GB2205646A

GB2205646A - Method of checking machine-produced artwork

Info

Publication number: GB2205646A
Application number: GB08713714A
Authority: GB
Inventors: Ian Douglas Gooch
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1987-06-11
Filing date: 1987-06-11
Publication date: 1988-12-14
Anticipated expiration: 2007-06-11
Also published as: GB2205646B; GB8713714D0

Abstract

Artwork is formed in a drawing area (11) on an artwork layer (10) of material, relative to at least two datum points (12, 13). A first check line (14) is drawn on the artwork layer (10) outside the drawing area (11) with a fixed spatial relationship relative to the artwork. A master layer is also provided carrying a second check line outside the drawing area in a position corresponding to that of the first check line on the artwork layer (10) and having a fixed spatial relationship relative to at least two datum points on the master layer. The artwork layer and the master layer are superimposed with their corresponding datum points in coincidence and the spatial relationship of the first and second check lines is compared. Two check lines may be provided on each layer, each inclined at a slight angle relative to each of two coordinate directions. <IMAGE>

Description

METHOD OF CHECKING MACHINE-PRODUCED ARTWORK Computer-controlled plotting machines are commonly used for producing artwork, particularly for the photographic processes used in the manufacture of printed circuits. Modern printed circuit boards have very high packing density which results in the need for accurate artwork to produce the closely-spaced tracks and lands. Since plotting machines readily operate with a repeatable accuracy of one thousandth of an inch (0.025mm), it is possible to plot points with this accuracy relative to one another. It is possible, however, for errors to occur in relating these points to a datum.Whilst this may not be of great importance if only a single piece of artwork is used, modern printed circuit boards are often made up from a number of superimposed layers, and it is essential that these should be correctly aligned relative to one another.

It is an object of the invention to provide a method for checking the accuracy of machine-produced artwork formed on a layer of material with reference to a datum.

According to the present invention there is provided a method for checking the accuracy of alignment of machineproduced artwork formed in a drawing area on an artwork layer of material relative to at least two datum points on that layer, which method includes drawing on said artwork layer outside the drawing area a first check line having a fixed spatial relationship relative to the artwork, providing a master layer of material on which is drawn a second check line outside the drawing area in a position corresponding to that of the first check line on said artwork layer and having a fixed spatial relationship relative to at least two datum points on the master layer, superimposing said artwork layer and said master layer with their corresponding datum points in coincidence and comparing the relative spatial relationship of the first and second check lines.

Preferably the first and second check lines are arranged to have slightly different angular orientations relative to the two datum points and are positioned such that when the artwork layer and the master layer are superimposed the first and second check lines intersect at an angle to one another.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an artwork layer bearing artwork and a first check line; Figure 2 is a schematic diagram of a master layer bearing a second check line; and Figure 3 shows the artwork layer and the master layer superimposed on one another.

Referring now to Figure 1, an artwork layer of material 10, preferably some form of transparent film, has a central drawing area 11 in which the artwork is drawn. Two datum points 12 and 13 are provided. Commonly one datum point 12 will be a hole which is interference fit on a datum locating peg, whilst the other datum point 13 is a slot which is an interference fit in one axis only. A first checking line is shown for each of the X and Y coordinate axes and comprises a line 14 drawn at a very slight angle (here shown exaggerated) to the coordinate direction. The check lines are drawn at the same time as the artwork in the drawing area 11 so that any alignment errors affect both the artwork and the check lines.

Figure 2 shows a master layer 20. This does not carry any artwork in the drawing area, but does have the second check lines 21, again one for each coordinate direction. These second check lines 21 are drawn at a slight angle to the coordinate direction in the opposite sense to the first check lines. The master layer has identical datum points 22 and 23 as does the layer of Figure 1.

Figure 3 shows the artwork layer 10 superimposed on the master layer 20 using the same datum points. Each of the first check lines 14 intersects the corresponding second check line 21 at twice the angle between the check line and the coordinate direction.

Any misalignment between the two layers in either coordinate direction is shown by the fact that the appropriate pair of check lines do not intersect in the middle. For example, Figure 3 shows correct alignment in the X direction but a misalignment in the Y direction.

In practice, by way of example, each check line may be 6 inches (approx. 15cms) long and set at an angle of 1 to 2 milliradians to the coordinate direction. The extent of misalignment may be measured, to a reasonable degree of accuracy, by dividing each check line by means of dots spaced 1/2 inches (approx. 1.3cms) apart. The error in alignment is indicated by the distance away from the centre that the two check lines of a pair intersect, the angle between the two lines being such that a shift of the intersection from the centre by 1 inch (2.5cms) indicates a misalignment error of 0.001 inches (0.025mm).

In practice, as has already been stated, the angle between the first and second check lines is very much smaller than is shown in Figure 3. The two lines in fact will overlap, with the intersection being shown by the thinnest point. This is in practice very easy to judge by eye.

It will be appreciated that each master layer relates only to the machine on which it was produced but may be used for checking all artwork produced on that machine.

It will be seen that only one of the two layers need be transparent. If used for photographic or printing purposes then the artwork layer will be transparent anyway. The master layer may also be transparent if desired.

Claims

1. A method for checking the accuracy of alignment of machine-produced artwork formed in a drawing area on an artwork layer of material relative to at least two datum points on that layer, which method includes drawing on said artwork layer outside the drawing area a first check line having a fixed spatial relationship relative to the artwork, providing a master layer of material on which is drawn a second check line outside the drawing area in a position corresponding to that of the first check line on said artwork layer and having a fixed spatial relationship relative to at least two datum points on the master layer, superimposing said artwork layer and said master layer with their corresponding datum points in coincidence and comparing the relative spatial relationship of the first and second check lines.

2. A method as claimed in Claim 1 in which the first and second check lines are arranged to have slightly different angular orientations relative to the two datum points and are positioned such that when the artwork layer and the master layer are superimposed the first and second check lines intersect at an angle to one another.

3. A method as claimed in either of Claims 1 or 2 in which the artwork layer and the master layer each carry two check lines, a separate one of which is aligned at a small angle to each of two coordinate directions.

4. An artwork layer suitable for checking by the method as claimed in any one of Claims 1 to 3 in which the layer carries artwork in a drawing area and a first check line outside the drawing area, the first check line having a fixed spatial relationship relative to the artwork.

5. An artwork layer as claimed in Claim 4 which carries two first check lines a separate one of which is aligned at a single angle to each of two coordinate directions.

6. A method of checking the accuracy of alignment of machine-produced artwork substantially as herein described with reference to the accompanying drawings.