I have conducted moisture removal rate determination using the methods outlined within IPC 1601 to compare the time required to remove 80% of the saturated weight from a given PWB using a 105 deg. bake versus a 90 deg. bake in a vacuum oven. I actually had to do this twice because a customer did not believe the results of the original bake qualification.
This particular PWB needed 8 hours of baking @ 105 deg. C to get to the 20% moisture level.
It required 7.5 hours of baking @ 90 deg. C. to get to the same level.
So the customer ordered us to run the test again at 105 deg. C in the vacuum oven.
It again required 7.5 hours to get to the same (approximate) level of 20%.
So we had to do this all again using different test specimens (different PWBs).
Same result.

We (and our customer) are now convinced that vacuum baking does very little to decrease either the temperature or the time required. It certainly helps, but not very much.
On the other hand, vacuum baking has been shown to possibly damage the PWBs if not done correctly. This is well documented.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of SALA GABRIELE
Sent: Friday, June 01, 2012 10:53 AM
To: [log in to unmask]
Subject: [TN] R: [TN] Vacuum Drying of PCB's

Hello  Rich,

by using Vacuum Drying method to dry-off the PCBs, compared to a classic baking method this one provides plenty of benefits in terms of time reduction for a cycle, vacuum helps moisture diffusion, also as we know moisture boiling point temperature under vacuum is lower then in open atmosphere that means thermal stress reduction on PCB/Laminates. 
Furthermore, by using Vacuum, other significant benefit is no oxidation of PCB metal surfaces, plus heavy Power consume reduction

In the other hands, cost and serviceability of vacuum machine should be more expensive then classic electrical oven. Also the cost should be higher.

Let's wait for someone  who experimented or are using VD.


Regards

Gabriele


-----Messaggio originale-----
Da: TechNet [mailto:[log in to unmask]] Per conto di Richard Kraszewski
Inviato: venerdì 1 giugno 2012 16.48
A: [log in to unmask]
Oggetto: [TN] Vacuum Drying of PCB's

I would appreciate anyone sharing their thoughts on vacuum drying of PCB's. 

What conditions do you use? (Time, temp and vacuum)

I realize it can be used to reduce required peak temperature but is there potentially any time reduction benefit to using vacuum?

I saw a paper from the "Electronic Components and Systems Division within German Electrical and Electronic Manufacturers´Association Working Group", entitled 

 "Recommendation & Guideline Values for drying for bare PCBs" 

The offered a rather paltry reduction in drying time of 60 minutes from 8 hours if vacuum was used. Doesn't seem like much of a benefit. 


Rich Kraszewski/PLEXUS 


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