The present invention relates to a method for pad-printing an electrically-conducting circuit onto a work piece.

For construction designs of the type in question found on the market, a method known as screen-printing is used to achieve electrical circuits on work pieces, preferable glass sheets, where the circuits conduct a current in, for example, a car windscreen that is to be heated. In addition, circuits are screen-printed to achieve alarm loops on glass that set off an alarm when the circuit is broken. Screen-printed circuits are also screen-printed on glass with a metallic coating where such circuits are screen-printed so that a part of the circuit ends up on the metallic coating, which can then be provided with current, especially to heat it up. This heated windscreen is used in vehicles, aircraft and boats. The screen-printing is done with the help of large machines. A net is tensioned over a surrounding frame. An image is created on the surface of the net that lets electrically-conducting ink through and that, with the aid of a scraper, is pressed through the net at the places where there are open holes in the mesh. At other places, the net is sealed by a sealing material. With this method, the mesh and the holes in between are far too coarse to achieve the best quality for the circuits, whereby pores and cavities occur in the electrically-conducting circuits so that a high rate of rejection exists during manufacture. In addition, screen-printing machines are large, expensive and cumbersome. Such a method is expensive, results in extra costs and takes along time.

Handling the frames, ink and the manufacturing is also costly and messy. Furthermore, the actual manufacturing is slow. To be able to screen-print different new circuits requires a complicated and time-consuming procedure since new frames have to be installed. If the work pieces to be screen-printed are curved, it is also difficult to get the net frame to seal tightly against them, whereby printing on uneven surfaces becomes difficult to achieve. Small work pieces such as the casing of electrical items are impossible to get at to screen-print, especially if one wants to print a number of different circuits, as found on a circuit-board. The screen- printing method is an open method, which means that all solvents are exposed to the surroundings, which requires costly and complicated ventilation equipment if the method is not to be too much of an environmental hazard. Nevertheless, with this method, the operator will always take in a certain amount of solvent. It is also complicated to achieve the actual image on the net frame. The screen-printing method also generates substantial waste as a large

amount of ink flows around in the frame. This ink deteriorates and must be replaced, which is costly, messy and stops production. Nor does the screen-printing method give an accurate location of the printed circuit, whereby the quality is not always good enough on the printed circuits. Screen-printing more than once onto one another to achieve a thicker circuit so that a connection surface in the form of, for example, a solder-friendly area is created so that one can connect connecting cables, components, contact devices, etc. , to the electrically- conducting circuit, is difficult if not impossible as this method is far too imprecise.

One object of the present invention is to eliminate the disadvantages exsisting in the method mentioned above by instead pad-printing the electrically-conducting circuit with an equipment and a method intended for the purpose, whereby small circuits and more complicated circuits that resemble circuit-boards can be pad-printed directly onto the work piece so that a circuit-board is not needed.

Thanks to the invention, an inexpensive and well-functioning means of pad-printing an electrically-conducting circuit has now been achieved, which is pad-printed on a work piece even if this has curves. When used, the circuit is connected to a source of current. This method is based on picking up an image from a cliche with a pad that is sufficiently soft and pliable. The cliche is a metal plate that is embossed with an image. The embossing is achieved by a simple photographic method that achieves recesses in the plate by etching. The electrically-conducting ink lies completely enclosed in a so-called ink cup, which is open at the bottom via a tight seal against the cliche. The ink cup is transferred backwards and forwards over the cliche by a pad-printing machine, whereby the cliche is coated with the electrically-conducting ink in the etched recesses, whereby an image is formed that constitutes a whole or partial image of the finally-printed, electrically-conducting circuit, which can include a number of circuits, whereby the image, according to one application of the invention, resembles partial images or a whole image of the circuit on a circuit-board. The pad then presses against the cliche, whereby the image is left on the soft pad. This then prints the image on the work piece in exactly the position desired. This can be repeated several times, whereby a whole image or partial image is printed on the work piece until the desired result is achieved with regard to the shape, propagation and thickness of the electrically-conducting circuit. Of course the work piece and/or the pad can move relative to one another in order to pad-print according to the invention. Pad-printing gives a very high image resolution, whereby very small work pieces can be printed. This is very useful when pad-printing circuits

in electronic items such as mobile phones, whereby many circuits can be printed directly on the casing of the mobile phone, whereby circuit-boards can be partly or wholly eliminated.

This creates the possibility of building very thin electrical items since the circuit-board, which occupies much space, can be left out, at the same time as one saves time and money during the assembly of, for example, a mobile phone. Due to the high quality of the electrically- conducting circuit that is printed on the work piece, no pores are formed in the circuit. This method is environmentally superior since the electrically-conducting ink is encased in the ink cup the whole time. In addition, there is no waste as only the electrically-conducting ink needed to create the image is used. The precision of the method is so great that one can pad- print several layers of circuit on one another and by one another so that one can build up the desired appropriate thickness and extension on the built-up circuits since one can print circuits next to and on one another with the highest quality. The machines for pad-printing are also cheaper and faster, and take up less space than screen-printing machines and they are besides easy to re-set to print other images of electrically-conducting circuits, whereby set-up times become shorter and adjustment less complicated. Furthermore, this method is very quick.

According to the invention, one can provide the machine with several ink cups, cliches and pads, whereby these can work together so that waiting times between picking up and printing the image are shortened by synchronising the movements of the ink cups and pads so that printing takes place with tighter time intervals. In addition, according to the invention, one can pad-print on work pieces with curved surfaces such as, for example, casings of different types that are suitable for electrical items such as, for example, mobile phones where curved surfaces are found and that have corners or edges that project up, which can also be found on sheet or slab-like items. According to the invention, this is possible to execute by building up partial images one after the other next to each other anywhere at all in the three-dimensional space against the work piece. The precision of the pad-printing means that one can easily print several layers on one another and anywhere at all on and by the electrically-conducting circuit, whereby an electrical connection area in the form of, for example, an easy-to-solder area or a good contact surface is formed and that is used to create a good connection to other components, cables and contact devices. In one preferred embodiment of the invention, the pad-printing machine is built up with pads that can move in a stepless manner in three directions controlled by a computer programmed for this purpose, whereby one pad-prints the electrically-conducting circuits directly on the work piece in a casing even if the surface of the work piece is curved. The same technique is used to achieve an electrically-conducting circuit in the form of a loop for electrically heating a vehicle windscreen that, in fact, frequently has

a curved surface. For manufacturing this, several cliches, ink cups and pads are needed in order to print several partial images. The work piece then lies on a working surface in a fixed position. The pads pick up different partial images at different cliches, whereby these are then printed next to one another until they form a complete whole image in the form of a complete circuit. Pad-printing alarm circuits or circuits for connections to electrically-heated windscreens can be performed by pad-printing according to the described methods. The primary advantages of the invention are that one has achieved an environmentally-friendly method that applies the circuit onto a work piece, even if the work piece is curved and small, that is quick and inexpensive, and that can eliminate circuit-boards in an electronic item since one can pad-print the circuit directly onto the work piece.

The invention is described in more detail below with the help of preferred embodiments, reference being made to the attached drawings where Fig. 1 shows a perspective view of an electrically-conducting circuit built up of several partial images and printed on a sheet-like item, Fig. 2 shows a perspective view of electrically-conducting circuits that are printed directly on the casing of a mobile phone.

As is more evident from the embodiment of the invention illustrated in Fig. 1, a complete whole image of an electrically-conducting circuit 1 is pad-printed from five different partial images a-e, each of which has a different appearance, which is why they are picked up from five different places at a number of different cliches and then pad-printed on five different locations on the work piece 5, which in this case is the sheet-like item 2. One can of course apply the invention by printing many more partial images than a-e, such as a-z. If the electrically-conducting circuit 1 is not too large, it can be pad-printed with fewer cliches and pads on work piece 5 to achieve a whole image. If it is small enough, it is sufficient to pad- print with a single pad and one cliche on one location to achieve a whole image that creates the circuit 1. By pad-printing a number of times on the same location, a thicker electrically- conducting circuit 1 is obtained. The partial image a shows a connecting surface 3 in the form of an easy-to-solder area or an electrical contact surface, whereby a good connection to other components, cables and contacts can take place. Joints 4 occur between the partial images.

These are well made since the pad-printing takes place with great precision, whereby no

electrical breaks occur at these places. What's more, the joints 4 can be printed to overlap to further improve their ability to conduct electricity.

As is evident from the embodiment of the invention displayed in Fig. 2, the work piece 5 is executed as a casing 6 for a mobile phone that has curved surfaces 8 in the form of corners, recesses, elevations, bridges, grooves, etc. Circuits 1 that resemble the circuits on a circuit- board and that have an extension that even extends over the curved surfaces 8 are pad-printed on the casing 6. The electrical connecting surfaces 3 are also shown here with different extensions in the form of rounded surfaces, rectangular surfaces, triangular surfaces or other shapes. If the casing completely or to a partial extent comprises an electrically-conducting material and is thus electrically-conducting, it can be insulated with an insulating surface, e. g. be surface treated with a non-electrically-conducting material, e. g. painted before circuit 2 is pad-printed, whereby the circuit is pad-printed on the surface-treated material.