I haven't seen anyone mention the aspect of vacuum oven baking that I have
used very successfully in the past.
In order to speed up the diffusion out of the substrate, the vacuum oven
usually has bleed ports which can be plumbed with dry nitrogen gas.
The nitrogen flow is adjusted only to the point that the pump can maintain
the 25 inches of water.
Effectively a flow of dry gas removes the water molecules from the surface
of the substrate and therefore permits the drying to be accomplished in
less time since equilibrium state is never reached.
I once had military pwbs which were full of moisture and used the vacuum
ovens to rapidly extract the moisture out of them.
At 03:58 PM 11/20/2007 -0600, Stadem, Richard D. wrote:
>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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