PCB manufacturing and BGA repair tips
1: Fault characteristics of industrial control circuit board capacitor damage and BGA maintenance
Faults caused by capacitor damage are the highest in electronic equipment, with the most common damage from electrolytic capacitors being the most common.
The damage of the capacitor is as follows: 1. The capacity becomes smaller; 2. The capacity is completely lost; 3. The leakage occurs; 4. The short circuit.
Capacitors play different roles in the circuit, and the resulting faults also have their own characteristics. In the industrial control circuit board, digital circuits account for the vast majority, and capacitors are often used for power supply filtering, and the capacitance used for signal coupling and oscillation circuits is less. If the electrolytic capacitor used in the PCB 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, the circuit is logically confusing due to the unstable voltage, and the performance is good or bad when the machine is working. Or can not open the machine, if the capacitor is between the positive and negative poles of the digital circuit, the fault performance 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 open the machine, and sometimes they can be turned on. When you open the case, you can often see the phenomenon of electrolytic capacitor bulging. If you remove the capacitor, measure the capacity. It was found to be much lower than the actual value.
The life of a capacitor is directly related to the ambient temperature. The higher the ambient temperature, the shorter the capacitor life. This rule applies not only to electrolytic capacitors, but also to other PCB manufacturing capacitors. Therefore, when looking for faulty capacitors, it is important to check the capacitors that are close to the heat source, such as the capacitors next to the heat sink and the high-power components. The closer to them, the greater the possibility of damage. I once repaired the power supply of an X-ray flaw detector. The user reflected that smoke had emerged from the power supply. After disassembling the chassis, I found a large capacitor of 1000uF/350V with oily things flowing out and removing a certain amount of capacity. Only a few tens of uF, it is found that only this capacitor is closest to the heat sink of the rectifier bridge, and the other distances are intact and the capacity is normal. In addition, there is a case where the ceramic capacitor is short-circuited, and it is also found that the capacitor is relatively close to the heat-generating component. Therefore, there should be some emphasis when looking for repairs.
Some capacitors are more likely to leak, and even hot when touched with a finger, this capacitor must be replaced.
In the case of a good or bad fault during maintenance, the possibility of poor contact is ruled out, and most of them are faults caused by capacitor damage. Therefore, when encountering such a fault, you can check the capacitors carefully. It is often surprising to replace the capacitors (of course, pay attention to the quality of the capacitors, and choose a better brand, such as ruby, black diamonds, etc.).
2. Characteristics and discrimination of resistance damage
It is often seen that many beginners toss on the resistance when the BGA repairs the circuit, and it is disassembled and welded. In fact, it is much more repaired. As long as you understand the damage characteristics of the resistor, you don't have to pay much attention.
Resistance is the most abundant component in electrical equipment, but it is not the component with the highest damage rate. Resistance damage is the most common with open circuit, and the resistance value becomes less common. It is very rare for PCB manufacturing resistance to become smaller. Common carbon film resistors, metal film resistors, wirewound resistors and fuse resistors. The first two resistors are the most widely used, and their damage characteristics are low resistance (less than 100Ω) and high resistance (above 100kΩ), and high resistance, intermediate resistance (such as several hundred ohms to tens of thousands of ohms). Very little damage; second, when the low-resistance resistance is damaged, it is often burnt and black, which is easy to find, and there is little trace when the high-resistance resistance is damaged. Wirewound resistors are generally used as current limiting currents with little resistance. When the cylindrical wirewound resistor is burnt out, some may be black or have a surface blast, crack, and some have no trace. Cement resistance is a type of wirewound resistor that may break when burnt out, otherwise there are no visible traces. When the fuse is burned out, some surfaces will blow off a piece of skin, and some will have no trace, but it will never burnt and black. According to the above characteristics, when checking the resistance, you can focus on it and quickly find out the damaged resistance.
According to the characteristics listed above, we can first observe whether the low-resistance resistance on the circuit board has black marks, and then according to the characteristics that most open circuits or resistance values become large and the high-resistance resistance is easily damaged. We can use a multimeter to directly measure the resistance across the high-resistance resistor on the board. If the resistance is greater than the nominal resistance, the resistor will be damaged. (Note that the resistance is stable after the resistance is displayed. In conclusion, because there is a possibility that the capacitor is connected in parallel in the circuit, there is a charge and discharge process. If the resistance is smaller than the nominal resistance, it is generally ignored. In this way, each resistor on the board is measured once, even if "missing" a thousand, it will not let go.
3. How to judge the good or bad of operational amplifier
The judgment of the operational amplifier is quite difficult for a considerable number of electronic repairers. It is not only the relationship of the degree of education. There are many undergraduates in the family. If you don’t teach, you will definitely not. If you teach, you will have to understand it for a long time. There is also a special The postgraduate of the instructor's frequency control is actually the same!), I would like to discuss with you here, I hope to help everyone.
Ideal op amps have "virtual short" and "dummy" characteristics that are useful for analyzing linearly used op amp circuits. To ensure linear operation, the op amp must operate in a closed loop (negative feedback). If there is no 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 as a comparator in the circuit.
From the figure we can see that no matter what type of amplifier, there is a feedback resistor Rf, we can check this feedback resistor from the circuit during maintenance, check the resistance between the output and the reverse input with a multimeter Value, if the large is outrageous, such as a few MΩ or more, then we can probably be sure that the device is used as a comparator. If the resistance is smaller than 0Ω to tens of kΩ, then check whether there is a resistor connected to the output and the reverse input. Between the ends, there is something to be used as an amplifier.
According to the principle of the virtual short of the amplifier, that is, if the operational amplifier works normally, the voltages of the same input and the reverse input must be equal, even if there is a difference, the mv level, of course, in some high input impedance circuits, the multimeter The internal resistance will have a little effect on the voltage test, but it will not exceed 0.2V. If there is a difference of 0.5V or more, the amplifier will be bad! (I am using the FLUKE179 multimeter). If the device is used as a comparator, it is allowed. The same direction input and reverse input end are not equal, the same direction voltage > reverse voltage, the output voltage is close to the positive maximum value; the same direction voltage < reverse voltage, the output voltage is close to 0V or negative maximum value (depending on Dual power supply or single power supply).
If the detected voltage does not meet this rule, then the device will be bad! So you don't have to use the substitution method, you can judge the operational amplifier is good without removing the chip on the board.
4, a small test of the multimeter test SMT components
Some SMT mounting components are very small, and it is very inconvenient to test and repair with a common multimeter pen. First, it is easy to cause short circuit. Second, it is inconvenient for the circuit board coated with the insulating coating to contact the metal part of the component pin. Here is a simple way to tell you a lot of convenience.
Take the two smallest needles, and fasten them to the multimeter pen, then take a thin copper wire in a multi-strand cable, tie the test leads and the sewing needle with a thin copper wire, and then solder them with solder. . In this way, when the SMT component is tested with a small needle tip, there is no short circuit, and the needle tip can pierce the insulating coating and directly smash the key parts, and no need to bother to scrape the film.
5, the circuit board public power supply short circuit fault repair method
In circuit board maintenance, if the short circuit of the common power supply is encountered, the fault is often large. Because many devices share the same power supply, each device using this power supply has a short circuit. If there are not many components on the board, the "earth" is adopted. In the end, the short-circuit point can be found. If there are too many components, it is up to luck if the "earth" can not reach the situation. Here, we recommend a method of comparing and using the method. With this method, you can find the point of failure very quickly.
It is necessary to have a power supply with adjustable voltage and current, voltage 0-30V, current 0-3A, this power supply is not expensive, about 300 yuan. Adjust the open circuit voltage to the device power supply voltage level, first adjust the current to the minimum, add this voltage to the power supply voltage point of the circuit, such as the 5V and 0V terminals of the 74 series chip, depending on the degree of short circuit, slowly increase the current. When the hand touches the device, when a certain device is heated, this is often a damaged component, which can be removed for further measurement confirmation. Of course, the voltage must not exceed the operating voltage of the device during operation, and it cannot be reversed, otherwise it will burn out other good devices.
6. A small rubber to solve big problems
There are more and more boards used for industrial control. Many boards use gold fingers to insert slots. Due to the harsh industrial environment, dusty, humid, and corrosive gas environments can cause poor contact failure. A friend may solve the problem by replacing the board, but the cost of purchasing the board is very considerable, especially for some imported equipment. In fact, you may wish to use the eraser to rub a few times on the gold finger, clean the dirt on the gold finger, and then try the machine, maybe solve the problem! The method is simple and practical.
7. Analysis of good and bad electrical faults
Various types of good and bad electrical faults may include the following in terms of probability:
(1). Poor contact
Poor contact between the board and the slot, unreachable when the cable is broken inside, poor contact between the plug and the terminal, and solder joints of the components are all such;
(2). Signal interference
For digital circuits, under certain conditions, the fault will be presented. It may be that the interference is too much, which affects the control system to make errors, and the individual component parameters or overall performance parameters of the board have changed, so that the interference is prevented. The ability tends to a critical point, causing a failure;
(3). The thermal stability of the components is not good.
From the perspective of a large number of maintenance practices, the thermal stability of the first electrolytic capacitor is not good, followed by other capacitors, transistors, diodes, ICs, resistors, etc.
(4). There is moisture, dust, etc. on the circuit board.
Moisture and dust will be conductive, have a resistance effect, and the resistance will change during the process of thermal expansion and contraction. This resistance value will have parallel effect with other components. When this effect is strong, the circuit parameters will be changed to make the fault. occur;
(5). Software is also one of the considerations
Many parameters in the circuit are adjusted by software. The margin of some parameters is adjusted too low and is in the critical range. When the machine operating condition meets the reason for the software to determine the fault, the alarm will appear.
8. How to quickly find PCB manufacturing components and materials?
Modern electronic products are diverse, and the types of components are increasing. It is more than a thousand. In the field of circuit maintenance, especially in the field of industrial circuit board maintenance, many components are unheard of, or even unheard of, and even if the components of a certain board are complete, However, in the computer, the information should be read and analyzed one by one. 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 the efficiency is too low, it will not be able to go with the pocket money.
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