[SI-LIST] Re: PCB Impedance Failure
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 11 Jan 2006 12:40:24 -0500
Clayton,
I was a little puzzled about two things you stated.
One, that the specified differential impedance was exactly twice the
specified single trace impedance. That requires no coupling between traces
of a differential pair (if they are routed on the same layers with the same
trace widths). Is that what you really intended? Since you got 86 ohms
diff. and 59 ohms s.e., your differential pairs must not have been routed
with nearly enough isolation.
The other is about the error on the test coupon. Can you trust any
measurements on those coupons?
Did your company actually specify 50 +/- 10%, or was that only a
recommendation? If it was in the spec, then I wouldn't even bother asking
about the risk; I'd send the boards back to the board vendor and tell them
to make new ones that satisfy what you paid them to do. The fact that they
were made wrong, is reason enough to be concerned.
Regarding the question of the impact on SI, that depends entirely on the
application, on how long the traces are (vs. risetimes and/or frequency
content), on the types of signals, on the chips used, and if and how lines
are terminated. Sometimes it makes little difference whether the actual
impedances are even close to the target. Sometimes it makes all the
difference. 59 ohms isn't that far from 50 ohms, but if you were using
leading edge technologies and pushing all the margins (which you probably
aren't, since you didn't use SI tools), it could indeed be significant.
Regards,
Andy Ingraham
> A board we just had fabricated failed the impedance test coupon. 59
> ohm vs 50+/- 10%. They could not measure the diffferential pairs
> because of an error on the coupon, but calculated 86 ohm impedance vs
> 100 +/- 10%
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