Rick,
Conan & I discussed that here too, and yes there is a cost adder for CI.
There has to be if you are asking the vendor the verify what they are
delivering is what was asked for.

There is a good argument that if process is controlled and you know what
your going to get by defining stack & traces etc. every time than yes
why not only define that knowing you gonna get xx ohm lines.

If you are only using very few vendors & you are using their standard
process or at least working closely with that vendor such that you know
what you will get delivered without having to specify it must meet this
CI, I could see some savings without problems. 
But if it is not written down anywhere and the vendor gets new
employees, and they decide to change material to save money etc I would
think there could end up being some problems down the road. Also if it's
not specified
on the dwg how would you have any recourse if there were a problem. It
all comes down to the relationship with your vendor.

It may not work even if dealing with only a couple vendors.
Fab house's get bought out, and vendors want to reap the savings of a
house they just bought and may want to transfer designs to that house.
We are now in that situation dealing with a very large fab vendor.
that is moving some of our boards to other facilities it owns (or just
bought). Now this should have been a transparent change to us one would
think. And in all honesty at first we were unaware this was being done
till we started getting a lot of questions about the designs. If you are
in this scenario trying not to document what you need this could be a
huge problem.
Or your gonna need to keep doing the same testing at new houses as they
want to move them. If you say you want to keep that design at the
particular house don't be surprised if price goes up next buy.

Bob


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Smith, Rick
Sent: Friday, May 02, 2008 2:56 PM
To: [log in to unmask]
Subject: [TN] FW: [TN] Cost of Controlled Impedance

Hi Charlie,

Thank you, I greatly appreciate your insights. Our process today is the
same as you state in your response. So far from what I'm gathering it
doesn't seem logical to change it.

I initiated my research based on some statements made by a colleague in
a lecture at Apex last month. He stated that he has not specified CI on
his drawings for many years to save costs of board fab. He went on to
say that he gets around it by working closely with the fabricator up
front and then specifying the stackup including material types on the
drawing. This is very similar to what I do with one exception, he does
not specify controlled impedance on the drawing where I go through great
pains to specify trace widths, spacings, reference planes, etc. He also
has the luxury of working with relatively few "approved" suppliers who
use the same materials so does not need to worry about Purchasing going
to other suppliers.

Anyways, a potential cost savings intrigued me but, I thought I should
do my own due diligence before changing our current process of
specifying CI on the drawing..

I created a stackup of a known good design and asked two fabricators to
model the impedance based on the data. This stackup specified a variety
of single-ended and differential impedance trace widths, the laminate
material and thickness, copper weight, core & prepreg, the usual stuff.
The impedance models that I received from both fabricators were
identical for 50% of the CI values. The other 50% of the CI values were
so close as to be negligible, ranging from less than 1% to 2%
difference.

My next step was to find out what the cost savings might be. I asked the
same fabricators what the cost difference between CI and non CI and the
answer from both was as stated in my original message to Technet. Being
a skeptic I felt it best to post to this forum to obtain a variety of
experiences.

So far I've learned that the performance of my test board should be the
same from both fabricators whether or not I specify CI on the drawing.
However, if there is no cost-savings advantage then it is better to be
safe-guarded by keeping the CI information on the drawing.

Again, thank you very much for your response!

Best Regards,
Rick



________________________________
From: Charlie McMahon [mailto:[log in to unmask]]
Sent: Thursday, May 01, 2008 2:44 PM
To: Smith, Rick
Subject: RE: [TN] Cost of Controlled Impedance


Hello Rick:

As your below comment states, yes; it is suprising....

As a supplier in New England of PCB's for 25 years for both rigid and
flex, when impedence is called out there is an additional cost adder. To
say otherwise or try to manipulate the process for so-called cost
savings is a strategy for pain.
CI is directly related to the tolerance % of the design. In addition, a
cost influence is in the dielectric constant of the material that is
required based on this design call out.

If you require a controlled impedence result that specifies a line width
and dielectric spacing to achieve, you need to address it with the
fabricator and detail same on the print. This PROTECTS you from the
fabricator saying later (when convenient) upon a board rejection that
the manufacturing tolerance was met and therefore they are not
responsible for the defect/reject.

Fianlly, to imply there is no added cost is mis-representing the facts
and is a dis-service to you. I could say more but I will close with
this....please specify on the print when a part is a CI board if it is a
design critical issue. And, as far as saving money, I believe (as I do
now) that DFM of the design should be done BEFORE release to manufacture
by the vendor working with the Client and that's you...together. This
should ensure a winning relationship for both parties with a cost
effective and profitable result. It is how I do business and frankly so
should you for the integrity and professionalism our business must
demand.

Best Regards,
Charlie McMahon
McMahon Sales Company
P.O. Box 1024
Windham, New Hampshire  03087
Tel:  603-432-3111
Fax: 603-432-6854
Cell: 603-401-4646
[log in to unmask]<mailto:[log in to unmask]>


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Smith, Rick
Sent: Thursday, May 01, 2008 2:21 PM
To: [log in to unmask]
Subject: [TN] Cost of Controlled Impedance

Hi All,

In an effort to save my company some $$$ on PWB's I began investigating
the cost of purchasing boards with and without specifying controlled
impedance (on the PWB drawing). Almost all of our boards specify
impedance control even if there are only a handful of transmission
lines.

I was surprised to learn from the two fabricators I spoke with that
there is no additional cost for impedance control other than a small
cost adder of about 2-5% if all panels require testing. I found this
very surprising.

Has anyone else had the same experience? Thanks much!

Regards,
Rick

Rick Smith
Manager, Engineering Systems & Services

 [cid:[log in to unmask]]
Access Transport & Supplies

15 Sterling Drive
Wallingford, CT 06492

Phone 203.639.7670
E-mail [log in to unmask]<mailto:[log in to unmask]>
Web http://www.arrisi.com




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