David,
Back in the 70's and 80's I did a lot of 2 sided boards using photographic methods, it worked very well. Back then you hand taped you master artwork at 2x and sent it to the photo house for reduction to net size. I was using Shipley AZ111 positive photo resist that was spin coated on the boards and etched in hot ammonium persulfate. The results were very good but the hand taping of the artwork was the major drawback.
To keep undercut to a minimum I had to make sure the emulsion of the film was in direct contact with the coated boards. Had to add a big note on the master artwork about which side of the artwrok was to face the lense of the camera to be sure they got it correct and the emulsion was on the correct side of the final film. I had a DYI exposure box using 3 18" VU fluorescent tubes for each side of the board. The board was clamped between two pieces of 1/4" glass. Exposure times were about 5 minutes. Looking back I am sure the glass absorbed a lot of the UV but it still worked fine. There was a local shop in Mountain View, that did much of the photo work for the electronics companies here in the valley and I could get 8x10 negatives for about $10 each. Because I was using positive resist I had to make a contact print of the negatives to reverse the image beforer exposing the board. For two sided boards, you just align the two films and scotch tape them together aroung the edge making a pocket and slip the board inside. Etching in fresh ammonium persulfate took 20 to 40 seconds then a dip in the electroless tin.
Drilling was a pain, I had a small 1.25" dia by 2" surplus DC motor that was attached to a small chuck salvaged from an old flex shaft tool. Powered it from my bench supply with a foot switch. The key here was to use DC braking on the motor by shorting out the motor leads with the foot switch to stop the motor before positioning the bit in the next hole. Used HHS drills that had to be frequently sharpened on a small carborundum stone. Tried carbide but they were too brittle for had use. I think the it was a 20V motor I ran on 40V so it turned a good RPM. Had to let it cool every once in a while.
Seems its hard to get good UV opaque artwork out of todays lasers or inkjet printers, lots of pin holes etc. I think one could print 2:1 artwork on a laser, then have it photo reduced 2:1. Print shops were a good source for camera work. Or even make a 2:1 camera from some plywood and a lense. I think front lighting the master would solve the opaque problem, if not go at it with a black sharpie.
Once you set up and get you process going its easy to make boards. Lately I have done a few simple PCBs with the Toner method, with mixed results, partly due to my Brother laser printer.
Craig
--- In Homebrew_PCBs@yahoogroups.com, David Griffith <dgriffi@...> wrote:
>
>
> I'm convinced now that I should give the photographic method a try and
> even try pre-sensitized boards. I'd like to do things the Right Way
> instead of going sloppo like I have been so far. That might explain my
> previous problems. Anyhow, I'll start with constructing a UV exposure
> box. The box and timer stuff seems straightforward enough, but what I'd
> really like is to make a gizmo that sandwiches the PCB and films between
> two sheets of glass or lexan. The idea is to put the things in this
> frame, adjust for registration, clamp it closed, and that's it. Insert
> into the UV box for exposure. I saw a page a couple years ago discussing
> something like this, but I can't find it anymore. Does anyone here smell
> what I'm cooking?
>
> I keep the boards small and one-sided, would a UV EPROM eraser work?
>
> --
> David Griffith
> dgriffi@...
>
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