[SI-LIST] Re: Resedn:Mitigating PCB fiber weave effect
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: "emcesd@xxxxxxx" <emcesd@xxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 26 Oct 2012 22:22:57 +0000
Hello Tesla,
I think the answer you're after is in the paper, but rather hidden. I
apologize that this finding wasn't clearer, but there were space constraints
and we couldn't detail every interesting thing we found!
In the paper, you'll find the following note:
"Also note that 45 degree routing is not as effective at mitigating the effects
as 13 degrees (see Table 2), and this might not be as effective for extremely
long routing lengths."
Table 2 then shows that a design rotated to 45 degrees reduced the max skew of
a 10" trace from 51ps to 11ps, while a design rotated to 12.76 degrees reduced
it to 6ps.
It's called out more explicitly in the PowerPoint (on slide 18), as well as our
suspicion of why 45 degrees isn't best, presented pictorially. We hypothesize
that traces on boards rotated to a large angle still have a significant chance
of aligning with the "knuckles" (where the weft and warp intersect), thus
reducing the mitigation effectiveness.
In short, we built boards rotated at ~13 and 45 degrees, to try both
approaches, and found the boards rotated 45 degrees didn't mitigate the effect
as well. This is a bit anecdotal, but agrees with the physics as I understand
them. I personally would not advocate a 45 degree rotation unless shown some
compelling evidence of its increased effectiveness and credible reasoning as to
why it would be more effective.
As far as applicability of rotation angle to the various weaves, figure 19
("Angle vs. Line Length to cross 2 Bundles") was drawn for the dimensions of
1080, as I recall. It indicates a rotation of 2 degrees, relative to that
weave, is sufficient to mitigate the effect. On top of that 2 degrees, we add
some more rotation to account for possible skew between the material and the
board edge, which we believe is about 5 degrees maximum. That gives us an
angle of 7 degrees to ensure the effect is mitigated. We believe that routing
traces angled 7 degrees relative to the board edge should mitigate the effect
for all reasonable scenarios.
We often actually route our traces at slightly different angles to make it
easier:
* 10 degrees is a nice round number
* ~11.31 degrees allows you to do angled routing but stay on the grid (if you
use a 1:5 or 5:1 rise/run ratio)
My understanding is that both Cadence and Mentor Graphics tools allow easy
angled (and "zig-zag") routing, others may also.
I hope this helps,
Jeff Loyer
P.S.
If some of you get multiple copies of this, I apologize - my e-mail's gone
wonky today.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Tesla
Sent: Thursday, October 25, 2012 10:02 PM
To: si-list@xxxxxxxxxxxxx; Scott McMorrow
Subject: [SI-LIST] Resedn:Mitigating PCB fiber weave effect
Hi,
Today read two two Designcon paper about PCB fiber weave effect
1 The Impact of PCB Laminate Weave on the Electrical Performance of
Differential Signaling at Multi-Gigabit Data Rates
Scott McMorrow
2 Fiber Weave Effect: Practical Impact Analysis and Mitigation Strategies
Jeff Loyer
For the method mitigating skew by fiber weave effect. Scott suggest that
use-Panel construction at 45 degrees to the weave.
Jeff suggest that A rotation of 13 drgree is very effective at alleviating the
problem and A 45 degree rotation is not as effective.
For the two methods, which is better? Does the rotation method to reduce fiber
weave effect applied to all kinds of fiber weave(106,1080, 2113 and so on) or
different dense fiber weave may need different degrees rotation?
Thanks.
Tesla
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