GB2177262A

GB2177262A - Making printed circuits

Info

Publication number: GB2177262A
Application number: GB08516477A
Authority: GB
Inventors: Martin Paul Mobberley; Maurice Lionel Apthorp; Maurice Clifford Heard
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1985-06-28
Filing date: 1985-06-28
Publication date: 1987-01-14
Also published as: GB8516477D0; GB2177262B

Abstract

An electrical circuit is formed by intaglio printing a pattern of conductive ink on to an insulating substrate (1). Intaglio printing of conductive inks enables the width and spacing of conductive tracks (7) to be controlled to a very high degree of accuracy and permits the height and profile of the ink thickness to also be controlled, thereby permitting raised contact bumps (8) to be printed. A number of identical circuits can be readily printed using the same intaglio printing plate. <IMAGE>

Description

SPECIFICATION Electrical circuits This invention relates to electrical circuits in which at least some of its conductors take the form of conductive tracks carried by an insulating substrate. Examples of such electrical circuits are printed circuit boards and hybrid circuits. In the former a rigid or flexible insulating board generally carries a thin adherent foil of copper which is selectively etched to leave a required pattern of conductors, and in the latter hybrid circuit the required pattern of conductive tracks is produced by printing conductive ink through a silk screen or the equivalent on to a ceramic substrate, for example. Often discrete elecronic components are mounted on the substrate to complete the electrical circuit.As electrical circuits become more complex the pattern of conductive tracks becomes more intricate and it is desirable to reduce the width of individual conductive tracks as otherwise the total area of the circuit can become very large. However, if the track widths and the spacing between adjacent tracks is reduced too much, then the quality of the circuit is impaired, either by the occurrence of unwanted discontinuities in the tracks or by short circuits between adjacent tracks. As the tracks become narrower so it becomes difficult to maintain and control a significant track thickness (i.e. height) so that the electrical resistance of the track may be greater than is desired.

The present invention seeks to provide an improved electrical circuit, and a method of producing it.

According to this invention, a method of producing an electrical circuit includes forming an electrically conductive region on the surface of an electrically insulating substrate by means of intaglio printing.

In intaglio printing the ink-carrying areas of a printing plate are recessed hollows below its surface so that the thickness of the layer of ink transferred to the substrate varies according to the depths of the hollows. The printing plate may take the form of for example, a cylindrical surface or a flat surface, and in this context the term ink embraces any settable fluid having predetermined electrically conductive properties. As both the lateral extent and the height of the printed areas are determined by the shape of the recesses in the printing plate the profile of the printed ink is very accurately controllable. Very fine lines indeed can be accurately and reliably formed, and the height can be controlled to give required con ductivity properties, or so that the profile of the ink itself can perform an electrical function.Thus localised raised ink bumps can be readily and integrally formed on a conductive track so as to provide contact regions for example. Furthermore, once the printing plate has been made it can be used a very large number of times to rapidly produce many electrical circuits having identical characteristics. The simplicity of intaglio printing contrasts with the previously used methods of circuit production such as those relying on selective metal etching or silk screen printing for each circuit, and these previously used processes give little or no selective control over the height of the conductors so formed.

The invention is particularly applicable to electrical circuits in which large numbers of the same circuit layout is required. The circuit may comprise conductive tracks which are formed by intaglio printing, and any discrete components required may then be added as a subsequent step. The size of the pattern of conductors is limited only by the size of printing plate which can be fabricated.

One convenient application of the invention is to the production of circuit strips for use with so-called tape automated bonding. Such a circuit strip consists of a repeating small pattern of conductive tracks which serve to interconnect semiconductor chips either directly to a larger circuit of which they are to form a part or to the terminals of a package in which each chip is mounted.

One such example of the invention is described with reference to the accompanying drawing in which Figure 1 shows one element of a circuit strip and Figure 2 shows a detail thereof.

Referring to the drawing, a thin flexible elongate plastic tape 1, carries on its upper surface adherent patterns of electrical conductors, only one pattern 2 of which is illustrated with identical adjacent circuits 3, 4 being represented by the broken lines. In practice the tape will be long and typically several hundred such patterns will be present on a single length of tape. The width of the tape may be in accordance with a standard 35mm photographic format, and it may be provided with indexing windows along its edge to permit use of a sprocket drive to position the tape as necesary. The tape 1 is initially free of conductors, and the pattern 2 illustrated is placed upon it by the intaglio printing of a conductive ink. An intaglio printing plate is recessed with localised hollows corresponding exactly as to width and depth of the required pattern.The depth of the hollow determines the thickness of the conductor which is subsequently printed, and it is possible to accu rately control the depths of the hollows which are cut into the surface of the printing plate.

The depth need not be constant along the length of a recessed groove but can be varied as required resulting in printed conductive tracks having heights profiled in accordance with a desired shape.The transverse width of the conductive track so printed is, of course determined by the width of the recesses in the printing plate.

In the case of the circuit pattern 1, each conductive track 5 consists of a contact pad 6, a link 7, and a contact pad 8 which in use contacts a recessed terminal of a semiconductor chip (not shown) and which is of greater height so as to project into the recesses of the chip. The detail of the longitudinal profile of a track 5 is shown in Figure 2. The pad 6 and the link 7 are of constant thickness (height) but the pad 8 is raised by an increased thickness of ink to constitute a contact projection or bump. The entire pattern 2 is formed simultaneously by a single pass of the intaglio printing plate.

Such a pattern can be readily printed in a repeating manner on to a long tape by means of a rotating cylindrical intaglio printing plate, and once the plate has been produced, production of the circuits is quick and inexpen sive: Suitable conductive inks are readily available and in general those inks which are used in the silk screen-like printing of circuits can be used in this application. Whereas, however, the resolution obtainable with the silk screen process is probably only about 70 microns with little control over the thickness of the ink which is printed, the resolution obtainable using intaglio printing is much better and typically about 20 microns.

Claims

1. A method of producing an electrical circuit including the steps of forming an electrically conductive region on the surface of an electrically insulating substrate by means of intaglio printing.

2. A method as claimed in claim 1 in which a recessed hollow of non-uniform depth is formed in an intaglio printing plate whereby the printed conductive region has a corresponding profile.

3. A method as claimed in claim 1 and 2 and wherein the electrical circuit includes a plurality of electrically conductive regions which are printed simultaneously.

4. A method as claimed in any of the preceding claims and wherein a cylindrical intaglio printing plate is used to print a repeating pattern of electrical circuits on to an elongate tape.

5. A method as claimed in any of the preceding claims and wherein the intaglio printing is used to form conductive regions having localised contact bumps.

6. An electrical circuit including an electrically insulating substrate which carries an electrically conductive region which is formed by intaglio printing.

7. An electrical circuit when made in accordance with any of claims 1 to 6.

8. An electrical circuit substantially as illustrated in and described with reference to the accompanying drawing.