iPhone not powering on.

I've been dealing with iPhone repairs for a few years but this one stumps me. I have a iPhone 6 that was initially mine and passed down to a few siblings and had a few screen and battery replacements. But on the last battery replacement the battery seemed to act weird. Put old one in was fine but didn't hold charge as well. The phone ended up just up and dying. Wouldn't charge. So I got the new battery from warrenty and tried that. Still won't power on. Got a known good battery still won't power on. Had different sceen that was good still won't power on. Replaced charging port still won't power on. Tried hard reset and all still no life. How do I figure if the logic board is my last issue or is the main issue? That's the only thing I haven't had to replace on a iPhone. Thanks in advance.