If you have an iphone that doesn’t turn on and doesn’t draw any amps through the usb charging cable, then one of the first things that you should check is a short on PP_VCC_Main, which is the main power line for iphones. Here’s a picture:
In the ZXW Tools picture, you can test for a short on VCC_Main by flipping the logic board over (on an iphone 6) and put your multimeter in diode mode to test for continuity. Continuity testing is essentially testing to see if one point on a logic board is connected to another point via something metallic. If the two points are connected, then your multimeter usually beeps to signify continuity.
For a short, a beep is bad because it says that one of the components, like a capacitor or a resistor, is not doing it’s job correctly. It’s good for us repair technicians because it tells us that something is broken along that power line.
Here’s a short video of me finding a short to ground on PP_VCC_Main on an iphone 6 and resolving the problem through deductive reasoning.
Since there are so many components on the PP_VCC_Main power line, it’s not very efficient to test every component one by one. If you do that, then the time cost outweighs the cost of the repair, so you have to find ways to maximize efficiency, if you want to make money in this business.