Buried vias should only be used where the density of the board justifies 
their additional cost.  In general buried vias allow the board to be made 
with fewer layers but it is like adding additional double sided boards to 
the build cycle.  The fabrication losses on the via layers are like the 
losses on double sided boards and are very dependent on the lines and space 
widths chosen.  They are helpful if Z axis expansion is a concern and if 
circuit run lengths are critical they allow interconnection without 
returning to the board surfaces.  Once laminated the losses are just like a 
comparable multilayer for the continuation of processing both at fabrication 
and assembly.  We once had many designs using them but found our fabricator 
supplier base to prefer added layers to buried vias as capacity in their 
shops plus dollar return per square foot processed are important to them. 
 It is like you take a multilayer shop and ask it to run double sided boards 
mixed in with the normal product.  The drill and plating time is used up and 
they do not get the dollar return for it.  Plus it seemed like when we had a 
internal fabrication shop and made buried via boards you were always short 
vias when you scheduled the lamination area and had to wait for them to be 
made.
 ----------
From: [log in to unmask]
To: [log in to unmask]
Subject: Users/designer of buried via product
Date: Monday, September 16, 1996 1:10PM

I am interested in obtaining any information from companies which have
designed and used printed circuit boards with buried via construction.
Specifically the success or failure of such designs, yield loss at the board
shop and assembly levels.  Thanks in advance for your advice.
Jenni Marcy
Sr. Sourcing Engineer
Storage Technology Corporation
email address:[log in to unmask]
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