PCB Technical - What are the skills and methods for repairing PCB boards?

With the rapid development of electronic products, the maintenance of circuit boards has become more and more common. This article puts forward some techniques and methods in PCB circuit maintenance.

When we get a circuit board to be repaired, we must first carefully observe its appearance. If the circuit board is burned out, be sure to carefully check whether the power circuit is normal before supplying power to the circuit board. The observation method is a static inspection method, usually following the steps below.

The first step is to see if the circuit board has been artificially damaged, mainly from the following aspects.

Check to see if the circuit board has dropped, causing the corners of the board to be deformed, or if the chip on the board is broken.

Check the chip sockets to see if there are special tools, because they don't have special tools.

Third, observe whether the chip on the circuit board is inserted into the socket, mainly to prevent the operator from inserting the chip in the wrong position or direction when repairing the circuit board. If you do not correct the error in time, when the circuit board is opened, it may burn the chip and cause unnecessary loss.

4. If there is a short terminal on the circuit board, check whether the short terminal is inserted incorrectly.

The maintenance of the circuit board requires theoretical and solid work, carefully and carefully through the careful observation of the repairman to determine the cause of the problem.

The second step is to see if the components on the circuit board are burned. For example, resistor-capacitor diodes are black and pasted. Under normal conditions, even if the resistor is burned out, its resistance will not change, nor will it affect normal use. At this time, you need to use a multimeter to assist in the measurement. However, if the capacitor diode is burned out, their performance will change in the circuit, and they cannot work normally, which will affect the normal operation of the entire circuit. At this point, new components must be replaced.

The third step is to observe the integrated circuits on the circuit board, such as 74 series CPU coordinating processor AD and other chips for roller cracks and black. If this happens, it is basically certain that the chip has been burned and must be replaced.

The fourth step is to see if the rope on the circuit board is peeled and burned. Are the deep copper vias separated from the pads?

Step 5: Observe the fuse (including fuse and thermal resistance) on the circuit board to see if the fuse is melted. Sometimes, because the fuse is too thin, it is impossible to clearly see the auxiliary tool multimeter to determine whether the fuse is damaged.

Most of these four situations are caused by the excessive current in the circuit. However, the specific reason is that the current is too large and needs to be analyzed. However, the overall idea of finding the problem is to first carefully analyze the schematic diagram of the circuit board, and then gradually derive its superior circuit according to the circuit where the burned component is located. Then, based on some experience accumulated in the work, analyze the most likely problem areas to find out the cause of the failure.

Before measuring, first, determine whether the circuit is based on an analog signal or a digital signal. For a circuit board with a schematic diagram, you can judge by looking at the schematic diagram. However, for a circuit board without a schematic diagram, the following two methods are usually used to determine whether the components on the circuit board have a microprocessor. Whether it is the early 8051 series or the widely used DSP, as long as there is such a chip on the circuit board, it means that there is a bus structure on the board. The digital signal must take up a large amount of digital signal to process it as a digital board. Second, there are not many chips that use 5V power on the observation board of a circuit board without a microprocessor. If many 5V power chips can also be repaired as digital circuits. The maintenance methods of digital circuits and analog circuits are different. Generally speaking, analog circuits are easier to maintain. But for digital circuits, because the circuit is suspended on the bus, there is no clear upper-lower relationship. Therefore, it is more difficult to maintain. The following focuses on the static measurement method of digital circuits. Maintenance is mainly carried out according to the following steps.

Step 1: Use a multimeter to check the short circuit between the power supply and the ground.

The method of inspection is to find a chip with a 5V power supply to measure two points on the diagonal (such as a 14-foot chip), measuring 7 feet and 14 feet.

Use a 16-foot chip to measure 8 feet and 16 feet. If there is no short circuit between the two points, the power supply is basically working normally. If there is a short circuit, it is necessary to find out the cause through investigation.

These steps are only the basic concept of power supply maintenance, especially complex circuit boards also require detailed analysis.

Step 2: Use a multimeter to measure the diode to see if it is working properly. Under normal circumstances, the positive and negative poles of a resistance gear measure tens to hundreds of euros. Under normal circumstances, diode damage is due to excessive current in the circuit causing the diode to break.

Step 3: Use the multimeter resistance file to measure the capacitance to see if there is a short circuit. The next step is to determine whether there is a problem with the component itself, or whether there is a problem with the circuit connected to it. In this way, the problem can be determined step by step.

The fourth step: also use a multimeter to measure the resistance of the transistor on the circuit board to determine whether it meets its logical performance. If the circuit board includes a bus structure, there is usually a row of pull-out resistors on the bus. Resistance measurement is a very important step. Through its quality, a preliminary inspection of the chip hanging on the bus can be carried out.

Online measurement methods are usually used by large-scale PCB manufacturers to facilitate maintenance, and usually establish a more common debugging and maintenance platform. It can easily provide the power supply needed by the circuit board and some necessary initial signals. The network measurement method mainly solves two problems. First, subdivide the problems found in the first two steps into problem components. Second, the problem of checking through the above two steps has not yet been resolved. The online measurement method is mainly carried out through the following steps.

Step 1: In this step, it should be noted that some circuit boards are not single power supply, and may require 5V and 12V24V, etc. Do not leak the power supply that should be added. When the circuit board is powered on, check whether the 74 series chip has burned components, if the components have burned. This means that the component may be damaged. After replacing the parts, check whether the fault of the circuit board has been resolved.

Step 2: Use an oscilloscope to measure the gate circuit on the circuit board to see if it is logical. If the output is not logical, it needs to be handled in two cases. One is that the output should below. The actual measurement of the high level can directly judge the damage of the chip. Another situation is that the output should be high level, the actual measurement cannot determine whether the chip is damaged, and the chip needs to be disconnected from the back circuit to measure again. Observe whether the logic judges the quality of the chip reasonably.

Step 3: Use an oscilloscope to measure the crystal oscillators in the digital circuit to see if they have output. If there is no output, the chip connected to the crystal oscillator must be removed as much as possible before measurement. If there is no output, the originally determined crystal oscillator has been damaged; if there is an output, the removed chip needs to be put back and installed in a test piece to find the location of the fault.

Step 4: The digital circuit with a bus structure generally includes a digital address control bus. Measure the third line with an oscilloscope to compare the schematic diagram to see if the signal is normal.

The network measurement method is mainly used to compare two good and bad circuit boards, and to find out the problem to solve the problem through the comparison. Thus, the maintenance of the circuit board is completed.

This article gradually elaborates some methods and skills in circuit board maintenance from three aspects: observation method, static measurement method, and dynamic measurement. With the development of modern technology, people pay more and more attention to various new instruments and equipment. But for those engaged in electronic engineering, relying on simple test tools, such as multimeter oscilloscopes, is still an essential skill for electronic engineers.