5
\$\begingroup\$

This question is a follow up to Can chips be sensitive to mechanical stress after an electrical incident?. It turns out that our problem was nothing to do with failed chips. It was the PCBs all along.

Background: We had a number of flexi-rigid PCBs fabricated and assembled by a company in the UK. We had several different designs of board made, but all had the same stackup: 6-layers of rigid at 0.8mm, and 4-layer flex.

When the boards arrived and we tested them, we discovered a couple of failed vias, which we fixed with jumper wires. Everything else seemed fine, and we put these boards into a longer term test. After more than 100 hours of testing at 70ºC with some mild vibration, we started to see some off behaviour from one of the microcontrollers on one of the boards. To cut a long story short, it turned out that four vias had failed under a QFN package, entirely cutting off that chip's ground!

4 vias failed under QFN

Question: How common is it that PCBs arrive with A) totally unconnected vias, or B) vias that fail after some time? Should we be highly suspicious of vias in general, or is this a freak manufacturing accident?

\$\endgroup\$
6
  • 1
    \$\begingroup\$ What's the via dimensions and PCB thickness? edit: PCB is 0.8mm \$\endgroup\$
    – Abdella
    Jun 28, 2022 at 10:37
  • 1
    \$\begingroup\$ I've had similar problems once, few years back. I could measure everything fine on the component pads on each side, but not on the vias connected to them. I scratched away the solder mask and everything looked ok, but there was no connection still. These were from one of the big Chinese manufacturers. \$\endgroup\$
    – Lundin
    Jun 28, 2022 at 10:42
  • 1
    \$\begingroup\$ It's quite interesting that all 4 failed vias are under one chip and not scattered randomly.. \$\endgroup\$
    – Abdella
    Jun 28, 2022 at 10:46
  • 1
    \$\begingroup\$ @Abdella maybe the exposed pad robbed copper from the via plating. \$\endgroup\$
    – Spehro Pefhany
    Jun 28, 2022 at 10:57
  • 2
    \$\begingroup\$ What's the drill diameter of those holes, and how close are they to your Fab's minimum spec. There is definitely a difference between what fabs "can do" and what they "can do reliably" \$\endgroup\$
    – pgvoorhees
    Jun 28, 2022 at 11:12

1 Answer 1

Reset to default
8
\$\begingroup\$

I have seen it a couple times (many years ago with smaller Taiwan suppliers). Cracking around the part where the through-hole plating connects to the copper on the surface. I believe it's caused by poor control of chemicals or purity in the production process.

Frankly, I would have discarded the entire batch upon finding bad vias. The later failures are almost inevitable. They probably discarded a bunch before shipping them too (assuming the boards were tested).

\$\endgroup\$
2
  • 1
    \$\begingroup\$ We would normally have discarded these boards, but during the pandemic (and still) components have been in short supply. Some of these boards had the last chips available. Also, the 8 week delivery time would have caused havoc with our scheduling. At least we were able to complete some of the mechanical testing with these boards. More are on their way from a different supplier. \$\endgroup\$
    – Rocketmagnet
    Jun 28, 2022 at 10:46
  • \$\begingroup\$ Understood. I guess that's a downside of getting the PCBA + fab from one source. \$\endgroup\$
    – Spehro Pefhany
    Jun 28, 2022 at 11:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.