PCB light box in a scanner shell

scanner-exposure-box

[Kizo] repurposed a flatbed scanner to use as an exposure box for making printed circuit boards. Exposure time is controlled by an AVR ATtiny2313 microcontroller. The device is connected to a separate display board to control four 7-segment displays using one shift register for each. Time is set in ten second increments and once started, switches on the lights with a relay. Once the right exposure time has been reached, the lights are switched off and a piezo speaker is buzzed. There’s no mention of they type of bulbs he’s using but they look like compact fluorescent with tin foil beneath as a reflector.

If these are just CFL bulbs, how will the performance compare to a light box based around a UV light source?

[Thanks Jake]

Comments

  1. Ben Ryves says:

    A nicely implemented project, and performance seems fine according to the project forum (and Google Translate) – “time highlighting the 2-2.5min and thus develop very quickly for about half a minute in 10% NaOH solution.”
    I wonder if there’s a way to make the exposure more even without adding more bulbs (not that it’s a problem for smaller boards)?

  2. Skitchin says:

    Those are definitely CFL’s, possibly mogul base or standard. Lights like those might be a few hundred lumen each. Mylar is better than foil, as it gives a more even reflection.

  3. Skitchin says:
  4. jcdillin says:

    That’s pretty fast, I use blacklight bulbs and I have to expose my boards for at least 6 minutes for them to come clean quickly.

    I was thinking of using the same flat bed scanner but just use UV leds.

  5. g3 says:

    there are cfl uv light sources, you can get them at the pet store for terrariums. they sometimes come in two varieties, “tropical” and “desert” depending on how much uvb you need. apparently some? reptiles need uv to process vit d for sth with calcium to make strong bones or what have you. they put out a hell of a lot more uv than you’ll get from any led, that’s for sure.

  6. googfan says:

    Those are not just cfl lamps. They are ultraviolet reptile lamps. About 30% of the light they output is UVB. 30% uva. and 40% white light.

    you can get them at petsmart. I just used one when i made some retr0brite to clean up a radio.

  7. Fallen says:

    If the scanner bed is glass it will block the UV.
    I’m pretty sure glass is opaque to UV.

  8. Boudico says:

    Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths while Silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm (Source: Wikipedia)

  9. jwstolk says:

    I have build a similar box a long time ago, a double sided one, so I could do both sides of the PCB at once, eliminating the possibility of shifting the masks when turning around the PCB.
    Some hints:
    -make the box 1.5 times as deep as it is wide.
    -use a cardboard template to form the aluminum foil in a parabolic shape, for an even distribution.
    -make the insides of the box black, or at least don’t put the foil there. (Light from the wrong direction is worse than a slightly longer processing time!)

    The trick is to get the light distribution very even over the whole surface, and to get the light perpendicular to the surface as much as possible. That makes it much easier to get a sharp image.

    I have successfully make double-sided PCB’s with 0.2mm traces on a 0.5mm pitch using that box.

  10. Lobo says:

    Great build! In the same vane I’ve got an old EPROM eraser kicking around. Apart from the health risks of exposure to short wavelength UV, would this light source work for photoresist PCB etching?

    My eraser is large enough to accept a decent size PCB and has a nice little timer built into the light-tight enclosure.

  11. svofski says:

    @Lobo: usualy photoresist is mostly sensitive to UVA, you can expose it even with regular lumnescent lamps (think office lamp with an U-shaped bulb). Chip eraser is more likely to be a quartz lamp in harsher UV range, which calls for using protective goggles and probably not very efficient for resist exposure.

    http://sensi.org/~svo/spdifpcb/dsc01711.html

    This is how I used to expose my PCB’s before I learned that you need longer wavelengths :) It worked, but much worse than little screw-in blacklight bulb I’m using now.

  12. GSV Ethics Gradient says:

    I thought fluorescent lamps and blacklight lamps were the same thing, the difference being that blacklight are coated to filter visible light, and fluorescent are coated to filter the UV?

  13. medix says:

    @Fallen, @Boudico – it depends on what’s actually *in* the glass. Glass isn’t just ‘glass’. There are many variations in composition, so you’d have to measure the amount of attenuation..

    Wikipedia really is a relatively poor source of information…

  14. Ashley says:

    You could build some kind of scintillometer to measure relative UV attenuation in a real ballpark way. take a light tight project enclosure, cut a window, add a camera style UV bandpass filter over the window, inside add a piece of material coated in uv reactive dye or something, and use a the photoresistive circuit of your choice to measure how much the UV reactive material glows. establish a baseline reading using a uv source of known intensity shone through the window, then measure with the material to be analyzed between the uv source and the window.

    just an idea

  15. Amos says:

    @GSV Ethics Gradient:

    Fluorescent lamps convert (most of) the UV light produced by the tube to visible with a phosphor coating. Blacklight tubes (i.e. not the regular incandescent ones) either use a different phosphor chemistry to produce specific wavelengths or just a filter to keep in the (more?) harmful shorter wavelengths (I don’t know which). There might even be a different gas/gases inside the envelope, too.

    And then there are the germicidal ones that have no coating of any kind (I have a “burnt-out” one on my desk right now). These are the ones that hurt/blind people and break down DNA in microbes.

  16. sean says:

    For better light distribution, you could use an aquarium lamp. They are a flat quad or sext tube array and available in 27-96 watts. The 96 watt daylight 6500 Kelvin tubes probably would drastically reduce the exposure time.

    The newer fluorescent outdoor flood lamps also use these flat tube arrays, and are readily available at any home DIY supply store.

  17. rublehousen says:

    Hi,i have a pcb lightbox with 9x 8w uv striplights, timer, cut out when opened switch,its profesionally made in a aluminium aircraft case. its no good to me- i was going to reuse the case. anyone know how much these are worth new/second hand? cant seem to find any info on them, is it worth selling or are these outdated/cheap enough to buy? the box would cost 30 quid to buy new, it weighs about 12kg and would cost nearly 20 quid to post inside the uk. thanks for reading and any advice much appreciated

  18. Guitar-Zan says:

    I made a similar exposure box using an old monochrome flatbed I picked up on Ebay for $10. For bulbs I used 6, 13w (effective 60w) Feit Electric compact fluorescent black light bulbs that I also got on Ebay for $5 each. I used a flat strip of mylar running underneath the bulbs as a reflector. All other interior surfaces were painted flat black. I get excellent results with 2 1/2 to 3 minute exposures. The entire box cost just over $50.

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