Harry Eaton wrote:
David Hart wrote:
5) Then do a DC Isolation test, to verify there is adequate electrical
isolation between nets that are not intended to be connected. Again,
called out in the Fab instructions, probably in some IPC standard, are
requirements for minimum isolation resistance and applied voltage. With
aggressive trace/spacing geometries, and traces stacked close to each
other, this is a hard spec to call out. Coombs goes into detail on how
geometries can effect pass/fail. The result can be either hard shorts or
leakage failures.
Here is where adding test pads can be useful in the manufacturing
environment. Simply connect to the test pads to test the isolation
between two nets. Potentially saves a lot of time.
Sorry, I don't see it. Since you must probe each component pin/pad to
verify connectivity (it does no good to see that it connects to the
"test pad" if it doesn't connect to the component), you already have
convenient places to probe for net isolation - just choose any point on
the net that you've already probed for continuity (i.e. a component pin
or pad). It is *less* convenient to use the test pad for isolation
testing because you must specifiy its coordinates *in addition to* every
component pin/pad.
The only thing that test pads will get you is a place to probe after the
components are installed (not really useuful for isolation testing) but
could be highly valuable as a scope or Voltmeter probe point, or you can
probe structures that don't otherwise connect to any components on the
board. For special structures that don't connect to anything on the
board (e.g. coupled line connected to an open-circuit stub) then test
pads are warranted. For everything else they should only be used
wherever you might want to probe the circuit in operation. For me this
covers a lot of signals, but far from every one.
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.
-Dan
