Hello Rimas,

Any reason for 2oz Cu? (about 3mil)
I'm sure that this would be difficult to etch to 7mil wide (always some
sideways etching even with spray etching)

Assymettric stackup could be a problem as mentioned.
A 'standard' 6-layer stackup would be;

foil
6mil prepreg
20mil core
5mil prepreg
20 mil core
6 mil prepreg
foil

Check with your manufacturer, sometimes they can build (and test) to
provide you with a required impedance - if you want a tight tolerance.
I usually try to design the tracks to achieve the required impedance
with a standard build. This can be hard if you want to keep the track
width small as you need to closely couple the reference plane (note that
the reference plane can be any DC voltage)Balancing this with the desire
to keep DC power planes close coupled to improve interplane capacitance
is hard. 
(even 6 layers starts to look difficult to achieve what you would really
like)

regards,

Phil.


-----Original Message-----
From: rimas [mailto:[EMAIL PROTECTED]]
Sent: Friday, 2 August 2002 08:36
To: Protel EDA Forum
Subject: [PEDA] question about layer stackup/controlled impedance
traces...


hello out there,

i'm about to have my first PCB design involving controlled impedance
traces 
manufactured in the near future (just a small run of protos).  I only
have 
controlled impedance traces on the top layer.  using the transmission
line 
impedance calculation tool at:

http://www.eskimo.com/~ultra/calc.htm

(which someone on this list pointed me to before, thanks!)

i've discovered that if I'm using FR4 and 7 mil traces with 2oz of
copper, 
that a dielectric width of 6.3 mils will give me a reasonably close 
impedance (52.6 ohms and i'm aiming to get 50)

now here's my question - is this a reasonable way to do the layer
stackup? 
(oh yeah, this is a 6 layer board and i'd like the resulting board to be

.063" wide)

(TOP)
core -> 6.3 mils
prepreg -> 12.6 mils
core -> 12.6 mils
prepreg -> 12.6 mils
core ->  18.9 mils
(BOTTOM)

this seems logical to me but being as that i am inexperienced and such 
things and have noone knowledgeable around here to ask i thought i would

ask for this group's wisdom.  thanks for any help, hope the 
non-directly-protel related question doesn't bother anyone.

-rimas


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