eagle - How do I find a short in an unpopulated 4-layer PCB?

I have a 4-layer PCB, designed in Eagle CAD 6.5. The stack-up is:

1. Signal


2. GND (ground)


3. DVDD (digital power)


4. Signal

GND and DVDD are solid planes, with vias connecting them to layers 1 and 4.

I have 4 PCBs. Three PCBs are bare - unpopulated, fresh from the fabricator.

In the bare boards (and the assembled one) there is a short between GND and DVDD. It could be a manufacturing defect, but since all 4 boards are bad, it's more likely it is a design problem.

I've manually examined the gerbers in gerbv to see if there are vias that connect to both GND and DVDD, but did not see any. But there are a lot of vias, so I could have missed one.

I've done an Electrical Rule Check (ERC) and Design Rule Check (DRC) - to look for problems. I get no unapproved errors. I've examined all the approved errors to look for problems - there are no overlaps.

How do I find the source of the short circuit?

Answer

Do you have any unplated holes or slots in the PCB's? I've previously specified some unplated holes on a similar layer stack, and found that the supposedly unplated holes were in fact plated and the plating was creating a short between the power and ground planes. A round file and a few minutes work quickly sorted the problem out.