Several S7 died completely

So i bought 6 s7 for cheap and most of them had good screens when inspecting. I also had one that was freezing when heating up so i have 7 in total. So i charged one that was in the best condition and got stuck on charging logo and after force restarting got serious artifacts on the motherboard. Opened the phone, screen still works. Removed wireless coil, speaker, antenna. Still ok. After switching to a working motherboard screen stops working. I tried everything but got no result. Same thing with every phone. When charging they slightly heat up after 10 - 15 min. So how can i make atleast 1 working phone? please help