To get an idea of metal thickness consider that a CD is coated with
50-80 nanometers of aluminum or gold. You can see through it when you
hold it up to a light source. 50 nm is one skin depth thick at 2
TeraHertz. Not much good for us. By the way, next time you get a CD you
don't need place it in a microwave oven and run it for 3 seconds.
Dave Cuthbert
From: [email protected]
[mailto:[email protected]] On Behalf Of Chris Maxwell
Sent: Friday, September 17, 2004 7:11 AM
To: John Woodgate; [email protected]
Subject: RE: Germanium & ITO Window shielding effectiveness
Whew,
Everybody is on the beam.
My two or three sentence reply has brought up a few questions. Let me
just say that I take no exception to anybody debating me regarding
electromagnetics...even though my surname is Maxwell. The genes have
thinned out over a few generations; so I make many "debateable"...even
"questionable"...or "wrong" judgements :-)
For Mr Woodgate: Did I mean mu-metal?
No, I meant monel. I put the question mark after it because I knew
that it was available as a shielding material; but unsure of its
permeability. According to Laird technologies website, monel contains
Copper Iron and Tin. I assumed that the Iron would impart a
permeability higher than 1.
Will mu metal shield low frequency better than monel? Sure. But I
haven't seen any mu-metal mesh screens for windows. I also don't know
how expensive a mu metal mesh screen would be if you could get one.
For Mr. Javor:
14 Ohms/square vs 14 Ohms /square inch... You're right, it's 14
Ohms/square. At what thikness of ITO? The usual thickness applied. I
don't apply it; you would have to ask someone that applies it...for
instance Chomerics (or whatever they are now called, these companies
change names faster than I change beard styles).
Will ITO provide some shielding? Sure 14 ohms/square will provide an
impedance mismatch with air. You'll get some reflection of plane waves.
But think about some of the other requirements of good shielding. A
well shielded enclosure needs to treat seams and aperatures so that they
won't radiate according to Babinet's principle. For instance, consider
a metal enclosure with an ITO coated window. As surface currents flow
on the enclosure, they will encounter the window. Most of the current
will flow around the window, since the conductivity of the metal is
orders of magnitude below the ITO. When the current flows around the
window, you have charge acceleration (it changes direction)...which
yields radiation. This is why I said ITO isn't so great for shielding.
It provides some, but not much.
Is 14ohms/square a higher conductivity than what's needed to dissipate
static.? Sure you could dissipate static with a lower conductivity
coating; but would it be as transparent as ITO? (Indium Tin Oxide by
the way for readers who are wondering). That's why I consider ITO a
good static dissipative coating.
How about some tips from experience, since we've used ITO:
1. ITO can cause rings to appear on a window due to refraction (you
know those little rainbow rings you see if you put a little oil or water
between two pieces of glass). This isn't always a problem. It can
happen if you sandwich an ITO coated window with another, or if you put
an anti-glare coating on an ITO coated window. We had the
anti-glare/ITO coating problem. Our display had little rainbow rings
all over it.
1-a. See above. We once had the rainbow ring problem and had a bunch
of windows we wanted to rework (i.e remove the ITO). The windows were
lexan with ITO coating. We had to scrap the windows since we couldn't
find a solvent that would remove the ITO while leaving the lexan intact.
2. ITO can be expensive, but any window shielding can be expensive.
3. ITO can be hard to bond to the bezel.
4. We were paying about 20-30 dollars for an ITO coated lexan window
over a 8" display. Once we passed compliance testing, we went back to
look at cost reductions. We tried a display windo with no ITO coating.
Our results were just as good (this doesn't mean it didn't provide any
shielding, it just means it didn't provide much; and the display wasn't
a problem area). We were able to eliminate it and bring our window cost
down to a few dollars.
5. ITO definitely has a low relative permeability; so it will have
problems with absorbing magnetic fields. ITO definitely has a low
conductivity, so it will have problems reflecting magnetic fields as
well. (it won't readily form eddy currents)
So, will ITO provide some magnetic shielding?...sure...but not much. To
me, it falls into the category of a marginal fix. If you have a window
that needs just a little bit of shielding to pass and you have no other
option...then ITO is OK.
However, if you need some serious shielding, you'll probably need to
look at a wire mesh window insert. If you need some serious magnetic
field shielding, then try to find a wire mesh window insert with a high
permeability.
Of course, these are just my own opinions.
Chris Maxwell
Nettest
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