Re: gEDA-user: Test pads in PCB

[Harry Eaton <bumpelo@xxxxxxxxxxx>]

Stuart Brorson wrote:

> As long as people are discussing vias in PCB, I'll raise a different
> question which occurred to me recently.
>
> Modern electronic manufacturing methods often include a testing step
> in which the stuffed board is checked for connectivitiy using either a
> "bed of nails" test fixture, or with a "flying probe" robotic
> probe.  These tests are totally automated, and are performed by very
> expensive machines manufactured specifically for testing PCBs.  Both
> types of test use little brass probes which look like pogo sticks 
> to touch little round pads you attach to each and every track.  These
> round pads are maybe 30 -- 50 mils in diameter.  The pads live only on
> the surface metal layers, and the soldermask is cleared away from the
> pad so that the probe may make contact.  Using these pads and your
> netlist, the machines can verify that your newly assembled PCB doesn't
> have any shorts, connections are correct, etc.  [1]
>  
Use a manufacturer with good equipment and they can do all the testing 
on the element connection points. i.e. where the components will be 
soldered. 30-50 mils is huge, far too expensive in board area to be 
seriously considered for every net. My employer uses such a test machine 
and there are never test pads added to the designs. We have probes 
capable of probing a 100 um copper area (because we sometime test hybrid 
substrates too) on our automated tester.

h.