How to diagnose a "no power" iPhone
The most common manifestation of a motherboard problem presents itself in a simple form, a phone that will not turn on. It is VERY common for a symptom to be the same for a multitude of different problems. The SYMPTOM can be no power, but the PROBLEM can stem from dozens or hundreds of different lines. Therefore, it is useful to try and categorize “no power” into something that narrows down the range of potential problems. To do this, there are a few tools that can help. The most commonly used tool by this author is the DC Power Supply.
A good first step to diagnosing “no power” is to see what happens when we connect DC power to the battery connector. With DC power connected, there are a lot of different possibilities that we can see. In a normal, working phone we will see an amp draw only after prompting the phone to boot that gradually rises from zero to around 1 amp.
Immediate amp draw (before prompt to boot)
If, when we connect dc power to the battery connector, we see an immediate amp draw, this indicates we have a main line short. Most commonly, a full amp draw (2a or more) indicates that we have a short on VDD_MAIN. However, we can also see this type of amp draw stemming from VDD_BOOST, BAT_VCC, SPKRAMP_TOP_VBOOST, and any other line that ties directly into VDD_MAIN through any components. To identify the offending line, a multimeter is used to check the different lines for connections to ground. Once the line is confirmed, the hunt for the individual component starts.
There are a few different methods used for identifying a shorted component, all of which can be viable. In essence, something must provide voltage and something must show a heat differential. This author uses a combination of DC power to provide voltage (thus also provided heat) along with freeze spray in order to reveal the shorted component. If we freeze spray the entire board, then provide voltage directly to the shorted line, that voltage will move to and heat up the offending component. This reveals the problem by melting the freeze spray at that component. We can then visibly see the melted component. Once the component is replaced or removed, the line is clear and the phone will boot.
This specific problem can be viewed in full, from the finding of the problem to the solution at this video: https://www.youtube.com/watch?v=Xr3lcxkkOK8&t=1s
Zero amp draw (after prompt to boot)
If we connect dc power and prompt the phone to boot but see zero amp draw, that can indicate a few different problems. The worst case scenario would be that the CPU is dead, however, that is rarely the case unless the board has been overheated by another motherboard tech. Most commonly, zero amp draw will indicate a shorted secondary power rail.