My local phone / computer repair shop just confirmed for me that the solder used to connect the micro USB port to the motherboard is a higher temperature solder than what's used for the other components, intentionally to make it darn hard (nearly impossible) to replace this “wear item” in order to sell more phones.
This shop won't even attempted this repair any longer after several failed attempts and they consider themselves experts at micro soldering.
Sucks that phone companies have all figured out this dirty little bit of truth (that we are not surgically precise in our connecting and disconnecting our cords) and so they are getting away with building in and selling planned failure.