From: Ingraham, Andrew ([email protected])
Date: Wed Nov 01 2000 - 10:29:10 PST
>>How many boards of each kind were built and tested, and found to be "very
>>good" versus "very very bad"?
>Two of each kind and the behaviour is very very consistent.
Two are better than one; but are the components on board "A" from the same
lot or datecode, and those on board "B" from a different lot or datecode?
(I'm just wondering what other differences there could be here.)
>The stackup is exactly the same...the decoupling is exactly the same...the
>component placement is exactly the same...
IC decoupling capacitors in EXACTLY the same place (with respect to IC
pins)? Same trace lengths to IC power and ground pins and cap pins?
All the dielectric thicknesses are identical, except for those necessary to
keep the signal trace impedance at 50 ohms?
Same thickness between adjacent power/ground planes? If you're using
stripline, the planes around the signal layer(s) would be further apart on
the 9 mil trace board, which would mean somewhat more plane/plane
inductance, less intrinsic capacitance, and a higher power system dynamic
impedance (below resonance) ... which doesn't explain why there would be
less ground bounce in that case.
>the routing is different but the routing the rules are exactly the same...
Could the rules have overlooked something? Are there any critical signals?
(What sort of signals are they, anyway?)
Just because the rules are the same, doesn't mean some signal won't be hit
by a lot more crosstalk on one board. 2X spacing allows plenty of
crosstalk.
>I did not measure and compare the ground bounce...this is just an
>observation of the end result..
Then exactly what DID you observe?
>I wanted to find out whether ground bounce on
>the board would significantly improve
>if I change the weight of copper
Ground bounce is an AC phenomenon. Changing the copper weight primarily
affects DC resistance and has little to no effect on high frequencies and
ground bounce.
>and whether you people think (based on
>experience) change in trace width contribute
>significantly to improve the quality of the signal...
Changing trace width from 9.0 to 7.5 mils, *while keeping the characteristic
impedance the same*, has only a small effect. The skin effect losses would
be a little more at 7.5 mils, so the edge rate might be degraded a little,
not much. I think it would have next to no effect on ground bounce (unless
the impedance *did* change).
Andy
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