First flckering,now not turning on.

I have an iPhone 6s that someone wanted me to fix. Said that lcd was not working. Swapped an new lcd and got flickering and stripes. Charged it for an hour and got 40% battery. Worked for a while. Friend took it home and said it flipped when charged,flickering screen and shutting down. He said there was a bad battery before. I took it back,swapped a new battery,nothing. Not turning on. Tried the old battery and lcd,nothing. Then changed the charging port,still nothing. I've tried everything,what is wrong with this phone? Soft reset not helping,tried for 5 minutes and still nothing. :(