Insulated and bare tabs of power devices

=20

Not necessarily available from a monolithic power amplifier.

?-)

The datasheet of the LM3886 discusses it at some length. It just doesn't mention any difference between the coated and uncoated types. My concern is based on the assumption that the two types have identical metal tabs and that the the insulated type simply has a layer of epoxy coating on the tab. If this assumption is true and both types are mounted _directly_ on a heatsink, then the coated type will definitely have an extra thermal resistance compared to the bare metal type.

When I asked about the case-heatsink thermal resistance, I meant the metal tab-to-heatsink Rth in _both_ types. I probably created some confusion by failing to make that clear. Sorry about that.

In general, yes. But there are a few partial exceptions like mica, beryllium oxide, anodized Al, silicone-rubber, thermal grease which are used for their relatively low thermal resistances. I don't have any data about the epoxy encapsulation used for semiconductors, but I'd expect them to have better thermal conductivity than other common plastics, though worse than metal.

Diamond?

snip

Diamond is better than silver. The relationship between thermal and electrical conductivity only holds for conductors.

Why can't we buy diamond heatsinks? Better yet, isotopically pure diamond heat sinks.

BeO is just about as good a thermal conductor as pure aluminum, and much better than common alloys. AlN is close to BeO.

There is only one actual exception, has excellent thermal properties comparable to copper, AND excellent insulation properties perhaps similar to mica. Is anyone brave enough to "out" this material?

BeO, or diamond, or better yet isotopically pure diamond.

AlN isn't bad, close to common aluminum alloys.

There are some forms of graphite are better than natural diamond, but aren't electrical insulators.

Like diamond?

Cheers

Phil Hobbs

Which is why I've been moving from the TO262 style to DFNs, and such, with heat slugs. They're much better packages. Smaller, too.

Read it again. He specified mica between the metal. It still might not be true, but your answer doesn't make sense.

It may be that they seriously tried to equalize Theta(j-tab) between the two for the first data sheet. No bets, no promises.

BeO is the only partial exception, look up the materials properties, and it has toxicity issues. The fundamental issue is to always keep the insulators very thin, mica does well at this but is fragile. Same with light anodize, heavy anodize can be a good compromise if sufficient care is taken not to break the anodize (it is still very thin). Grease helps but has a bad tendency to leak / vaporize away. Epoxy and silicone usually hurts thermal transmission. It is quite simple to look up the thermal properties of the various materials and calculate the effects.

?-)

=20

I find it more fun to actually get the numbers.

Several materials here:

BeO and AlN here:

Cheers, ?-)

welding)

as

Oops. But mica sure isn't a top thermal conductor (it is a glass).

?-)

welding)

as

Ralph beat you to it. Kind of costly though. All compared, look to AlN.

?-)