- Issue Time
- Nov 16,2021
When a printed circuit board fails, if the cause is not clear in time, its normal use cannot be guaranteed. This article will specifically introduce the method of detecting PCB faults.
With the development of modern technology becoming more and more mature, various new instruments and equipment are constantly emerging, and detection methods are also emerging endlessly. However, using the following methods and relying on simple inspection tools, such as multimeters, oscilloscopes, etc., to measure and repair printed circuit boards is still an essential skill for an electronic engineer.
When we get a PCB circuit board to be repaired, we first carefully observe its appearance. If the PCB circuit board is burned, before powering on the PCB circuit board, be sure to carefully check whether the power circuit is normal, and then power on after ensuring that it will not cause secondary damage.
The observation method is a kind of static inspection method. When using the observation method, the following steps are generally followed.
1. Observe whether the PCB circuit board has been artificially damaged. This is mainly from the following aspects:
(1) Check whether the PCB circuit board has been dropped, causing the corners of the board to be deformed.
(2) Observe the socket of the chip to see if it is forcibly broken.
(3) Observe the chip on the PCB circuit board. If it has a socket, first observe whether the chip is inserted incorrectly. This is mainly to prevent the operator from inserting the chip in the wrong position or direction when repairing the PCB circuit board. If the error is not corrected in time, when the PCB circuit board is powered on, the chip may be burned out, causing unnecessary losses.
2. Observe whether the components on the PCB circuit board are burned out. For example, whether the resistors, capacitors, and diodes are blackened or muddy. Under normal circumstances, even if the resistor is burned, its resistance value will not change, its performance will not change, and normal use will not be affected.
At this time, a multimeter is required for auxiliary measurement. But if the capacitors and diodes are burnt, the performance will change, and they will not be able to play their due role in the circuit, which will affect the normal operation of the entire circuit. At this time, new components must be replaced.
3. Observe whether the integrated circuits on the PCB circuit board, such as 74 series, CPU, AD, etc. chips, are bulged, cracked, burnt, or blackened. If this happens, it is basically certain that the chip has been burned out and must be replaced.
4. Observe whether the traces on the PCB circuit board are peeled, burned, or broken. Whether the sinking copper hole is out of the pad.
5. Observe the insurance (including fuse and thermistor) on the PCB circuit board to see if the fuse is blown. Sometimes because the fuse is too thin to see clearly, you can use an auxiliary tool-a multimeter to determine whether the fuse is damaged.
Most of the above situations are caused by the excessive current in the circuit. But what is the specific cause of the excessive current requires specific analysis of specific issues.
For most PCB circuit boards, the problem cannot be found through the previous observation method. We need to use a multimeter to orderly measure some main components and key points on the PCB circuit board to find and solve problems.
Before the measurement, we must first determine whether the circuit is based on analog signals or digital signals. For a PCB circuit board with a schematic diagram, it can be judged by looking at the schematic diagram. But for PCB circuit boards without schematic diagrams, generally, the following two methods are used to judge.
(1) Observe the components on the PCB circuit board to see if there is a microprocessor on the PCB circuit board, whether it is the early 80, 51 series or the widely used DSP, as long as such a chip appears on the PCB circuit board, It shows that there is a bus structure on the board, and the digital signal must occupy a large part, so it can be treated as a digital board.
(2) For PCB circuit boards without microprocessors, observe the onboard components and see if there are many chips that apply 5V power. If there are many 5V power chips, you can also use them as a digital circuit for repair. The maintenance methods for digital circuits and analog circuits are different.
Generally speaking, the maintenance of analog circuits is simpler. You can deduce step by step to find out the problem. But for digital circuits, because the circuits are all hung on the bus, there is no clear upper-lower relationship. Therefore, maintenance is more difficult. The following only focuses on the static measurement method of digital circuits. Maintenance mainly follows the following steps.
Use a multimeter to check whether there is a short circuit between the power supply and the ground.
(1) Find a chip powered by a 5V power supply and measure two points on the diagonal (for example, a chip with 14 pins, measure 7 and 14 pins; for a chip with 16 pins, measure 8 and 16). If there is no short circuit between the two points, the power supply is generally working normally. If a short circuit occurs, you need to find the cause through the troubleshooting method.
(2) Use a multimeter to measure the diode and observe whether it works normally. Under normal circumstances, the resistance range is used to measure the positive and negative poles. The positive phase measurement is tens to hundreds of ohms, and the reverse phase is one thousand to several thousand ohms. Generally speaking, when the diode is damaged, the diode is broken down due to the excessive current in the circuit.
(3) Measure the capacitance with the resistance file of a multimeter to see if there is a short circuit or open circuit. If there is, it means that there is a problem with this part of the circuit. The next step is to determine whether there is a problem with the component itself or the circuit connected to it. The method is to solder one foot off the suspicious component to see if the component is open or disconnected. In this way, the problem can be determined in one step.
(4) Use a multimeter to measure the integrated circuits, transistors, resistances, etc. on the PCB circuit board to see if they meet their own logic performance. If the PCB circuit board includes a bus structure, generally there will be a pull-up resistor row on the bus. The measurement of the resistance row is a very important step. Through its quality, the quality of the chip hanging on the bus can be preliminarily tested.
The online measurement method is generally applied to manufacturers who produce PCB circuit boards in batches. For the convenience of maintenance, manufacturers generally build a more general debugging and maintenance platform, which can conveniently provide the power supply required by the PCB circuit board and some necessary initial signals.
1. Power on the PCB circuit board. In this step, it should be noted that some PCB circuit board power supplies are not single, and may require 5V, plus or minus 12V, 24V, etc., do not add the power supply that should be added.
After the PCB circuit board is powered on, touch the components on the PCB circuit board to see if there are hot components, and focus on checking the 74 series chips. If the components are hot, it means that the components may have been damaged. After replacing the components, check whether the PCB circuit board failure has been resolved.
2. Use an oscilloscope to measure the gate circuit on the PCB circuit board and observe whether it conforms to the logical relationship. If the output is not logical, it needs to be treated separately in two cases. One is that the output should be low level, and the actual measurement is high level, which can directly judge the chip damage; the other is that the output should be high level.
The actual measurement is low, and it cannot be judged that the chip has been damaged. It is necessary to disconnect the chip from the following circuit, measure again, observe whether the logic is reasonable, and judge whether the chip is good or bad.
3. Use an oscilloscope to measure the crystal oscillator in the digital circuit to see if it has an output. If there is no output, you need to remove all the chips connected to the crystal oscillator as much as possible before measuring. If there is still no output, it is preliminarily determined that the crystal oscillator has been damaged; if there is output, you need to put back the removed chips one by one, install one by one, and test one to find out where the fault is.
4. A digital circuit with a bus structure generally includes three lines of number, address, and control bus. Use an oscilloscope to measure the three-way bus, compare the schematic diagram, observe whether the signal is normal, and find out the problem.
The online measurement method is mainly used for the comparison of two good and bad PCB circuit boards. Through the comparison, problems are found and solved. So as to complete the maintenance of the PCB circuit board.
If you want to know more about printed circuit boards after reading the above, you can get a comprehensive solution by contacting us.
As a professional PCB product manufacturer, PCB QUICK can not only provide users with diversified choices but also formulate effective solutions. In order to control the quality of the products, we are equipped with a professional quality inspection system and a comprehensive management team, which can control the quality of the products in an all-round way. If you want to buy our products, please contact us immediately!