Alexis,
The impedance value is set by the relationship of the signal trace width and its distance through the specific dielectric with its inherent characteristics to the return path of that signal, (i.e. ground plane, etc.). If you change the core thickness of the board, you must also verify that the relationships of the signal to return path have not varied, otherwise you will get unexpected electrical performance results from the board. If the fabrication drawing specifies the cores and layering structure, I would not deviate from that without an ECO. The comments about test are very valid, you may be unwittingly requiring your engineering test department have to re-do a complete DVT (Design Verification Test) all over again and loose what ever money the board vendor wished to try to save you on this particular run of boards.
I would not give the PCB board shop the authorization to deviate. Find another vendor or get this one to sign up to the fabrication drawing requirements.
Best regards,
Bill Brooks
-----Original Message-----
From: Alexis Meehan
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Sent: Thursday, June
05, 2003 1:50 PM
To:
[log in to unmask]
Subject: [DC] Core
thickness/emissions
Recently a board shop asked to use a different thickness of core material than what
we called out on one of our boards (they wanted .009 instead of .006, and .006 instead
of .008). Our engineer was reluctant to agree because EMI testing had already been
done on the prototype and had passed with the cores we called out. Does core thickness
have a noticeable effect on emissions? I haven't seen this as a problem before. Any
feedback would be appreciated. Thanks.
~~~~~~~~~~~~~~~~~~~~~~
Alexis
Meehan
PCB Design Manager
NOKIA
Nokia Internet Communications
Desk Phone 650.625.2124
Cell Phone 650.892.2572
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