Best clear material for diy pcb exposure box

[Dan Moos]

Those of you successfully doing do it PCB with the photo method: what clear material do you suggest for cleaning the transparency sheet to the PCB? I have been using a piece of glass from a picture frame, with a heavy object to hold it tight, but there has still been issues with uneven clamping of the glass to the board letting light in from the sides.

I'd like to make a purpose built framem of sorts.

Does acrylic work well?  I like the idea of a material I can easily drill holes through. Sadly, the only stuff I have found locally is treated to not pass UV (to protect photos). Obviously that's no good.

Where would you get acrylic (or some other cut-able, UV passing material)?

Can glass be drilled easily?

Also, I have been using an aquarium light fixture with four T5 HO 10,000k 24  watt bulbs. They are 21 1/2"  long. I realize those aren't specifically UV, but they have been kinda working. I get fast exposures, and I'm still thinking the inconsistency is due to my glass not pushing hard enough, consistently across the PCB. That said, is there a bulb out there that is better, that would fit this fixture?

Finally, if I build a box, should the inner material be light reflective, or absorbent? My gut says absorbent, so the only light comes from the bulbs above. That way no light gets in from the side, which I think is part of my trouble.

My results are tantalizingly close to the kinda specs I need! Every attempt is just a section or two on a mid sized PCB from bring usable, so I know I can do it. 20/20 track/clearance for sure, and sections of 10/10 that came out well.

My current project is a 6502 single board computer. It's working well on a bread board at 1 megahertz. I feel a diy double sided PCB is reasonable to attempt for this.

[jpanhalt]

Some people use a few drops of water between the transparency and the PCB.  Of course, if you use an inkjet, you need to have the ink surface up.

I use single thickness window glass (two pieces) and clamp the board and transparency(ies) between them with low-tension spring clamps.  Be sure the window glass is not coated to be "non-reflective," as that coating will absorb near UV.  I put the image side (toner or ink) against the PCB.

John   

[sleemanj]

I use glass (from an old scanner top, indeed, the entire top of my large box is the top of a scanner).

Glass : Artwork : PCB : Two Layers of Spongy Foam Shower Mat : Lid

Across the lid I have a wooden bar, and across the bar I have a velcro strap, the strap loops around handles on the side of the box, and thereby I can cinch the bar down tight and the velcro holds it in place.

The foam shower-mat layer holds everything in place and provides an even-enough pressure to push the PCB down flat against the artwork.









I use tracing-paper for the artwork.

[jmelson]

I use 2 pieces of thin Plexiglas I got on eBay.  I glued a tiny "tygon" type plastic tube around the edge of one to act as an O-ring.  (They sell this on eBay to protect fish hooks.)  I drilled and tapped a hole for a fitting to connect to a diaphragm pump to suck it down.  Works real well.  I do a lot of double-sided stuff, so I want both sides clear.  I glue the two artworks together with a piece of scrap board material as a spacer, at one edge.
Then, slip the sensitized board between the artworks and expose on both sides without releasing the vacuum.

My exposing box has 4 24" filtered blacklight bulbs (the black ones) for 405 nm peak radiation.  It works with 1 minute exposure per side.
To answer your other part, I glued aluminum foil to the inside of the box behind the blacklight tubes.  it gets crinkled when you handle it, so it breaks up and evens out the reflections a bit.

Jon

[DTJ]

According to this page 'glass' passes 90% of UV and PMMA acrylic passes 92%.


http://www.pmma.dk/Acryl_kontra_glas.aspx?Lang=en-GB

I wonder if all acrylics are equal?

Some hardware stores sell polycarbonate sheet as well which may be better mechanically as it doesn't crack.

[jmelson]

Acrylic is acrylic.  Plexiglas and Lucite are two tradenames for it.  It is polymerized methyl methacrylate, fairly closely related to super glue.
It will eventually craze when old, but should last for decades.  I finally had to make a new one, after using my vacuum frame for probably over 40 years.  It still worked fine, but looked a bit ugly.

The lamp is a custom-made wooden box (a sheet of plywood with 4 pieces of 1 x 3" something lumber.  I got 4 pairs of sockets for the fluorescent lamps and 4 starter sockets and ballasts.  The lamps were bought on eBay, there are lots of people selling them.  I put crinkled-up aluminum foil against the inside of the box as a reflector.  I didn't do anything special to "crinkle" it, it just got that way when I put it in.

So, I put the bottom piece of plexi on the table, place the PC board and artwork on it, and then put the top sheet on.  I start the vacuum pump and press down on the edges of the plexi to help it seal, and then it pumps down, making the artwork conform to the board.  Expose the top for 1 minute, flip, expose bottom for 1 minute, and then let sit in the dark for 15 minutes so the polymerization of the resist can complete.

It is really shade tree mechanic stuff, but works quite well.

Jon