On Thu, Dec 04, 2014 at 09:13:06PM -0500, Jason White wrote:
> Hello, I am considering a project where a PCB with two MOSFETs is
> potted in a plastic box with a (unintentionally) thermally
> insulating plastic potting compound. There will be a small copper
> pour beneath the MOSFETs to provide a limited amount of heat
> sinking.
>=20
> Two questions:
> 1) There will be a small air pocket in the bottom of the product in
> which there will be no potting compound. [Any thoughts are welcome]
> Would it be thermally "better"/cooler to have the copper pour
> contacting this air pocket or might it be "better" to have the
> MOSFETs in this air pocket with their copper pour contacting the
> potting compound? (see attached diagram if the wording is confusing)

It depends on the thermal resistance of the potting compound, the air
pocket, the thin plastic shell, and the temperature of the air or
liquid beyond the shell and potting.

> 2) Is there/(could it exist) a practical upper limit that a
> temperature may reach before the plastic shell and potting would
> start to dissipate built-up heat from he MOSFETs? What I am
> imagining is that very little to none of the heat will escape the
> plastic and that this thing will quickly start smoldering.

Piece of string type question.

Both the shell and the potting will transfer heat, but the rate at
which they do so will relate to the temperature difference and the
insulation characteristics of the material.  The higher the
difference, the more the transfer.

This can all be calculated (not by me), or done experimentally, with
sensors inserted.

"Smoldering", or undesired chemical reactions, will occur at different
temperatures for different materials.  The MOSFETs will have maximum
operating temperatures, and maximum storage temperatures.

--=20
James Cameron
http://quozl.linux.org.au/
--=20
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