When you receive a faulty board, as mentioned before, you need to understand what the board is used for, what year it was made, and whether there are any obvious damaged components. Sometimes, the faults are not immediately apparent, such as micro-cracks in component pins, slight corrosion, abnormal solder joints, slight warping of surface-mounted components, or broken wire turns.
These types of faults are not easy to detect, and when conditions permit, you should use a magnifying glass (10-20x) for observation, preferably a good quality one.
Initially, you can check similar components; the more you observe, the better you will know which components to focus on, such as high voltage or high current components, or some sensitive components. Furthermore, based on the type of fault or phenomenon, you should examine specific circuits, such as temperature measurement circuits or power distribution circuits. In summary, it is a process from the overall view to the local details.
If you still cannot see any faults under the magnifying glass, you will need to use a multimeter for measurements. However, you will first encounter a “dilemma”; most industrial circuit boards have protective coatings (three-proof paint) on both sides, making it difficult to get a reading.
At this point, you will need to use a scalpel to clear specific areas of the circuit or certain components before measuring. Be careful with the force applied; the less you remove, the better, to avoid damaging the original circuit. Usually, it is sufficient to clear the pins.
However, sometimes even after cleaning and obtaining readings, it can still be difficult to determine if a component is good or bad, especially for resistive components, which can be easily influenced by external circuits. At this time, it is important to follow a principle: if it can be measured without disassembly, do not disassemble; if you can remove one pin, do not remove two or more, as disassembly is generally more difficult than reassembly. Frequent disassembly can easily damage the original board.
Additionally, while measuring, you can make some assumptions, such as if a certain component is faulty or has failed, what chain reactions it might cause, and then verify the results based on those assumptions.
Moreover, you should power on the board for testing. Before powering on, measure the power circuit to avoid short circuits. For predictive powering, you can set the output current low (e.g., 0.1A); if any anomalies are detected, disconnect the power immediately. During the power-on period, check for components that are overheating (using thermal imaging) or compare them with similar circuit boards to identify faulty components.
As mentioned earlier, disassembly is more challenging than assembly, and achieving a steady hand requires considerable time and practice; persistence is the best remedy.
Let me share two common scenarios for desoldering. First, for through-hole components with fewer pins, you can directly use a soldering iron and a solder sucker to remove them. The solder must be fully melted, so the temperature of the soldering iron must be well controlled (350-400°C, adjusted according to the size of the solder joint), which is the simplest method. If there are more pins, you can use an electric solder sucker for removal.
In the above situations, you may encounter a problem where oxidized solder joints are difficult to melt. In this case, you will need to use flux. Nowadays, soldering oil is commonly chosen. Flux is effective in removing the oxide layer and lowering the melting point.
For surface-mounted components (including some multi-pin chips), you can use a hot air gun and tweezers for removal.Control the temperature of the hot air gun (usually set between 320-400°C) and the airflow, and the heating time must be flexibly managed. Once the solder balls under the chip pins are completely melted, you can use tweezers to remove it, avoiding prolonged high temperatures that could damage the component.Before starting, it is also advisable to apply some flux.
There are many videos online about this type of work, and some instructors explain it very well with innovative and unique methods that you can learn from. In the future, I will also share some good short videos related to electrical work, as I cannot do this myself.
Today, I will keep it simple. This is just a casual reflection, and if there are any inaccuracies, please feel free to leave a comment for correction.