Re: The goo inside of IGBT modules.

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Steve,

At 10:52 PM 9/14/2004, you wrote:
>Hi,
>
>Since there seem to be a handful of experienced power engineers
>around, i was just curious, what is the true function of the gel type
>stuff they put inside of big power semiconductors (ive seen it in
>IGBTs as well as SCRs).

"Experienced power engineers" use the gel for a wide variety of practical 
jokes!  If you throw it on the ceiling above you pal's chair, it will fall 
on him after a few minutes.  A paper towel stuck to it makes nice "lawn 
darts" for walls and ceilings too! :-))  It can be left around in odd 
places for people to unexpectedly grab onto...

>My thoughts were that it was to help contain
>the explosion when they fail.  Also, that it may prevent moisture from
>getting on the dies and shorting it all out (since the module casing
>is not completely air tight).

The gel has no thermal use.  It just protects the die and bond wires from 
moisture and thermal expansion and cycling.  It also vastly increases the 
flash over voltage on the device surface.  It does allow for an airspace 
above the die that does actually "cushion" the explosions (as opposed to a 
full hard plastic case).  There is really no way they could fill large 
devices with solid plastic since it would shrink during cure and the 
thermal expansion over such a large area would crack it all up.


>But what was more important to me was whether or not the goo serves
>any use in removing heat from the dies?  Im not sure if this gel is a
>thermal insulator or a conductor really... its hard to say.  Does
>anyone know for sure?

It is somewhat more thermally conductive than air.  But there is no "sink" 
for the heat to go to.  It's thermal contribution is slightly more than 
zero (I don't think they even bother to spec it...).  We always used Dow 6101:

http://hot-streamer-dot-com/temp/Dow-6101.pdf


E-bay is certainly the best place to find big IGBTs for the hobbyist.

Cheers,

         Terry


>Steve
>
>PS. anyone have any IGBT bricks for sale?  Im always looking for
>spares for my big coil!  Contact me off-list.