Hi Glenn,
Personally, I'd be very wary of doing this, mainly for the points that
Werner raised. I'd add that in my experience it is very difficult (if not
impossible) to try to determine acceleration factors to mechanisms driven
by humidity. A major reason for this is that the amount of moisture either
on a surface or absorbed by a material generally is not linear with
respect to %RH, and that several humidity-driven reactions require some sort of
critical level of moisture before they kick in. You may cause some
reactions to be operative in a HAST chamber which may never occur in an
infinite life under "controlled" conditions.
I tend to question blanket "acceleration factor" statements which cover a
broad range of potential mechanisms. Kind of unlikely that they'd all be
governed by the same one! Tempting to try to apply, though.
I don't really know what kinds of tests I'd propose to do. I'd be hard
pressed to do a HAST test and would be cautious about doing any kind of
humidity-based tests. I would probably consider checking out solder joint
reliability after such a long period of time,
particularly if your joints are on the thin side. I sympathize with your
situation!
Best of luck,
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA
[log in to unmask]
----
glenn.pelkey wrote:
>Hi Werner,
> Thanks for taking the time to respond to my query. Let me
expand on
>the rational behind our plan.
>
> Our product is planned to sit in a controlled environment for up
to 20
>years before being activated. We found an article that addresses a
similar
>storage condition for plastic encapsulated microcircuits. But, the
product we
>manufacture has more components than just PEMs (i.e. chip caps and
laminate
>substrate). The article gives an acceleration model that represents the
>influence of temperature and humidity and sample test condition of 130 C
and
>85 %RH. We wanted to avoid a 130 C temperature due to the Tg of the
base
>laminate...getting too close for comfort. That's when we looked at
lowering
>the temperature to 120 and increasing the humidity to maintain the
>acceleration. It sounds like you're suggesting 98 %RH may be too hard
to
>control. Maybe we should look at lowering it to 85 %RH.
>
> So, I guess I'm looking for any objection to following the same
test
>methodology for this more complex package. Now with a condition of 120
C and
>85 %RH
>
>Thanks again for any input.
>
>Glenn Pelkey
>Quality/Reliability Engineer
>
>
>Ref: IEEE Transactions on Reliability, Vol. 45 No. 1, 1996 March. "The
>Appropriateness of Plastic Encapsulated Microcircuits in a Specific
>Wooden-Round Application," John R. Gardner.
>
>[log in to unmask] Wrote:
>|
>| 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]
>|
>
>
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