Chosen Solution
I had the iPhone 6s Plus since new over 4 years ago. The screen went black while using it with plenty of battery left and won’t power back on nor did iTunes recognized it. I even tried the soft reset by pressing the home and power buttons for a few seconds with no success. The original battery was less than 80% of max capacity. Replaced the battery with the iFixit part with the same results of not powering on. What else could be wrong with it?
I would try disconnecting the screen and cameras and see if iTunes recognises it as it could be a faulty part causing it. But I would say the phone has developed a short since the replacement battery should have booted the phone. You will need a multimeter and board view software to diagnose which line has failed. I would start with PP_VDD_MAIN, and PP_BATT_VCC Since these phones are getting a bit long in the tooth it might be time for an upgrade if you do not have the correct equipment to diagnose and repair the problem as it will cost about $100 for a shop to repair it.
Check the U2 Power Charging ic 1610A3 Tristar Chip
