Brian,

You want to specify what is important to the designed product, and leave as
much of the construction methodology to the fabrication facility. Obvious
items are overall board thickness, copper thickness on each of the internal
layers (external should have been covered in a note), and layer stackup and
identification. If a specific dielectric spacing between layers is important,
then it should be specified as well.

If your engineer has properly modeled and simulated a controlled impedance
design, then you may wish to control the construction by specifying conductor
widths and layer dielectric spacing requirements. Normally, you would specify
the impedance of a given signal on a given layer along with an appropriate
tolerance. This will allow the fabricator to adjust dielectric thickness and
conductor width for variations in his process, thereby assuring yourimpedance
requirement is met.

Most important is not to tell the fabricator how to build the product such as
in foil lamination or core lamination, but give him your end product
requirement. To be most cost effective, the construction option should be left
to the fabricator.

Regards,

Gary Ferrari
Executive Direcor
IPC Designers Council
860-350-9300

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