John Anselmo wrote:
        I would agree with Phil.  If you need a controlled dielectric,
     specify that on the print, not the type of prepreg to use.
        1 ply 1080 has a nominal thickness of 3.0 mils (based on resin
     content and glass).  So no, the supplier cannot add as much or little
     resin as they want.  But, there can be up to a 10% variation in this
     thickness.
        Specifying prepreg severely limits the board manufacturer options,
     and increases the chances of the board not meeting your requirements.

     John Anselmo
     Hadco - Tech Center Austin
     512-246-5616

______________________________ Reply Separator _________________________________
Subject: Re: [TN] PREPREG
Author:  Phil Crepeau <[log in to unmask]> at smtplink-hadco
Date:    2/7/00 10:06 AM

hi,

i would strongly recommend that, if at all possible, you leave these details
up to the board fabricator.  and here's why: if the board doesn't meet your
end product requirements you can blame the fabricator.  if you specify the
details, you will be responsible if the board doesn't meet your end product
requirements.

best of all, you should be involving the board fabricator at the initial
stages of you design.  and by the way, i am not involved in the board
fabrication business.

phil

-----Original Message-----
From: Brunker Ed [mailto:[log in to unmask]]
Sent: Monday, February 07, 2000 8:37 AM
To: [log in to unmask]
Subject: [TN] PREPREG

Is there anyone out there who can help me with understanding prepreg. If we
specify a given prepreg, (1080) does that give us any control over board
construction. I'm told that 1080 specifies a glass matt. So does this mean
that for a given glass matt the resin added to it can be as little or as
much as a supplier wants and the characteristics of that resin can be
whatever? So sheets of prepreg can be any thickness and have any flow
characteristics.
Also for mobile phone usage what are people using in FR4 to achieve
approriate dielectric characteristics? Are there a range of prepregs being
used or just 1080?

Regards
Edward Brunker

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As an OEM,using Controlled Impedance Printed Wiring Boards in our designs,
I agree with John and Phil.We specify number of layers,specific controlled impedance
with tolerance and overall thickness target with tolerances.If you are realistic with
the target impedance and tolerance and  have not pushed any limits with line width
and spacing,then the normal variation of the Resin/Glass Ratio should not cause you
to miss your design window.
There are two things that we do to make the above statements true.We qualify our
Vendors and we do Signal Integrity Analysis on the Designs prior to their release.
Dewey Whittaker 
-- 
Dewey Whittaker
Honeywell -- Air Transport Systems Division
US Mail: P.O. Box 21111 MS: M16A2 Phoenix AZ 85036
Phone: +1 602 436 436-2766
NSFnet: [log in to unmask]