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UV lightbox with UV leds physics?

Discussion in 'Electronic Design' started by [email protected], May 17, 2007.

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  1. Guest

    I saw an discussion at:
    http://www.diyaudio.com/forums/showthread/t-87555.html

    Claiming an rebuilt scanner with 40 UV leds will properly expose an
    pcb with the speed of 2cm/minute.

    Known:
    2 cm/minute
    40 UV LED in line.

    Assumed:
    Scan area = A4 width = 210 x 297 mm
    P_led = 3.8V*.04A (mentioned in the text)
    t = 297/20*60
    lambda = 400 nm

    Math:
    Q = 3.8 * .04 * t * 40 = 5417.28 Joule
    A = .210*.297 = .06237 m²

    PCB needs 86857 Joule/m² to be properly exposed.
    Ignoring inefficiency of the LED, distance to pcb and it's beam angle.

    If a board with each LED responsible for an 30 mm² area on a 100x160 mm board
    is built it will need to shine for:
    (.100*.160*86857)/(3.8*.040*60*((100*160)/(30**2))) = 8.57 minutes

    Correct?

    Especially how much energy is actually needed to properly expose an pcb board
    photoresist..?
    I'm interested in knowing practical numbers for this type of application.
    And if the boards gets sharp?
    Would pulsed UV led be more efficient?

    More links:
    http://hackedgadgets.com/2006/10/31/uv-led-pcb-exposure-system/
    http://www.elektor-electronics.co.uk/default.aspx?tabid=29&forumid=5&postid=1369&view=topic
     
  2. D from BC

    D from BC Guest


    Sacrifice 1 photoboard.
    Make a test jig with masking and expose areas with various exposure
    times.
    From that you can derive a scan rate.

    Cameras takes pictures with a flash.. If the photo resist acts like
    film then pulse overdriving the UV leds may reduce the exposure time.
    That will increase the scan rate too..

    Damn..I should have kept my old paper scanner...I could have converted
    it into a PCB exposer :(
    D from BC
     

  3. And the problem with using the sun is...?

    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
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  4. D from BC

    D from BC Guest

    The sun is not as fun as UV LED's... :)

    However...I should mention..I use a 500W halogen for PCB exposure..
    Quick, cheap, simple, easy to get...

    Some people just order the PCB from the PCB fab house.
    Heck..some people (managers) don't even order a PCB...
    "Hey you! Yeah... Order a PCB for me.."
    D from BC
     

  5. I like to work at night when its cooler, and no one s bugging me on
    the telephone?


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  6. James Arthur

    James Arthur Guest

    Do they have sun in Sweden this time of year?

    But seriously, we have to save all that solar energy for Al Gore.

    Best,
    James Arthur
     
  7. Guest

    Sacrifice 1 photoboard.
    I wanted to avoid experiments initially so I can just do the math and figure
    out how to dimension the build.
    I think the photo flash is more like send all the energy in one go to
    exploit short exposure times. While the LED pulsing would be to exploit high
    peak values. Question is if the peak energy is more efficient than using
    mean energy ..?
     
  8. D from BC

    D from BC Guest

    I think junction temperature, LED current and LED voltage affect the
    lumens/watt efficiency.
    See the datasheet.. I dunno what wacky things a UV LED will do..

    I'd guess taking an LED beyond it's spec current will reduce its
    lumen/watt efficiency..
    The junction heat spoils the performance.
    But it still will be brighter!..for the moment..and that might
    increase the exposure efficiency.
    Exposure like this..
    POOF...move...POOF...move...POOF...move
    The LED's get to cool down during the moving period..

    But if the exposure time is long (more *flux/second) then I think
    continuous max UV LED current would be better.

    *flux/second <<Just made that up..dunno if it's a real unit..
    (The brightness of a spot light that moves.)

    Here's an idea..
    Take a fiber optic cable, couple it to a UV led.
    Then use a converted ink jet printer to move the fiber optic cable
    around..
    This would print negative photo PCB's. No mask required. :p
    D from BC
     
  9. Guest

    The junction heat spoils the performance.

    Maybe mount them in a heat sink then. Without obscuring the light..
    Though LED body is usually mad up of plastics wich is a poor heat conductor.
    Have three sections of LED's, then light them in sequence?
    Why not just mount some LED's in the printer head.
     
  10. redbelly

    redbelly Guest

    Since you now have a ballpark figure of 8-9 minutes to work with, and
    this Nordic person at diyAudio was able to make an exposure in 2
    minutes, it's time to move on to the next phase: experimentation, as D/
    BC suggested.

    Try using a single uv led and see how much area it will expose on a
    board. Vary the distance and exposure time. Then you can figure out
    how many led's you need, how close to space them, and how long to make
    the exposure.

    Good luck,

    Mark

    p.s. did Nordic (from diyAudio) ever say what wavelength his led was?
    There is almost certainly a strong, nonlinear wavelength dependence
    here. What somebody did with a 390 nm led could take orders of
    magnitude longer (or be impossible) to do with a 400 nm led. At this
    point, the best way to find that out is to try with what you have.
     
  11. joseph2k

    joseph2k Guest

    How about converting a 24-pin type printer, far less issues with the
    existing drive electronics.
     
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