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minimum UV exposure time

Discussion in 'Circuit Simulation & PCB Design' started by mik3ca, Jul 13, 2018 at 7:39 PM.

  1. mik3ca

    mik3ca Member

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    I recently wasted 2/3rds of a circuit board and the last third barely passed in quality.

    The first attempt failed with some traces missing but I could guess because the T5 insect killing lights haven't warmed up yet. I exposed it for 5 minutes and then developed for about one minute using a plastic container of 1 part of local sodium hydroxide (aka MGchemicals "developer") mixed with 10 parts water.

    The second attempt also failed the same way but with traces missing in other places. The only thing I did different this time was increase exposure time to 11 minutes.

    Now for the third attempt, I exposed for 5 minutes then this time, I developed for about one minute using a plastic container of 1 part of sodium silicate (DP-50 developer from Kinsten) and 4 parts of water. Yes I followed their directions. The board looked decent in the solution. So I took it out and after etching, it seems the traces were legible yet instead of pure dark tracks, I get them grainy, meaning there are thousands of little dots (like spots the same color as that of a circuit board without the copper layer) inside the traces.

    In all three tests, I did the following:

    1. For the artwork, I went to a local print shop to print the art dark with a professional lazer printer on a sheet of transparency and the image comes out very dark.

    2. For exposing, I used this UV exposure unit. I had the knob turned all the way to the right in all tests.

    [​IMG]

    Details of that unit are found here: http://ca.asc365.com/newproductdetail.asp?productid=010031&typeid=2005

    Things were working better before the lights that came with the unit eventually burned out. Sadly there were no labels to the original lights so I ended up researching and ended up buying UV lights from the UK.

    The listing is located here: https://www.ebay.ca/itm/6x-8W-T5-Ul...nsect-Fly-Killer-Mosquito-Zapper/322199166926

    I replaced every light with the ones from ebay before running all three tests.

    I think I read somewhere the 368 in the model number 368BL means 368nm which is between 350 and 400nm (the range for PCB presensitized layer). So the only thing that comes to my mind is that I should probably warm up the unit first with no board for 5 minutes, then after expose my board for 2 1/2 minutes only since I'm using light in the PCB layer range and probably the lights that were originally included in the unit were of higher wavelength?

    or is 2 1/2 minutes still too long?

    I'm a bit confused, but I don't want to have the developer strip away the good tracks.
     
  2. gophert

    gophert Active Member

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    Location:
    Western, PA (USA)
    It is normal to tinker a bit to find the right conditions for your boards and light source. First some basics...

    1) make sure your printed surface on the film is touching the copper. You may have to print "mirrored" image to the transparency. If you don't do this, the photomask is one transparency film thickness away from the board and you will get light leakage.
    2) use binder clips to hold a glass plate onto your board to pinch the transparency tightly to the board and prevent any ripples in the plastic from allowing light leakage under your traces.

    3) testing correct timing....
    A) make a test pattern of some traces About a 25mm x 100 mm with 0.010", 0.015", 0.020"... traces that run 100 mm length.
    Set up your board to etch but slight a very thin piece of 25mm x 150 mm foil between the board transparency (and position so that only 10 mm of board is exposed to light). Most of the board should be uncovered.

    B) set the board into the light box and expose for 30 seconds. The pull the foil to expose 10mm MORE board.
    C) repeat "B" (above) 9 more times until foil is completely pulled from the board.
    (You now have a board exposed at 30 second intervals form 30 seconds to 5 minutes.

    4) Develop per instructions EXCEPT, wear good rubber gloves and use a soft fine-grained make-up remover sponge to gently "wash" the exposed photo-resist from the board. Don't go by time, just pull it out as soon as you see a nice crisp version of your circuit. Usually 30 seconds to a minute with the nice surface agitation.

    5) Dry the board and look at the quality of the traces. You may need to repeat above but leave the board in the box an extra 5 minutes after the last step to get 5:30 to 10minute exposure.

    6) Pick the conditions that work best.

    Note that one thickness of the foil is between the photomask (transparency) and board so you may not have perfectly crisp lines but you should be able to determine which is the "best" range quite easily. Also, the range may be broad. I use a simple "cold" color fluorescent bulb at 50 mm from the tube and get the same great results from 12 to 16 minutes. I use two stacked layers of Transparency from an HP laser printer. On HP printers, the big areas don't darken as well as narrow lines for some reason so I have to stack.

    Finally, if you have supper dark prints and ritually no UV light coming through, then exposure time is a no issue - go as long as you want. Since you are experiencing over-exposure issues, you may not be as dark as you think ( or you may need to review the basics in steps 1 and 2). I usually just copy-paste my circuit to have two on one sheet and, after printing, just cut them and stack them so I have no wasted materials and no extra cost.

    Good luck.
     
  3. mik3ca

    mik3ca Member

    Joined:
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    I always do this.

    That might be an option of the past but I'm using a UV exposure unit (see picture in my question). The unit itself has metal clips that allows me to press the board next to a glass plate
     

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