Electrical – STM32 MCU gets shorted

I've decided that breadboarding ahead of time is a waste of time unless there are aspects of a part's performance that you need to measure yourself.

I even think building the board in pieces is a waste of time. The initial hardware debug just takes a day or two. Any subtle issues won't show up until software integration anyway. I'd rather debug one circuit (the complete one) than several ad-hoc reductions of the circuit.

Just build the thing! Assemble the whole board and power it up. Look at the supply voltage. How many mA does it draw? Which part is burning hot? Which part is warm? Then try to burn firmware on to whatever microcontroller is on the board. Then start writing firmware. Get the clock going and toggle a pin. Get serial (or whatever) communication up. Now write test programs for each peripheral. Then build the production test fixture and start writing the "real" firmware.

Edit:

On finding stuck rails

If a supply rail is stuck to ground, (measures 0 ohms with an ohmmeter), power it up on a bench supply. Set the voltage normally and the current limit to a few hundred mA. Print out the PCB design on paper, and find a DMM that measures microvolts. Measure microvolts starting at the supply terminals, and write down the voltage drops on the PCB printout. By looking at microvolt differences, you can trace exactly where the current is going without depopulating or chopping up the PCB. This technique is a better than using an ohmmeter because you can pump a lot of current through the circuit, more than any normal ohmmeter will supply.

A similar technique works for finding shorted traces on an otherwise populated board. Run the board a use a scope to find "digital" traces that are in an in-between voltage range.

Finding a short on a board can be a challenge. One technique involves injecting a fairly high current into the board and then using a micro voltmeter to profile the voltage drops across the board to zero into the key area. I've tried this several times with mixed success.

One time some 15 years ago we had done a board spin on a fairly complex board that was full of digital, telcoms, audio and video in a mixed signal design. A half dozen of the new boards were built and in initial testing we noticed injection of digital noise patterns into the audio output from the board. It became clear that someplace on the board a digital GND was shorting into the audio GND. (Note that the audio GND and digital GND had to be tied together at one point under the audio chip). Finding the improper GND connection ended up being a fruitless exercise over more than a day of investigation. In the end I determined that I needed to sacrifice one board and used a Dremel tool to grind the board surface in two places down into the digital GND plane and the audio GND plane. Two 10 AWG solid copper wires were then soldered down to the GND plane copper (that took a lot of heat to achieve). I then shorted the two heavy copper wires across a fully charged 12V car battery allowing many many Amps to flow into the board. It took about 12 seconds for the short to reveal itself. A small plume of smoke rose up out of a via hole. In this case it was an improperly placed digital via that encroached on the audio GND plane pour. PCB DRC failed to flag an error because as far as it was concerned GND = GND.

The lesson learned was that it is a good idea to consider isolating two GND areas like this through some ferrite beads. Sometimes zero ohm resistor components can be used for the same thing. This makes the two GND areas separate nets on the PCB and the DRC checking can flag shorts.