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Subject:
From:
Keith Calhoun <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Keith Calhoun <[log in to unmask]>
Date:
Fri, 19 Mar 2010 14:11:04 -0400
Content-Type:
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I have been involved in many MIL-SPEC environmental test programs.  We
always saw condensation during the temp/humidity testing.  We would just
design for this event, either sealed desiccated housings for our RF
sections or conformal coating, weep holes, etc for the rest of the
system.

Keith S. Calhoun
Product Engineer
SoPark Corporation
3300 South Park Avenue
Buffalo, New York 14218
Phone:  716-822-0434 Ext. 237
Fax: 716-822-5062
email: [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
Sent: Friday, March 19, 2010 1:39 PM
To: [log in to unmask]
Subject: Re: [TN] condensation

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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