In this video I show my typical procedure while dealing with an iPhone 12 Pro that has spontaneously stopped working and has developed a motherboard problem. It is good to have a plan of action for each type of problem you encounter.
My first step is always to check the DC power supply amperage draw before and after prompt to boot.
In this case, I see small amperage spikes after prompting the board to boot. If you were to see this problem (without viewing further into the video) do you know what you would do?
My next step was to check the power rails for shorts. Small amperage spikes are often caused by some short power rail. In this case, all the power rails I normally check seemed fine.
I could have also checked for voltage, but if a pmic output voltage is missing (but not short) I usually see a "brain dead" amp draw rather then a small spike (brain dead meaning small but steady amp draw). This is not a universal rule, but it is most often the case.
After measuring power rails I split the interposer to view the board components further.
This is when I found a loose Boost Coil. Loose Boost coils are a very common problem on the 12 series. It is likely this is the most common cause of spontaneous board problems on this series. Knowing signature flaws for each model is important, but only comes with experience.
This was an easy find because I knew where to look. You would be surprised how many CPU swaps are performed as a result of something as simple as a loose coil. If you don't know where to look, this problem can easily slip past you.