The iPhone 6s doesn’t even have a U2 IC. The charging circuit isn’t based on a single chip, you have several interconnected and each of them is surrounded by components supporting its functioning. When/if the phone gets overpower/overvoltage a number of things can go bad and get burned. Changing arbitrarily one specific IC without knowing what’s broken is in my opinion unprofessional and just a gamble, besides a waste of time and money.