A couple of weeks ago, I encountered a very peculiar issue with a batch of printed circuit boards.
At first glance, the PCBs looked very well made, and we decided to proceed with component assembly.
During both SMT and PTH assembly, we didn’t encounter any major difficulties and moved on to electrical testing.
During ICT testing, thanks to nodal impedance testing, we noticed potential issues on nets where PTH connectors were mounted.
A very strange situation… especially since the same potential issues were found on many other boards.
After ICT testing, we found that out of 50 produced pieces, 15 had this potential issue (30%).
What to do? Was it actually a problem or a repetitive false error?
Performing a simple test with a digital multimeter in short-circuit detection mode, I noticed that the connector’s net was shorted to ground.
I immediately thought the connector was broken… unusual but possible!
I replaced the connector (after checking if its pins were shorted)… tested again with the multimeter for shorts, and it was still present!!!
At this point, I removed the connector again and carefully examined the pad under the microscope.
And yes… there was indeed a problem with the PCB.
The solder mask isn’t centered relative to the rest of the board, and the underlying plane (in this case, the ground plane) is exposed in certain areas.
This way, when performing PTH soldering, it’s possible for the solder to attach not only to the connector but also to the ground plane, creating a short circuit.
I’ve already asked my supplier for an explanation of this phenomenon and am awaiting their response.
I’ll keep you updated…
