Laser decapsulation - not so fast

After dreaming for many years about a cleaner way of chip decapsulation, I finally got my hands on a MOPA fiber laser. But like many things in this life, it apparently is more complicated than it seems. I went through ~20 chips trying to find pulse energy threshold that still eats through plastic, but keeps microchip alive. Unfortunately, at least for glass-filled plastic, I don't see any opening in the process window.


The issue is that while plastic is gone, so does all content of the chip. if you don't zoom too close - it looks like it's a success, die & bondwires are there, but it's a total failure. Alternatively, glass spheres just cover everything and "protect" material below.




Any hints, tips or tricks how to make it work? Or it only works for non-filled plastic?
I am thinking about air assist, oblique angle, increasing spot size (=larger focal distance lens).