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PCB weird pattern over traces

Discussion in 'Circuit Simulation & PCB Design' started by Wirth's Law, Jan 17, 2018.

  1. Wirth's Law

    Wirth's Law New Member

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    Hello all,

    I was looking at a couple of used circuit boards, and on it I found several high voltage traces (120/240 VAC) with a strange pattern overlayed. Instead of being normal, solid green traces, they had a gray and dashed pattern on top, with the remaining copper covered with green solder mask like normal. It was as if the solder mask for this part was perforated in order to leave an exposed pattern of tin solder when the board was finished. I've attached a rough sketch of what it looked like.

    Checking Google, I found some pages on circuit board features called thieving and stitching. But, I'm not convinced either of these are what I've seen on this trace, although they have similar purposes. I can guess at the moment that this might have something to do with thermal management, since the exposed solder should offer less thermal resistance to ambient temperatures, and because these traces carry the most current on this board.

    So has anyone seen this before? Is there an industry-recognized name for this pattern/technique? And what purpose does it have?

    mystery_pattern.png
     
  2. Ian Rogers

    Ian Rogers User Extraordinaire Forum Supporter Most Helpful Member

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    Stitching is used for applying more via's to a trace to allow current to flow.

    Thieving is just mainly for economy... If you don't remove all copper the A) the etchant lasts longer B) I suppose it's quicker to etch!!

    At least that's my take... Stitching is used more now with SMT devices to run traces on both sides of a PCB
     
  3. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I do/have designed power supplies for some very low cost companies.
    At my house I use 2oz copper or a power supply because the thick copper caries high current and it helps with temperature. (pulls heat away from hot components)
    In China 2oz copper is to much money so they go past 1oz and go to 0.5oz copper. They have a problem that the traces on the PCB get hot from current and lift off the board.

    What I do is to remove the solder mask from the high current areas and let the copper build up solder. (from the solder flow process) There is a very thin layer of copper on bottom and thick globs of solder over the top. This reduces the resistance of the trace. It also acts like a heat-sink for the trace. The increased surface area radiates heat better. Just removing the solder mask will slightly help the heat but copper should not be exposed directly to air and your finger. The manufacturing engineers say the solder blobs should be over only 50% of the trace so the boards do not warp. I have not see the warping with 100% solder build up but I only see the prototypes.

    While you said "high voltage traces" it is more likely they are "high current traces".
     
    • Informative Informative x 1
  4. dave miyares

    Dave New Member

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  5. Wirth's Law

    Wirth's Law New Member

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    I guess that answers a few questions then. All I'm missing is an industry name for this technique. I'll have to look up where this pattern might rank in effectiveness.
     
  6. Beau Schwabe

    Beau Schwabe Member

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