Re: 10kV here and 10kV there ...

MassiveProng wrote:
On 7 Feb 2007 13:04:49 -0800, "whit3rd" <whit3rd@xxxxxxxxx> Gave us:


On Feb 5, 3:52 am, "Winfield Hill" <h...@xxxxxxxxxxx> wrote:

Martin Riddle wrote:


Bertan also used RTV, but they allways forgot to evacuate the moisture
when filling. They had many QA issues, probably the reason they sold
out to Spellman.

Can we not assume that the tubes of GE clear RTV from the
hardware store are free from troublesome moisture?

Certainly we cannot. The common RTV adhesives require
moisture to cure, and emit acetic acid when they do cure.


Common SINGLE part RTVs.

Most mixed two part jobs are a lot more "neutral" in that respect.
They DO cure faster in humid climes though.

We vacuum our down, so I am reasonably sure all the water is gone.

Neither moisture nor acetic acid are acceptable here.


I never liked the taste of the two together much either. :-]


Potting compounds use other kinds of chemistry for this
reason.


If one places common heat shrink into many two part RTV compounds,
it will inhibit the cure. It is the sulfur content in the shrink
tubing. Kynar doesn't do it. Our compounds rep told us about it
after we experienced "issues".


If you try to exclude moisture and use the
hardware-store adhesive, you get a gooey uncured fill material.
Using the real commercial potting compounds, you get good results
and pay lots more money.

The old C64 computers had a potted power supply that was
famous for early failure; the potting compound often
shrank and cracked, fracturing components as it did so.


That sounds like an epoxy "solution". Epoxies are usually a lot
more firm after hardening. The problem is that their thermal
expansion characteristics causes huge shear forces across circuit
elements. Stycast is real good for shearing SMDs. One has to get the
right stycast, as there are several varieties. (about 4).

have you ever looked at the translucent sticky muck inside IGBT modules? I dont know what its called, but its very sticky - we referred to it as "Japanese snot".

Cheers
Terry
.

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