I was working with IPC to try to find a metric we could use to quantify the
"goodness" of the various spread styles but I doubt that will come to pass
in a meaningful timeframe. In order to accomplish that, you need to measure
the skew mitigation vs. some measure of "spreadness" and I don't think
that's reasonably possible. In our experience, the amount of skew varied
significantly between boards and traces on the same board, so any studies
have to have rigorous statistics applied, including having a large number of
panels built and measured for each weave type being analyed. I doubt that
will happen, though we can continue hoping.
I think that looking at a cross-section and then assigning a skew value to
that observed weave is not going to be realizable. Instead, you're probably
forced to rely on others' experience with particular weave types, if you use
that method to mitigate the Fiberweave Effect.
For my information, can I ask folks why they don't just use "zig-zag"
routing? It's what we primarily adopted at Intel in order to have a robust
and cost-free solution. The drawback we were running into when I left was
that there was a limit to how much you could apply it - at some point you're
forced to run through pin fields, etc. and are forced to route parallel to
the board edges. On the other hand, the studies that I saw indicated that
modern receiver equalization took care of the Fiberweave Effect much better
than when our original study was published, so 2-3" of skew due to the
Fiberweave is inconsequential for modern interfaces.
If "zig-zag" routing isn't a good option for some applications, wouldn't
just rotating the board 10 degrees be more palatable than constraining your
material type?
Jeff Loyer
Signal and Power Integrity Product Manager, Altium
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Boris Bakshan
Sent: Thursday, December 31, 2015 2:17 AM
To: buenoshun@xxxxxxxxx
Cc: Lee Ritchey; SI; scott@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Glass Weave effects and Cross sectioning
Hi,
Thank you all for your informative answers.
If we focus on the subject of this correspondence, I can't conclude anything
on X-Y surface where the weave type can be determined by doing a cross
section. Is that correct?
On Thu, Dec 31, 2015 at 5:19 AM, Istvan Nagy <buenoshun@xxxxxxxxx> wrote:
Lee,
How come those fabrics yield better result than the 2116?
I have a photo album of glass fabrics from different vendors. The
1067,
1035
seems more loose than the 2116.
http://www.buenos.extra.hu/download/PCB_MATERIAL_LIBRARY.xls
For one vendor:
Measured gaps and coverage:
Fabric Xgap% Ygap%
106 74.05405405 50.48543689
1067 38.11188811 9.48905109
...
3070 14.18918919 11.26760563
2116 18.02325581 16.1849711
...
You see the 1067 has much bigger gaps than 2116.
I followed my intuitive idea that the more dense the fabric on the
photo, the more uniform it is and the least skew we get. Is there
anything wrong with my concept? Am I missing something? The percentage
of 2layer/1layer/0layer areas would determine the uniformity and density.
Layer, I mean layer of glass thread on top of each other. The 0layer
gaps have dimensions of 1.3…3mil (somewhat smaller than trace
width), the weave pitch ranges from 14mil…26mil (several times the
trace width & diffpair gap), while the impedance controlled PCB traces
are 3.5mil…7mil (typically
4.5mil) wide and gaps are like 5-8mil (signal pitch 10mil). Typical
high-density high layer count designs I have seen are like this. So,
none of the fabrics have small enough dimensions to appear smooth to
the traces while having large % gaps. Therefore I relied on the gap
percentage.
Scott,
To achieve the 14mil-18mil signal pitch, you need thick prepreg/core
layers like 6-8mil thick, then the whole stackup is thicker and needs
more layers due to lower density (18mil diffpairs vs 10 mil ones take
more space). So my
18 layer 93mil stackup would become 26 layer and 200mil thick. Is this
what you are suggesting? pitch-matching would need 2x thick PCBs. Nice
theory, but I am not sure about the practicality. On the other hand...
if this is proven practice, I might consider it for my next designs.
Some form factors like ATCA have fixed board thickness, and increasing
layer count by 50% might be hard to justify. We have 200-300 10Gig
diffpairs on my latest board design, although we did not measure skew
on it, just on-die captured eye diagrams. Small company, no SI-team...
"know the fill direction of the laminate that... "
- I will keep that in mind. Somehow it has to go into the fab drawing
gerber file I guess.
So, gap-percentage, pitch-matching, documenting fill direction and
12+deg rotation...
Best regards,
Istvan Nagy
Principal Hardware Engineer
-----Original Message-----
From: Lee Ritchey
Sent: Wednesday, December 30, 2015 2:05 PM
To: 'Istvan Nagy' ; 'SI' ; bbakshan@xxxxxxxxx
Subject: RE: [SI-LIST] Re: Glass Weave effects and Cross sectioning
Scott listed my preferred weave styles in another reply. And, yes we
do very well when we control the weave type. Better than any of the
other methods that have been proposed.
Just had a PCB with 200+ 10 Gb/S paths with the worst skew being 2 pSEC.
-----Original Message-----
From: Istvan Nagy [mailto:buenoshun@xxxxxxxxx]
Sent: Wednesday, December 30, 2015 1:35 PM
To: SI <si-list@xxxxxxxxxxxxx>; bbakshan@xxxxxxxxx; Lee Ritchey
<leeritchey@xxxxxxxxxxxxx>
Subject: RE: [SI-LIST] Re: Glass Weave effects and Cross sectioning
Lee,
What is on your list of good enough weaves? Are those provide much
better results than 2116? The nicer weaves are not as available as
2116, i am talking about series production of networking and computing
boards, not test boards.
Do you expect the skew problem to be resolved mainly by glass style
selection?
Regards, Istvan, mobile
*** Sent from my BLU smartphone device *** On Dec 30, 2015 9:07 AM,
Lee Ritchey <leeritchey@xxxxxxxxxxxxx> wrote:
would
I have had severe skew problems with 2116 at 10 Gb/S. It is not on
my list of good weaves.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Istvan Nagy
Sent: Monday, December 28, 2015 10:53 PM
To: bbakshan@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Glass Weave effects and Cross sectioning
Hello,
First getting on this topic of spread glass, everyone is looking for
the miracle spread that has no skew issues.
You can choose the glass style as a number, or to take it further
and choose a material manufacturer who promises mechanical spread or
square wave or other features too.
In reality the traditional glass styles (106, 1080) were kind of
"bad", newer more denser weaves (2116, 1501) are "better", but there
is no "excellent" fabric.
I just try to use 2116 for layers with 2Gig+ signals. So you specify
glass style, and material name, that's mainly what you can do about
it, before building the stackup for design and manufacturing.
You need to have the detailed datasheets listing all available glass
styles of your chosen material. Get glass fabric photos from your
material vendor (like Bill from NanYa), so you can see how much
difference those different fabrics (numbers) have in
covered/uncovered areas. The board area consists of "no glass",
"1-layer glass", "2-layer glass" parts. If you imagine a simple
cloth fabric, you would know what I mean. The best material has the
smallest percentage of "no glass" area and the largest percentage of
"2-layer glass" area, that
be the most uniform (logically thinking).
"If not, how can I guarantee that the right type of weave was indeed
selected when building my board?"
- instead of "they" building your stackup and board, you have to
determine the stackup before they manufacture it. If you just let
fabs and who knows who manufacture whatever for your board designs,
then many things can go wrong.
Regards,
Istvan Nagy
-----Original Message-----
From: Boris Bakshan
Sent: Sunday, December 27, 2015 10:16 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Glass Weave effects and Cross sectioning
Hi experts,
I've been reading much on glass weave effects and its contribution
on skew.
If I encounter a skew problem, can I say anything on weave type
,whether it is mechanically spread or not by doing a cross section
of my traces?
If not, how can I guarantee that the right type of weave was indeed
selected when building my board?
Thank you all!
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