I considered using hot air, but the chip is very close to a CC2520 radio chip (QFN package) and a PIR sensor, neither of which I want to expose to much heat. While researching alternative techniques I stumbled across Chip Quik. At €15 (available from Mouser and Farnell/Element14) I figured it was worth giving it a try.
Basically this is low melting point solder. It takes long enough to solidify that you have enough time to wet all the pins and push or lift the chip.
The kit comes with about 1m of this special solder wire, a tube of flux and alcohol wipes to remove the flux residue. The procedure is simple: apply flux to the pins. Then apply this solder. Run the soldering iron around all sides and just push or lift off the chip.
This is a video I recorded from my USB microscope of my very first practice run. The board is a controller from an old 3.5in floppy drive.
It works as advertised! I was surprised at how easy the chip lifted off. The next step which I didn't show is to clean off the pads with soldering wick. Then use the supplied alcohol wipes to clean the flux residue from the PCB (and the chip if you need it again).
What is this alloy? What is its melting point? The packaging made no mention of this. Being the curious type I got my Steinel heat gun and started at 150°C expecting to be working up the temperature scale. Instant liquid! 120... still liquid... 100 ... 80 ... 60 .. and finally at the 50°C it solidified. Wow... that's not much higher than the melting point of butter! Doing a bit of googling I found a page on their site which has some details: it's a bismuth, tin, lead and indium alloy with a melting point of 60°C (139°F).
Conclusion: if you need to remove a chip without disturbing neighboring components, this seems like great product to use.