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.

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