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February 2006

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Subject:
From:
Donald Vischulis <[log in to unmask]>
Reply To:
Donald Vischulis <[log in to unmask]>
Date:
Tue, 14 Feb 2006 21:56:18 -0600
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Probably not.  50 ohms and 5% tolerance with a std dev of 1.5 ohm and a perfectly centered distribution has a Cpk of 1.11.  The actual results are dependent on the complexity of the board and the materials of construction.  For a reasonably complex design, centering is going to be a challenge and holding the standard deviation at 1.5 ohms is unreasonable.  You'll find more than a 3 ohm variation in readings with the same lot of boards on the circuits.  10% is a more common tolerance with 5% pushing into the sort and ship acceptable product realm.

Have you performed a sensitivity analysis on your design?  How much can line width variations or distance between layers affect the theoretical impedance?  What is your supplier's process capability for holding these parameters on your design?  Keep in mind that your design can create localized resin rich areas with a different Dk than other areas of the board.  This will increase variation on a single reading.

Additional complexity to consider regarding the measurement system:  Do you expect the measurements to be taken on a test coupon or on board circuitry?  Are you looking at impedance on surface or buried circuits?  There is often more variation in surface circuits than in  buried circuits.  How much variation is permissible on a single measurement - believe me when I say that the trace is seldom a flat line.  How much variation is designed into the circuitry?  There is seldom a perfect design situation on active circuitry.  What is the gage R&R?  I'll bet it consumes more than 10% of the tolerance band.  How well are the various measurement systems correlated?

At the end of the day, you need to determine what performance advantage found with a tolerance this tight and how does the impedance tolerance compare to the other circuit elements?  Do you really need a specific impedance value or do you need an impedance that is consistent from lot to lot (bet it's really the later)?

Don't mean to rain on your parade, but you should collect data on your present designs to get a feel for the typical variations found on production parts.

Don Vischulis

-----Original Message-----
>From: Glenn Pelkey <[log in to unmask]>
>Sent: Feb 14, 2006 1:05 PM
>To: [log in to unmask]
>Subject: [TN] Impedance Spec +/-5% and Cpk of 1.33
>
>Hi folks,
>
>Is the industry capable of this requirement?
>
>Impedance +/-5% with a demonstrated Cpk of 1.33.
>
>This could apply to Single Ended 50 ohms or 100 ohms Differential.
>Also, this is on a Flex Circuit.
>
>Thanks,
>
>Glenn
>
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Search the archives of previous posts at: http://listserv.ipc.org/archives
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