Decapsulation is a failure analysis step performed to open a plastic package for internal examination.
The Focused Ion Beam (FIB) tool can cut away (mill) material from a defined area with dimensions typically in square microns or deposit material onto it.
FIB and Decap is used to analyse or fix the chip.
For example, FIB can change the metal in the chip.
And befor the FIB, the package must be get rid of.