Roger wrote:
I was visiting a local SGC dealer years ago as he was troubleshooting a
brand new antenna tuner that was not working. When he opened the
plastic case, that special RTV acidic smell almost knocked us over and
all the tinned circuit board traces and exposed wiring were corroded
and black. SGC claimed they used the wrong type of RTV on a batch of
tuners that was not for electronics use.

Most likely because they closed the cases before the silicone had cured,

That outgasing from a think layer will go on for many days which would
make it impractical to use in this case.

not realising they were trapping the acetic acid vapour inside. The same
can happen in balun boxes and the like.

In this case it would also depend on how much RTV, or how thick the
layer of RTV. As I said, the outgasing will go on for quite some time
and the thicker the longer it outgases. I'd not use the acid smelling
version from either producer on PC boards.

It would be interesting to know if there have been *any* cases that
didn't also involve some kind of closed situation, or possibly a layer
that was too thick to let the vapour diffuse out.

A thin layer should cause no problem if it is given time to outgas,
but a thick layer will most likely eventually ruin the PC board.

Agreed; but that isn't really the fault of the silicone material. It's
more like "misuse".

But why do we use silicone for coax/antenna sealing anyway? Anything for
which you might use silicone could probably be done a lot better using
hot-melt glue.

Ordinary hot-melt glue (amorphous polypropylene) has good RF properties,
is non-corrosive, is very sticky, seals very well to metals and
plastics, is flexible, has no problems about being used in thick layers,
sets in minutes rather than hours, has at least as good weathering
properties as silicone, can be removed by cutting, peeling or melting,
and is very economical.

You can often use it in the backyard without electrical power, because a
typical glue gun gives up to 5 minutes working time after having been
unplugged.

There's more information in the archives of this group, thanks to Barry
Ornitz, WA4VZQ.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek