Chosen Solution
Hi. I recently bought an iPhone 6 on eBay, but it won’t turn on. After opening it I checked the battery voltage and it was dead. I purchased a new battery and replaced it, but when I try to power it on nothing happens. Both interior and exterior of the phone are mint condition, no signs of water damage (marks are white, no corrosion), no signs of unscrewing. I tried charging it for a while, or connecting it to iTunes, but it won’t detect it. Any ideas of what it could be?
This is a belated answer so hopefully it can be of use to folks who search for and find this question. I would start by testing a bare-bones setup. Start by disconnecting the battery first (always!). Then disconnect the digitizer, LCD, front camera, rear camera and home button extension flex (see this guide). Now re-connect the battery; you should only have the battery and Lightning Dock flex connected. Connect a known-good, preferably Apple-original Lightning cable to an iTunes enabled computer. If you have access to a multimeter, you could measure the battery first. If it reads ~3.8V - 4.2V, then it is good. Does iTunes recognize your phone? If so, then you should re-connect one flex at a time until the phone stops working. I would start with the Home button flex first, then add the rear camera, LCD & Digitizer then from camera flex. If the bare-bones setup does not get recognized by iTunes, then you most likely have a logic board issue. You could send it (or find a local) to a repair shop that specializes in micro-soldering repair.
