Brandon,

First I can tell you that we have gotten our specs down so that there is absolutely no

change to the material thickness. All the boards tested were accurate and we began

having board built without the test and had some of those tested here. They were also

100 within spec if not right on the head.

I'm not "The man" on this issue but I have spent a lot of time with our specifications.

Our specs (to an extent) area at www.robertsondne.com . They are modular so that

you can choose between a couple different ones. There is also a calcualtor, based

and tested on several others including polor. The advantage is that you can

calculate up to 12 layers at once instead of chasing your tail with the 2 layer versions.

(resource.xls) it also allows you to build your stackup and record it along with other

settings.

First there is the material itself. IPC spec material tolerance is only 1mil for prepreg

and 2 for core material. (check for yourself) If you do the math any more will be out of

impedance tolerance. +/-10% for 50Ohm. I provide a spec that the material may

change the value of the material tolerance (+/-)

Second if possible, sandwich a prepreg between two planes. Use this to adjust to

whatever thickness remains. You don't want to adjust the dielectric thicknesses

to account for the board thickness, so this is the best way I have found.

Third, trace tolerance is +/- 1mil. If you do the math, that is all that is allowed,

so a board house running advanced technology will be required.

Fourth specify the impedance tolerance and the Er value that your calculations

were based on so the material is matched up. (I have to say that because I

get different stories from fabricators)

 

Test for the first few you do and find out the "fall out" amount and get feedback

on what the actual stackup was to verify future values.

 

Seperate you minimum trace note from your controlled impedance note to clarify

things. Some manufacturers like to calculate things for themselves. I use 8 mil

traces for controlled impedence and 6 mil min trace. (don't want them to do

calculations for 6mil)

 

 

Make very sure (very dam sure) that they call you if they need to make the thicknesses

different than specified. It's more than a matter of who is right, but documenting what

was done.

 

 

Also there a few combination you will find in the trace/dielectric thickness 8mil trace

on 1 oz copper with 12mil dielectric has been working for me.

 

There are several different stackups, but find one that works best and stick with it.

Controlled impedence is normally done with high frequency so at first, practice

sandwiching your signal layers with plane layers to eliminate crosstalk then move

on to trace-trace stack ups. (I will stay with plane-trace-plane for a long time)

 

 

I don't take credit for this program because I riped everyone off to make it.

I would like to thank them all for the cullmination of their information.

I'm working on making this a stand alone program for users of any layout program.

 

I hope this helps. If so pass it on.

 

Chris Robertson
[log in to unmask]

Senior Designer
Lockheed-Martin Services Inc.
4912 Research Dr.
Huntsville, AL 35805
(256) 722-2626

 

 

Good Day!

  We are having a raging debate about specifying controlled impedance on our fabrication drawings.

  The main part of the debate is over laminate thickness specifications.  I would like to know how other companies are handling these issues. 

        Are you specifying the individual laminate thickness?

        Are you specifying the individual laminate thickness tolerance?   If so, how do you handle deviations between different fab houses beyond the tolerance?

It seems that every time we use a different fab house, they like to use their standard laminates.  While this is also a benefit to us, it causes problems when our boards are microsectioned and the board doesn't agree with the print. 

We will continue to specify trace width, copper thickness and impedance, all with tolerances.

Any input will be appreciated!

        Brandon Luther

        Dataram Corp.

(609) 799-007  x2310