Hi George,
Somewhere along the way, Andy's message got somewhat mixed up. There are
essentially three possible failure modes for PTH/PTVs. Listed in order of
decreasing importance, they are (i) barrel fracture, (ii) interconnect
separation (post separation), and (iii) corner cracking. Some of the steps to
minimize one failure mode may increse the failure potential in one of the
other modes.

All of these failure modes are primarily driven by loading during soldering
processes, in particular manual processes. Thicker PTH plating will reduce
(i), but increase (iii) and to a lesser degree (ii). Thinner, or no, inner
layers near the PWB surface will increase (iii) and (ii), because of
increased land rotation and thus larger bending moments at the attachment
points of surface and innerlayer lands; there is no significant effect on
barrel stresses (see Iannuzzelli, DEC). Thinner inner layers increses (ii)
also because of a reduced attachment area to the PTH barrel/cross-ection of
the foil.
Putting all of this in a practical context: by using a high Tg PWB polymer,
using E3 (HTE) copper foil, avoiding manual soldering operations with high
maximum temperatures, and having good production quality, you should have no
real adverse effect by switching to 1/2 oz. foil.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]