[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Re: gEDA-user: Test pads in PCB

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.

harry