>There are two schools of thought associated with the implementation of
>impedance controlled signals in a PCB fabrication package and can be applied
>for both stripline and microstrip impedance controlled impedance signal
>traces:
>1) Calculate the required trace width and dielectric spacing(s) and specify
>the dielectric spacing(s) on the fabrication drawing (preferrably in the
>cross-section view of the laminates). Keep into mind that the resultant
>impedance is affected by tolerance shifts in a) trace widths (typically
>+/-0.001), b) dielectric constant of the laminate and c) dielectric spacing
>(+/- 10% of material thickness). A call to your PCB fabricator can narrow
>down the dielectric constant of your laminate, as certain laminate vendors
>have certain dielectric constant "windows" specific to their product. These
>"windows" of laminate dielectric constant tolerance vary among manufacturers,
>but remain consistant within lots of a common laminate vendor. You may
>consider specifying the laminate source in your notes as well.
>2) Calcualte as in #1. Specify the required impedance of the controlled nets
>in a fab drawing note. Let the vendor work out the details concerning the
>dielectric spacing and trace width. This method is the more expensive and
>does not necessarilly give any tighter toleranced results, as the tolerances
>mentioned in #1 still apply. The reason for the increased cost is simply the
>ownership of the results. Give that ownership to the vendor and you will have
>to compensate them accordingly.
>
>Note: Since the trace width tolerance is based on etch tolerance (typically
>+/- 0.001), make your impedance controlled trace as wide as possible to
>minimize the overall percentage of deviation tolerance (e.g., 0.001 deviation
>from a 0.005 trace = +/- 20%; the same deviation applied to a 0.020 trace =
>+/- 5%). If you cannot achieve the required impedance from stripline (GND
>plane above and below trace), consider using microstrip (GND plane below or
>above trace only). Microstrip can achieve higher impedance results from the
>same dielectric spacing and works very well for most video and RF
>applications.
>
>----------
>From:  Donna Perry[SMTP:[log in to unmask]]
>Sent:  Friday, August 28, 1998 4:31 PM
>To:    [log in to unmask]
>Subject:       [DC] stripline specifications
>
>Happy Friday!
>
>NEEDED:  What specs/notes should be added to the fabrication drawing for
>producing stripline circuits?
>
>TIA to all who respond.
>Donna
>http://www.radioconnect.com
>
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