Hi Kai,

You should be able to protect the chip from ESD by grounding all the leads. You might fab something that clamps around/over the chip which does this, and which leaves a window (stencil) open to the area you want abraded. I have an Airbrasive micro bead blaster for such delicate work, in particular for cleaning electron filament housings, and I can tell you it would handily obscure the stamps on any plasic chip. If you don't have (access to) such a unit, you might be able to experiment with something like a sacrificial hobbyist's air brush.

On the other hand, if you already have, and are very proficient with, a milling machine, ground the workholder surface. Clamping the chip to the workholder surface should provide simultaneous grounding of all the leads. If you need to protect the leads from clamping forces, which is to say provide strain relief, use conductive foam. Just make sure that you dissipate any static buildup on the package surface before you remove it from the clamps.

The point is that you can protect the chips from ESD. Still, for concern over mechanical stresses of the package transferring to the lead frame and the chip itself, I would still vote for bead blasting.

Joe