If I had to guess I would say the majority would agree on 125 deg C.  This
is the temperature that the dry bake is done and it could obviously be done
quicker if the bakeout temperature was higher.  However the world has
settled on 125 deg C so the packaging engineers design their packages to
handle a quick ramp to 125 deg C.  Above that and your on your own if you
rework without a dry bake step.  I'm not saying it won't work but you will
have to qualify the process (and assume the risk) if you heat the package
body to more than 125 C without a long dwell at 125 C like dry bake
provides.  You may want to consider an abbreviated bake out of 8 hours.  If
you know the components are not "saturated" you may be able to qualify a
shortened bake time.  Many of the semiconductor companies do this after
electrical test if they know the exposure is predictable.

My experience tells me that it is the package temperature that is of concern
here.  The localized heating of the leadframe can usually be ignored.  Its
the plastic and anything else inside the package which can absorb moisture
that causes the problem.  Be sure to use a fast acting thermocouple when
taking the body temperature measurements.

I hope this helps

Russ Winslow


-----Original Message-----
From: Yves Trudell <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Friday, November 13, 1998 1:46 PM
Subject: [TN] Moisture Sensitive Devices


>With respect to a saturated moisture sensitive device, what is the maximum
>temperature (and ramp) that the device can reach before it becomes damaged
>(delaminated)?  Most references just say that you shouldn't put it through
>reflow. Our ovens are set to max out at 215 deg C. So, if I brought the
>saturated device up to say 200 deg C (which is less than our reflow
profile)
>then would I damage it?  If I had to guess, I'd guess that we should be
safe
>as long as we're below the glass transition of the plastic.  But  If there
>is a general "magic" temperature for a saturated device, is it only
>important that the plastic over the die stay below this temperature, or is
>it the plastic over the die and leadframe (this is important for repairs
>using soldering irons or hot air pencils to touch up solder joints).
>I'm not looking for a quote of an acceptable oven profile for smt devices
>that are not saturated.  The reason that I'm talking about saturated
devices
>is with respect to PCBAs that may spend time in a debug queue that
>eventually get hit with a hot air pencil to touch up solder joints.  We
>don't want to send delaminated devices out the door.
>
>And NO, I don't have a Sonoscan... although I wish I did.
>
>Yves Trudell
>Nortel, Wireless Networks Calgary
>Quality System Engineering
>
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