Fifty Years in Engineering

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Fifty Years in Engineering

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By: Stewart Da’Silva
Senior Designer, Product Design

22nd February 2017

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pcb_layouttoolsIn 1966 whilst serving my apprenticeship as a mechanical design draughtsman, I was assisting a senior draughtsman on a design that required a simple power supply. This product was going to be manufactured in medium volume and he suggested that maybe I would like to investigate the possibility of using a Printed Circuit Board to connect it up instead of using wire to make the connections. This was my first introduction to PCBs.

He had really set me a challenge as no-one in my company had used one before. Anyway, suffice to say that that very first layout of mine was constructed using an ink pen, rule and compass using Indian black ink on white Bristol board. To increase accuracy of the finished PCB, it was drawn 4:1 scale. As is probably obvious to the reader, mistakes whilst drawing the PCB usually necessitated starting from scratch again. The next stage was arranging for an industrial photographer to generate a 1:1 positive film from the 4:1 artwork that could be used by a printed circuit board manufacturer to fabricate the PCB.

I finished my apprenticeship in 1968 and not long afterwards started work as a mechanical designer in a ’Contract Office.’ This was essentially a design house offering design capabilities to companies that did not have the necessary skills or had to outsource projects due to a high volume of work.

taped-artwork_black-tapeIt was here that I learnt the basis of PCB design. Things had moved on, although designs were still only single or double sided. Instead of ink on Bristol board, the initial design was drawn, again at a scale of either 2:1 or 4:1, on a stable semi-transparent plastic film that was placed over a similar transparent film with a 0.1inch matrix printed on it that was fixed to an A0 drawing board. This grid was used as a guide for the PCB layout.

If the PCB design was double sided, the usual convention used was blue pencil for the component side and red pencil for the solder side. Once completed and checked this pencil layout was flipped over and secured over another grid that was in turn attached to the surface of an A0 size light box. A translucent film was positioned over.

lightbox1Using pre-cut adhesive backed-tapes and pads of various sizes and following the red colour of the layout that was under this sheet as a guide, the solder side of the PCB took form as the designer built up the artwork. When the solder side artwork was complete it was removed from the light box together with the pencil layout. The artwork was flipped over and secured once again to the light box, another plastic sheet was placed over this and again, using the pre-cut pads, the designer aligned these with the pads on the completed solder side artwork. Once all the pads were positioned, the solder side artwork was removed and this ‘pads only’ component side was again placed over the now turned over pencil layout and the blue colour followed to tape up the component side. The two sides of the finished and checked artworks were then sent to an industrial photographer who generated a 1:1 artwork from the originals.

red-blue-artworkThe next step that the industry took was to use only one piece of stable plastic sheet instead of two. The pre-cut black pads were still used but instead of black tapes, transparent blue and red tapes were used and were placed on opposite sides of the sheet. The industrial photographer would then attach filters such that only the red or the blue traces appeared as black when he created the 1:1 artworks. This may seem a small step but it did mean that alignment of both sides of the PCB artworks was guaranteed as exactly the same pads were used.

Read part 2 of Stuart’s blog here.

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Further Reading

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