PhotoResist PCB Etch

Lawrence Lile says:

I went round and round with all kinds of iron-on PCB transfers, and other unsatisfactory methods. Started out with a laundry marker drawing PCB's by hand back in the '70's. Do some studying and set yourself up to make photoetched PCB's, you won't regret it.
I use Kepro^ for photoetching supplies. They have a nice tutorial on etching your own board, if you want to save some money. Prices are great. They have a kit called KEPROCLAD, a 4"x6" single sided board with chemicals. I buy them in bulk. They have a nice tutorial on making PCB's at:
http://www.kepro.com/howto.htm
My etcher consists of a 2 gallon plastic laundry tub (for secondary containment) with a 2 quart plastic ice container inside. The ice container holds ferric chloride (it was a handy size - no I don't use it for ice) The whole thing is heated with a hair dryer stuck in the side of the laundry tub (heat to 110 F) and stirred with a little low pressure compressed air line in the bottom of the etch tank (not absolutely necessary to stir, just makes it go faster). I put it together for about $15, and I can make a board in 20 minutes. It really doesn't take much equipment to make your own PCB, a glass tray and the proper exposure lamp is really all you need.

Mask

The mask required for the exposure of the photo sensitive boards can be produced in a number of ways. Printing on transparency film will work if the printer can produce a VERY opaque print. Laser printers typically do not achieve this, but good results have been obtained with InkJet printers. Different ink colors may have different levels of UV blocking, so black isn't always the best color. MISPro Yellow is best. Durabright Magenta seems to work well, or Magenta / Yellow. UltraChrome black is reported to work just fine. Some printers also print with a higher resolution with colored inks than they do with black. Many find medium to dark green works well. Test each color to see which works best with your printer.

Laser printed transparencies typically do not cover well enough to make an opaque mask, even after several passes. jpanhalt^ offers a possible "fix" for Laser printer users:

The method I developed uses a black dry erase marker {Expo brand} to make the image on the transparency more opaque. ... Just paint over the entire laser transparency on the toner side, let it dry, and wipe off with a soft tissue like Kleenex. ... It does not smudge any of the lines or other details. It simply fills in the grain on the transparency. ... Just lay on a nice wet layer like when painting with water colors. The transparency image is slightly softened by the solvent in the ink and can be scratched by the felt tip if scrubbed. If completely dry, you can put on a second layer, which I have not found to be necessary, unless I miss a few spots. ... The dried marker comes off in tiny flakes. These can scratch the surface, if they get caught beneath the tissue and you scrub back and forth. Just wipe in one direction, then another, etc. Fold the tissue as needed to get a clean surface.

Some InkJet printers can even print directly on PCB stock if it is thin and small or be modified to print on larger boards. The trick here is to get the ink to adhear to the stock and not bead or run. The use of a wetting agent such as break fluid solves the problem for bare PCB stock, but stock coated with photosensitive film may be damaged by this sort of treatment.

Inkjet films made for silkscreens are said to work better, producing a more opaque print, than transparency film. e.g. Fixxons Waterproof Silk Screen Positive Film.

After printing on a transparency, or other transfer media, the ink or toner side MUST be placed directly against the stock; otherwise the thinkness of the media will allow the exposure light to shine back up under the mask, resulting in a fuzzy edge and traces that are thinner than expected, or even broken.

Photosensitive Stock

For low-quality boards it is possible to spray them with a photoresist-in-a-can product if you do it inside a *clean* container to keep dust and imperfections from creeping onto the board.

Just buy ready made boards with photoresist for high-quality boards.

Typically for professional PCB work the photoresist is a dry film that is applied, not sprayed or poured on, and is a negative photoresist. The liquid stuff has a shorter storage life, may have to be refrigerate, and is typically spun on (at least for wafer processing).

Dry film is available and can be laminated to standard PCB stock. The difficult part is keeping air bubbles and other things from being caught between the film and the PCB surface. You can keep the air out by applying the film under water, sort of like a decal, or by keeping the film OFF the pcb until the laminator presses it down

DJ Delorie dj at delorie.com says:

I flip the PCB over and lay the film next to it, with a 1/2 inch overlap, and press the overlap part down. Now I flip the PCB over and fold the film along the attached edge. Feed the attached edge into the laminator and IMMEDIATELY grab the other two corners of the film and peel it off the PCB. Pull gently away from the laminator, and apart, to keep the film smooth, and pull up some to keep it off the pcb. You want the film's first contact with the PCB to be at the laminator's roller, so that no air bubbles are trapped.

Placement: During the exposure, the mask must be held directly and consistently against the stock. Any wrinkling of the mask or space between the mask and the board will result in the exposure light shining up under the edge of the mask, resulting in fuzzy edges and traces that are thinner than expected, or even broken. Methods of holding the mask tightly against the board include quartz or glass sheets (note that glass will reduce the UV reaching the photoresist) but the best results seem to have been obtained by using vacuum. A needle or fine tube connected to a vacuum pump is inserted along the side of the stock, between the mask media for the top and bottom masks in the case of a double sided board or between the mask media and the surface on which the stock is laying. Tape is applied around the edge sealing the space against outside air. As the air inside is removed, the mask media is pressed securly against the stock.

Light/Exposure

The optimal wavelength for photoresist exposure is usually 350nm which is near UV (400nm is visible purple). The 'blacklight' lamps output maximum here. However, The Gxxx lamps emit far UV light, useful for erasing EPROMs bugs and your eyes, but have enough emission at 350nm to expose photoresist (not to mention that they are about 5 times more efficient than blacklight lamps and that photoresist is also sensitive down to to 253nm).

Look for fluorescent bulbs with markings that start with G = germicidal/bare glass, uv transparent (this is what you want for boards). Danger: these emit short-wave UV which can cause eye and skin damage. For example, H-G4T20BLB(3) is a UV light (G) with (4) watt capacity and (T20) means four pins, two at each end, 20mm apart from each other which is the standard.

The exposure of the photosensitive stock is the most difficult part of the process to predict. Differing sensitivities of photoresist and lights of different strengths will require experimenting with the exposure and development until a reliable result is obtained.

Text Exposure: Every setup will be different, do an exposure test with a scrap before trying with the real thing. Cover all but one end of the PCB with a shade and then move the shade back, exposing more and more of the PCB at regular intervals (e.g. 30 seconds to 2 minutes depending on how strong a light you think you have). When the last bit has been exposed for the interval you choose, stop and develop the scrap.

Examples: (your times will vary, do an exposure test first):

500W mercury vapor lamp (standard lightning lamp), getting around 18 minutes at a distance of 20cm. Or a mercury vapor yard lamp with the outer frosted glass envelope removed, leaving only the inner quartz bulb.
500W Gxxx lamp for 2.5 minutes at 18"
A pair of 9W Tropical Aquarium "Blue" bulbs for 12 minutes at 6"

Double sided PCBs

Photoetching is one of the best ways to achieve perfect alignment on double sided boards.

Peter L. Peres says:

...to achieve registration, register the two artwork sides with out the board, insert a piece of cardboard between them outside the artwork area, and staple them together. Then slip the board into the 'folder' and the folder into the frame described above. The cardboard serves as spacer, it should be as thick as the board. Instead of staples you can use fast glue. I have used double sided sticky tape (foam type) instead of the cardboard. Works great and the foam compresses between the glass sheets so results are near perfect (as near as they can be with this method).

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