Debugging Vcc GND shorts on PCB

[EEEnthusiast]

Friends
I just wanted to share one method to debug Vcc to Ground shorts on PCBs.

Many a times, we get a board back from assembly and find that the supply is shorting to ground. The bare PCB does not have the short, so it can only come from some component. If the VCC and GND are both power planes, and there are hundreds of components like ICs, decoupling caps soldered on them, it is very difficult to debug this.
For e.g. if a 0 Ohm resistor is used instead of a capacitor. Often, thermal analysis would not yield the point of short.

This may be a known method to many, but I just want to share it with anyone who is new.

Steps
1. Connect the VCC and GND points from the board to a power supply which is set to CC mode.
2. Feed a reasonable current which will not melt the board. 1A is a reasonable value for many boards.
3. Use a multimeter in voltage mode. Connect the negative terminal of the multimeter to the ground terminal on the PCB.
4. Probe multiple Vcc points with the multi-meter and record them on the gerber file. The L1 layer or Top Assembly layer printed on paper is the easiest way to record the voltages.
5. The readings will usually be in several milli volts.
6. After recording sufficient points across the board (10 to 20 points will suffice in most cases), draw a contour of the equipotential points.
7. The centre of the contour with the lowest potential, will be the point of the short.
8. The current flowing on the Vcc plane will cause a voltage gradient with its minima at the point of short to the Ground.

This is the cheapest way to detect the short. This is suitable for dead shorts. Shorting through ICs can be debugged by this, but there is a risk of component damage.

Comments are welcome...