Circuit Board Repair Technology

With the rapid development of electronic technology, electronic products are constantly updated. Since there are always various reasons for the failure of electronic products during the production process, there is electronic testing, and testing must be inseparable from testing systems.

The test system is composed of multiple circuit boards. Once the circuit board fails, the system cannot work normally. Since each circuit board is expensive, it must be repaired if a fault occurs. The key to repair is analysis, and the process of analysis is the process of finding faults. Each circuit board is composed of one or more functional modules, and each functional module is composed of many components, and there are close connections between these components, so it is an important task to find faulty components. Very difficult thing.

If you want to repair the circuit board, you must conduct in-depth research on the circuit board, fully understand the structure of the circuit board and the functions of each module; then there are maintenance methods, and good maintenance methods can make maintenance more effective.

1. Maintenance method of circuit board

The principle of circuit board maintenance method is to look first and then test, first easy and then difficult. The faulty circuit board must first be visually inspected to understand the fault phenomenon through visual inspection. Understanding the fault phenomenon will help to analyze the fault in the next step. The analysis process includes asking, watching, smelling, and measuring. The question is to ask the user about the failure, whether it is caused by improper use or a sudden failure during normal operation, and whether the circuit board has been repaired by others before and which components have been moved. The second thing to look at is to carefully observe the electronic components to see if there are obvious signs of discoloration and burnt, whether there are virtual soldering, reverse soldering, etc., and if necessary, you can use a magnifying glass to observe. Then smell, smell if there is any peculiar smell, once the integrated circuit (especially the high-power device) is burned out, it will smell like rotten eggs. The last thing is to measure is to measure, to analyze and find out the faulty device through measurement, and the measurement is divided into static measurement (that is, measuring the circuit board without power) and power-on measurement. The simplest measurement is to use a multimeter to measure the resistance between the power supply and the ground of the circuit board. Usually, the resistance of the circuit board should not be less than 70Ω. If the resistance is too small (only a few ohms or a dozen ohms), it means that there is Components are broken down or partially broken down.

2. Classification of circuit board faults

The scope of failure includes: local failure and global failure. Partial failure refers to the abnormal operation of one or several functional modules of the system; global failure refers to the operation status of the entire system.

Faults are divided into stable faults and unstable faults. Stable refers to failures caused by functional failure of components, short circuits, etc.; unstable refers to: poor contact and poor function of components.

3. The working principle of the circuit board

This paper takes the DPS (measurement device voltage source) board as an example, its main functions are: (1) During the test process, it provides two device power supplies for the device under test according to the applied conditions, one of which has a voltage range of 0~50V, and the other The voltage application range is -50V~0V. (2) Measure the operating current of the device power supply, and the measured value is up to 5A.

The device power supply board is composed of DPS1 and DPS2 two-way device power supply. Its main components are: logic control unit, decoding control unit, data converter, voltage and power amplifier, current clamp control unit, current sampling unit, differential amplifier , Relay control unit.

The working methods of the DPS board are: pressurization (FV), pressurization flow measurement (FVMI). In the FVMI mode, the driving voltage value is provided to the input driver through the digital-to-analog converter, the output current is sampled by the sampling resistor, converted into a voltage value through the differential amplifier, and then the current value is read back by the analog-to-digital converter. The clamp value can be adjusted according to the load Setting, the clamping current plays the role of current limiting protection.

The logical block diagram of the DPS board can be drawn by understanding the structure of the real diagram, as shown in Figure 3. Drawing a logic block diagram can make us understand its working process more clearly, which can provide great help for maintenance.

Combined with the working principle and composition of the DPS board introduced earlier (the DPS board is composed of two circuits, one for positive voltage and one for negative voltage), we can see that these components provide positive and negative power supplies. Knowing this kind of information is very helpful for analyzing the circuit board. For example, if the positive voltage cannot be added on the DPS board, but the negative voltage can be added, the positive voltage can be repaired by referring to the negative voltage.

When the DPS board fails, it is firstly detected statically. Since the integrated circuit uses direct coupling, there is a certain DC resistance between the other pins of the integrated circuit and the ground. This determined DC resistance is called the internal equivalent resistance of the pin, referred to as R, You can use a multimeter to measure the internal equivalent DC resistance of each pin to judge its quality. If the internal equivalent resistance R of each pin is in line with the standard (we can take a good integrated circuit and measure the internal resistance of each pin with a multimeter If the equivalent resistance R is used as the standard), it means that the integrated circuit is good; otherwise, if the difference from the standard value is too large, it means that there is a fault inside the integrated circuit. One point must be paid attention to when measuring, because there are a large number of non-linear components such as triodes and diodes inside the integrated circuit, it is not possible to judge whether a resistance value is measured in one direction during measurement, so the test leads must be exchanged and measured again to obtain positive and negative values. To the two resistance values, only when the forward and reverse resistance values in R meet the standard can it be concluded that the integrated circuit is intact.

In the case where the fault source cannot be found in the static detection, only the DPS board is powered on for on-the-road maintenance. If you want to power on the DPS board, you must extend the DPS board, as long as you extend the DPS board, you can power on and analyze it.

In actual maintenance, first measure the voltage of the pin when the power is on. If the voltage is abnormal, you can disconnect the pin connection and measure the voltage of the terminal to judge whether the voltage change is caused by the external components or inside the integrated block circuit; The DC equivalent resistance R between the external circuit and the ground is judged externally. Usually, the DC equivalent resistance (in-circuit resistance) between a pin of an integrated circuit and the grounding pin is measured in the circuit, which is actually the total DC equivalent resistance connected in parallel between R and R. In maintenance, the measurement method of on-circuit voltage and on-circuit resistance is often used in combination. Sometimes the on-circuit voltage and on-circuit resistance deviate from the standard value. It is not necessarily the damage of the integrated circuit, but the damage of the peripheral components that makes the R outside abnormal. , resulting in anomalous on-circuit voltage and on-circuit resistance. At this time, only by measuring the DC equivalent resistance inside the integrated circuit can we judge whether the integrated circuit is good or bad. According to actual maintenance experience, it is not necessary to solder the integrated circuit from the circuit board when measuring the internal DC equivalent resistance of the integrated circuit on the road, as long as the abnormal pin of the voltage or on-circuit resistance is disconnected from the circuit, and the grounding pin is connected to the circuit at the same time. The circuit board is disconnected, and the other pins remain as they are, and the positive and negative resistance values in R between the test pin and the ground pin can be measured to judge whether it is good or bad.

4.Draw the circuit diagram

Now many manufacturers do not provide the circuit diagram of the circuit board, but if you want to conduct power-on analysis on the circuit board, there is no way to conduct power-on analysis without the circuit diagram, so you have to draw the circuit diagram yourself. To draw a circuit diagram, you need to know the following points:

(1) To understand the working conditions of related integrated circuits, first find out the signal input pins and signal output pins of the integrated circuits and the corresponding power pins and ground pins. The power supply pin of the integrated circuit is used to add the DC working voltage output by the rectification and filtering circuit to the internal circuit of the integrated circuit to provide DC power for the internal circuit of the integrated circuit; and the ground pin is used to connect the internal circuit of the integrated circuit The ground wire of the integrated circuit is connected with the ground wire in the circuit of the whole machine, so that the current of the circuit in the integrated circuit forms a loop. Therefore, the power pin and the ground pin are the two most basic and important pins for the normal operation of the integrated circuit.

The signal input pin is connected to the circuit output pin of the previous stage, or connected to the signal source of the whole machine circuit, so that if you understand the signal input pin of the integrated circuit, you can move from the subsequent stage to the previous stage. for analysis. In many troubleshooting of circuit boards, for example: no voltage can be applied, no current can be applied, etc., only after the signal output pin of the integrated circuit is determined can the next step of maintenance be carried out, so find out the signal in the integrated circuit. Input and output pins are very important.

(2) According to the manual of electronic components, write down the working characteristics of the IC (truth table, working characteristic curve, working voltage, input and output pins), and then simply arrange the positions of the relevant ICs according to the context of the structure diagram . Finally, use a multimeter to measure the relationship between the front and back stages of the IC, so that the connection between them can be simply drawn.

5. Circuit board module unit

1. Logic control unit

The logic control unit is the main control unit of the circuit board, which is equivalent to the "CPU" of the circuit board and is a programmable logic device. The main function is to control the relay unit, current clamp control unit and measurement unit by logic level.

Failure analysis:

(1) Since the level from the "logic control unit" is not enough to drive the relay, it must be driven by the drive circuit, so this should be taken into account when the relay does not work properly (logic control failure, drive circuit failure, failure of the relay itself).

(2) When the system is powered on, if the "circuit board cannot be found" phenomenon occurs, it can basically be concluded that the logic control unit on the circuit board is faulty.

2. Digital-to-analog converter (D/A)

The digital-to-analog converter converts the digital signal given by the controller into an actual analog quantity to control external devices. When the DAC is working normally, the output analog voltage is proportional to the input digital quantity, so when the input and output are not proportional, it can be determined that the DAC is invalid.

3. Current clamp

The main function of the current clamp is to protect the subsequent circuit and the device under test. The current clamp on the DPS board is to protect the voltage and power amplifier of the subsequent stage. Because there are positive and negative power supplies on the DPS board, the clamping current should also have symmetrical positive and negative circuits.

It can be seen from Figure 5 that the front stage connected to these two diodes is the clamping part, and the lower-stage operational amplifier circuit connected to this can be found. Use the diode characteristics of the multimeter to measure the relationship between the input and output of the lower-level op-amp circuit, and it can be known that the function of the op-amp circuit is to form a voltage follower. The voltage follower has the function of impedance transformation. The so-called impedance transformation function means that after being amplified by the voltage follower, its output voltage is approximately equal to the electromotive force of the signal source, but its output resistance is very small. This circuit is often used as an input stage, an intermediate stage, and an output stage. class.

Fault analysis: When the DPS board is powered on, 5V is added and 5V comes out, but when it is added to more than 5V, it cannot be added (it is always 5V). At this time, the diode used for clamping the input stage of the follower can be removed. , power on again, if the detection returns to normal at this time, it means that the operational amplifier circuit used for clamping is faulty; if the fault persists, conduct reverse analysis to check the previous level, here it should be noted: as long as the current does not enter the closed loop, it can be Open the output pin of the op amp circuit for measurement.

4. Current sampling unit

Current sampling is composed of sampling resistors and relays with corresponding gears. Current measurement (MI) is the output current is sampled by sampling resistors. After sampling, it is converted into voltage through a differential amplifier, and then the current value is read back by an analog-to-digital converter.

Fault analysis: The current is divided into many gears by the sampling resistor. If there is an error in a certain gear, first use a multimeter to statically measure the resistance of the sampling resistor. If the measurement is normal, then measure the relay of the corresponding gear. If the measurement is also normal, Then we have to look at the differential amplifier of the subsequent stage. If the measurement is normal, it can be concluded that the logic controller is faulty.

6. Precautions for circuit board maintenance

Before maintenance, anti-static work must be done first, such as wearing anti-static overalls and shoes, and wearing an anti-static wrist strap. The workbench for maintenance should also be treated with anti-static treatment, such as adding an anti-static table.

Pay attention to the safety of electricity use, including the maximum bearing capacity of the circuit board when powering on and maintaining the circuit board; when you need to contact strong electricity, you must pay attention to personal safety and take measures to prevent strong electricity.

With the continuous development of electronic products, the types and frequency of various equipment failures have also been greatly increased, so the requirements for maintenance personnel are also increasing, especially for those who are new to circuit board maintenance, many people are very confused at the beginning. I don't know where to start, and I don't know what to learn. In fact, if you want to learn circuit board maintenance well, you must have a certain foundation in digital electricity and analog electricity. Theory + method + practice + summary, so that we can deal with all kinds of faults that occur on the circuit board.

The above is the maintenance method of the test system circuit board introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. for you. If you have software and hardware function development needs for smart electronic products, you can rest assured to entrust them to us. We have rich experience in customized development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad, including MCU, voice IC, Bluetooth IC and modules, wifi modules. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth wifi development, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS solutions, the development of Bluetooth audio, the development of children's toys, and the development of electronic education products.