Heat sink compound

[Frederic Breitwieser]

>A salesperson at AutoZone tried to sell me some heat sink
>compound stating that it was a dialectic.  Of course I challenged
>the statement and he told me that he had used heat sink compound to
>insure good connections.
>Is heat sink compound a dialectic?  Curious minds want to know!

Most heat sink compounds are diaelectrics, which is an
electronic insulator, but a thermally conductive substance. 
Most components mounted on a heat sink either use the heat
sink as a thermal dissapation device, or, if the heat sink
is part of the circuit (in the case of much larger
electronic devices sometimes), the part is bolted to the
heat sink to ensure a reliable electronic connection as well
as thermal conductivity.

While in college, I worked for a small manufacturing company
in NJ, where I had the opportunity to assemble electronic
motor brakes.  The devices varying in size from a 120V 5A
capacity up to a 660V 120A capacity often relied on the heat
sinks (which were 1/4" aluminum "C" channel about 2' long,
8" wide, and 2" high on the larger models) as a primary
conductor.  What I found "interesting" is they were using 0
guage cable, yet holding the parts to the heat sinks with
severely undersized screws, which are severely narrower than
the wires they crimped and bolted to the heat sinks.  The
point of the story is they apparently had the same
impression as your autozone friend, that the heat sink
compound conducts electricity - the larger devices this
company just attempted to introduce into the market kept
failing due to the limited current capacity of the small
mounting screws between the part and the heat sink - since
the compound doesn't conduct electricity very well.

Aside from the "goo" type of heat sink compounds, there are
a few other choices.  THere are these little mica washers
typically used over the years underneath TO-3 transister
cases, acting as an insulator for electricity, but a
reasonable conductor of heat.  Not as good as the compounds,
but they are easier to work with.  Some manufacturers have
had problems with human-assembled products because some
products receive more heat-sink compound than others, and
depending on how far to the limits the unit was designed,
"Pop" goes the unit.  Radio Shack experienced this with
their home receivers back in the late 70's for a brief
period.