PCBA Tech - Seven methods commonly used in PCBA maintenance

Based on the maintenance and analysis experience of PCBA over the years, we have classified the maintenance methods into seven categories. Today, we will not make a detailed analysis of the PCBA circuit diagram, but roughly outline several methods that we often use in the maintenance analysis.

1. Visual method

Visual inspection is the basis of all maintenance. When we get a piece of defective PCBA, the first thing we need to do is to conduct a comprehensive visual inspection of the PCBA to see whether there are broken wires, open, short, cold welding, missing parts, multiple pieces, wrong pieces, kneeling feet, parts displacement, monument, back whitening, deformation or burning, especially whether there is tin on the back of the PCBA? In addition, it also depends on whether the defective PCBA is repaired, whether the BGA solder joints are regular, and whether there are traces of soldering iron movement in other parts. Visual inspection is the first and very important step of maintenance analysis. Many people often ignore this point. After getting a piece of defective PCBA, they can't wait to start checking it with multimeter, oscilloscope and other detection tools. When we spend a lot of time to find the problem, we find that the problem can be easily solved with a little visual inspection. At this time, it is too late to regret? Most long-term maintenance and F/A personnel have these experiences and lessons. In fact, some pcba have been repaired for several days without any clue. Eventually, often unconsciously, the problem is solved visually. Therefore, we should pay special attention to this point in the actual maintenance analysis to avoid taking detours.

2. Comparison method

As the saying goes, there is no difference without comparison. Comparison is also a maintenance method that we often use. We tend to have some signal or value deviation in the process of measure maintenance, but we are not sure whether it is abnormal. The best way is to take a good part for control measurement. Sometimes, we tend to compare more than a dozen with samples or to get a stronger evidence.

The comparison can be divided into the following aspects:

1) It can be divided into:

a. Comparison of resistance measurements;

b. Comparison of voltage and waveform measurement;

c. Comparison of current current measurements.

2) It can be divided into:

a. Comparison of multiple and missing parts;

b. Comparison of wrong parts;

c. Comparison of different manufacturers of the same part

3) It can be divided into:

a. Comparison of functional tests at different stations;

b. Comparison of functional tests of defective products;

c. Function test comparison of different specifications of fixtures;

d. The comparison and comparison of functional tests of different external devices is a shortcut, which can help us quickly find problems. By comparing different maintenance methods, we can find the key to the problem timely and accurately.

3. Touch method

Strictly speaking, the touch method is also known as the temperature sensing method, that is, you can directly feel the temperature of the chipset or other components on the PCBA with your hand to visually judge whether the PCBA works normally. This method can be used for actual maintenance analysis and function test of production line. In the PCBA production process, for those PCBA whose equipment is hot but whose function test is normal, we have an example of letting the operator touch the equipment to determine the quality of PCBA. In the process of repairing and analyzing bad PCBA, we often have such experience. At present, we cannot find the real cause of some problems, but when our hands sometimes unconsciously touch some BGA or chips, we will feel that they are very hot and will soon heat up.

4. Hand pressing method

When repairing some bad PCBA, mainly unstable PCBA, we may encounter some problems that cannot be measured when measuring signals. At this time, we can press some chips on the PCBA by hand, mainly BGA closed components. See if it's okay. If it is good, we can say red ok bga or try to replace this chip. The manual pressing method is mainly applicable to such undesirable phenomena as bgaopen, cold welding or tin cracks. It can be used for power on test and normal resistance value test. Its advantage is that it can reduce the tedious steps of relevant measurement and maintenance, and greatly shorten the maintenance time. The manual pressing method is mainly applicable to such undesirable phenomena as bgaopen, cold welding or tin cracks. It can be used for power on test and normal resistance value test. Its advantage is that it can reduce the tedious steps of relevant measurement and maintenance, and greatly shorten the maintenance time. Manual pressing method also has certain stress. For a mature maintenance analyst, when pressing the PCBA by hand, the force should be appropriate. Do not use too much force. Do not lift one end of the PCB, and then press the BGA with your hand to prevent the PCB from powering down or disconnecting. When testing whether there is cold welding when BGA is powered on, the top of BGA must be pressed by hand to achieve better results. When a signal is connected between two BGAs, and we measure that its resistance (diode value or resistance value) is too large, and there are no other components between the lines for us to open, how can we determine which BGA is poorly soldered? At this point, we can first take a simple and effective method, that is, press two related BGAs with your hand. When we press BGA, its resistance returns to normal, indicating that there is a poor welding. Press repeatedly from different directions to get results. One or two times may not be effective, but don't lose heart. Try again and press several times, and you will find the answer soon. It is worth reminding that we can judge the open welding or cold welding of some BGA by manual pressure, but not all the open welding or cold welding can be determined by manual pressure.

5. Circuit breaking method

The open circuit method is commonly used to measure the resistance of PCBA elements. The so-called open circuit method refers to disconnecting the circuit and checking separately. It is usually used to judge cold welding, open circuit, short circuit of some parts, or too large or too small resistance at a point. In the process of disconnecting the circuit, we sometimes need to remove some resistors, inductors, transistors or other components, and sometimes we need to pick up some chip pins. The typical application of open circuit method is to eliminate the drain resistance near the chip to measure the upper limit signal. It must be emphasized here that when some signals are connected between BGAs, and there is no small part in the middle of the line for us to disconnect, we must not cut the PCB wiring to find unauthorized fault points. Because the company has strict appearance requirements for PCBA products, it not only did not eliminate the fault, but also spent a lot of time and took many detours, sometimes even scrapped PCBA due to too many BGA rework times. It is not uncommon for the above examples to be popular in routine maintenance operations. As an excellent maintenance analyst, every step should have rigorous logical thinking ability and accurate judgment, clear mind, clear thinking, and orderly analysis, so as to achieve twice the result with half the effort.

6. Decontamination method

As the name implies, is it to remove the dirt on the PCBA? There are some undesirable phenomena on the PCBA, such as excessive battery leakage current or poor function test of other external equipment. Before measuring or replacing parts, we can visually check the suspected place for Wuxi slag and other impurities, and clean them when necessary. This move will have a strange effect on some undesirable phenomena.

7. Replacement method

The replacement method we mentioned refers to the maintenance method adopted when the resistance, voltage and waveform measurements are invalid, because not every defective PCBA can be visually detected with detection tools. The replacement method also has certain principles. We can decide which to replace first and which to replace later according to our understanding of various pcba and maintenance experience. Secondly, the measurement of some key point signals can also be used as the basis for our replacement. In actual maintenance, for defective PCBA that are difficult to find at present, our replacement is a typical application of the replacement method. As mentioned above, we will only use the replacement method when we can't find the problem point, but in the actual maintenance, we should not replace the BGA or other chips at any time without thoroughly checking the relevant lines due to laziness or temporary negligence. This usually affects its functionality. In addition, excessive magnetic flux will also cause excessive leakage current of the battery. Of course, some excessive magnetic flux can be detected with a multimeter. During maintenance, we should not forget this. In a word, the maintenance and analysis methods of PCBA are various. We should not only master various maintenance methods and analysis methods, but also learn and apply them flexibly. For beginners, this is not overnight. We must calm down, work hard, guard against arrogance and rashness, and maintain a calm mind to make a difference.