[SI-LIST] Re: Effects of steam
- From: Kevin.G.Rhoads@xxxxxxxxxxxxx (Kevin G. Rhoads)
- To: John.Barrett@xxxxxx ("John Barrett"), si-list@xxxxxxxxxxxxx
- Date: 25 Nov 2008 07:17:59 -0500
---"John Barrett" <John.Barrett@xxxxxx> wrote ---
> Thanks to everybody for both the online and offline
replies + some contacts to follow up.
> What has been straining my electronic engineer's brain is
the concept that steam is not the same as water. Water, as
has been pointed out, is a terrible dielectric but steam has
a dielectric constant of 1.01 and negligible electrical
conductivity and, in a superheated environment, I'm assuming
it never becomes water - it's been helping me to think of it
as "water gas", rather than steam and therefore to look at
it as a gas absorption problem. I've found a link
(http://jjap.ipap.jp/link?JJAP/46/1553) which attributes
increases in polyimide conductivity (and, therefore, I
suppose loss tangent) on moisture absorption to increased
charge mobility and charge density linked to release of
impurities but does superheated steam have the same effect?
If it all comes down to molecules of water, whether they
come from liquid water, unsaturated or superheated
atmospheres, can we expect the effects to be the similar?
>
Yes -- expect the effects to be similar (disclaimer, I'm
not a chemist or a materials person, just an EE, but ...).
Your presumption about liquid H2O vs. gaseous H2O is
correct, as far as it goes. It really applied to water,
in whichever phase, in isolation. Water as it effects
the circuit boards is most definitely NOT in isolation.
The temperature/pressure phase diagram for H2O applies
only to H2O in isolation. On surfaces there can be
adsorbed layers that act quite differently. If you look
into the electrochemistry literature, in the Debye layer
immediately adjacent to a surface, water behaves much
differently (including lower dielectric constant) than
in bulk. Then there is the compact vs. diffuse double
layer -- and behavior is different in both.
Now in your circuit board substrate, electrical properties
will modulate as the hydration of the material changes.
As has already been pointed out, loss will likely become
significant, as well as changes in the lossless propagation
behavior.
If you really need to work well in such an environment,
I would suggest limit yourself to a 2 layer board on a
non-porous, inert substrate -- possibly a glass-ceram
or highly dense, non-porous ceramic. If you need more
than 2 layers, assembly a stackup mechanically - not by
chemical bonding. A coating may also be useful, but it
will be hard to find a coating that is completely
non-reactive AND non-absorptive in a superheated steam
environment. Perhaps a highly hydrophobic silicone?
Get some help from a real material scientist ...
Perhaps post on one of the Usenet electrochem groups...
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