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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Tue, 20 Nov 2007 15:58:50 -0600
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Correct. 
I should have clarified that the vacuum did not have much effect at both
the temperatures I was tyically baking at, and the amount of vacuum
being evaluated, which was 1/2 atmosphere. Perhaps at much lower or much
higher temperatures and with more of a vacuum the effect would be
greater. 
I cannot release my own process evaluation data, but I have found some
of the other studies and attached them below. I will try to find the
others and post them also.

In addition, the rates of adsorption and diffusion are listed in
IPC-EG-140, I believe.

http://www.calce.umd.edu/articles/abstracts/1999/ingresslam.htm

Moisture solubility and diffusion in epoxy and epoxy-glass composites -
all 3 versions >
LL Marsh, R Lasky, DP Seraphim, GS Springer - IBM Journal of Research
and Development, 1984 - portal.acm.org
... Moisture solubility and diffusion in epoxy and epoxy-glass
composites

Also



-----Original Message-----
From: Thayer, Wayne [mailto:[log in to unmask]] 
Sent: Tuesday, November 20, 2007 3:19 PM
To: TechNet E-Mail Forum; Stadem, Richard D.
Subject: RE: [TN] Vacuum Baking of MSDs

Richard-

Do you have some moisture loss curves to peruse, with and without
vacuum?

The article which Ben has referenced below is actually in favor of
vacuum baking.  The quote Ben has identified appears to be mildly out of
context.  Just prior to that quote the author states that vacuum was the
most convenient way to reduce relative humidity.  So it seems the author
is making the point that it's not vacuum actively "sucking" the moisture
out, or that it is dropping the boiling point below 100C, but that it
drops the relative humidity to an extremely low value, which is
important because the rate of moisture removal drops way off when the
moisture in the package gets close to being in balance with the
surrounding relative humidity.

Wayne Thayer 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, November 20, 2007 3:28 PM
To: [log in to unmask]
Subject: Re: [TN] Vacuum Baking of MSDs

It depends.
That statement appears to be very true, and is in agreement with
exhaustive qualification results done here and at other companies.
Below 100 deg. C, the boiling temperature of water, the rate at which
moisture is driven out of a component (or a circuit board for that
matter) rapidly decreases. Vacuum does little to increase this rate of
removal. This can vary, however, for different component types and also
for different PWB types, but the curves across the entire spectrum are
very similar. This is why many companies use a standard bake
time/temperature of 8-12 hours at 105 deg. C. I have found that this
bake process nearly always results in very acceptable drying times
without being so aggressive as to damage or reduce the solderability of
components and PWBs, with the possible exception of immersion tin
finishes on PWBs. My experience has been that even immersion silver
finishes hold up very well with this bake time/temp combo with very
little reduction in solderability, as long as they are properly packaged
and stored prior to baking in preparation for assembly and reflow. See
IPC 4552.

The referenced study mirrors qualification results I and others have
done in the past. All of the studies I have seen show that after
approximately 80% of the saturated moisture content is removed, the
charts showing the rate of moisture removal change from a sharply
vertical upward curve to a very gradual horizontal taper with or without
vacuum. 80% of moisture removal takes place in the first 8-12 hours of
baking, again with or without vacuum. Typically, the time to remove the
last 20% is more than twice as long as the first 80%, and vacuum does
little to improve that. All of these studies seem to show that if you
get at least 80% of the moisture out of either a component or a circuit
board, and keep it out by proper dry storage until just before assembly
and reflow or rework, you will very likely have no component or board
delamination issues. So vacuum has little added value.

I need to qualify the above by saying that with lead-free reflow
temperatures, even a moisture content of 20% can cause serious damage.
As Werner Engelmaier has pointed out on this forum several times, the
amount of water pressure does not rise linearly with increase in
temperature, it is an exponential rate. So even a small increase in
reflow temperature can result in a tremendous increase in internal steam
pressure, enough to bring a component over the edge of delamination, or
result in a PWB experiencing damaged vias from Z-axis expansion, even
though it may not fully delaminate. And this can go undetected until the
assembly enters its service life in the field.

But there are times when any baking can harm certain components. Perhaps
only in these exceptional cases should a vacuum bake be considered, as
it may shorten the exceptionally long bake times enough to make it
worthwhile. Another case where vacuum baking may be helpful is in the
case of components such as certain silicon chips that require a long
bake time even above 100 deg. C in order to achieve a moisture content
below 5%. You would be surprised at how a piece of glass can suck up
moisture from the air, and how little moisture (even 7%) will cause it
to crack during standard reflow. I have seen vendor-recommended
vacuum-bake times of 72 hours at 125 deg. C. for 1/4" square flip-chips.


They turned out to be justified.



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gumpert, Ben
Sent: Tuesday, November 20, 2007 12:27 PM
To: [log in to unmask]
Subject: [TN] Vacuum Baking of MSDs

Anyone want to make a comment (to support or refute) on the following
statement regarding moisture sensitive parts?

"Contrary to some claims, vacuum was not found to accelerate the rate
that moisture is withdrawn from a plastic package."

From the following article:
http://circuitsassembly.com/cms/content/view/5610/95/

Ben

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