Re: [SI-LIST] : Different board vendors different impedances

rbmccammon@mmm.com
Thu, 25 Mar 1999 13:15:31 -0600

Kim Flint wwrote:
>At the PCB West conference yesterday, during the
>course on Materials for High Speed Design (which
>was quite interesting, I wasn't familiar with
>much other than FR-4, but that's another topic....)
>a fellow in the audience happened to be from a .
>major fab house and made some rather interesting
>comments that seem to pertain to your questions.

>One was that FR-4 material is made by a variety
>of vendors, and it is not all the same. A fab
>house will be taking those differences into account
>for their controlled impedance calculations,
>depending on the material vendor they use.

That's what my vendors tell me also.

I often have a lot of lumped loads, which have considerably
more capacitance than the trace between them. In those cases
I opt for fixed separation, regardless or Er which gives me repeatable
inductance between my lumped loads.

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