I used to be an engineer, and later an operations manager, for one of
the largest manufacturers of semiconductors in the world. Most of our
components were plastic encapsulated with materials far more
sophisticated than the PVC, nylon, polypropylene, etc, commonly used in
general purpose enclosures and yet we knew for a fact that moisture
would very quickly penetrate the package and get to the silicon. The
key was to make sure that the manufacturing process was very clean and
that the encapsulating materials had very low ionic impurity levels so
that when the moisture did get to the silicon chip it wouldn't create
leakage paths and corrosion. In the early years of such semiconductors,
one of the more popular molding compounds was silicone based. It was
quite clean for those times but also very porous relative to other
materials ... it didn't do a good job of keeping out moisture but
neither did it trap the moisture in, and some of our most reliable
products used silicone for years before better materials were developed.
Anyone who thinks a plastic case is going to keep out water vapor is
kidding themselves, and the more tightly sealed it is the greater the
chance for it to eventually cause problems.
73,
Dave AB7E
On 9/27/2010 1:10 PM, TexasRF@aol.com wrote:
> And further, if you don't let the housing breathe, that condensation can
> accumulate and eventually fill it with water.
>
> Trying to seal a box like that will never prevent vapor laden atmosphere
> from entering the box but can seal condensation from escaping. Air apparently
> will travel through much smaller spaces than water.
>
> I have seen this process take place inside hollow Heliax center conductors
> and in a few days flood the downhill connector with water droplets. In this
> case, the answer was sealing the center pin as if it were a water pipe by
> use of plumbers tape around the center pin before installation.
>
> If you could actually totally seal the box in question, you might be able
> to prevent the problem. But, the problem is: you can't. And if you could,
> you have to evacuate all air from the box otherwise there will be some water
> vapor and condensation of that. You can't win!
>
> 73,
> Gerald K5GW
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|