Hi Wayne,

Pass a test at 95%?  The problem that comes to my mind is that one cannot reliably create a 95% R.H. environment.  40C/93% R.H. is pushing the limits of (any) national metrology.  85/85, while used for decades, is not traceable because the environment cannot be controlled well enough.  If you have trouble falling to sleep, NCSL RISP-5 is the international standard for intrinsic/derived RH.  Thunder and NIST will get you to 72/85.
Anyhow, the point is: at elevated temperatures, the dewpoint is too close to the nominal system temperature.
As boards and assemblies have relatively low thermal diffusivity, 95% and 100% can't reasonably be differentiated.
From my experience with T/H/B testing, I would say that this far outweighs the surface phenomena.
If you want to pass a 95% test, you should assume that there will be condensation. 

Chris



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne - IIW
Sent: Thursday, March 11, 2010 9:36 AM
To: [log in to unmask]
Subject: [TN] condensation

Anyone know of a reference for RH thresholds for moisture condensation on an uncoated assembled PCB?  My intuition tells me to expect that condensation will initiate in tiny cracks at RH levels below 100%.  I found some references for this effect with some ceramics, but nothing on PCB assemblies.  I'm trying to advise my customer to conformal coat if he really wants to pass a test at 95%.

Thanks,

Wayne Thayer

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