I suspect there are simpler ways, but this is commonly done with a TDR -
Time Domain Reflectometer.  A simple version is used to locate breaks in
cables, a good one can find a discontinuity in a trace to within a few
mils.  Probably more expensive than you'd like.  A TDR is also use to
measure pcb impedance - your pcb fabricator should have one and would
probably be willing to test the board, since it's possibly an error in
their fabrication.

You can DIY a TDR, but it takes a high speed scope and pulse generator.
Something like this gives the idea:
http://www.allaboutcircuits.com/projects/build-your-own-time-domain-reflect=
ometer/
   but that's only good for cables.  To cut things really fine takes a much
higher speed scope and pulse generator.

On Sun, Nov 20, 2016 at 11:40 PM, Richard Prosser <rhprosser@gmail.com>
wrote:

> Harold,
> I've used a signal generator and a scope to find breaks in cables.
> Might work on PCBs provided the trace is not too buried. On cables it
> works best if all other conductors are grounded - not as easy on a
> pcb.
>
> I think I used frequencies in the low 100's of kHz area.
>
>
> RP
>
> On 21 November 2016 at 15:23, Harold Hallikainen
> <harold@mai.hallikainen.org> wrote:
> > We occasionally find an open PCB trace (or perhaps via). Of course, the
> > problem is on an inner layer. We can tell from continuity measurements
> > that we have a problem between one via and the next. But where is the
> > break? At one of the vias, on the trace? Is there a tracing device wher=
e
> > we can inject a signal at a via and then follow it down the trace? I've
> > seem something like this for tracing cable pairs in telephone cables. I=
s
> > there something like this to find a problem in a PCB?
> >
> > Thanks!
> >
> > Harold
> >
> >
> >
> > --
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