Brad,
I will address how I did things back in Medieval times below.
First, I will tell you how to get natural color FR-4.
I did a little detective work by going to the website of Frys
electronics (frys.com), a US electronics chain that I know carries
PWB materials and chemicals. Their website shows that they carry
board material and chemicals from MG Chemicals. A trip to their
website, mgchemicals.com, shows that they sell non sensitized and
photo sensitized single and double sided FR-4 material. Further
thrashing around their site yielded some training videos, one of
which is called "The Etching Process." Lo and behold, the color of
the board that comes out of the etch tank is "natural." From this,
I conclude that their boards are just the color you want them to be.
The website also lists world wide distributors for their materials.
As a double check, you could always ask them what color the boards
are.
There were several ways we made our boards back in the Dark Ages.
Many commercial boards were made by using photo sensitized boards
and exposing/developing them with a process similar to processing B
& W prints (Photolithography). Others were made by silk
screening etch resist onto the board, drying it and etching. I did
photo process by buying non sensitized boards and using an aerosol
spray photo resist from one of the chemical companies. The actual
material was Kodak KPR, a negative acting photoresist. I prepared
my artwork 1:1 on clear acetate film using precision cut tape and
pad patterns from Bishop Graphics. The artwork was a positive of
the final copper pattern. Everything I have described so far was
very expensive. The artwork was contact printed onto Kodalith
film. This is a lithography film that displays almost no gray
scale. The output from this was a negative with dense blacks and
clear areas. The negative was contact printed onto the sensitized
board and exposed to UV light. The developer washed away the parts
of the resist where the copper was to be etched away.
That explains how it was supposed to work.
Problems in the garage lab:
1. The KPR spray had to be done in semidarkness. It had a purple
dye, so it could be seen in low light. I used a low wattage yellow
bug light for my safe light. It was very difficult to spray a thin
coating of uniform thickness on the board in the low light. Uneven
thickness interfered with the development process, so that parts of
the board didn't clean off properly in development. KPR was also
available as a very thick liquid. This would be dispensed into the
approximate center of a board mounted horizontally on a spinner.
With vacuum holding the board down, it would be spun until the
resist distributed itself evenly on the board, and all the
surroundings. This probably worked quite well, but was too
difficult to implement in the garage.
2. Kodalith was an excellent product when it was fresh.
Unfortunately, the smallest quantity I could buy it in was 25
sheets. Since I was making boards only occasionally, it took me a
long time to use this up. After some period of storage, the dense
blacks became dark grays, and no longer performed adequate masking.
3. Ditto for the photo processing chemicals.
4. On top of this, the development process required some reasonable
degree of temperature control. I was doing this in an unheated
garage in New York, winter and summer.
5. My sources of UV light were varied. I tried the bulbs that were
used for lighting up fluorescent posters in Discos. They worked,
but the light was too uneven. I tried the recommended photoflood
bulbs in a 15" reflector. This worked, but it was intensely hot,
and the board had to be moved far enough away that the resist would
not melt and stick to the negative. The lamps only had a four hour
life, and if you jostled them at any point the filaments broke. Did
I mention that they were also expensive. I built a small light box
with 4" UV fluorescents. This worked but was limited to very small
boards.
6. The photo process gave excellent alignment for two sided boards,
but the problems of etching both sides evenly made it a grim
undertaking. I won't even try to describe the handstands I did to
get that to work.
Getting a really good board out of this was as much a matter of luck
as science. Most boards had at least a few jumper wires repairing
traces, and a few scratch marks where bridges had to be removed.
We also had resist pens that were like blunt Sharpies, and gave out
with a black tar. This worked but was very sloppy, and was only for
one off boards. Other processes for one off boards was the placement
of pads and tape directly on the copper, and then etching. This was
very tedious, and suffered from undercutting where the tape went
over the pad.
It wasn't easy in the bad old days.
(Another) Harvey
Thanks
for this very excellent explanation. It’s doubly hard
being a newbie but also trying to understand how
things were done back in the day.
Because
this is a replica, the correct look is very important
to me. It would have been much easier to take the
offerings available on mouser or ebay.. but the
colours look wrong and the boards themselves too
modern. Further, for some bizarre reason, most of the
manufacturers don’t bother taking a picture of the
actual PCB.. just the copper side, so you can’t ever
be sure what color it is. It’s funny how one does
that and then they all do.
So
yeah, the reason I went to these guys is, it looks
like we have the right ‘grain’ and right colour. I
don’t care so much about fire resistance, but it seems
like all the G10 I’ve found in my color is FR4
regardless.
When
you guys were doing these boards in the 70s.. they
were single side copper clad photo sensitive right? I
tried asking Don exactly what format the boards he
used came in (ie if they were just the PCB or had
copper affixed), but he tends to be cryptic in his
responses.
Hi guys,
I have been following this group for many months,
and I am very excited to see the quality of the
discussions presented here.
I have not contributed until now because I had
little to add. I made boards as a teenager in the
'60s and then into the '70s. In those days I used
G-10 board. A some point FR-4 crept in, and
became the standard for PWBs. Recently, I have
become interested in making boards again, and am
delighted to have discovered this group.
My 2 cents: I think the train has become derailed
on the subject for G-10 vs. FR-4. It seems to me
that the stated goal was to get boards that look
like Don Lancaster's green G-10. There are
several companies making unclad G-10 and FR-4.
You only need to look at the websites to see that
both are the same color when purchased as
natural. The only difference between them is the
addition of a small amount of Bromine to make
the FR-4 boards flame retardant. Check this out:
http://www.acculam.com/data-chart.html. Other
places on this website (Accurate Plastics, inc)
show samples of the material, and it can be seen
that the color is the same. The color of the G-10
from American Micro Industries is the same.
The "green" color is called natural. Back in
those days, the only color that glass/epoxy boards
came in was natural or "green." The shade did
vary between manufacturers, probably because of
differences in the epoxies they used.
Since the color is the same and the goal is to
provide the same looking boards, I think that the
solution is to use FR-4, and specify "natural"
color for the base material.
(Another) Harvey
