Yves,
The "magic" temperature for the inside of the component is 100C. 18 grams
of water (a mole, if you remember any of your chem) occupies 18 cc, while 18
grams of steam is going to occupy more than 22.4 liters! I say more than,
because we usually talk of a mole of gas occupying this amount of space at
what we call standard temperature and pressure (STP), which is 273K (0C) and
760 torr (1 atmosphere), but since at 0C water is not a gas, we need to look
at a higher temperature, so let's pick 100C, the first point at which
the water is all gas (Chemists, don't get picky on me wrt the all bit.)
Since gas expands with temperature, the amount is more than 22.4 L, how much
more I am too lazy to figure out right now. So even with a fraction of a
gram of water in a supposedly sealed but water absorbing component you are
going to get a 125 increase in volume, and thus CRACK! (or should I say
POP!).
So for Class 3 and poorer BGAs (and TSOPs too I might add) you have got to
watch your incoming handling, storage, time between machine introduction and
reflow and time between reflow and wave solder and rework is even another
kettle of fish.
Hmmm, I'm hungry. I'm outa here.
regards,
Bev Christian
Carrier Networks
Belleville
Nortel
> ----------
> From: Trudell, Yves [WNS:W460:EXCH]
> Sent: Friday, November 13, 1998 3:55 PM
> To: [log in to unmask]
> 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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