[SI-LIST] Re: Let's talk laminate induced skew

Bill, I have done measurements on weave induced skew & what I have discovered 
that there are lot of factors that come into play.
  1) resin content 2) type of glass 1080,2116 etc 3) dielectric material  4) 
construction of board 5) MEASUREMENT technique.
  I put measurement technique in bold because based on how you are measuring 
skew you could end up with slightly different numbers & also the measurement 
system has to be repeatable because you are looking for differences in ps.
  I would start with material selection, selcting the glass & resin content, 
talking to material vendors & fab vendors for their opinion. Then I would make 
different sets of boards each having different experiments like dielectric 
material, rotating the traces w.r.t weave i.e. different angles and last of all 
measurement method. I would use TDT, TDR and S-parameter method to get 
measurements to corealte to each other.
  Also as you might have read in these papers that by implementing all the 
right techniques on the PCB you will still have weave induced skew & top of my 
head it is 1ps/inch. I would also look at active compensation techniques in IC 
to minimize weave induced skew i.e. dynamic skew compensation for P & N in 
chips.
  I did make lot of measurements & by implemenitng some of the techniques above 
I did get skew numbers which followed the lenght of the trace. I could not do 
all the minimization because of manufacturing or implementation issues.
  Bob
Lee Ritchey <leeritchey@xxxxxxxxxxxxx> wrote:
  Bill,

This is traceable to the kind of weave in the glass. Some styles are much
better than others. The most popular choices have been 106 glass and 1080
glass, both of which have this problem. There is at least 1 weave style
that has very little of this problem. We documented this in my next book
which is due out in March of this year.

Lee Ritchey


> [Original Message]
> From: Loyer, Jeff 
> To: ; 
> Date: 1/19/2007 7:21:53 AM
> Subject: [SI-LIST] Re: Let's talk laminate induced skew
>
> Shameless plug - I'll be presenting a paper on this at DesignCon.
>
> 6-TA2: Fiber Weave Effect: Practical Impact Analysis and Mitigation
> Strategies
>
> Jeff Loyer
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Bill Dempsey
> Sent: Thursday, January 18, 2007 4:48 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Let's talk laminate induced skew
>
> Hi All,
> =20
> I've been reading a couple of these papers published on laminate induced
> skew of trace pairs. I know that several contributors to this forum are
> the
> authors of those papers and I would like to dig into this problem a
> little
> bit more.
> I am getting ready to build a backplane for a customer that has 40" of
> trace
> and mulitple pairs running between two connectors. They would like to
> reach
> 6.5Gbps on these traces but when I look at some of these published
> figures
> for just p-to-n skew I am worried about being able to have a reasonable
> eye
> opening at the receiver (without EQ or pre-emphasis).
> =20
> Can I take a poll from the community and ask who else has measured data
> for
> laminate skew? What did you see?
> How many people out there are running 40" or more at greater than 3.125
> Gbps
> using FR4 (or similar material) with success? Did you compensate for
> laminate skew?
>
> Lastly, how much does a Rogers 4350 dielectric exhibit this tendency?
> =20
> Thanks,
> Bill
>
>
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