Neal, my assembly area is controlled to 35% RH, 20 deg C and as a result, I
also wondered if I could extend the floor life (exposure time) of moisture
sensitive devices.  Internal Nortel sources "figured" that I could safely
double my exposure times.  I looked at the logistics of how much I was
baking and what benefits might result from doubling the exposure time:
*       Save $ due to reduced baking "events".  I expected this to be
significant, however it wasn't, primarily due to the fact that we bake-out
devices in-house. Only about $3500 savings per year.
*       Improved inventory management, where devices that should be
assembled are sitting in an oven being dried out.  I had no dollar value for
this.
*       The potential (although it didn't apply to me) to eliminate baking
all together, and to not have to track/manage exposure times if you show
that you should never reach over-exposure based on doubling the exposure
time.

I also considered the downside of doubling the exposure:
*       The "buffer" that exists with the current exposure limits would be
gone.  For example, we only verify that moisture sensitive devices aren't
over-exposed at the start/end of a shift or the setup for a new build.
Devices could become over-exposed during the build.  Having the buffer in
place eliminates the need to manage/track exposure to a higher degree that
would prevent "in-shift" over-exposure.
*       We would be deviating from industry standard exposure times which
(although justified) could open a whole new can of worms if we were to run
into delamination issues.
*       Changing exposure limits away from industry standards could also
send the wrong message to the shop floor, since it was a challenge to
establish and execute a good program for the control and tracking of
moisture sensitive devices.

Being a quality engineer, I chose not to double the exposure time and to
stick with the standard; however, if a "must use" situation arises, the
buffer can be used as reason to deviate from the standard exposure time.

I have a couple of papers that I could fax to you (as far as I can tell,
they aren't confidential):
*       "Diffusion Model to Derate Moisture Sensitive Surface Mount ICs for
Factory Use Conditions". Published in 1995. Written by researchers out of
AT&T Bell Laboratories.
*       "Moisture Absorption and Desorption for Plastic Packaging".
Published in 1995.  Written by researchers out of IBM Canada (Bromont).

The quality of print isn't very good on these documents since they were
faxed to me, but if you want a copy, just send me your fax#.

Hope this helps.

Yves Trudell
Nortel, Wireless Networks Calgary
Quality System Engineering


> -----Original Message-----
> From: Neal W Driscoll [SMTP:[log in to unmask]]
> Sent: Wednesday, September 23, 1998 9:02 AM
> To:   [log in to unmask]
> Subject:      [TN] Moisture Sensitive Components
>
> Technetters,
>
> Industry standards (IPC, JEDEC....) for the "floor life" of components
> that
> have been shown to be moisture sensitive are valid in environments with a
> temperature of < 30 degrees C and a relative humidity(RH) of < 60%.  It
> also states that ambient conditions exceeding 60% RH will require the user
> to determine safe exposure times for these conditions.  Does anyone have
> any experiences with ambient conditions above 60%RH and the affect on the
> published floor life for the component?  Also, does anyone have any
> experiences showing that if the RH is below the 60%, the floor life could
> actually be longer that the published time frame?
>
> Any input would be appreciated.  Thanks for the help.
>
>
> Neal Driscoll
> Senior Manufacturing Engineer
> Rockwell Collins Avionics
>
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