Steve,

Here is some more information that will explain and support what I said
earlier. There are always many ways to skin a cat, but only one that is the
industry standard.

The proper guidelines for bag sealing are actually specified in the JEDEC
standard EIA/JEP124 section 5.2.1 Packing moisture-sensitive components :
"...Partially or lightly evacuate the bag to reduce packaging bulk and heat
seal the bag as close to the end as possible following the heat sealing
equipment manufacturer's guidelines. The bag should not be completely
evacuated since this will reduce the effectiveness of the desiccant and
possibly damage the content..."

As far as I know there are two technical reasons to explain this :

1. With a tight vacuum, the desiccant is squeezed between the bag and the
stack of trays. There is no surface contact with any air that would allow it
to pull moisture from everywhere inside the bag.

2. A high vacuum will slowly pull the outside air and the humidity that it
contains through the bag. Over time the moisture content inside the bag will
increase faster than with no vacuum.

I am aware of some experiments that were made by TI a couple years ago. It
showed that when dry bags were opened, after 1.5 years, bags with low vacuum
had less than 10% RH inside, while bags with high vacuum had 15% RH inside.

Of course you could use a vacuum sealer with very low vacuum but not all
machine types can be set at low values. Many people use machines originally
designed for the food industry and even at the lowest setting they will pull
too much vacuum. Also I have seen that over time people tend to crank up the
vacuum level (that warm and fuzzy feeling once again). And trust me, you can
pull a high vacuum inside a dry bag. When you can read the marking that is
embossed in your JEDEC tray you know you're in trouble...I personally think
that it is safer to use a standard heat sealer and ask your operators to
remove the excess air before sealing.

And yes I know, many suppliers are not aware of this very important
specification. Feel free to point them to the right document.

Regards,

François Monette
Cogiscan Inc.
Tel : 450-534-2644
Fax : 450-534-0092
E-mail : [log in to unmask]
www.cogiscan.com



Date:    Wed, 1 May 2002 20:39:32 EDT
From:    [log in to unmask]
Subject: Re: vacuum seal and MSD

Hi Earl!

Nice procedure. But this thread has raised a few questions that I'm trying
to
sort out.

There have been a few responses that have said that vacuum sealing is
actually not a good thing. Then there was the response that you shouldn't
use
dessicants when vacuum sealing...these differing views have come from well
respected people. Which only confuses me more...

I originally thought that vacuum sealing was probably about the best thing
you could do to preserve shelf life...but now I'm not so sure...or shall I
say from the reponses from a few have posed some questions, and now I'm not
so sure....

Is there anything out there that can settle the questions about this issue?
Anyone?

-Steve Gregory-

------------------------------

Date:    Wed, 1 May 2002 22:52:44 EDT
From:    [log in to unmask]
Subject: [TN} Vacuum Sealing...

Why is this there this disparity? Don't understand it at all..

Why is there so many differing opinions concerning component storage?

Geeze, this is really weird! We don't know about this subject at all, do we?

-Steve Gregory-

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