In this video we are showing how to fix doesn't power on iPhone 6s by re soldering HDD IC.
The question is how does i diagnosed this phone?
The user informed me that the phone was bricked by dropping it.so first i Connect it to iTunes and it connected automatically in DFU mode with no serial number. Indeed the serial number is programmed in HDD(flash chip).so it indicates that the issue was with HDD(flash chip) because the phone was dropped by user. It means the HDD(flash chip) might be disconnected with PCB so I removed the HDD(flash chip) and resoled (reball flash chip) it.
The whole process of resoled you can see in this video. After this process the phone turned on directly without any software interference.
NOTE :- This work should be done by only professional technicians other wise you gonna brick your phone