iPhone 6 not turning on issue

Just about anything could be wrong...you will need to isolate the issue a bit more if you hope to get a better answer. Q1403 is responsible for switching PP_VCCC_MAIN from either the battery or usb connection. That means that anything downstream that is pulling excess current will over heat the MOSFET.

You need to check VCC_MAIN & BATT_VCC to see if they are (partially) shorted and if not then go and look at all of the voltage rails generated by the PMIC (U1202). Something is pulling excess current, you need to find out what it is.