Odd MOSFET Behaviour

Hi All,

I frequently poor epoxy over some logic level MOSFETs (PSMN2R8-25MLC, PSMN2R4-30MLD etc).

Recently I have a couple acting very strangely. Sometimes it struggles to power up. Sometimes it won’t turn off. Sometimes it acts fine.

Can the epoxy itself be the problem?

The Gate it is driven by an ATTINY pin.
There is a 900 Ohm resistor between the ATTINY and the Gate.
The Gate has a 13K Ohm pull-down resistor to ground

The Gate is less than a millimeter from the Ground, so I might expect them to turn off if they had a short to ground, but they often fail to turn off.

Any suggestion comments are welcome. Has anyone else had this happen with similar MOSFETS?

Did you test it before potting?

Yes it was working well enough before the epoxy, and I’ve used similar epoxy before.

So I wonder if it something that will happen sometimes, or if it should (as I thought) never be a problem.

George at Taskled says some compounds are capacitive and slightly conductive which might be a borderline issue. Or it could be a contaminant in that batch of epoxy.