Visual inspection

Most of the time, your eyes are more important than your brain. Always inspect the edges of the logic board! 95% of the time the problem is somewhere in that area.
There is no greater time-sink for the new technician than over-intellectualizing a problem. Trying to decode what every single signal abbreviation means and trying to reverse engineer how an undocumented IC works, when the problem is staring you in the face. Do you need to know what a rusted, green, rotting probe point is for in order to fix it? NO!

Thinking before acting is a great thing, and one should always be thinking. However, when it comes to board repair, thinking almost always comes after seeing.

What you see will tell a story, and that story will often teach you a lesson for the next board. If you have no wifi, and have corrosion on a probe point - you now know that the signal going through that probe point is related to wifi. Corrosion, rust, burned traces, and green dust are beautiful little markers that alert you to where your issue is. Without the corrosion there, you might have never found that this signal has anything to do with wifi. With the corrosion, you now know that this area is broken. Once you repair the damage and see that wifi is back, you can add to your knowledgebase that AP_RESET_L has something to do with wifi. AP. "Ah - ACCESS POINT - THAT'S WHAT AP STANDS FOR!"

Your eyes figured out what was broken before your brain knew that AP_RESET_L mattered. As a new technician, your eyes will be doing far more work than your brain, and will also serve as a teacher.

Let's take the board below as an example. Knowledge of the onewire circuit and its role in creating a green light on the charger, would be necessary if using your brain for this board repair. Knowledge of how a logic gate works and the ability to see the data on a logic analyzer would help to see what is going on on the SYS_ONEWIRE line. To fix this board on a brains level, requires quite a bit of knowledge and experience.

To fix this board on an eyes level, one must simply scrape the mask off the traces on each side of the probe point circled in yellow, tin with solder, and put a jumper over it. You need not know the voltages that must be present, or the purposes/identities of the many components. You don't even have to know what the circuit does.

This doesn't mean you shouldn't go back after you're done to try and learn. See if you can infer how the signal/voltage that was missing as a result of the damage might be the reason that the machine is presenting the problem it has. This is a never-ending journey.

This below may be fixable via a reflow of the chip.



Working components/solder joints that won't work for long:



This probe point and component have corrosion of the worst kind - it is just good enough to last for testing, but will fail when the customer starts using it. This solder joint will not stand the test of time, besides perhaps at L2 Computer with their 30 day warranty. This will come back for warranty service.

Since the corrosion was not addressed before ultrasonic cleaning, it is much more difficult to spot, as the green lump of dust has been washed away by the cleaner. It is imperative to address most corrosion before ultrasonically cleaning the board - or, to make a note of where it was, so you will know to come back and check the area later.

Once the board is ultrasonically cleaned, and the evidence of the spill removed - the corroded areas are more difficult to spot. Out of sight, is out of mind!

The component & its solder pad are important - the probe point isn't. Why does this probe point not matter?

Look at the earlier picture. The bad probe point has a signal passing through it - in that case, from a connector, to the chip on the bottom right. Here, the probe point is for testing/measurement purposes only. That path goes to P3V42G3H_BOOST - which only goes to that one capacitor, and nothing else! The schematic and the board view below confirms that pin 3 of U7090 goes to pin 1 of C7094, not any other component/chip. The trace is already intact from the chip to the capacitor. The probe point that is under the capacitor serves no functional purpose. However, the capacitor & its solder pad do, and must be reworked.

Another working component & solder joint that will not last.

The liquid damage was repaired in one area, but the board was cleaned before it could be fixed in another. This joint technically conducts electricity, but if you smacked this component with the tweezer, it would immediately come off the board. This is less of a solder joint and more of a pile of sand waiting to be blown away by a breeze.