One of the steps we use to diagnose faults on devices is to freeze them to -60 which can identify components generating excessive heat. In the video below we have an iPhone 6S that came to us “dead” (no power or charge).
We inspect all parts of the logic board at x75 magnification to look for any damage or liquid ingress. If there is nothing obvious, we then freeze the board with a chemical that takes less than 5 seconds to reduce the temperature to -60.
A voltage is then applied direct to the board to see if any of the components overheat, if we see something get hot very quickly this is most likely the component causing the issue.
In this case, a capacitor was short circuit resulting in the phone being not able to power on or charge. Once the component is replaced, the iPhone powers up normally and is fully functional.
These types of components are smaller than a pin head therefore require specialist tools.