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I2C copper trace lost continuity

Discussion in 'PCB Layout, Design and Manufacture' started by sambady929, Feb 11, 2020.

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  1. sambady929


    Feb 6, 2020
    I have a unit that's been operational for a little over a year. Something went wrong and suddenly the LCD is no longer getting the I2C clock signal. Shorting the LCD's pin directly to the I2C SCL pin of another chip gets it working again.

    I found a discontinuity between a via underneath the MSP back to the MSP pin. Everywhere else on the line has continuity.

    At first I thought maybe the via wasn't properly connected all the way through the board. I lifted the MSP off the board and the via is fine. I carefully scratched off the solder mask and saw part of the copper trace has just vanished. You can see the discontinuity in the middle of the first image linked. The trace to the pin is fine, the via is fine, everywhere else is fine, but between that via under the MSP and the trace to the pin there's a small but clear discontinuity of no more than 25um across the entire width of the trace.

    Has anyone seen something like this with a clock line? Trace width is 8mils, should be okay, it's been working for a year. Is this a one-off manufacturing defect, or a likely error in design/manufacturing that I can avoid?

    Any and all advice/help is appreciated, thanks in advance!

    Link to images
  2. HellasTechn


    Apr 14, 2013
    Are you sure you scratched the PCB after the line went open ? It sure looks that the line was scratched open.
    davenn likes this.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    It sure does look so.

    It could be a one-off manufacturing error - the design looks generally o.k.
    To reduce the risk of overetching during pcb fabrication use teardrops in these places. This is a proven method.
  4. sambady929


    Feb 6, 2020
    100% sure. I scraped it off very carefully perpendicular to the line, and in sections. I first scraped off only part of the via, it didn't have continuity then either. Unfortunately I don't have pictures before it was all scraped off.
  5. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir sambady929 . . . . .

    It certainly appears that some caustic-alkalinic-acidic residue has really done a number on your Cu foil conductors in the fine YELLOW . . . . erosely encircled area.
    As is being compared to the QUITE normal, BLUE circled vias in the top right corner.

    The worst case seems to be the item #54 12C SCL line which rises and then slants off to the left to connect with the via at its end and . . . then is just eroded open. Refer to the disrupted CERISE bar on the foil path, or its companion , like LIME GREEN bar on the drawing.
    The far left YELLOW marked in via, also looks marginally corroded . . .but . . .not so BIG TIME, like the right sides A B C three YELLOW companion vias are.

    He say . . .
    It sure looks that the line was scratched open.

    Me say . . .
    It sure looks to me, like that foil path was chemically eroded open.

    The ROBIN EGG BLUE boxed in area appears like there might be a single strand of tinned solid wire sloping upwards at an upward 10:30 position, then bending to the left, but I see no further passage beyond the closer left side of that foil.
    In the ORANGE boxed in area, that appears as having some wire solder . . . or a tinned bare wire . . . laying across the area, across several foil fingers.

    You might wet a cotton swab / Q-tip with distilled water and then use it for a VIGOROUS scrubbing / sampling within the worst appearing of the YELLOW encircled area. Then use a swimming pool Ph test kit to see which direction you need to go to neutralize the boards contaminating deposits.
    Then totally clean the areas, the best possible. Clean off the contaminated solder at vias . . . and then refresh with neew solder .particularly YELLOW A B C and re tin and reflow the solder joints . . . . . . and / also any bad looking foil runs.



    73's de Edd. . . . .

    My dental hygienist is OH- SOOOOOO cute. Every time I visit, I eat a whole package of Chocolate Oreo cookies while waiting in the lobby.

    HellasTechn likes this.
  6. HellasTechn


    Apr 14, 2013
    Now that you mentioned it does remind me of some short of over-etched pcb.

    My respect to Sir 73's de Edd.
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