Most useful UV lightbox size?

Discussion in 'General Electronics Chat' started by sirket, Mar 9, 2009.

  1. sirket

    Thread Starter Member

    Mar 7, 2009
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    I'm building a UV lightbox and I'm curious what size people think would be the most useful. Right now I'm considering an exposure area of either 6x8 or 8x10 inches.

    The latter box offers 67% more working area- but it would be significantly more expensive. I'm just not sure if it would actually be useful for the types of projects most people do at home.

    I'm using UV LED's for this project. I've done numerous tests using a variety of UV LED's and all of them have worked reasonably well. LED's in the 390nm wavelength seem to work best. Arranged tightly enough I get very nice, even, consistent exposure on all of the different presensitized boards I tested. The LED's themselves are not expensive in quantity, and even the cheapies from Hong Kong available on eBay worked fine during my testing. (I tested all the LED's before use- every so often I had one that was noticeably darker than the others and I tossed it).

    The problem? The PCB I need to mount the LED's gets expensive fast. In protype quantities it's about $55 per board for two 6x8 boards (it's a double sided lightbox). That cost jumps to $75 for an 8x10 board (plus the cost of additional LED's). Those costs drop dramatically in quantity so I'd rather design a board and controller people could make in quantity- hence my question- 6x8 or 8x10?

    I estimate I can drive about 640 LED's using a 12 volt (13.6 really), 4 amp power supply I have handy. 4 LED's x 3.4V drop = 13.6 volts. 4 amp = 160 sets of 4 (at 25ma per set). That gives me a maximum of about 300 LED's per side- enough for either size. 6x8 would require about 140 LED's per side and the 8x10 would require about 240 per side. (If my math is off please let me know- I'm more than a little tired at the moment).

    Any thoughts?

    Has anyone else done any testing of UV LED's for lightbox use? If so what were your results? What model LED did you use? Output? Size? Wavelength? Voltage drop? Viewing angle? What resists did you test it on?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    I've got a Kepro BTX-200 that uses fluorescent bulbs. It's given good results for something like 30 years. That is five 2 foot tubes for a 26' X 26' exposure area. I would think that fluorescents would be more cost effective (haven't changed a tube yet). They may be a better wavelength, too - my exposure times are 80 seconds.
     
  3. sirket

    Thread Starter Member

    Mar 7, 2009
    11
    0
    I've used several commercial lightboxes myself- all with good results. In this case I was thinking something a lot smaller and easier to store. 8x10 is about the largest size I have room for and about the largest size I can see somebody needing for home use. Size is part of the reason I think 6x8 might even be more useful for the home user.

    If several people bought boards- then the cost with the LED's would be well under $100. (400 LED's can be had for as little as $32 on ebay- probably less if you haggle).

    The 390nm wavelength seems to work perfectly for exposing resist- I'll check into the wavelength of any bulbs I can find and see where they fall. Any idea what bulbs your unit uses?

    Depending on density and current I get exposure times of about 2 - 3 minutes. I can probably make that faster if I push a little more current or pack things a little more densely.

    Unless you get a good deal on a used unit- or find a tanner in the trash- building a lightbox can be costly. By the time someone buys bulbs, starts and sockets, they'll probably spend as much or more than they would for LED's- assuming the board cost can be brought down.

    Is this the sort of thing anyone would be interested in or should I just build my own unit and sit quietly in the corner? :)
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    The bulbs are GE #F15T8 - BL. The BL is for Black Light. Emission is around 356 nm. Everybody seems to have them.

    Construction costs depend on contents of the junk box. I have several fluorescent sockets around, but only 40 watt ballasts. Make a long, skinny box with those.

    The two-sided dodge is to use fixed reference pins at the edge of the clear area. You can place your negative on those pins and do accurate exposures of each side.

    I've even used staples to hold one edge together with the old eyeball lining up pads and vias. You carefully place the negatives over the double sided board and use Scotch removable tape to hold the far edges and flop carefully. I got better than 80% good results that way.
     
  5. sirket

    Thread Starter Member

    Mar 7, 2009
    11
    0
    356nm is a better wavelength. 390 is on the high end but my tests show it works well too. The nice thing about the LED's is that they are highly directional so you don't get incident rays leaking under the film. I've never really had a problem with light boxes either- but it's still nice to know.

    I'm hoping to put together something that anyone can build- even if they lack a junk box full of parts.

    Yep- I've never really had a problem doing double sided exposures this way. It's just that if I'm going to build something- I might as well build a double sided unit and save myself the trouble :)

    That said- any opinions on size? Do you often make boards larger than 6x8 or 8x10?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    No, I'm making nothing larger than 6 X 8 lately. My eyes are so bad that drilling pads gets to be pretty tedious, but I can't handle SMD well enough to go that route.
     
  7. sirket

    Thread Starter Member

    Mar 7, 2009
    11
    0
    Good to know. I can't remember the last time I made a board bigger than 6x8 myself so I'm not sure if most people really need anything bigger than that.

    Anyone else have any feelings one way or the other?

    That sucks. My eyes have never been the greatest but PCB work has never been a problem.
     
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