PCB Tech - Detailed PCB soldering repair technology

1. Fault characteristics and maintenance of engineering PCB capacitor damage

Failures caused by capacitor damage are the highest in electronic equipment, and damage to electrolytic capacitors is the most common.

The performance of capacitor damage is: 1. Capacity becomes smaller; 2. Complete loss of capacity; 3. Leakage; 4. Short circuit.

Capacitors play different roles in the circuit, and the faults they cause have their own characteristics. In industrial control circuit boards, digital circuits account for the vast majority, and capacitors are mostly used for power supply filtering, and less capacitors are 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 will be no voltage output; or the output voltage is not well filtered, and the circuit is logically chaotic due to voltage instability. No matter the machine, if the capacitor is connected between the positive and negative poles of the power supply of the digital circuit, the failure will be the same as above.

This is especially obvious on computer motherboards. Many computers sometimes fail to turn on after a few years, and sometimes they can be turned on. Open the case, you can often see the phenomenon of electrolytic capacitors bulging, if you remove the capacitors to measure the capacity, Found to be much lower than the actual value.

The lifetime of a capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the lifetime 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 the capacitors that are close to the heat source, such as the capacitors next to the heat sink and high-power components. The closer you are to it, the greater the possibility of damage.

In the case of ups and downs during maintenance, except for the possibility of poor contact, most of the failures are generally caused by capacitor damage. .

2. Characteristics and discrimination of resistance damage

It is often seen that many beginners are tossing on the resistance when repairing the circuit, and it is dismantled and welded. In fact, there are a lot of repairs. As long as you understand the damage characteristics of the resistance, you don't have to spend a lot of time.

Resistor is the most numerous component in electrical equipment, but it is not the component with the highest damage rate. Open circuit is the most common type of resistance damage. It is rare for the resistance to become larger, and it is rare for the resistance to become smaller. Common ones include carbon film resistors, metal film resistors, wire wound resistors and insurance resistors.

Wirewound resistors are generally used for high current limiting, and the resistance is not large. When cylindrical wire wound resistors are burnt, some will turn black or the surface will burst or crack, and some will have no traces. Cement resistors are a type of wire wound resistors, which may break when burned out, otherwise there will be no visible traces. When the fuse resistor burns out, a piece of skin will be exploded on the surface, and some have no traces, but it will never be burnt or blackened. According to the above characteristics, you can focus on checking the resistance and quickly find out the damaged resistance.

3. How to judge the quality of operational amplifiers

No matter what type of amplifier, there is a feedback resistance Rf, so we can check this feedback resistance from the circuit when repairing, and use a multimeter to check the resistance between the output terminal and the reverse input terminal. If it is too large, such as If the resistance is more than a few MΩ, we can probably be sure that the device is used as a comparator. If the resistance is less than 0Ω to several tens of kΩ, then check if there is a resistance connected between the output terminal and the inverting input terminal. It is used as an amplifier.

According to the principle of virtual shortness of the amplifier, that is to say, if the operational amplifier works normally, the voltage at the same direction input terminal and the reverse input terminal must be equal, and even if there is a difference, it is at the mv level. Of course, in some high input impedance circuits, a multimeter The internal resistance will have a little effect on the voltage test, but generally it will not exceed 0.2V. If there is a difference of 0.5V or more, the amplifier will undoubtedly be broken! (I am using a FLUKE179 multimeter)

If the voltage in the same direction is less than the reverse voltage, the output voltage is close to 0V or the negative maximum value (depending on dual power supply or single power supply).

4. A small trick to test SMT components with a multimeter

Take the two smallest sewing needles, (Deep Industrial Control Maintenance Technology Column), close them to the multimeter pen, and then take a thin copper wire from a multi-strand cable, and tie the pen and the sewing needle with the thin copper wire. Together, use solder to solder firmly. In this way, there is no risk of short circuit when measuring those SMT components with a test pen with a small needle tip, and the needle tip can pierce the insulating coating and directly ram the key parts without having to bother to scrape the film.

5. Troubleshooting methods for short circuit faults of PCB public power supply

In circuit board maintenance, if you encounter a short circuit of the public power supply, the fault is often serious, because many devices share the same power supply, and every device using this power supply is suspected of short circuit. If there are not many components on the board, use "hoe the earth" After all, you can find the short-circuit point. If there are too many components, it will depend on luck whether the "hoe the earth" can hoe the situation. A more effective method is recommended here. Using this method will get twice the result with half the effort, and the point of failure can often be found quickly.

6. A small eraser can solve big problems

More and more boards are used in industrial control. Many boards use golden fingers to insert into the slots. Due to the harsh industrial site environment, dusty, humid, and corrosive gas environments, the boards are likely to have poor contact failures. Friends may have solved the problem by replacing the board, but the cost of buying the board is very considerable, especially the boards of some imported equipment. In fact, you might as well use an eraser to rub the finger repeatedly, clean the dirt on the finger, and try the machine again. The problem may be solved! The method is simple and practical.

7. Analysis of electrical faults in good times and bad times

In terms of probability, various electrical faults with good times and bad times may include the following situations:

1). Poor contact

2). The signal is disturbed

3). Poor thermal stability of components

4). Moisture, dust, etc. on the circuit board.

5). Software is also one of the considerations

8. How to quickly find component information

Modern electronic products are diverse, and PCB components are becoming more and more diverse. In circuit board welding and repair, especially in the field of industrial circuit board repair, many components are unseen or even unheard of. In addition, even if a certain board has components on hand The information is complete, but you need to browse and analyze the information one by one in the computer. If there is no quick search method, the maintenance efficiency will be greatly reduced. In the field of industrial electronic maintenance, efficiency is money. If you can’t get along with efficiency, you can’t get around with pocket money. .