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Scratches/scoring on photo etched PCB

Discussion in 'PCB Layout, Design and Manufacture' started by Ethan Engle, Oct 13, 2014.

  1. Ethan Engle

    Ethan Engle

    5
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    Oct 13, 2014
    First post in this forum!

    I am in the process of etching a PCB using the photo-resist method. After developing the PCB, I have discovered that it is covered with small scratches that have gone through the resist layer and exposed bare copper. I have attached a photo to show an example. I am sure that etching the board will break traces that intersect with these scratches.

    I got my PCB from Jameco, which I would think is of good quality. Are these photo-resist PCBs really that sensitive to even small bits of dirt that rub on it from the transparency, developing solution container, or drying cloth? Has anyone else encountered this issue and found a good solution?

    And on another note, does anyone have any suggestions for patching the resist over the traces before I etch the board? I was thinking of masking the board with tape around the traces, then applying nail polish.
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,131
    1,842
    Nov 17, 2011
    Welcome to our forum.

    1) Yes, photo resist can be sensitive. It depends on the kind of dirt.
    2) You can repair the traces before etching using an etch resist pen. A paint marker may work, too, but you should run a test etch with some spare PCB first to verify the paint really resists the etchant.
     
  3. hevans1944

    hevans1944 Hop - AC8NS

    4,199
    1,988
    Jun 21, 2012
    Those scratches are bad news and will definitely result in open traces.

    Assuming normal care in making the exposure through the transparency followed by developing and drying, those scratches were probably on the un-exposed photoresist when you received them from Jameco. Did you carefully inspect the coated boards before exposure? Was this a positive photoresist developed in an aqueous caustic solution (typically a diluted solution of sodium hydroxide) and rinsed in water after development? Was the developed board allowed to air-dry in a dust-free environment or did you use a drying cloth? I would not have used a drying cloth on a recently developed board.

    Years ago (1969 to about 1978) I used to purchase negative-photoresist presensitized coated boards from Kepro, which is no longer in business although others have taken up the slack. The boards arrived packaged in heat-sealed thick plastic envelopes. I think multiple boards in a package had paper separators.

    All of our artwork was made at 1X scale (actual size) as a positive image using Bishop Graphics crepe black tape and die-cut "puppets" (representing holes and components) transferred to a Mylar sheet. Then the sheet with circuit applied was contact printed onto Kodak Kodalith film and developed in A-B Kodalith developer. The negative was carefully inspected for pin holes and scratches after drying. Any defects were repaired with a Koh-I-Noor Rapidigraph technical drawing ink pen. Finally the dried negative was placed in contact with the sensitized board and exposed for fifteen minutes or so in a light-box using an ordinary photo flood lamp.

    We developed the exposed photoresist in trichlorethylene solvent, which was not kind to the skin. I don't think trichlor is used for this purpose today. We etched the boards in a heated ferric chloride bath, at first by hand agitation over a hot-plate, later with a Kepro spray etching tank.

    The entire process was messy and time consuming and not easily adapted to double-sided boards because our exposure frame did not accommodate registration pins. A few technicians did make some double-sided boards, but plated-through holes were out of the question. Nevertheless, the process worked and produced one-offs that looked semi-professional. We even dipped the finished boards in an electroless tin-plating solution Kepro sold for that purpose, but I was never satisfied with the results of that.

    Eventually I moved on to more complex PCBs and used local PCB houses (all of them now out of business) to manufacture them. Today I would use any of several on-line PCB manufacturers, many of whom offer free schematic capture and circuit board layout software. A few years ago I purchased PADS software, which I charged off to the customer, and did their PCB designs with that, e-mailing Gerber files to one of the on-line PCB manufactures. The advantage of that approach is double-sided boards with plated-thru holes, multiple internal layers with blind vias if needed, a silk-screened legend on the component side, solder mask on both sides (if required), and gold-over-nickel electroplated edge connectors (if needed).

    It is apparent that your board was designed with PCB layout software as a single-sided board, with most of the copper serving as a ground plane. This obviously lends itself to home manufacturing. Nice touch using thermal isolation pads for the ground pads. However, I don't see any bypass capacitors mounted near the Vcc pin connections on the IC sockets. Or maybe you purchased sockets with bypass capacitors already installed. Anyway, congratulations on a professional looking design. If you can afford it, I would have the board made by an on-line PCB manufacturer. It's a shame to spend all that time on a layout and get less than professional results at the end.

    During the several years I spent making single-sided PCB prototype boards as described above, I never saw the kind of scratches your photo exhibits. We didn't exactly have a "clean room" to perform the exposures, just an ad-hoc photographic dark room, a large closet really, with a light outside the door to warn other lab personnel that it was in use. We took normal precautions to avoid contaminating the artwork and the negatives with dust and hair, but nothing other than that. I think your presensitized boards were probably scratched before Jameco sent them to you. If you purchased more than one, check the others carefully.
     
  4. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    I am using the photo-resist method and have never seen scratches in the resist. I have used boards from MG chemicals and boards from China off Ebay with good results. My main problem tends to be copper left behind on the edges of the board or from dust during exposure.

    I do not touch the sensitized surface at any time until after etching. It goes from exposure to developer, rinse with water then right into the etching tank.

    Bob
     
    hevans1944 likes this.
  5. Ethan Engle

    Ethan Engle

    5
    0
    Oct 13, 2014
    Thanks for this great info. I'm using a positive-type board. And I did use a paper towel to dry the board, which I saw used in the tutorial I was following, but as I was drying the board I was doubting whether it was a good idea. When working with future boards I'll make more of an effort to keep my transparencies clean and handle the board as little as possible. My hunch is that there was some dirt on the transparencies that rubbed against the resist as I was positioning it for exposure.

    This is my second photoresist etch. I'm doing the same circuit with some design improvements. My last PCB came out with these same scratches and I had to repair several traces with a silver tip pen: very unsightly. As for touching up the traces that need it, I don't have much experience with etch resist pens, or other such methods. I think I'll try out a few on a scrap piece of copper clad to see which will work best for my thin traces.
     
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