Inge is far too kind with those words! Brian Ellis does not have
endless knowledge and remember that a little learning is a dangerous thing!
He has set out the basics very well with the possible exception of one
factor; epoxy resins, like FR-4, do contain ionic salts which are
generally hygroscopic. With electrical resins, the salt content is very
low but not negligible and it could be that you have the odd molecule
sitting neatly on the surface as a nucleus to absorb sufficient moisture
to start the formation of a droplet. Of course, this is all part of the
energy equation is that nucleus cannot form without the correct thermal
conditions.
Of course, not all FR-4 resins are the same and some may contain more
ionic salts than others, but I can make no recommendations in this
respect. As a rule of thumb, those with the lowest dielectric constant
may be better than others, but this is not an inviolable rule.
Brian
Hernefjord Ingemar wrote:
> Wayne,
>
> it's not easy to make a reference, because all is about surface energy. Microscopically seen, two boards (non coated)may seem exactly the same, but one board can have a nanometer film of e.g. a oil, which will result in quite different surface energy. There may be numbers of reasons for the surface energy to vary, even if it's all about FR-4 or about solder mask or whatever. The cohersive forces that makes droplets to form is also depending on the microstructure, and even here, I think there are large variations. Moreover, the polar water molecules (that will make a droplet)are depending on the polarity of the object, which means the board can be more or less repellent. I'm afraid I can't help you, but there is a TN guy with endless knowledge. He may develop this topic.. if he got time. Brian Ellis.
>
> /Inge
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne - IIW
> Sent: torsdag 11 mars 2010 15:36
> 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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