Paul
Just to correct your statement
"Then the boards were exposed to either direct sunlight or a bulb that had heavy IR content in it's light."
The resist is not sensitive to infra red (IR), it is sensitive to ultraviolet (UV).
Malcolm
I don't suffer from insanity I enjoy it!
Mene, mene, tekel, upharsin
The writing is on the wall.
Ha-ktovet al ha-kir
--------------------------------------------
On Wed, 9/30/15, palciatore@... [Homebrew_PCBs] <Homebrew_PCBs@yahoogroups.com> wrote:
Subject: [Homebrew_PCBs] Re: G-10 questions
To: Homebrew_PCBs@yahoogroups.com
Date: Wednesday, September 30, 2015, 2:30 PM
OK, I made PC boards in my garage back in the 70s.
I used a variety of techniques.
First, you wonder how the copper got on the
boards. Well, even back in those dark ages, you just bought
them that way. The factory put the copper on them: I never
even considered the possibility of doing it myself. Oh, they
did sell pens that drew conductive lines, but I never
trusted them so I did not even try them. It was a conductive
paint, not solid copper.
You could buy either plain copper clad boards
or sensitized copper clad boards. The sensitized ones had
the photo resist already sprayed on them in a nice even
coat. You needed a high density negative or positive,
depending on the type of resist on the board. It was before
the days of inexpensive computer CAD so I made 2 times
actual size layouts using graph paper with a 1/10th grid of
"drop out" lines. That means the lines did not get
picked up on the negative or positive transparency which was
the next step. I used drafting aids like black adhesive tape
in various line widths and self adhesive doughnut pads and
IC patterns. All of these were available from drafting
supply stores. I carefully placed them on the grid with due
care for trace separation. My primary tool was an Xacto
knife.
Then that artwork
was taken to a drafting or blueprint company who had a copy
camera. That was a camera that had the lens, film holder,
and art holders all mounted on floor tracks. It could make
negatives or positives in exact geometric ratios. Since my
original was 2X, I had 1/2 size negatives made. Of course,
you could make your own negatives but then you had to work
with 1X artwork on transparent plastic and a contact print
frame.
Then the boards
were exposed to either direct sunlight or a bulb that had
heavy IR content in it's light. They were developed
using a solvent solution and allowed to dry. Then placed in
an etchant until the excess copper was removed. Then the
resist was scrubbed off with steel wool or dissolved with a
different solvent.
I also
liked to use a tin plating solution to improve the
solderability. Dip, wait two minutes, remove and rinse.
Finally, they were drilled
with the various sized drills needed for the individual
pads.
Or you could buy the
plain boards and spray the resist on from spray cans. A
company called Kepro had all these supplies, but they no
longer exist.
Or you could
use those self adhesive pads directly on the bare copper
board to form the pattern on. This was OK for a one-off
board, but tedious if you needed several. And the etchant
could get under the traces where they overlapped a doughnut
hole due to the height of that doughnut pad.
Another way was to start with
the bare copper board (one side or two) and use a perf board
to mark the pad locations. Then drill them. Then use a
resist pen or a Sharpie marker to draw the foil pattern.
Etch, clean, tin plate and use.
Others used other techniques. You could make a
Zerox copy of the artwork and use an iron to transfer the
toner to the bare copper board. I believe some people still
do this today. Some retouching with a resist pen or Sharpie
was generally needed.
Like
I said, many ways, even back in that ancient era of the 70s.
I assume you are trying to
do this in an authentic manner, but back in the 70s I
usually saved the PCB process for boards where I needed four
or more identical boards. If I only needed one, two, or
three I usually used the Vector slit and wrap, wire wrap
pens. I built many professional projects that way. I made
wire lists and was very careful in making the connections
properly. I never had a bad WW board.
Paul A.
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