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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