> >desired finished board thickness. The general consensus was that
> >unless there is a good reason for specific materials and thicknesses
> >(for impedance control) it would be best to let the vendor decide
> >the best stackup for a particular design.
> >
> >only himself to blame. If he calls out only the desired impedance
> >and lets the vendor "tune" his process for it, the vendor is to
> >blame if it comes out wrong.
> >
> >instead of just "throwing it over the wall". Yet when you try to get
> >educated, you are told to "leave it to the vendor".
> >
> The 2 different approaches are not totaly opposed to each other. I 
> theoretical, not the exact process in each individual shop. The vendor
> himself must be allowed to make adjustments for his particular 
> Document the "END" product, do NOT document how the vendor gets there.
> 

	All,

	Our attempt at reconciling these worlds relies on two method-
	ologies:

	First, many applications do not need specific impedances, only
	uniformity, and hopefully within a rational range.  For these
	cases we call a controlled stackup, using commonly available
	materials.  This method provides uniformity layer-to-layer and
	lot-to-lot.

	In situations where a genuine value is needed, we can only
	"open a window" for the fabricator to apply their specific
	needs.  So we specify the end product impedance, but have done
	several stackup values to be certain what we ask is possible.
	When the windows get too narrow, we'll involve the fabricator
	prior to routing.

	Best regards,
-- 
 
      Jeff Seeger                             Applied CAD Knowledge Inc
      Chief Technical Officer                      Tyngsboro, MA  01879
      [log in to unmask]                               508 649 9800

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