Electronic – PCB plated through hole short-circuit

pcb-fabricationshort-circuit

We have a board built in the thousands, and several have (apparently) short-circuited a plated through hole (PTH) to an adjacent pour (see photo, note the other thermal damage is probably due to thermal conduction of via internal trace running to the PTH).

The PTH to pour clearance is 7mil, and the PCB manufacturer and assembler is big name CM!

Any ideas about possible causes or lines of inquiry to pursue?

—followups

  • This problem has occured in ~0.05% percent of the builds.
  • the damaged PTH is where the short is suspected to an INTERNAL pour.
  • I had assumed that 100% of these PCBs were electrically tested. perhaps
    I should not assume.
  • the applied signals are from a <50V battery which is rated for ~20A, but if we assume ~3kv / mil dielectric withstand we are nowhere near a breakdown even with some voltage ringing given our 7mil clearance.
  • Failures are found during sub assembly testing in a pretty organized / clean facility, and occurs upon connection of power, not during use.

enter image description here

Best Answer

On such wide question I can only give research questions.

  1. Validate if the board layout is correct.

    • Check if the continuous voltage present provides enough clearance and creepage.
    • Check for any surges or spikes, the re-validate the the board design.
    • Check for current rating and track/via within normal conditions.
    • Check for current ratings in abnormal conditions. (add fuses is necessary)

  2. Validate the Assembly is correct.

    • Are there lots of tiny solder balls after soldering? Improve reflow/solder process.
    • Is there manual soldering used? Too hot, too much, leftover flux of bad kind?

  3. I can't see if there used to be a component where the board has vanished, what if this component is abnormal or is used incorrectly?

  4. Is the operating environment clean (pollution degree) and how is moisture?

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