PWB testers like the IntegraTest machine we use measure the capacitance of a
net to identify it. In this way it reduces the exhaustive test time to
something doable. With a totally dumb machine you'd have to test every pad to
every other pad. Typically the test begins with the continuity test where it's
verified that each pad in a net has the same capacitance and has a minimum
conductance between each pad. During the "shorts" test only those nets with
approximately the same capacitance need to be resistance verified.
John

Terry Harris wrote:

> On Wed, 25 Sep 2002 20:59:10 -0700, Dennis Saputelli wrote:
>
> >so now that this aspect (impedance) is raised does that mean that as
> >part of a bare bd test if impedance control is required that it could or
> >would be specifically tested?
>
> I called it an impedance test but I really don't know what it is doing. If
> you just measured the capacitance between a net and a plane the vast
> majority of opens and shorts would produce a significant difference.
>
> The tester only compares whatever it measures against the 'gold' board, it
> is saying this board is like a good one, not measuring any particular
> parameters.
>
> >i always thought that that was 'as designed'
> >i.e., if specific traces were called out as 50 Ohms or whatever that
> >as a part of a 'normal' bare board test that they would be verified as
> >well?
>
> Controlled (transmission line type) impedance is a who different can of
> worms. I imagine others here know more about it than I do.
>
> I as far as I know testing is limited to test traces (usually outside the
> PCB outline). You can buy trace impedance measuring equipment (which seems
> very expensive for what it does) so the board house can measure your test
> traces. The problems start when what you measure is not what you want.
>
> Firstly you can't measure it till the board is finished and if it is wrong
> the board (and most likely the whole batch) is scrap.
>
> Secondly the trace impedance is mostly determined by the designers stack up
> and trace widths, if the designer gave you something that is going to be
> about 100 ohms there is no way the board house can make it 50 ohms (unless
> you are happy to have your 1.6mm board come back as 1mm ?).
>
> The board house does have some control of prepreg and core thicknesses,
> material dielectric constant, and control of trace widths through etching
> and plating.
>
> The board house I use looked into offering controlled impedance boards and
> decided against it for the time being. Apart from the expense of the test
> equipment how do you cost a board that you might have to scrap and remake
> three times? To be good at it they would have to gain a lot of experience
> and put a lot more effort into material and process control than they need
> to currently. They were also worried about customers ignorant of the
> issues.
>
> Cheers, Terry.

--
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>John M. Cardone       Electro-Mechanical Dsgn. Engr. Grp.
>M/S 278-100           Mechanical Engineering Section, 352
>4800 Oak Grove Dr.    NASA / Jet Propulsion Laboratory
>Pasadena, Ca 91109
>Tel: 818.354.5407     MailTo:[EMAIL PROTECTED]
>Fax: 818.393.6400     Cell: 818.653.7818
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


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