As a Tech, what are the most common PC-Board Failure Culprit?

As usual, no answer fits-all. Every situation is different. It may depend on industry and type of components used. Different industries use different types of components which may have different types of failures.

Then there is automated assembly vs. human assembly.

Is it pure PCB, or mixed PCB and wiring harnesses.

Typically, on proven automated lines, errors are low percentage. But, when assembly lines are humans, the rates of errors will tend to be greater. And these kinds of errors are usually wrong wiring or wrong components or reversed polarities, etc.

But, if you are saying assembly is not the main issue, vs. component failure, then you need to look at the types of components used in the product.

Printed circuit boards with high-density runs and multi-layers are often prone to "bridges" between circuit traces. This is a PCB board mfg failure (etching process). In good proven process, the % of bad boards is probably fairly low. (unless there is a sudden process problem)

After the PCB boards, the soldering process is next on list of problems. Is the soldering process automated or human? If the wave soldering process is proven and bugs worked out, then again the % of bad joints and bridges is relatively low. But, when soldering is by humans, the % is random and unpredictable (just like people tend to be). In which case it comes down to training and worker reliability.

Wiring harnesses and connectors, again come down to worker training and reliability. These are people and procedure issues.

But, if we are strictly talking about component reliability issues, I would focus more on the types of components used.

Passive components tend to be more reliable than active components.

In other words, resistors, capacitors, LEDs, diodes, chokes, transformers, etc tend to be highly reliable with low failure rates. This assumes the circuits are well engineered and all components are operating well within design parameters. Poorly engineered circuits may be more prone to failure due to components being exposed to extremes such as too high currents, voltages, and even enviromental issues such as temperature, etc.

Active components tend to have higher failure rates than passive. This may include transistors, ICs, and switching circuits. But, now days, component reliability is much higher than years gone by. Actually, one of the biggest reasons of active component failure may be due to ESD (electro static discharge) which is usually a result of mis-handling by workers. ESD can damage components without even realizing it. This is where proper assembly and handling procedures are extremely important. Troubleshooting ESD damaged components can be difficult and often requires a well trained technician to locate. ESD failures are often intermittant and difficult to find.

Using temperature is often helpful to isolate intermittant failures. In other words, heat guns placed over strategic areas can cause failure to come and go to help with location (being careful not to overheat). Also, "cold spray" is also helpful troubleshooting aid to help locate temperature sensitive intermittant problems. Good techs have various tricks in their bag to assist in troubleshooting - but this usualy comes with experience...

You seem to be talking about trouble-shooting production issues versus field failures, which in a perfect world would not occur prior to shipment. I've done both types, but here are production issues. Where you are on the QC chain is important to where you start. Then you ask a series of questions, and explore both answers mentally until you decide which is more plausible. Repeat as necessary.

Most production failures I encountered were boring or frustrating. On boards assembled via robotics, solder bridges or joints not soldered at all were the prime suspect in most cases.

If they're stuffed by 12 yr. olds on Friday night at the end of the 16 hr shift, look for components stuffed backward and poor soldering. They're tired. Give 'em a break.

Either way, a thorough visual inspection pays off in the long run.

Next is Ohm's law. It's usually the power components, eliminate the power as a suspect first.

If you still have a problem after that, it should be a good learning experience.

If you're working on multi-layer surface mount boards, reworking them incorrectly will cause more problems and waste time. If you can't fix it on the first try correctly, scrap it. Good luck.