating which is itself relatively thin. However, the width of one of these slots may extend over more

only one mesh of the sieve.

  It cannot however be excluded that there are other possibilities of producing narrow tracks with another process using a less thixotropic paste, a less fine sieve (for example made up of about 70 threads per cm), a coating of the less or thicker sieve, Despite the narrowness of said tracks, which can hardly be discerned, a heating power is obtained comparable to that of the usual screen printed tracks or heating zones. Nevertheless, the thickness of the tracks, although increased, is kept within acceptable limits. The tracks obtained using the method described here thus have a maximum thickness on the surface of the glass, measured after baking, of approximately 35 μm, more generally of the order of to 25 μm, while the thickness conventional track is about 12 .mu.m. This greater maximum thickness can be obtained inter alia thanks to the highly thixotropic pastes used according to the invention which have the ability to recover very quickly their initial viscosity after printing on the glass. Thanks to the method according to the invention, it is therefore possible to significantly reduce the width or dimensions of the individual electrically conductive tracks by applying, by printing, a thixotropic paste containing at least 35% by weight of silver, having good properties of flow for a high shear gradient and containing very small particles, by means of a particularly fine sieve, made up of threads of material known per se, arranged in such a way

  so the opening of the mesh is small.

  The importance of the thixotropic nature of the paste should be clarified with a view to its application by screen printing. During the application, the shear stress undergone by the dough is high enough to cause a sharp and sudden drop in viscosity which allows the dough to pass through the openings of the sieve and to deposit on the substrate forming the pattern of the tracks. Pastes which are suitable for this purpose are defined by a ratio of the viscosity in the absence of shearing (starting viscosity) to the viscosity under shear stress (under the conditions of screen printing) which varies from 50 to 1000 or even more, for example up to 1300-1500. For comparison, this

  ratio is between 2 and 10 for the usual screen printing pastes.

  It is also important that after depositing on the substrate, the paste regains a viscosity value as close as possible to that of departure in a very short time (recovery time) but also that this value remains stable over time. In this way, the drawbacks associated with the creep of the dough are avoided, in particular an increase in width and a decrease in the thickness of the printed tracks, drawbacks which become all the more important as the thickness of the dough deposited is high. . In general, it is advisable to choose a paste whose recovery time is less than 1 second, of

  preferably in the order of a few tenths of a second.

  With a less thixotropic dough or with larger particles, it is not possible to obtain the conductive tracks having the indicated width because the dough cannot pass through the narrow openings of the sieve. In addition, it is also not possible to envisage producing narrow conductive tracks with a paste whose recovery time (or the transition time between the almost fluid state under shear and the normal, practically solid state) is too high to allow tracks to remain stable immediately after

removing the sieve.

  When the silver content of the paste is more than 50%, the tracks whose width is less than 0.25 mm can be used as tracks

  heating, without temperature increase to the rated power allowed-

  sible. Tracks with a lower silver content, for example of the order of

  %, are rather used as an alarm and / or antenna.

  The combined use of a thixotropic paste made up of very small particles and a fine mesh screen makes it possible to print conductive tracks with excellent resolution. In addition, by increasing the silver content in

  thixotropic paste, we can reduce the final thickness of the tracks.

  Although particularly suitable for silver-based pastes, the invention can be extended to pastes containing metal particles capable of meeting the required electrical conduction criteria, such as particles of

copper or gold.

  The glazing obtained according to the process of the invention, although being provided with narrower tracks, has electrical conduction properties comparable to those of glazing provided with tracks obtained by conventional screen printing, all other things being equal (number tracks, distance between tracks, layout, ...). In the case of heated glazing in particular,

  a similar heating power is reached with the same number of tracks.

  The method according to the invention allows, to a large extent, to overcome the constraints which impose to vary the cross section of the tracks according to the location they occupy on the glazing. Such a constraint exists for example on glazing of trapezoidal shape where the tracks located in the upper part are shorter than those in the lower part, which requires adapting the section of the tracks in order to maintain a comparable heating power over the entire glass surface. This constraint is found with glazing where maximum heating power is sought in the zone corresponding to the driver's field of vision. In this case, the conductive tracks have a greater width in the vicinity of the lateral edges than in the center of the glazing. By proceeding under the conditions of the invention, it is possible to form tracks of identical width without this resulting in a significant difference in the uniformity of the heating power. In this regard, there is every reason to believe that the decrease in electrical resistance results from the fact that the paste has a high silver content and that it is deposited over a greater thickness. The maximum admissible temperature values at the level of the collecting bars are largely respected without the need to apply a significant thickness of screen-printing paste with high conductivity. Most often, the temperature near the busbars is 15% lower than the maximum allowed temperature and does not exceed 50 C, under normal conditions of ambient temperature, for a nominal power not exceeding

  450 Watts for a supply voltage of 11 to 14 Volts.

  The production on an industrial scale of the glazings obtained in accordance with the process of the invention makes it possible to save on the screen printing paste, in particular when the latter is rich in silver (content greater than or equal to 80%). This is true for the production of heating tracks and even more so in the case of tracks intended to function as an eVou antenna alarm. In accordance with a first preferred embodiment, a thixotropic paste is used which has a silver content greater than 35%, preferably% and better still 70%, the essentials (at least 98%) of the constituent particles of which have a dimension less than 25 m, preferably 12 m, and a

  screen printing sieve with at least 90 threads per cm and a coating thickness

  ment of at least 30 µm, preferably 50 to 100 µm. This procedure allows to deposit on the substrate, by printing, in a single pass, a relatively large thickness of screen printing paste compared to the thicknesses that are usually obtained. The deposits thus obtained have, after firing, a thickness of less than 35 μm, more generally of the order

from 15 to 25, um.

  The sieve is used in this realization with a thickness of re

  garment larger (more than 30 µm) than that of sieves with an equivalent number of threads used for the intended application (generally less than 10 µm). This

  sieve can be obtained by the photographic technique, known per se, which

  is to cover the surface of the screen with a layer or a photoresist resin film

  culable and to operate by projection of a slide in order to reproduce the pattern of printing on the sieve. In the present case, a presensitized photocrosslinkable resin is used which is capable of forming a coating which is sufficiently solid to resist the action of the screen printing doctor blade, in a very short time, of the order of

  seconds or less. For comparison, with a photocrosslinked resin

  classic lable, it is necessary to have a high exposure time, of the order of

  5 to 6 minutes to obtain crosslinking throughout the thickness of the coating.

  By "presensitized photocrosslinkable resin" is meant here a pre-crosslinked resin.

  which comprises one or more low molecular weight polymers capable of reacting

  gir under the effect of light to form a cross-linked network. Photoresist resin-

  the presensitized lable can in particular be implemented in the form of an emulsion layer deposited directly on the sieve or of a film supporting said resin, this film being moistened before being applied to the surface of the sieve. By limiting the duration of exposure, it is thus possible to prevent the effects of stray light near the edges of the printing mask, light which causes unwanted crosslinking of the resin. This results in a reduction in the size of the pattern impressed compared to that of the mask, on the face subjected to exposure which results in screen printing, by a reduction in the amount of paste deposited on the substrate and a less good resolution in printing

tracks.

  It is advantageous to choose a sieve whose edges corresponding to the printing patterns are substantially parallel in thickness so that the opening devoted to the passage of the dough remains substantially constant from one face to the other of the sieve. In any event, it is desirable that the variation in the opening between the two faces of the screen, for the same reason, be less than 20%, and preferably 10%. For comparison, with photocrosslinkable resin

  previously cited classic, the high exposure time necessary for the reticula-

  does not allow the desired opening corresponding to the printing pattern to be obtained.

  on (reduction of opening or even complete blockage).

  The nature of the wires constituting the sieve is not critical. Preferably, the threads are made of polyester and each thread consists of a single thread (monofilament) of

  diameter between 30 and 60, um, preferably 40 and 50, um.

  The doctor blade which makes it possible to press the paste through the screen printing screen can be a standard doctor blade having a right-angled, chamfered or rounded printing edge. The use of this latter type of racie makes it possible to obtain a certain increase in the shear tension or a reduction in the viscosity of the dough during the passage through the sieve. Preferably, the doctor blade is made of a material of the polymer type, for example a polyurethane, having a

  Shore A hardness between 65 and 85.

  Thanks to the invention, it is possible to obtain conductive tracks having a surface resistance of less than 2.5 mOhm per square for a thickness of 10 .mu.m

  after firing, which corresponds to a resistivity of less than 2.5, uOhm.cm.

  In addition, the cord tracks have a satisfactory abrasion resistance, even when their thickness is large. This is attributed to the greater densification of the silver particles in the dough during baking. The following describes the manufacture of a heated glazing for a

  use as a heated rear window of a motor vehicle.

  The thixotropic screen printing paste contains 80% silver, 4% glass frit and 16% of a medium which has the function of facilitating application on the substrate. All the particles contained in the paste have a size less than 15 μm. The ratio of viscosity without shear stress to viscosity under

  shear stress under the conditions of rig rig rap is equal to 200.

  The sieve is made of a 100 T fabric sold by SEFAR which contains 100 threads per cm, each thread being made of a single polyester thread having a diameter of 40 µm, and has a mesh opening equal to 58 µm . The sieve is covered with a presensitized photocrosslinkable emulsion layer of, μm thickness and the pattern corresponding to the printing mask is reproduced on the sieve by the photographic technique (exposure time: 150 seconds; power of the blade : 7000 W). The edges of the covering at the level of the patterns reproducing the tracks are parallel, which means that the opening for the

  passage of the dough is constant from one side to the other of the sieve.

  The screen pattern is printed on a glass sheet by means of a polyurethane doctor blade at right angles to a Shore A hardness equal to 85. With a printing speed of 20 m / min and an out-of-contact contact of 8 mm, tracks are formed which, after a baking cycle (from room temperature to 650 ° C. in 150 s) have a width of 0.2 to 0.22 mm and a thickness of 15 μm. Glazing provided with current busbars has the same performance in terms of resistance and heating power as a glazing having an identical number of conductive tracks, arranged in the same way, obtained by serial

  classic spelling (width greater than 0.5 mm).

  The manufacture of a heated glazing according to another variant of the invention is described below, but also for use as a telescope

  heated rear of a motor vehicle.

  Although the usual doctor blades have a sharp right-angled printing edge, with which the dough is pressed through the sieve on the surface placed under it, it has been found preferable to carry out the invention by using a modified printing edge of the doctor blade allowing a certain corner effect. We have not yet found an explanation for this effect resulting from a chamfer or rounding of said printing edge for this specific configuration, but we can assume that there is a reciprocal effect with thixotropy - that is to say the drop in viscosity by increasing the shear stress exerted on the medium

  thixotropic- screen printing paste.

  The speed of printing by the doctor blade is slightly reduced compared to the usual methods due to the need to pass the dough through slits or openings of sections considerably reduced compared to the sections of the screen linings. n us. In addition, the tam is out of contact (ie the distance between the freely stretched tam and the

  substrate to be printed, here the glazing) is fixed at 10 mm.

  With these materials and dimensions, a resistivity less than 2.5, u. * Cm

can be obtained after cooking.

  As the paste, the product H 669 from the company DuPont is used, the viscosity of which is equal to 17 Pa.s (Pascal x sec). It contains only particles of size less than or equal to 10, um. It has a ratio of viscosity without shear stress to viscosity under shear stress in

  screen printing conditions equal to 100.

  The sieve is made of a 95 T. fabric marketed by SMTi SpA, Italy, which contains 95 threads per cm, each thread being made of a single polyester thread having a diameter of 40 µm, and has a mesh opening equal to 65, crm. The sieve is covered with a presensitized photocrosslinkable emulsion layer of approximately 16 μm in thickness and the pattern corresponding to the printing mask is reproduced on the sieve by the photographic technique. The edges of the coating at the level of the patterns reproducing the tracks are parallel, which means that the opening for the passage of the dough is constant from one face to the other of the sieve. The slots formed in the coating have the widest width

narrow of about 250, um.

  The pattern of the sieve is printed on a glass sheet by means of a polyurethane doctor blade, the edge of which is bevelled at 45 x 0.2 mrn and has a Shore A angle equal to 65. With a speed 0.35 m / sec printing and 10 mm out of contact, tracks are formed which, after a baking cycle (from room temperature to 650 C in 150 s) have a width of 0.2 to 0.22 mm and a thickness of 12-15 µm. Glazing provided with current busbars has the same performance in terms of resistance and heating power as a glazing having an identical number of conductive tracks, arranged in the same way, obtained by conventional screen printing (width

greater than 0.5 mm).

Claims (20)

  1. A method of manufacturing electrically conductive tracks on a transparent substrate, according to which an electroconductive paste is applied, by screen printing, to the surface of the substrate to form a predetermined pattern of tracks and said tracks are baked, characterized in that the 'is used a thixotropic paste having a ratio of viscosity without shear stress to viscosity under shear stress under the conditions of screen printing of at least and having a silver content greater than 35%, and a sieve whose coating is provided, at least partially, with slits the narrowest width of which is at most equal to 0.25 mm + approximately 0.05 mm so that the width of the smallest of the electrically conductive tracks formed by printing is less or equal to 0.3 mm.   2. Method according to claim 1, characterized in that the paste is made up to 98% of particles having a size less than or equal to 25 µm, and in   that the sieve includes at least 90 threads per cm.   3. Method according to one of claims 1 or 2, characterized in that one   uses a thixotropic paste having a silver content greater than 50%, preferably 70%, and of which at least 98% of the particles have a size less than 12 .mu.m.   4. Method according to one of claims 1 to 3, characterized in that one   uses a sieve having at least 95 threads per cm and which is applied by printing individual conductive tracks whose smallest width is less than 0.25 mm.   5. Method according to one of claims 1 to 4, characterized in that one   uses a screen with a coating of greater or equal thickness at 10, um.   6. Method according to claim 5, characterized in that the thickness of the   coating of the sieve is between 50 and 100, um.   7. Method according to one of claims 1 to 6, characterized in that the   sieve is provided with a coating whose edges correspond to the patterns   are substantially parallel in thickness.   8. Method according to one of claims 1 to 7, characterized in that one   uses a sieve, the coating of which is obtained by exposure to a resin   presensitized photocrosslinkable.   9. Method according to claim 8, characterized in that the resin is deposited in the form of an emulsion or a film supporting said resin applied to the surface of the sieve.   10. Method according to one of claims 1 to 9, characterized in that one   uses a doctor blade having a Shore A hardness of the order of 65 to 85 and an edge right angle printing.   11. Method according to one of claims 1 to 8, characterized in that one   uses a doctor blade having a Shore A hardness of the order of 65 to 85 and an edge   45 chamfered or rounded print.   12. Method according to one of claims 1 to 10, characterized in that   we use a thixotropic paste having a silver content greater than or equal to% and that we form individual conductive tracks of which the smallest   width is between 0.1 and 0.25 mm.   13. Use of a printing sieve comprising at least 90 threads per cm, preferably 95 threads per cm, to form electroconductive tracks according to the   Method according to one of Claims 1 to 12.   14. Use according to claim 13, characterized in that the sieve has   a thickness greater than or equal to 30 μm, preferably 50 to 100 μm.   15. Transparent substrate, in particular a glazing, comprising electrically conductive tracks applied by printing on the surface by screen printing, characterized in that the minimum width of the tracks is less than or equal to 0.3 mm, preferably 0.25 mm.   16. Substrate according to claim 15, characterized in that the tracks are composed of a screen printing paste having a silver content greater than 35%.   17. Substrate according to claim 16, characterized in that the tracks are composed of a screen printing paste having a silver content greater than %, preferably 70%.   18. Substrate according to one of claims 15 to 17, characterized in that   the tracks protrude above the surface at a height of less than 35 µm.   19. Substrate according to one of claims 15 to 18, characterized in that it   furthermore comprises two current collector bars between which the conductive tracks extend, at least portions of which are less than or equal to 0.3 mm in width, the temperature in the vicinity of said bars is not 50 ° C. normal conditions of lover temperature, for a rated power does not exceed 450 Wags for a Jensen dial_ amp.   20. Use of Subra1 Solon one of the 15 lg claims while   qua adage au10mobe chauMant antenna ou Karma.