PCB Blog - Precautions related to PCB Board layout

Capacitor failure

Faults caused by capacitor damage are common in PCB board electronic equipment, especially electrolytic capacitor damage. Capacitor damage is manifested as the capacity becomes smaller, the capacity is completely lost, leakage, and short circuit. The functions of capacitors in the circuit are different, and the faults caused by them also have their own characteristics: in industrial control circuit boards, digital circuits account for the vast majority, capacitors are mostly used for power supply filtering, and there are fewer capacitors used for signal coupling and oscillation circuits. If the electrolytic capacitor used in the switching power supply is damaged, the switching power supply may not vibrate and there is no voltage output; or the output voltage is not well filtered, and the circuit is logically confused due to unstable voltage, which is manifested as good or bad when the machine is working or open. If the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the fault is the same as above. This is especially obvious on the computer motherboard. Many computers have been used for several years, sometimes they can't be turned on, and sometimes they can be turned on again. When you open the case, you can often see the phenomenon of bulging electrolytic capacitors. If you remove the capacitors, measure the capacity. , found to be much lower than the actual value. The life of the capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the life of the capacitor. This rule applies not only to electrolytic capacitors but also to other capacitors. Therefore, when looking for faulty capacitors, you should focus on checking capacitors that are close to the heat source, such as capacitors next to heat sinks and high-power components. The closer you are to them, the greater the possibility of damage. Therefore, attention should be paid to maintenance and search. Some capacitors have serious leakage and even burn your hands when touched with your fingers. Such capacitors must be replaced. In the case of good and bad faults during maintenance, except for the possibility of poor contact, most of the faults are caused by capacitor damage. Therefore, when encountering such failures, you can focus on checking the capacitors. After replacing the capacitors, it is often surprising.

Resistor failure

It is often seen that many beginners toss on the resistor when repairing the circuit, dismantling, and soldering. In fact, it is repaired a lot. As long as you understand the damage characteristics of the resistor, you don't have to spend a lot of time. Resistors are the most numerous components in electrical equipment, but not the components of the damage rate. Resistor damage is common in the form of an open circuit, it is rare for the resistance value to increase, and it is very rare for the resistance value to decrease. Commonly used are carbon film resistors, metal film resistors, wire-wound resistors, and fuse resistors. The first two types of resistors are widely used, and their damage characteristics are that the damage rates of low resistance (below 100Ω) and high resistance (above 100kΩ) are relatively high, and the intermediate resistance (such as hundreds of ohms to tens of thousands of ohms) is extremely high. Less damage; Second, when the low-resistance resistor is damaged, it is often scorched and blackened, which is easy to find, while the high-resistance resistor has few traces when it is damaged. Wirewound resistors are generally used for high current limiting, and the resistance value is not large; when cylindrical wire-wound resistors burn out, some will turn black or the surface will explode, crack, and some have no traces; cement resistors are a kind of wire wound resistors. , it may break when it is burned out, otherwise, there will be no visible traces; when the fuse resistor is burned out, some surfaces will blow up a piece of skin, and some have no traces, but they will never be scorched and blackened. According to the above characteristics, you can focus on checking the resistance and quickly find the damaged resistance. According to the characteristics listed above, we can first observe whether the low-resistance resistors on the circuit board have any traces of burning black, and then according to the characteristics of most open circuits or increased resistance when the resistance is damaged, and the high-resistance resistance is easily damaged, We can use a multimeter to directly measure the resistance at both ends of the high-resistance resistor on the circuit board. If the measured resistance value is larger than the nominal resistance value, the resistance must be damaged (pay attention to wait until the resistance value is stable before drawing a conclusion, because there may be parallel capacitor elements in the circuit, and there is a charging and discharging process), if the measured resistance value is If the value is smaller than the nominal resistance value, it is generally ignored. In this way, every resistor on the circuit board is measured again, even if one thousand is "killed by mistake", one will not be missed.

Op-amp failure

It is difficult for many electronic repairers to judge the quality of operational amplifiers. It is not only related to the level of education. I would like to discuss this with you here, and I hope it will be helpful to everyone. The ideal op amp has the characteristics of "virtual short" and "imaginary off", which are very useful for analyzing op amp circuits in linear applications. In order to guarantee linear operation, the op-amp must work in a closed loop (negative feedback). Without negative feedback, the op-amp under open loop amplification becomes a comparator. If you want to judge the quality of the device, you should first distinguish whether the device is used as an amplifier or a comparator in the circuit. According to the principle of the virtual short of the amplifier, that is to say, if the operational amplifier works normally, the voltages at the non-inverting input terminal and the inverting input terminal must be equal, even if there is a difference, it is at the mv level. The internal resistance will have little impact on the voltage test, but generally, it will not exceed 0.2V. If there is a difference of more than 0.5V, the amplifier must be broken. If the device is used as a comparator, the non-inverting input and the inverting input are allowed to be unequal. If the same voltage > reverse voltage, the output voltage is close to a positive value; if the same voltage < reverse voltage, the output voltage is close to 0V or a negative value (depending on dual power supply or single power supply). If the detected voltage does not meet this rule, the device must be bad, so that you can judge whether the op-amp is good or bad without having to use the substitution method, without removing the chip on the circuit board.

SMT component failure

Some SMD components are very small, and it is very inconvenient to use ordinary multimeter test pens to test and repair them. First, it is easy to cause a short circuit, and second, it is inconvenient to contact the metal parts of the component pins on the circuit board coated with the insulating coating. Here is a simple method that will bring a lot of convenience to the detection. Take two sewing needles and place them close to the multimeter pen, then take a thin copper wire in a multi-strand cable, use the thin copper wire to tie the pen and sewing needle together, and then solder them firmly. In this way, there is no risk of short circuits when testing those SMT components with a test lead with a small needle tip, and the needle tip can pierce the insulating coating and directly hit the key parts, so there is no need to bother to scrape those membranes.

Short circuit fault of public power

In circuit board maintenance, if there is a short circuit of the public power supply, it is often a big problem, because many devices share the same power supply, and each device using this power supply is suspected of being short-circuited. If there are not many components on the board, the short-circuit point can eventually be found by using the method of "hoeing the ground"; if there are too many components, it will depend on luck whether the "hoeing the ground" can reach the situation. A more effective method is recommended here. Using this method, you can get twice the result with half the effort, and you can often find the fault point quickly. There must be a power supply with adjustable voltage and current, voltage 0-30V, and current 0-3A. Adjust the open circuit voltage to the power supply voltage level of the device, first adjust the current, and add this voltage to the power supply voltage point of the circuit, such as the 5V and 0V terminals of the 74 series chip, and gradually increase the current depending on the degree of short circuit. Touch the device with your hand. When you touch a device with obvious heat, this is often a damaged component, which can be removed for further measurement and confirmation. Of course, the voltage must not exceed the working voltage of the device during operation, and it cannot be reversed, otherwise, other good devices will be burned.

Board failure

More and more boards are used in industrial control, and many boards use gold fingers to be inserted into slots. Due to the harsh environment of the industrial site, the dusty, humid, and corrosive gas environment is easy to cause poor contact failure of the board. Many friends may solve the problem by replacing the board, but the cost of purchasing the board is very considerable, especially for some Boards for imported equipment. In fact, you might as well use an eraser to wipe the gold finger several times, clean the dirt on the gold finger, and then try the machine again. Maybe the problem will be solved. The method is simple and practical.

electrical failure

Various electrical faults, good and bad, probably include the following situations in terms of probability: poor contact: poor contact between the board and the slot, when the cable is broken inside, the connection is blocked, the line plug and the terminal are not in good contact, the element Device soldering, etc. belong to this category; signals are disturbed: For digital circuits, faults will only appear under certain conditions. It may be that the interference is too large, which affects the control system and causes errors. There are also individual components of the circuit board. The parameters or overall performance parameters have changed, which makes the anti-interference ability tend to the critical point and cause failure; the thermal stability of components is not good: from a large number of maintenance practices, the thermal stability of electrolytic capacitors is the first, followed by Other capacitors, triodes, diodes, ICs, resistors, etc.; moisture, dust, etc. on the circuit board: moisture and dust will conduct electricity and have a resistance effect, and the resistance will change during thermal expansion and contraction. The value will have a parallel effect with other components. When this effect is strong, the circuit parameters will be changed and faults will occur; software is also one of the factors to consider: many parameters in the PCB board are adjusted by software, and the margin of some parameters is too low to be critical. When the operating conditions of the machine meet the reason for the software to determine the failure, then the alarm will appear.