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Debugging Your PCB

Your company’s new circuit board does not work, and it’s your job to find out why. Where do you start? First, remember the best advice in solving a problem is to cut it in half, and then cut that in half. By cutting your problem in half, you will constantly reduce the size of the problem and more easily determine where the actual fault lies.

Step 1: Debugging Your PCB

A useful trick is to consider building two PCBs:

PCB

Design Fault

Test PCB 2 and compare to the original circuit board. If both boards have the same problem, then it’s a design fault in either the PCB or the circuit.

PCB/Faulty Component/Assembly Defect

If PCB 2 works, you have a bad PCB, faulty component or an assembly defect in PCB 1.

- Resolution: Inspect both boards carefully going over each solder joint and part. Also check the direction of diodes and large caps or any component that has polarity.

Have a PCB design personnel do a thorough net comparison between the designed circuit drawing to the actual PCB Gerbers.

- Test the PCB, or a minimum the problem area, with an ohmmeter

- Check the circuit drawing and verify that the PCB trace/via connections are correct

- Use a yellow marker on the circuit drawing and identify each correct connection (this will help eliminate confusion)

Intermittent Board

One of the most difficult faults where the circuit works, then stops.
A can of Freeze-It will pinpoint a bad via or connection, or even a bad component
Freeze each small section of the board with the spray. It will pinpoint the area quickly by either causing the circuit to work or fail. PCB 2 can help the test. If it works, then the intermittent board is a PCB fault

Wrong Impedance

This is a tough one to find at the test bench
PCB manufacturers use an off the panel coupon, located along the side of the PCB panel to measure the impedance of a set of predetermined trace widths and spacing TDR (time-domain reflectometer) will at least show a particular trace impedance on the actual circuit.
TDR may be hard to relate to the PCB test coupon
CAF (conductive anodic filament) growth only appears after the circuit is powered up and usually takes a few days to weeks to rear its ugly head.
The DC power sets up a small plating tank when moisture is present. If the circuit worked and then stopped, look for a little tree branch-like shorts that grew between DC traces.

Hidden electrical problems within a PCB

Wicking of the electroless solution after drilling causes high resistance shorts between vias and close traces
Inner – layer intermittent shorts are created when thin prepreg is used
A silver scenario can also happen during solder mask where an overwhelming trace breaks off and shorts to an adjacent trace

These problems that causes faults on an operation circuit are numerous and not necessarily caught by the PCB shops electrical testing.

POWER SUPPLY

Noise Problem

An oscilloscope will show noise. Move the probes around and check the power right at the chip V++ or solder on bypass capacitors right to the chip power and ground to see if it fixes the problem.

Thermal Issues

An oscilloscope will show noise. Move the probes around and check the power right at the chip V++ or solder on bypass capacitors right to the chip power and ground to see if it fixes the problem.

Component, Values, etc

If problem persists, look at the parts used by the initial designer. Make sure that it was not substituted by a part that is not quite the same.

- Keep the layup peg board that the initial engineer used

- Scope various signals and clocks between the two circuits and find, or at least understand, the problem

- Check the values, manufacturer part numbers and data codes on the physical components

Grounding

This can be a major reason for a new circuit not to work, especially when processing high-speed circuits and tricky clock signals with high power switching
Running wires from grounds to other chip grounds at various points can show you where the signals are going wrong.
Ringing and noise can be a major problem when the grounds are not right

Low Resistance Connections

A trick to measure the voltage across a connection by placing one lead on the wire and one lead on the screw terminal.
It must have almost 0 mV
Any voltage across the contact will be indicative of a current loss causing measurable voltage

Outside Electromagnetic Noise/Radio Signals

These signals comes in large amount of spurious RF signals and can affect testing specially on high-speed circuits
Make a simple Faraday field enclosure by covering a cardboard box with copper or metal mesh and join all corners, as well as the ground
Re-test the circuit

Think like special agents on NCIS. The signs of why the circuit is not working may be staring you in the face; they might be yelling at you, “Hey you! It’s a signal integrity problem. Add a few bypass capacitors and I will work.”