There are some basic best practice procedures when dealing with a phone that will not turn on. The first thing that I tend to check is what happens when your DC power supply is connected to the phone. If your phone immediately starts to draw amperage before prompting the phone to boot, then you have a main power rail short. In this case, the phone did nothing before prompt to boot, which is normal behavior at that point.
After prompting the phone to boot, this happens:
When prompting the phone to boot, sometimes it stays at zero, sometimes it flashes .43a, sometimes it even flashes other numbers like .13a. What this tells me is that the phone wants to start but cannot even get to the initial start up sequence. This typically indicates a different shorted power rail other then the main.
From prior experience, the most commonly spontaneously shorted line on the iPhone 8 series is 3v0_nand. A quick measurement of this line shows that it is indeed short to ground. To find which component is the culprit, voltage is injected into the shorted line on a frozen board to create heat at the shorted component and reveal the problem:
Remove the shorted component:
The phone will now boot to full functionality with data in tact:
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