I currently use DuPont Riston 4713 dry film resist. Lately, I've been getting it from Think-n-Tinker. I was lucky enough to get a dry film laminator at a good price. For lower volume, I would suggest you buy the board pre sensitized with the dry film resist. There might be someone closer to you who supplies similar material. I tried using the liquid resists, like Kodak KPR, and had very erratic results. It was also hard to tell when the resist gave a good print, because it was nearly clear. The Riston has a dark color, so you can easily see the traces and pads, and any defects can be touched up. Removal of resist is easy with an X-acto knife. A black Sharpie pen fixes pinholes. The main benefit of the dry film is it is remarkably tough, as you will find out if you have to touch it up with an exacto knife. I won't even try the peel-off techniques, I know they won't work for the 2-sided work I'm doing.

Exposing Light Source

The resist hardens (cross polymerizes) under mercury-band UV light (357 and 406 nM). I have 4 24" filtered black light fluorescent tubes spaced with about 1/2" between the tubes. I have an aluminum foil reflector behind the tubes. I expose the resist for about 1 minute with this setup. If you use a longer exposure, it starts to leak under the artwork film and fill in the places that should etch away.

Exposure Frame

For best results, the emulsion side of the master film should be held in close contact with the resist surface. I use a home-made vacuum frame. It consists of 2 sheets of 1/8" Plexiglas. One sheet has a 1/8" NPT angle pipe fitting threaded into it, for the vacuum hose. The other sheet has a ring of .060" plexiglas strips glued around the edge. Just outside this ring is a piece of very thin (about 1/8" OD) Tygon plastic tube, used as an O-ring. The ends are glued together, and it is tacked down in a few spots with contact cement. The vacuum pump is a small diaphragm-type pump. it really doesn't need to provide much suction, due to the large area of the sheets. Mine is about 12 x 18", large enough to do just about anything I'd be likely to do. Right now, I expose one side at a time, but the films are aligned, so I can flip the vacuum frame over without risking that the board will slip out of alignment with the side already exposed.

Note that the dry film resist should be exposed with the cover sheets on, and then held in a dark place for 15 minutes before removing the cover sheets and developing.

Developing the Resist

I develop in Sodium Carbonate for about 3 minutes, at 35-40 C, with very light wiping with the bare fingertips on the resist. Some people are sensitive to the developer, and say a sponge can be used safely. Wash the board very thoroughly, or any softened resist can dry back onto the bare copper. I wipe again with the fingertips during the rinse step. Etch in any acidic etchant, but not alkaline, that will dissolve the resist. Ferric Chloride works, but is messy. I'm looking at Cuprous Chloride. A spray etcher is best, spraying 35 C FeCl2 on both sides of a double sided board at the same time for about 5 minutes. This resist is so good, you can use a dip tank, but it takes about 17 minutes, and the edge definition is poorer. You need constant agitation if you dip or float the board on the etchant. The resist is stripped with NaOH (Lye).

A Trick for Aligning Artwork

I align the two sides of a double-sided board's negatives by gluing them together to a strip of scrap board. I have a plastic sheet which is .060" thick, same as a PC board +/- a few thousandths of an inch. I put it between the two negatives and align them on a light table until the holes line up. I glue the PC board scrap along one edge with rubber cement, with the clear plastic still between the films. I put a heavy weight (perhaps a kilogram or so) over the glue, and then make a final alignment. I usually get registration of .015" or better, depending on what equipment was used to make the films. Note that laser printers are not very accurate, and so any board bigger than, say, 100 mm (4") on a side will not line up at all the holes. You need to take your master artwork, however you make it, and make a negative on high contrast film, such as Kodalith (Lithograph-type) film. The resist is low contrast, and so the negative must be very high contrast, or you will have light leaking through the areas that are supposed to be black, and causing the resist to not all wash off there. That will ruin the board.

I have modified a Calcomp plotter to hold a light-pen (optical fiber), and write on litho type film at 1:1 size. It's a haywire contraption, but it does work. This looks like a positive, black lines on a clear background (like a laser print would look) and so I then have to reverse it by contact printing onto the same type film. This improves contrast a bit, too. Then the films are contact printed with the UV light to the circuit board resist.

I have recently built a 1000 x 1000 DPI photoplotter, using a diode laser to write the artwork on the film. Software can make it positive or negative, so i eliminated the step of contact printing. The accuracy is MUCH higher, so all the holes of 2-sided artwork line up across even a large board.