old pcb method, vintage pcb, fr4, bakelite

[gain-wire]

Abouth old-days PCB fabrication method.

OK, now this topic is probably going to frustrate some readers because it's like asking someone how to make a spoon. Whatever. As Iam very interested in building vintage-looking projects, I also want them to be built using the methods of the time period they are supposed to belong to. So, I am looking for information on how printed circuit boards used to be made in the late-sixties to seventies.

More precisely how was a pattenr copied, say, you had an amplifier circuit you wanted to copy on a board four times. HOW was this done?? The only thing I can come up with was that you would take your artwork, photocopy it as many times as you wanted (I AM ASSUMING that photocopiers existed in that period) and then cut out the circuit and essentially past them next to one another onto a master paper and then copy that to a master celluloid sheet, which you would then use to do the UV etch method.
AM I too far off here. Just for history's sake, I would really like some info here, please can somebody help me out here?
Maybe someone who worked at Tektronix, EICO or Heathkit can educate me on this subject.
Thank you very much!

[poobah]

Originally,

PCB's were laid up on a light table using clear Mylar film. The light table usually had a grid sheet so you arrange things relatively precisely. The work was usually performed at 2 or 4 times actual size.

Small, precut donuts and tapes were used for traces and pads. These tapes were made from black crepe paper so you could bend them into curves etc... very much like masking tape.

Often multiple layers of mylar were used to avoid redundant effort. For example, a 2-sided board was usually made from 3 sheets. All the pads or "donuts would be placed on one sheet... called the pad master. Another sheet would then be layed over the master, this is where the tapes were laid to represent traces for one side of the board. Repeat for the other side of the board. Even for single sided boards a separate pad master was used.

Once all these lovely sheets were done, you took them to a special photoshop. They would hang the sheets on a light table with a vacuum frame. They would then use film the same size as the actual finished board. Using different combinations of sheets, they would create film negatives for the different layers of the board. They would also create a negative of the pad master.

These negatives were then photo'd again to create film positives. These were "the master films". Why the pad master? when making the film positives, the pad master was deliberately over-exposed, this would make the pads larger... this film would be used for the solder mask on the final PCB. Yet another pad master would be created without any exposure tricks.

All these films went to the circuit board shop... this is where the "step and repeat" work was done. The PCB shops used a special type of film that was less sensitive to light. In this way the S & R work could be done in low light (red) conditions. The final working films were usually negatives. And there is bunch of different methods for exposing and developing the PCB material.

Why the pad master?... again? once a full size, S & R'ed working film was created, a program had to made for the drilling machine.
The film was laid on a light table and a special magnifier with a crosshair was positioned over each pad. Once "bomb-sighted", a button was pushed, the computer logged the position and punched the tape that stored the drilling program.

All of this SUCKED! I don't even think they make circuit boards like this in the Steppes of Mongolia anymore. I'll bet you can still find the special tapes and precuts on the Russion black market. The company that owned that market here in the states bellied up in the early nineties after some crazy employee buyout thing to try and save their jobs.

All this stuff was pricey as hell. A grid for light table was $200. Mylar sheets were $20. Little boxes of donuts were $5... as were the rolls of tape.

How to replicate the old look? What makes 'em look old is the gently curving tracks of copper. Do your boards using modern tools. Export the files in DXF format. Replace the traces one by one with splines (curvy-lines). Convert the splines to polylines and assign a width. Bring all this back into your PCB environment and make your gerbs from there.

[Bone]

In my day - late 60s - we used to draw the circuits in black Indian ink on a smooth card called Bristol Board, 10 times full size. This was after laying out the circuit in pencil on graph paper!

I once had to draw a multilayer board, 8, if I remember for a flight recorder. Each layer was on a separate piece of card. You had to be accurate - try erasing this ink without leaving a trace.

The cards were then photographed and negatives were made the correct size to expose the board.

Tony

[poobah]

Are you talking about the Rapidograph pens?

ugh...

[Christer]

We did a double-sided board once, by using precut tape of the type Poobah mentioned, but this particular PCB manufacturer wanted the layers on the same film, using red and blue tape for the different layers. I assume they then used very strong coloured lights and filters to separate this into two different films photographically. But this was c:a 1980, so it is perhaps to modern?

For home use and single layer, I also used the black type of tape and precut symbols. Or sometimes, with less success, the special felt tip pens sold to draw the pattern by hand.

[poobah]

That red/blue stuff you couldn't turn corners with... you had to miter all the corners with a knife.

Were you working at 1:1 scale?

[Christer]

Originally posted by poobah
That red/blue stuff you couldn't turn corners with... you had to miter all the corners with a knife.

Were you working at 1:1 scale?

Fortunately, I wasn't doing it myself. We had a very patient guy who did it all the tape work. I don't know about the corners, but you are probably right. It wasn't the crepe type of tape.

Guessing from memory, we probably did it twice the sice of the final PCB.

I think the board maker messed it up anyway in the end, so he had to do a new batch for us, at his own expense.

[anatech]

For home use.

Layout the holes using graph paper (10 or 20 lines per inch). Tape to PCB material and use a center punch to mark all your centers. Drill with appropriate drill size, I use a pin chuck as well to hold these things.

Now you remove the paper, clean the PCB material with steel wool followed by thinners of some kind. Using the pen of your choice, connect the dots and fill copper areas. Let dry and etch. Rinse well and use thinner to get rid of the ink. Now you can tin the board, or use solder coat. If you want more than one, place the board you just did on a couple more and drill using your first as a guide. This is before populating it of course!

I like the "bluing ink" pens I think PC33 by Lenbrook.

Yeah, it stinks doing it that way.

-Chris

[poobah]

I never make boards.

I take the CAD/CAM girl at my board shop to lunch at least twice a year. She puts my proto's on the same panel with somebody elses proto's and I get them free... well sort of...

[Christer]

Originally posted by poobah
I never make boards.

I take the CAD/CAM girl at my board shop to lunch at least twice a year. She puts my proto's on the same panel with somebody elses proto's and I get them free... well sort of...

Hope she's not the ex girlfriend of the boss there.