Mechanical shock: failed. Chip destroyed along with package. Will test with smaller hammers.
Thermal shock: mixed. Heating with blowtorch until red then dropped in water. Reasonably effective with DDR2 memory chips (large die, thin package). Separation of die from interposer still difficult.
Boiling in rosin soldering flux: failed. Was tipped off that boiling colophony (aka rosin) could destroy epoxy encapsulation. Sadly rosin flux was ineffective against all ICs tested. Will test with pure rosin when it arrives.
Paint stripper w/ methylene chloride*: success! Sanded flip chip from bottom until underfill was reached. Underfill treated with paint stripper and removed. Solder bumps still attached. Will test with other ICs when able. WARNING: stripper will build pressure if contained! Now using full face shield and goggles after accident!
*methylene chloride to be banned from paint strippers in US. Will test non MC strippers.