The inspection, processing skills and the complete understanding of the formula of PCB board maintenance are arranged as follows by jhao PCB board factory:

Three methods of PCB board maintenance and inspection

1、 First look at the quantity and then the quantity

The circuit board to be repaired should be visually inspected first. If necessary, it should be observed with the aid of magnifying glass.

Mainly see:

1. whether there is any broken line and short circuit; Especially, whether the printed board connecting wire on the circuit board has the phenomenon of breaking and sticking;

2. whether the relevant components such as resistance capacitance inductive diode triode are disconnected;

3. has anyone repaired it? What components have you moved? Whether there are some problems such as false welding, missing welding and reverse insertion.

2、 First outside and then inside

If the situation allows, it is better to have a good circuit board as the board to be repaired as the reference, and then use the double Bang VI curve scanning function of the tester to conduct good and bad comparison test on the two boards. The starting point of comparison test can start from the port of the circuit board; Then, the comparison test of the meter and the inside, especially the capacitor, can make up for the defect that it is difficult to detect the leakage of the capacitor on line.

3： Easy first, then difficult

In order to improve the test effect, some technical treatment should be done to the repaired board before online function test on the circuit board to minimize the impact of various interference on the test process. The specific measures are as follows:

1. preparation before test

Short circuit crystal vibration (note that the two feet can be short circuited to the four foot crystal oscillator. Remember that the other two feet are power pins in general, and do not short connect them!) For the large capacity electrolytic capacitor, the next leg should be welded to open the circuit, because the charge and discharge of the large capacity capacitor will also bring interference.

2. the device is tested by exclusion method

In the process of online test or comparative test, all devices passing the test (or relatively normal) shall directly confirm the test results and record the test results. If the test fails (or the comparison is beyond the standard), it can be tested again. If it fails, the test results can be confirmed first, so that the test is carried out until the device on the board is tested (or compared) and then the devices that fail to pass the test (or the comparison is over poor) can be processed.

Some test instruments for devices that fail to pass the function online test also provide a more normal but practical treatment method: because the power supply of the test instrument to the circuit board can also be added to the corresponding power supply and ground pin of the device through test clamp. If the cutting edge is carried out on the source foot of the device, the device will be separated from the circuit board power supply system.

Then, the device is tested online; Because other devices on the circuit board will not be powered up and interference will be eliminated, the actual test effect will be equal to the accuracy rate of "quasi offline test".

Seven techniques of PCB board maintenance

1. appearance inspection method

That is, through the old method of looking, asking, smelling and cutting, observe whether there is burning place on the circuit board, whether copper is applied to have broken place, smell whether there is peculiar smell on the circuit board, whether there is bad welding place, whether the interface and gold finger have mildew and blackening. When asking the customer about the failure phenomenon and the process of the fault, the focus of the fault can be focused on some parts. Through the above treatment, we can often find some problems.

2. general killing method

All components will be tested once, and the defective components shall be replaced to achieve the purpose of repair. If the components that cannot be detected by the instrument are encountered, the replacement method shall be adopted, that is, whether it is damaged or not, new components shall be used instead, so as to ensure that all the devices on the board are good and achieve the purpose of repair. The method is simple and effective, easy to start, and the technical level of the engineer is not high, but the operation requires high care and responsibility. Otherwise, some devices do not remember the position when they are removed, and the wrong installation will lead to failure of maintenance. In addition, the method can not be used to solve the problems such as the hole is not available, copper deposit is broken, potentiometer is not adjusted properly. This method cannot be used when encountering chips with programs and data.

3. comparison method

The comparison method is one of the most common methods for maintenance of circuit boards without drawings. Practice proves that it has very good effect. Through the comparison with the good board state, the purpose of finding out the fault is achieved, and the abnormal is found by comparing the curves of each node of the two boards. However, many times we don't have a good board to compare, but this does not mean that we can not use this method. For example, we can find the circuit with the same properties in the circuit board, for example, there are three same interfaces in a board, and the three interfaces have the same circuit, we can use the curve of three identical interfaces to compare each other to judge the problem. When we can't find the above mentioned situation, we can still find the circuit with common properties, such as the bus in the circuit usually has the same curve. At other times, we can not judge whether an IC is damaged or not, and we may also use scanning curve to compare with good IC to achieve the purpose of judging. In short, we need to use our brains to bring the contrast to the extreme. Curve scanning is really useful in practice, but some beginners don't understand its working principle and complain that the instrument has no practical value. So I have seen many companies in the same industry basically use circuit board maintenance tester, and call it a good job. Of course, the merchants are also suspected of exaggerating the function. In fact, in many cases, when scanning the integrated circuit of COMS process, the curve of each scan is different, which makes it impossible to judge. This reason is caused by the high impedance of CMOS and the failure of drain channel after charging the inter junction capacitance. Therefore, it needs rich experience to judge CMOS integrated circuit. Finally, it is necessary to add that curve scanning is the peripheral device of scanning. If the scan is integrated circuit, it is the components connected to the pins in the scanned IC. 90% of the IC damage is peripheral device damage. However, in case of non peripheral device damage, the problem can not be found in scanning curve, and the device will have problems when it works.

4. state method

State method is to check the normal working state of each component. If the working state of a component does not conform to the normal state, there is a problem with the device or the affected part. For example, when we check CPU, we will check whether crystal oscillator works normally, reset signal is normal, etc. The input and output logic of the gate is correct to judge whether the device is normal or not. When using this method, the input is often forced to stimulate the signal. For example, the signal generator is used to send a signal to the input end of the operational amplifier, and the output signal amplitude and distortion are measured by oscilloscope to judge the operation and discharge problems. The input level of logic gate circuit is forced to be pushed up or pulled down, and the output level is measured to be in accordance with the logic of the chip, so as to judge the quality of the chip.

State method is the most accurate method of all maintenance methods. Its operation difficulty is not mastered by general engineers. It needs rich theoretical knowledge and practical experience. In order to meet the state requirements, engineers must try their best to test the circuit better than curve scanning, Whether it is the peripheral circuit or the peripheral circuit of IC, it can be found.

5. substitution method

Replace all IC with new ones until they are fixed. IC is becoming cheaper and cheaper now. 74 series and 4000 series chips are less than 1 yuan / chip. Instead of checking problems together, we should replace them all. This method is a helpless move, and its disadvantages are obvious. The effect should be lower than that of the kill method. At least, the kill method should judge the device. If it is accumulated for a long time, I will have some experience on some circuit boards. The repair rate of this method is determined by luck rather than effort. Even if it is repaired, I don't know where the fault is. But this method still has its advantages. If a circuit board or electronic device can be used to verify whether it is normal or not, we can replace IC one by one. When the fault disappears when it is replaced, it can be proved that the one just replaced is broken. If it is a direct plug-in IC, it is recommended that you install an IC seat immediately after removal. After all, it is only a few cents. There is also a fatal problem in this method: it is necessary to ensure that the IC you bought is good. If you accidentally encounter bad IC when purchasing, it will not only repair it well, but also enlarge the failure and increase the difficulty of repair.

6. grounded circuit method

The method of using the method of building up circuit is to make a circuit. The circuit can work after installing the missing integrated circuit, so as to verify the quality of the tested integrated circuit. For example, in order to judge the quality of a 555 integrated circuit, we can build a time-based oscillation circuit made of 555 circuit, and then put the tested integrated circuit in the circuit. If the circuit works normally, It is proved that 555 is OK, and vice versa.

The method can reach 100% accuracy, but the tested IC has many models and complex package, so it is difficult to build all the integrated circuits into one set of circuits.

7. principle analysis

This method is to analyze the working principle of a board. For engineers, some boards, such as switching power supply, can know the working principle and details without drawing. For engineers, it is very simple to know the maintenance of the parts that know the schematic diagram. However, for some relatively complex and rare equipment, without the ready-made schematic reference, we can reverse engineer it manually, draw out the schematic diagram, and then repair it with the schematic diagram, which can be said to be a great success.

The method requires great patience and careful attitude of engineers to complete. One effort can benefit forever. However, whether the method is really adopted depends on the value and quantity of maintenance and the difficulty of doing the work.

The maintenance tips of PCB board

A common circuit board, with a circuit combination of ten million

The character recognition is the key to the change of the component appearance

Resistance capacitance is the most common and damage is common

Resistance is easy to change, capacitance leakage and leakage

Inductive transformer is coil. It is easy to test and see on / off

Diode and triode, measure the PN junction forward and reverse

MOS tube and SCR, trigger test is the key

All of the above are discrete parts, IC for many years

Simulator has operation and amplifier, and virtual end is used to judge

The optocoupler is isolated at the front and rear stages. The damage is tens of millions

Digital devices are often seen in front of the 40 and 74 standards

And analog to digital converter, which is very hard to test

Don't forget ROM and CPLD. Burn and write depends on programmer

CPU, single chip microcomputer, timing logic instrument

All kinds of sensors, the damage probability is the first

Well, the above is about PCB board maintenance of the three inspection methods and seven processing skills and tips to fully understand, hope to be able to help you; We provide multi-layer PCB, PCB, PCB proofing service, welcome to consult!