iPhone 6 not booting

The first thing to consider is that possibly one of the IC replacements didn't work out, either through improper soldering or a defective component. The PMIC is not underfilled but is still a trickier job than a Tristar or Tigris.

At this point, you need to start probing the circuit to see if anything obvious needs to be addressed. Check all of the PMIC outputs. You could check for shorts, do diode mode measurement or read the voltage of the powered-on logic board. Whatever technique you prefer is fine, they each have their advantages and disadvantages. Don't neglect the Reset and I2C lines.

You should also check the entire Tristar and Tigris circuits.