It's not just the gap that is important for skew reduction. What you do
not see in a 2D view from above the weave is the thickness profile of the
weave. It looks more like an elliptical bundle for older fabrics, and like
a mush more spreadout and flattened bundle for modern mechanically spread
fabrics. Because of this, there are 3 distinct regions of fabric that meld
into each other, each with a different dielectric constant when viewed down
the z-axis, A high resin region, or gap. A single fiber region, with
higher Er. And a double fiber crossover region, or "knuckle", that has
even higher Er. Er variation is pretty bad when compared to modern fabrics.
Scott McMorrow
R&D Consultant
Teraspeed Consulting - A Division of Samtec
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
On Thu, Dec 31, 2015 at 3:22 PM, Istvan Nagy <buenoshun@xxxxxxxxx> wrote:
Gap is the area that is not covered with glass, there is only resin there.
You can see on the photos (in my excel file, second tab) as rectangle empty
areas. The 1067 has bigger gaps than 2116, very visible on the photos.
So, you are saying the glass thread cross section is worse for one
material than for another? Or is where the 2 diagonal threads cross and
they somehow flatten? Do you have some drawing that explains it?
-----Original Message----- From: Lee Ritchey
Sent: Thursday, December 31, 2015 9:42 AM
To: buenoshun@xxxxxxxxx ; 'SI' ; scott@xxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Glass Weave effects and Cross sectioning
I don't know what you mean by "gap". The reason that 2116 is not good is
that the weave results in peaks and valleys where the valleys are filled
with pure resin and the peaks are almost pure glass.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Istvan Nagy
Sent: Wednesday, December 30, 2015 7:20 PM
To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>; 'SI' <si-list@xxxxxxxxxxxxx>;
scott@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Glass Weave effects and Cross sectioning
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 1.3â¦3mil (somewhat smaller than trace width), the weave
pitch ranges from 14milâ¦26mil (several times the trace width & diffpair
gap), while the impedance controlled PCB traces are 3.5milâ¦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:
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 would
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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