Homebrew PCBs

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






On 3/18/2016 9:03 AM, 'Brad' unclefalter@... [Homebrew_PCBs] wrote:
>
> Thanks for this very excellent explanation.  It\u2019s 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\u2019t bother taking a picture of the actual PCB.. just 
> the copper side, so you can\u2019t ever be sure what color it is.  It\u2019s 
> 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 \u2018grain\u2019 and right colour.  I don\u2019t care so much about fire 
> resistance, but it seems like all the G10 I\u2019ve 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.
>
> *From:*Homebrew_PCBs@yahoogroups.com 
> [mailto:Homebrew_PCBs@yahoogroups.com]
> *Sent:* Thursday, March 17, 2016 12:03 PM
> *To:* Homebrew_PCBs@yahoogroups.com
> *Subject:* [Homebrew_PCBs] Re: Custom G10 copper clad
>
> 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
>
>