Samsung s5 doesnt turn on, main board problem?

I had my S5 for 1.6 years now. I have to admit that I did abuse it several times by continuing to use the phone until the battery is completely dead. Not sure if it was the culprit for my troubles.

Last week I was using my phone and the charge I remember was 70%. Then suddenly the phone turned off and I have not being able to bring it back up. I tried to plug it to the charger and keeping it there for couple of hours, didn't work. I tried putting a another battery, didn't work. The phone was never water damaged or subjected to major physical abuse. Phone does not show any indication at all ( ie no display or lights working, no vibration or sound)

Went to Samsung service center and asked them to look at it. They told me they couldn't power it on, so most probably need to spend around $300 for a repair. So, I am trying to fix it by my self now and save the money.

I think its the main board but I realized there is a power pcb as well. So I am not sure which one to replace. Can some one advice me on this?