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Re: gEDA-user: Test pads in PCB

Stuart Brorson wrote:

I am very pleased to see how much intelligent discussion my little
comments have sparked.  This is a very educational thread.



Hopefully I'm not putting words in Stuarts mouth, but we're really talking about 2 different things in this thread. Some of the comments here are related to testing a bare board (no components soldered down yet). In that case, the component pads are probably sufficient.

The 2nd case, which I think is the one Stuart was initially asking about, is for automated testing of a board which has been already assembled. In that case, you really do want test pads on all the nodes and preferably on one side of the board. We used to do that at a place I worked and it was pretty cool. They'd stick a fully assembled analog/mixed signal board into the tester and catch nearly all of the boards with manufacturing (placement/soldering) defects.



Thanks, Dan.  I am indeed talking about testing fully assembled
boards, not raw PCBs.  That's why probing the component pads isn't
possible or applicable.

Stuart



Analog or digital? If digital, you could use boundary scan - if your parts are jtag. What about some level of in circuit tests (ICT), followed by system checkout at run-time via diagnostics?

Not sure about analog designs though - not enough experience there. But you could specify bed of nails tests to verify that the components are correct value, and indeed soldered in place.

I've seen both ends of the spectrum regarding board testing. At one employer we tested all the boards on a teradyne bed of nails machine. Worked really well. They only do this early on in the production phase - once the machines are programmed correctly, and the soldering profiles are correct, these things generally work 98% of the time. If you are using a schlock house, then I suppose your mileage will vary :-) Following this phase, board failures are found in functional in-system tests. Those that fail go back to the teradyne for debug.

Where I work today - they have 0 test coverage from the board house, and we pay dearly for lousy workmanship. Somehow the management doesn't see it that way - but that's a whole other story.

gene