Benny Nilsson is correct in his comments. There are several factors that
come into play when determining total board thickness and tolerance, as
well as layer to layer thickness and tolerance. End usage of the pwb
invokes numerous design requirements for such factors as: Controlled
impedance, available materials, process variations, voltage requirements,
physical usage, etc.. Therefore, I feel this needs to be determined by the
designers and reviwed with the fabricators for produceability. These
thickness and tolerance requirements must be given in the drawing or P. O..
IPC-D-325 paragraph 4.2.5 and Table 4-2.A.4 show the need to identify these
requirements in the master drawing. Additionally, IPC-2221 Table 5-1 under
Board Thickness gives some basic guidelines for board thickness and it's
effect on fabrication. Other than possibly further enforcing within
IPC-2221 the need to identify these requirements on the drawing, I don't
see any other action necessary at this time. As a pwb fabricator, I can
report that most customers do a respectable job of providing this
information. We usually only need to contact the customer for these
requirements on quick-turn or engineering type orders. Oddly enough, these
are the orders that really need to have this information on the drawing to
reduce processing time. I'm sure I've missed valid points concering this
issue, and will watch for those points to be raised.
Tom Kemp
Collins Printed Circuits/ Rockwell Collins
Q. A. Manager