Hi Glenn,
A couple of comments:
(1) there is now way I know of that any HAST of the type you describe can be
related to 20 years of operation,
(2) the water vapor pressure at 120C, 98%RH is huge and is likely to cause
all kinds of things to happen to your PWAs that would not happen in realistic
conditions,
(3) you must have an incredibly well controlled humidity chamber in order not
to get condensation somewhere in the chamber--and then of course all bets are
off anyway,
(4) a HAST really is designed to serve a totally different purpose than an
accelerated reliability test; HAST is designed to be used on prototypes to
make a design more robust, while accelerated reliability tests are designed
to cause damage by the same damage mechanism expected in operation or storage
on an accelerated basis WITHOUT the introduction of any extraneous damage
mechanism(s).

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]