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> sample decapsulating, milling die from the plastic package with CNC, then etching with 104 percents boiling oleum

Then etching with... what?

Oleum. Fuming sulfuric acid.

https://en.wikipedia.org/wiki/Oleum

And apparently the percentage is strength relative to (non-fuming?) sulfuric acid (i.e. neat sulfuric acid without sulfur trioxide added) which is why it’s over 100%.

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