EE and EE technicians learn early it can difficult to locate a short circuit on a PCB.
New people to electronics will sooner or later be faced with the dilemma - how do I find a short circuit.
Failed components and poorly etched circuit boards can be the cause of short circuits.
Many short circuits are at such low resistance, you cannot use a common DVM’s ohms range to find them.
One method for finding a PCB short circuit:
Inject low voltage, ~0.3VDC, low current, ~300mA, between two shorted PCB traces.
A DVM, set to the 200mV range, is used to track down the location of a short between these traces.
0.3 volts is low enough as to not damage components including ‘input protection’ diodes.
300 milliamps is small enough that board traces are not damaged.
In the image below we have a short between +5V and GND.
We connect the above-mentioned power supply between point Z and point B.
Our DVM ‘negative’ lead is left stationary at point A, our DVM ‘positive’ lead is moved along the shorted trace.
Note: the RED bottom surface is our GND plane.
The actual millivolt readings are not important, it is the relative measurements that are.
We conclude that the short is somewhere near point H.
Looking through a microscope, we see an unetched copper filament connected between point H, +5V and our GND plane.
Keep in mind, a short circuit can be between: signal trace to signal trace, signal trace to Vcc or GND, inside components themselves, and between component conductive surfaces, screws and chassis.
Note: we often inject our low voltage trace current between a trace, the location which is not critical, and with Vcc and GND connected.
This method is sensitive enough to be used in tracing short circuit problems with flooded copper pours.
Since the voltage readings at point D is the same as point C, F is the same as E and I is the same as H, these branches have no current flow and are not the problem.
Our board is turned over and examined under a microscope.
See discussion at the following link for a DIY power supply schematic, post #670.