Killer PCBs

Step 6 Expose

Place the laminated PCB between the pre-registered transparencies and tape securely in place. Tape a Stouffer 21-step sensitivity guide over an open section of PCB if you plan to calibrate your exposure process. You want to press the artwork tightly to the PCB to prevent light from leaking under your traces. You can do this with two sheets of glass, or, preferably, you can use a vacuum bag, or vacuum frame. You can then place this assembly in bright sunlight for about 5-8 minutes per side, or use another UV source of your choosing. Note that, despite the tight fit of PCB & artwork, collimation (making light rays parallel) is important to achieve fine traces--refer here for instructions for constructing a collimated UV light source.

Comments

[drleavsy]

I suggest to use 100-200 UV LED (12V power supply) in the form of array as it was described in http://www.instructables.com/id/How-to-make-a-printed-circuit-board-PCB-using-th/, then there is no need to take care about collimation and about fragile tubes and stuff. I believe the time of exposure can be really short about 1 min as it described in aforementioned source . 

[john-louis]

incoherent,

Thanks for that, I have had a look and see what you mean about the channeling / frame construction. I have been thinking about the glass vs bag approach and think maybe a double sided frame with glass on one side and film / thick transparrent plastic on the other - kind of like yours but transparrent plastic - may give the best results as one side can stretch and "vacuum form" to the shape of the board.
I will measure up and start playing when I get back from uni in a week (last week of exams =D)
Regards,
Alex

[john-louis]

Hi, thanks for replying!
Alex is indeed my real name, john louis the middle bits =D
I have spent a good while thinking about this frame, I need to do some .5mm pitch surface mount stuff soon so was planning a complete UV box rebuild - the current one is a classic scanner enclosure (using lid for pressure) and philips solarium lamp job. I plan to use an atachie case with a raising lid (on collapsable telescopic style stilts) with the UV lights in the bottom of the case, along with vacuum pump and ballists and (possibly) a collimator - if I can figure out what is really needed for one.

A removable double sided glass vacuum frame for doing double sided work near the lid / stored in the lid - but slidable up and down the stilts.
I was thinking of getting two really simple picture frames and milling a trough (moat) around the outside of the glass and then putting a thin rubber seal around the outside of the trough, then threading a fitting into the trough from the outside, allowing the glass surfaces to almost touch when vacuumed together, hence working kind of like a flower press on the PCB. My only worries is that the glass may crack or that because neither of the surfaces have any "give" the requires pressure will not have the desired effect...
From your description I assume the U section channel is on the top bit (with the plastic sheeting) and the pipe tapped into the U channel?

Cheers,
Alex

[incoherent]

Alex,

Here are some pics of my vacuum frame. Incidentally, I scanned a pre-registered, double-sided transparency while working on the PCB instructable and found the scan made it appear to be very poorly registered (see 4th pic in the series). I think this was due to the very-wide-angle nature of the scanner optics as well as the the separation of the layers despite the lid being closed. The effect could have been caused by total internal reflection even if the layers were tightly pressed together, but I never tried it again with a vacuum bag/frame to diagnose the problem. The point is that the scanner lid may not press as effectively as you might think. Oh, yeah (ADD moment here), you might be able to make a kind of hybrid vacuum frame/bag by placing a transparent plastic film between the glass and the frame.

Regards,
incoherent

[incoherent]

I forgot to mention that you can try out the vacuum bag approach too. This may be even better than a vacuum frame in that the flexible bag presses the artwork to the board instead of non-conforming glass (especially good for doing solder mask where you've already got an uneven surface on the board). The vacuum bag presses so tightly (and conformingly), that if you hold it up to the light, you can see little bumps where little particles of dust are trapped between the board and the bag! Just get a large ziplock-style bag (the kind without the little plastic slider). Next get a spool of thread, non-silicone adhesive/caulk and some tubing (fishtank tubing seems to work well). Working from the inside, poke the tubing through a corner of the bag oposite the zipper and pull all but 2" through the hole. The bag should be big enough to lay flat on the exposure unit without the zipper or tubing under the glass. Apply some caulk inside the bag around the tubing in the corner, then use the thread to tightly bind the bag to eliminate leaks. Next place your PCB/artwork sandwich in the bag and zip it up, then vacuum out the air (leave the vacuum running throughout the exposure process to keep it tight). You can also use a vacuum bag such as this for doing fiberglass or carbon fiber composites to tightly squeeze the layers together and squeeze out the excess polymer--just be sure the vacuum won't suck it up!

[incoherent]

john-louis/Alex (if that's your real name:), I've been working on a video instructable for the vacuum frame, but, of course, that means a whole new process. I'll summarize though: Go to the local home improvement shop and ask about window screens. They should have extruded aluminum channels designed making window screens. They are about 3/16" x 1/2" (5mm x 12mm) with a u shaped channel along the inside. You'll also need the plastic corner pieces and the rubber strips used to mount the screen to the frame. You'll also need ~4mil plastic sheeting, foam tape (the kind used for weather stripping), and non-silicone adhesive/caulk. I think that the stuff I used is acrylic-based, but they don't say for sure--it smells like acrylic, though. Cut the channels about 1/4" smaller than your glass sheet on both sides and top/bottom. I drilled holes along the inner edge all around and plumbed brass tubing through the corner pieces to ensure that the vacuum would flow evenly with trapping pockets of air (although this step might be overkill). Run foam tape around the bottom (just outside of the holes (if you made them). Finally liberally coat the foam tape and all joints with the acrylic caulk (no silicone, because silicone doesn't stick to silicone--go figure...). You'll need to make multiple layers (and keep them kind of thin so that it will set up. Use a crayon to coat the glass sheet with wax and place the frame on the glass after applying the caulk--this will ensure a smooth, flat surface for your seal. Keep applying adhesive until you stop all the leaks. I hacked a fishtank air pump to to draw a vacuum on one side and pump on the other. just figure out where the air intake is and try to hack it to feed from your vacuum tubing. I used part of an old pill bottle to make a sort of plenum which I sealed over the air intake on the bottom of the pump. I'll try to get the instructable together just as soon as I figure out how to dub the video with narration. Good luck, incoherent

[incoherent]

BTW, I put a gauge on it and it is drawing about 5 kPa (even though it still has a few leaks). This works out to about .7PSI, or about 75lbs of force pressing the 9x12 inch frame to the glass, of which about 8lbs is evenly distributed over the 3x4 inch PCB.

[john-louis]

Hi, I was wondering if you had any info / advice on making a vacuum frame similar to yours, or just generally more info about your specific uv exposure unit, especially the frame and collimation grid. Does the bag inflate to push the material down or vacuum out the air to pull the top lid down? Many Thanks, Alex