I'm adding a heatsink between a transistor that was previously screwed to the chassis. Can I place the mica or otherwise insulation patch between the heatsink and the chassis, leaving the transistor in contact with the heatsink it's self? The heatsink isn't wide enough to fit the isolation pads I have, and I worry that if it is important to isolate it, and I trim down a pad, there will be a risk of contact to bare metal.
The risk you are creating is a bigger (and more accessible) piece of 'live' **metalwork.
If it was a TO220 for instance normally bolted down (with insulating washer) then just the tab is 'live' **, but with a heatsink added on the device side of the insulation you've created a bigger 'live' target ....
whcih may 'interfere' with other metalwork in the product.
If the washer is fitted between the device and heatsink, the heatsink is isolated.
(** Live - at the potential of the device tab)
If it was a TO220 for instance normally bolted down (with insulating washer) then just the tab is 'live' **, but with a heatsink added on the device side of the insulation you've created a bigger 'live' target ....
whcih may 'interfere' with other metalwork in the product.
If the washer is fitted between the device and heatsink, the heatsink is isolated.
(** Live - at the potential of the device tab)
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If you can't touch the heatsink when the enclosure is closed and the tab of the transistor (normally the collector) is at a reasonably constant DC voltage, then it will probably work fine. If there is signal on the collector, then your heatsink may become an antenna to radiate the signal to places where you don't want to have it. (Besides, you will have the capacitance of the heatsink loading a signal node, but that's probably not the biggest issue.)
Of course there's always exceptions to avoiding it: https://www.emcstandards.co.uk/files/emc_techniques_for_heatsinks_july_2010.pdf
