An oven is (typically) the best place to allow a circuit board to cool down, because the average oven chamber contains very little moisture while it is going from, say, 105 C to 23 C. At that point the board should either be processed through reflow within a couple of hours or moved to a drybox until ready for processing.
I have checked this by installing a humidity datalogger with alarms in an oven to qualify the practice. Even in older ovens that are not very well sealed, where the blower intake is open to factory air, the air inside the chamber remains well below 30% RH for at least 20 hours in a factory kept at (for this example) 55%RH. I have tested Shel Lab models, Blue M ovens, and many other ovens with the same results.
So it is a good practice to leave the boards in the oven while cooling down, and then move them to a drybox or into production as soon as they have reached room temp. Of course, you should verify your own ovens hold a dry environment, and for how long.
So, to answer your question, it is true that a hot board moved immediately into a drybox will re-absorb some moisture simply because it is cooling down during the short time it is exposed to 55RH, but the fact is that the hot board will cool down to 23 C in about 10-15 minutes so it will see very little reabsorption during the short cooldown. Most circuit board assemblies will recover at least 80% of their saturated moisture content within 4-6 hours if left out (at 55%RH). While it is true that the rate of reabsorption is higher during the time they go from 105 C to 23 C, that is such a short time it does not make a significant difference.
And contrary to popular belief, immersing the board in water for a short wash does not significantly increase the moisture reabsorption rate. FR-4 and polyimide PWBs absorb moisture from humidity at a faster rate than from direct contact with water. Try it yourself and see.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Friday, February 16, 2018 7:12 AM
To: [log in to unmask]
Subject: [TN] IPC 1601 board cooling
There is a clause in the IPC1601 that doesn't seem right to me. It says:
The material (PCB) should be cooled as rapidly as rapidly as possible in a
dry environment. Hot material in a humid environment will rapidly reabsorb
moisture. For best results, cool in the oven if possible . . .
Usually, my intuitions align with science. But, it seems counter intuitive
to me that a hot absorbent material would absorb moisture form a cool humid
environment faster than an absorbent material at the same temperature as
the humid environment.
Wouldn't it make more sense to take the boards from the oven to a cool dry
environment?
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