Give 4 vendors board stackup information with a suggested thickness,
line width, and a required impedance to match, and you'll probably get 3
different possible stackups, with one vendor saying they can't do it
within a .062" thick board. Reasons vary, but the major one is the wide
variation in acceptable dielectric constants for FR-4. From what I've
gathered talking to some vendors, FR-4 just means flame retardant
fiberglass with a dielectric constant somewhere near 4, but that can be
varied between 3.x and 4.y based on the exact construction of the
insulating layer. x and y may vary per vendor.
Apparently there are a very large number of degrees of freedom in
construction, so much that every vendor has a different idea of how to
vary them to meet a given impedance.
-- Laurence Michaels magnus frater spectat te No solicitation, please.
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