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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