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PCB Design In The 1970's

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RonaldSmith View Drop Down
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    Posted: 21 Nov 2020 at 2:56am

Thank you for this post , it's very informative and helpful to all new and existing industry experts.


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Tom H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 30 Apr 2018 at 2:52pm

Back in the early 1970’s, when I started my PCB design career, every component package was through-hole. We manually sketched the pads and traces with red and blue pencils on vellum parchment paper using a 10th inch grid pattern on a drafting table using a T-square and triangle and circle templates. 

When the sketch was completed, the pencil drawing, and grid sheet was placed on a flat light table. We overlaid a sheet of mylar on top of the pencil drawing and manually placed sticky donut pads, made by Bishop Graphics and using an Exacto knife to create the Pad Master. Then a new sheet of mylar placed over the pad master and pencil sketches. We used 0.062” wide black tape and an Exacto knife to create the traces for the top and bottom layers. All the designs were 2-layer boards that were laid out 2:1 scale and reduced to 1:1 scale using a giant precision camera to create negatives of the outer layers. 

The pad master was over-exposed to create the solder mask swell. The PCB’s where created using the negatives to apply a temporary mask that protects parts of the copper laminate from acid and leaves the desired pattern. This etching process is still used today, but back in the 70’s it was a manual process using gloves, tongs and timers. The solder mask was manually applied to the PCB using a screening process. The primary goal of solder mask back in the 70’s was to protect the bare copper traces from oxidation and the acronym SMOBC was created. The solder mask swell was between 0.20 mm (8 mil) and 0.25 mm (10 mil) annular ring. 

We were not concerned about solder bridging because the through-hole components were hand soldered. A manually sketched assembly drawing on mylar was placed into an ammonia tube with blueprint paper to generate copies. The blueprint assembly drawing was used to indicate polarity and package part number or value and was hung from a wire using alligator clips in front of the person doing the soldering. All of the processes from schematic diagram to PCB layout to fabrication and assembly were all performed in the same building. 

The EE engineers would manually pencil sketch the schematic on paper and the PCB designer would use India ink on mylar using schematic symbol templates to create the master schematic. The Bill of Materials was manually created on paper. There was no netlist and the PCB designer who drew the schematic had to use it to lay the PCB traces. The PCB holes were manually drilled using a drill press and the holes were un-plated. The vias had to have a bare copper wire crimped and soldered on both sides.


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