PCB Tech - PCB circuit board repair techniques

1. Visual inspection method

That is, through the old methods of looking, asking, smelling, and cutting, observe whether the circuit board is burnt, whether there is a broken place in the copper coating, whether there is a peculiar smell on the circuit board, whether there is a bad soldering place, the interface, the gold finger Whether there is mildew and black phenomenon, etc. Ask the customer about the fault phenomenon and the process of the fault, often focusing on certain parts of the fault. Through the above processing, some problems can often be found.

2. Killing method

That is to check all the components again, find the defective component and replace it to achieve the purpose of repair. If you encounter a component that cannot be detected by the instrument, use the substitution method, that is, replace it with a new component regardless of whether it is damaged or not. Finally, all components on the board are guaranteed to be good, and the purpose of repair is achieved. This method is simple and effective, easy to use, and does not require high technical level of engineers, but requires a high degree of care and responsibility during operation. Otherwise, some devices will not remember the position when they are removed, and the wrong installation will occur. It is easy to cause maintenance failure. In addition, this method is helpless for problems such as blocked vias, broken copper, and improper adjustment of potentiometers. This method cannot be used when encountering chips with programs and data.

3. Contrast method

The comparison method is one of the most commonly used methods for repairing circuit boards without drawings. Practice has proved to have very good results. The purpose of detecting faults is achieved by comparing the status of the good board. The abnormality can be found by comparing the curves of the nodes of the two boards. . However, many times we do not have a good board for comparison, but this does not mean that we cannot use this method. For example, we can find circuits with the same properties on the circuit board. For example, a certain board has three identical interfaces. If the three interfaces have the same circuit, we can use the three identical interfaces to compare the curves with each other to judge the problem. When we can't find the situation mentioned above, we can still find circuits with common attributes. For example, the buses in the circuit usually have the same curve. Sometimes we cannot judge whether an IC is damaged or not. We may also compare the curve with a good IC by scanning the curve to achieve the purpose of judgment. In short, we need to use our brains to maximize the contrast. The curve scanning method is really useful in practice, but some beginners do not understand its working principle and complain that the instrument has no practical value. Therefore, I have personally seen many companies in the same industry use the circuit board repair tester basically, and shouted. Of course, the merchants also have the suspicion of exaggerating this function. In fact, when scanning the integrated circuit of the COMS process, the curve of each scan is different, resulting in a situation that cannot be judged. The reason is due to the high impedance of CMOS and the charging of the inter-junction capacitance. Since there is no release channel afterwards, it is necessary to have a wealth of experience when judging CMOS integrated circuits. The last thing to add is that the curve scanning method scans peripheral devices. If the scan is an integrated circuit, it is the components connected to the pins inside the scanned integrated circuit. 90% of the integrated circuit damage is caused by the damage of the peripheral device. In case of damage to non-peripheral devices, the problem will not be found when scanning the curve, but problems will occur when the device is working.

4. State method

The state method is to check the normal working state of each component. If the working state of a certain component does not match the normal state, there is a problem with the device or its affected parts. For example, when we check the CPU, we will check whether the crystal oscillator is working normally and whether the reset signal is normal. Whether the input and output logic of the NAND gate is correct or not can be judged whether the device is normal. When using this method, it is often forced to give an excitation signal to the input. The signal amplitude and distortion are used to judge the problem of the op amp. Forcibly push the input level of the logic gate circuit up or down, and test whether the output level matches the logic of the chip, so as to judge the quality of the chip.

The state method is the most accurate method of all maintenance methods, and its operation difficulty is not that ordinary engineers can master. It requires a wealth of theoretical knowledge and practical experience. In order to meet the state requirements, the engineer must do everything possible. The method is superior to the curve scanning method for circuit testing, and both the peripheral circuit and non-peripheral circuit problems of the IC can be found.

5. Substitution method

Replace all ICs with new ones until they are repaired. Nowadays ICs are getting cheaper and cheaper. The 74 series and 4000 series chips are less than 1 yuan per piece. Rather than checking the problem piece by piece, it is better to replace them all. I personally think this method is a helpless move, and its shortcomings are obvious. Its effect should be lower than that of the pass-killing method. The pass-killing method must at least judge the device. If accumulated over a long period of time, it will affect some circuit boards. Some experience, and the repair rate of this method depends entirely on luck rather than effort, even if it is really repaired, it is not known where the fault is. However, this method still has its merits. If a certain circuit board or electronic device can be used to verify whether it is normal on the spot, we can replace the IC piece by piece. When the fault disappears when a certain piece is replaced, it can be proved that The piece that was just replaced is broken. If it is a direct-insertion packaged IC, it is recommended that you install an IC holder immediately after removing it. After all, it only costs a few cents. Another fatal problem with this method is that you must ensure that the IC you bought is good. If you accidentally encounter a bad IC during purchase, not only will it not be repaired, but the fault will be enlarged and the repair difficulty will increase.

6. Circuit method

The circuit method is to make a circuit by hand, and the circuit can work after the integrated circuit is installed, so as to verify the quality of the tested integrated circuit. For example, in order to judge the quality of a 555 integrated circuit, we can build a 555 The time-base oscillation circuit made by the circuit, and then the integrated circuit under test is placed in the circuit. If the circuit works normally, it proves that 555 is no problem, otherwise it is faulty.

This method can reach 100% accuracy, but the tested integrated circuits have many types and complex packaging. It is difficult to build a set of integrated circuits.

7. Principle analysis method

The method is to analyze the working principle of a board. For some boards, such as switching power supplies, engineers can know the working principle and details without drawings. For engineers, it is very simple to repair things that know the schematics. However, for some relatively complex and rare equipment, there is no ready-made schematic diagram reference to manually reverse engineer it, draw the schematic diagram, and then repair it with the schematic diagram, which can be said to be invincible.