Flexible Printed Wire Board

This invention relates to crossover conductors or wires in a flexible printed wire board.

In a printed wire board including a flexible printed wire board, conventional crossover wiring methods as shown in FIG. 3A and B have been widely used. The materials of construction are generally similar to the materials used in, prior art, U.S. Pat. No. 4,287,394 (Hargita et al.). As part of these configurations insulation 5' is printed/formed on a desirable part of a wiring pattern 2, 3, 4. The wiring pattern 2, 3, 4 is printed by etching copper film or conductive paste which has been placed on a flexible or hard substrate 1. A crossover wiring pattern 10 is printed/formed over the insulating layers 6, 7, 8 (collectively 5') which cover a lower wire 2 of the wiring pattern 2, 3, 4. The crossover wiring pattern 10 connects two terminals (end or connecting portions)/wires 3a, 4a/3, 4 of the connected conductive/wiring pattern 2, 3, 4 on the surface of the substrate 1 across the insulation 5' covering the lower wire 2. In order to obtain the good insulation between the lower conductive/wiring pattern (wire) 2 and the crossover conductive/wiring pattern 10 it is necessary to have an insulating film 5' of a relatively thick predetermined thickness. Because the predetermined thickness cannot be obtained by the application of a single insulation layer, the required predetermined thickness of insulation 5' is obtained by repeating the insulation application procedure two to three times stacking the insulation layers 6, 7, 8 on top of each other. This stacking has been performed conveniently and easily by repeating the insulation application procedure using the same pattern mask several times.

But the above conventional crossover technique has problems because a step is formed at the edge of the crossover insulation between the top of the uppermost insulating pattern layer 8, and the surface of the substrate and the surface of the terminals/wires 3a, 4a/3, 4. This step is relatively large. The crossover wiring pattern 10 is printed/formed across this relatively large step. After the conductive/wiring pattern 10 has been printed/formed it is often found to be disconnected at the point where the wiring pattern crosses the step. Or, even if the wiring pattern 10 across the step is not immediately disconnected, the thickness of the crossover conductive/wiring pattern 10 applied across the step is thin and the conductive/wiring pattern 10 crossing the step is often disconnected during handling, and therefore the reliability of the connection across the step has been poor. These problems also appear in the flexible printed circuit boards where the substrate 1 itself is bent often.

The object of the present invention is to provide a flexible printed circuit board having a high reliability without the fear of disconnection. The present invention comprises a flexible printed insulating wire board with a conductive/wiring pattern formed on the flexible board/film, part of the conductive pattern is coated with an insulating layer. The materials of construction are generally similar to those described in the prior art above. The insulation coated portion of the conductive/wiring pattern forms a lower conductive/wiring pattern. Insulation covers a portion of the lower conductive/wiring pattern to provide a crossover location for a crossover conductive/wiring pattern. The crossover conductive/wiring pattern connects at least one pair of wires of the conductive/wiring pattern to each other across the insulation over the lower conductive/wiring pattern. The insulation is applied in several layers, however unlike the prior art, the same insulation mask is not used for each layer. At the edge of the insulation the mask pattern for each successively applied insulation layer overlaps the previous one partially and has a smaller area than the previous layer such that the edge of each insulation layer forms a step in what appears to be a set of stairs where the bottom landing coincides with the substrate or the conductive/wiring pattern formed on the substrate and the top step is the top of the top insulation layer. When a hole is formed in the insulation to provide access to a connection on the substrate then the edges of the insulation layers are stepped from both inside and outside the hole. The present invention provides the above configuration, so that the insulation pattern layers form a cross section which is smoothly slanted, so that when the crossover conductive/wiring pattern is printed/formed it is not disconnected by a tall step. The smooth transition between insulation layers prevents the crossover conductive/wiring/film from becoming extremely thin, thereby assuring reliability for a long period of time.

The foregoing and other features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention and as illustrated in the accompanying drawings.

FIGS 1A and 1B show one embodiment of the flexible printed circuit board according to the present invention, FIG. 1A is a plan view and FIG. 1B is a central cross sectional view of FIG. 1A;

FIG. 2A and 2B show a second embodiment of the present invention, FIG. 2A is a plan view and FIG. 2B is a central cross sectional view of FIG. 2A.;

FIG. 3A and 3B show the flexible printed circuit board of the prior art, FIG. 3A is a plan view and FIG. 3B is a central cross sectional view of FIG. 3A.

In FIGS. 1A & 1B, a flexible substrate 1 has a wiring pattern (2, 3, 4) printed/formed thereon. A lower conductive/wiring pattern 2 is coated by an insulation 5. A connected conductive/wiring pattern having wires 3 and 4 spaced and insulated in a plane from the lower conductive/wiring pattern 2 is a part of the wiring pattern. End portions(terminals) 3a and 4a of connected conductive/wiring patterns 3 and 4 are the portions connected with the crossover pattern 10 and have enough area to perform reliably. An insulation 5 is comprised of three layers, in which first insulating layer 6, second insulating layer 7 and third insulating layer 8 are in turn stacked on each other, and are printed/formed to coat said lower conductive/wiring pattern 2. These insulating layers 6, 7, 8 have holes 6a, 7a, 8a formed respectively therein at places where the connection portions 3a and 4a of connected conductive/wiring patterns 3 and 4 are exposed. Collectively these holes 6a, 7a, 8a form a perforation 9 of the insulation 5. Thus, the edge pattern of each successive insulating layer is offset from the one below it by 0.2mm, between two insulation layers that is, a width of 0.2mm of the edges of the lower insulation layer can be observed after the upper insulation layer is applied. Also, the holes 6a, 7a, 8a in each layer of insulation 5 are successively widened by 0.2mm respectively. Accordingly, the insulation 5 has a slanted surface, which is smoothly thickened with increasing distance away from the insulation's edges. A crossover wiring pattern 10, connects both crossover portions 10a together. Each crossover portion 10a is connected with a connecting portion 3a and 4a of the connected conductive/wiring patterns 3 and 4. Also, the crossover portion 10a at the center of the insulation 5 is printed/formed to be placed on connecting portions 3a and 4a of connected conductive/wiring patterns 3 and 4. Thus, as described above, the edges of the perforations 9 on the insulation 5 is a surface smoothly formed and slanted towards the local medial axis (varied insulation patterns have areas where the edge patterns converge and diverge from adjoining edges, points which are approximately equidistant from each adjoining edge form a local medial axis) of the pattern, so that the crossover pattern 10 traversing each of the edges is not extremely thin at the edges of the insulation 5 and the crossover conductor/wire/film 10 thickness at these edges is sufficient to assure that reliability is obtained. Also, conventional flexible circuit boards have often experienced failures. These failures result from the fact that the insulation 5' uses the same full mask pattern for applying the lower insulation layer 6 as it uses for applying the upper insulation layer 8 and thus the mechanical stiffness of the insulation 5' is high and as a result the insulation 5' does not bend easily; when the flexible printed circuit was bent, the insulation 5' separated from the substrate and as a result the lower conductive/wiring patterns or the connected conductive/wiring patterns were disconnected. Contrasted with the flexible printed circuit board of the present invention which is formed so that the upper layers 7, 8 of the insulation 5 have a reduced mask pattern area wherein the thickness of the insulation 5 increases towards the local medial axis of the pattern of the insulation. Whereby the insulation 5 is not and the inconvenience described above is avoided.

FIG. 2 shows a second embodiment of the present invention. This embodiment is different from the first embodiment in that the perforations 9 are not designed within the insulation 5, and the crossover pattern 10 and the connected conductive/wiring pattern 3, 4 are connected to each other outside of the boundaries of the insulation 5. The actual effect of this configuration is the same as that of the first embodiment.

The first embodiment has insulation which is applied in three layers 6, 7, 8, but the number of layers may be modified according to the amount of insulation 5 required. As described above, the present invention is a flexible printed wire board provided with a wiring pattern 2, 3, 4 formed on the flexible board/film 1, in which a part of the conductive/wiring pattern is coated with a number of overlapping insulating layers 6, 7, 8 to form a lower conductive/wiring pattern 2, at least one pair of wires of the connected conductive/wiring pattern 3, 4 which are spaced and insulated from the lower conductive/wiring pattern 2, are connected across the lower conductive/wiring pattern 2 by a crossover conductor/wiring pattern 10 formed on the insulation 5. At the step edge of the insulation 5, the edges of each succeedingly lower insulation layer stickout beyond the insulation layer immediately above it by a predetermined amount, thereby forming steps which provide a smooth transition from the top insulation layer 8 to the substrate 1 or an end portion of a conductive/wiring pattern 3, 4 on the substrate adjacent to the lowest insulation layer 6. Whereby the crossover conductive/wiring pattern 10 traversing each of the edges does not become extremely thin even at the edges, and it is possible to obtain a crossover conductor/wire/film 10 thickness sufficient to assure that reliability is maintained.