Its not beyond possibility that the soldering iron used has built up a static electric charge and that has been discharged through the chip.
Way back when I was doing mods to pcbs for BT, we had specially earthed soldering irons to prevent just such damage.
Edit to add following:
I regularly 'earth' my soldering iron by touching it on an earthing plate on my workbench.
This is simply a small piece of brass connected to the earth pin of a 3 pin plug - all other pins are removed to prevent inadvertent contact with Live and/or Neutral (in case anyone was worried), though if you want something a bit more sophisticated, these are available:
http://www.ebay.co.uk/itm/Bench-Mount-Dual-Ground-Point-Anti-Static-Wrist-Strap-Connector-/291577687509?epid=1338367404&hash=item43e3626dd5:g:C4gAAOSwXk5ZcCHZ
I also use one of these (or similar) when handling any ESD sensitive electronics:
https://www.ebay.co.uk/i/401109928443?chn=ps&dispItem=1&adgroupid=41767405657&rlsatarget=pla-327666149029&abcId=1129006&adtype=pla&merchantid=9702594&poi=&googleloc=1006674&device=c&campaignid=861364981&crdt=0
Usual disclaimers etc - not recommendations, just examples.