A New Technique for PC Layout Digitization

Copyright (i;) IFAC Computer Aided Design Indiana . USA 1982

A NEW TECHNIQUE FOR PC LAYOUT DIGITIZATION K. H. Muralidhar, B. S. Chandrashekar, C. N. Ajit, S. Rajaram and A. Prabhakar CAD Group, Central Labs(R&D}, Indian Telephone Industries, Bangalore-560 016, India

Abstract. A new technique to automate the digitization of printed circuit layouts is described. An optical scanner acts as the input device for digitization and the data so input is processed using the thinning and tree spanning techniques of picture processing. The PC layouts are treated as a special class of pictures whose properties help easy implementation of the process with a low cost scanner and minicomputer system. Being optical, the technique is fully compatible with the Bishop Graphics Color scheme thereby providing a very powerful bridge for conversion of manually laid artwork to computer generated artwork. The technique has been extended to other areas of engineering documentation. Keywords. Pattern recognition; computer graphics; optical character recognition; PC layout digitization.

INTRODUCTION The use of Interactive Graphics System for generation of printed circuit artwork has become a standard practice in the electronics industry. Such systems accept as input, either the circuit schematic information serving as input to the Autoroute program, or the manually designed layout information digitized into the system. Though various types of automatic routing software exist, quite often, especially in the telecommunication industry, recourse has to be taken to manual layout design to meet critical design conditions. In such cases, the existing practice is to prepare a blown up version (2X or 4X) of the layout and 289

digitize it manually into the system. This step forms a major bottleneck in the process of artwork generation because of the huge amount of dat. that has to be input, especially in the case of highly complex layouts. Moreover, being manual, the method is prone to errors. The technique to be explained in tnis paper, overcomes the limitation by automating the digitization process. The technique employs a low cost, low resolution optical scanner with helical raster as the input device.

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Fig. 1. Flow cnart of the technique PC LAYOUT AS A PICTURE PC layouts in this technique, are treated as a special class of pictures having the following properties. (a) Picture information is binary in nature. (b) The layouts consist only of landing pads and piecewise linear tracks. (c) All landing pads lie on grid

points. (d) The end points of each linear segment. of a track lie on grid points. These properties are in-built in any of the layouts designed with existing practices and do not impose any restriction on the prepara·t ion of the layouts.

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A New Technique for PC Layout Digitization

THE TECHNIQUE A drum scanner having 250p p1xel size and 1024 pixels per line has been used for implementation of the technique. The scanner generates an 8 bit word for each pixel which can later be thresholded through software to two level information. The data is written onto a magnetic tape and using the color separation facility, a separate file is created for each of the layers of the PC layout.

EXPERIMENTAL RESULTS A number of layouts have been processed using this technique and the technique is found to have great

In case of hand written layout the two layers are shown in Red and Green and (TM) color separation is done using KODAR filters No. 29(Red) and No. 58(Green). The landing pads are also in Red color and form part of the first f1le. Bishop Graphics(TM) artworks are scanned with filters No. 29(Red) and No.47 (Blue). Notice that in this case, a third file with a very high thershold and without any filter is also generated to extract the information about landing pads. The flow chart for processing the data thus recorded on the magtape 1s shown in Fig. 1. Each file is first processed for the track information and later, the landing pads information is generated by mapping onto the unthinned versions of the appropriate files. Finally the information so processed on the PDP-ll minicomputer system running RSX-I1M, is consolidated into a drawing file and fed to the Interactive Graphics system. In fact, in some of the Interactive Graphics systems, even the processing of the scanned data can be incorporated because they allow concurrent working of FORTRAN jobs and Graphics operations, thereby giving further advantages in terms of reduced turn around time.

Figure. 2 advantage over the manual methods. Fig. 2 shows the photumaster of a layout digitized using this technique. Both sides have been supe~imposed for convenience. With the present technique the layout required about 25 minutes for scanning and pre-processing followed by 30 minutes of editing at the IGS. The same layout when

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digitized using the manual methods, required 60 minutes of digitizing time followed by 45 minutes of editing at IGS. This clearly brings out the advantages of the new technique over the existing manual methods.

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track information. The track information for component side is in the file corresponding to Fig. 4. Frintouts have also been taken for a small portion of a Bishop Graphics artwork and the three outputs are

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Figure. 4 The scanner printouts for a small portion of a double sided hand written PC layout are shown in Figures 3 & 4. Figure 3 corresponds to the Red filter and Fig. 4 to the Green. The original draft has not been shown due to lack of color printing facilities. It may be noted that the file corresponding to Fig. 3 will have landing pads information also in a ddition to pattern side

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presented in Figures 5, 6 & 7. It may be seen tha~ while the process of extraction of track information for the two sides is very much similar to that in the case of hand written layouts, the landing pad information in tnis case is extracted from a third file corresponding to the printout of Fig. 7. This file is also used to generate the NC drill +-aoe.

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Extensive tests have been taken with respect to variations in writing and color of the inks in case of hand written drafts. Results have shown that the technique can accomodate lot of variation in preparation of layout. While property (d) of PC layout helps the use of a low resolution scanner, property (a) allows variations in the intensity of lines and also color of the inks. Any noise in the picture which may persist even after thersholding gets automatically rejected by virtue of properties (b) & (c). It has been found that the drafts can be prepared with fibre tip pens, having tip diameter of 10 to 20 mils. CONCLUSION Over six years of experience with computer aided artwork generation at Indian Telephone Industries,Bangalore, has shown that the new technique is an extremely powerful alternative to the existing methods of manuai digitization especially in the case of highly complex layouts. The technique has become very attractive from the point of view of hardware cost since it uses only a low cost scanner. Successful attempts have also been made in reading other types of engineering drawings like circuit schematics into the computer using the optical scanner. ACKNOVILEOO EM.t:;NT The authors thank Mr. M.S. Jayasimha, General Manager, R&D Indian Telephone Industries Limited, Bangalore for his kind permission to publish this paper. The authors are also grateful to Prof. G. Krishna OT Indian Institute of Science, Bangalore for permitting them to use the scanner.

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REFERENCES Deutsch, E.S. Comments on a line thinning scheme. Comput. J.12 412 R. Stefanelli & A. Rosenfeld (1971) Some Parallel Thinning Algorithms for Digital Pictures. JACM, 18, 255264