iPhone 6 not booting

There are so many potential issues that could cause a "dead phone". There is no silver bullet or "solution", you're going to have to work hard on this one.

If you suspect a logic board issue, check PP_BATT_VCC, PP_VCC_MAIN (which you already did) and PP5V0_USB. I would start by checking to see if those rails are shorted to ground. If one of these rails is shorted to ground, then you will need to identify what is causing the short. It could be a bad decoupling capacitor, conductive debris or defective IC that is directly supplied by those rails.

Then you move onto the PMIC and check the voltage rails it generates:

• The PMIC generates ~15 voltage rails. They are all important (for obvious reasons) but the ones to check first are as follows: PP_CPU, PP_GPU, PP1V8_SDRAM, PP1V2_SDRAM, PP_VAR_SOC, PP0V95_FIXED_SOC, PP3V0_Tristar, PP3V0_NAND, PP1V8_ALWAYS and PP1V0.
• The PMIC also generates, what I would consider secondary, yet still important voltage rails for the following sub-systems: PP3V0_MESA, PP1V8_VA_L19_L67, PP3V0_PROX_ALS, PP3V0_PROX_IRLED, PP3V0_IMU, PP3V3_USB. PP3V3_ACC is not necessary.

If you still don't have any shorts, then look at the outputs from the Baseband PMU.