jeez Terry - that's a great response, thanks

so is it fair to say that a bed 'o nails would do better at finding 
random shorts or is it still more dependent on the software and test

BTW, i've seen the little 'nicks' on the SMD pads, that is a good 
telltale sign, but i always assumed that they were from the nails and 
not a flying probe 
but i don't know for sure

i had no idea about the impedance test as an indication of the veracity 
of the connectivity, that makes sense though, i guess this is indeed a
wonderful world

so now that this aspect (impedance) is raised does that mean that as 
part of a bare bd test if impedance control is required that it could or 
would be specifically tested?

i always thought that that was 'as designed'
i.e., if specific traces were called out as 50 Ohms or whatever that 
as a part of a 'normal' bare board test that they would be verified as

or would that be considered an additional test call out ?

Dennis Saputelli

Terry Harris wrote:
> On Wed, 25 Sep 2002 16:58:42 -0700, Dennis Saputelli wrote:
> >so how does a 'flying probe' test really work?
> >i understand the general idea of a couple of probes walking around the
> >comparing connectivity to a 'netlist' made from the gerbers
> >but it seems to me and i think i read somewhere that this is better at
> >finding opens than shorts
> I know the guy who runs a flying probe tester at my board shop.
> Depends, checking continuity between all 'terminal' nodes of a net is
> pretty easy.
> Testing for shorts between a net and every other net on the PCB would take
> forever.  The test preparation software runs an adjacency algorithm trying
> to identify which other nets a particular net might be shorted to. This
> software hasn't been perfect and they had at least one instance of shipping
> boards with an undetected short.
> For boards with planes the tester they have can do a high frequency
> impedance analysis against the planes. It continuity checks one PCB to
> ensure it is 'gold' and measures 'something' at high frequency (I guess a
> MHz or so) with a single probe on each net. Subsequent boards are mostly
> tested using the impedance test, a single probe on each net being way
> faster. They find this testing method pretty reliable.
> If you are worried about your boards not been tested, if they are surface
> mount you can look for tiny holes left in the pads from the probes. I don't
> know if "bed of nails" testers leave the same indication.
> They recently got an optical inspection system which is very impressive. It
> tests almost as fast as you can load boards. It does actually inspect
> against gerber data with a rather complicated rule system for what is
> acceptable. It presents anything dubious to the operator on a video
> display. It was impressive to see the tiny nicks in tracks or bits of
> copper or dirt it picked up. It's like a manual inspection with a
> microscope but 1000 times faster and doesn't miss anything.
> They got it especially for inspecting the inner layers of multilayers (to
> avoid the waste of putting a faulty layer though subsequent processing) but
> it is so fast and effective they now seem to put all but the most basic
> jobs through it.
> Cheers, Terry.

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