1. Visual method
Visual inspection is the basis of all maintenance. When we get a bad PCBA, the first thing we need to do is to conduct a comprehensive visual inspection of the PCBA to see if there is any disconnection. Open. Short, cold welding, missing parts, multiple parts, wrong parts, kneeling And parts displacement, tombstone, anti-white, deformation or burnt, etc., especially pay attention to whether there is tin on the back of the PCBA? In addition, it depends on whether the defective PCBA has been repaired, whether the BGA solder joints are regular, whether other parts are Traces of soldering iron movement. Visual inspection is the first and very important step in maintenance analysis.
Many people often overlook this point. After getting a piece of bad PCBA, we can't wait to start checking it with multimeters, oscilloscopes and other testing tools. We didn't find out until we spent a lot of time finding the problem. The bad point can be easily solved with a little visual inspection, and it is too late to regret it? Most long-term maintenance and F/A personnel have these experiences and lessons. In fact, some PCBAs have been repaired for days without a clue. Eventually, often unconsciously, the problem is solved visually. Therefore, special attention should be paid to this point in the actual maintenance analysis to avoid detours.
2. Comparative method
As the saying goes: There is no difference without comparison. Comparison is also a maintenance method that we often use. We tend to measure some signal or value deviation during maintenance, but not sure if this is abnormal, then the best way is to take a good part against the control measurement, sometimes with the sample or to get a stronger evidence, we tend to Compare a dozen or so.
The comparison can be divided into the following aspects:
1. According to the measurement method can be divided into:
A. Comparison of resistance measurements;
B. Comparison of voltage and waveform measurements;
C. Comparison of current current measurements.
2 According to the appearance can be divided into:
A. Multiple and missing parts comparison;
B. Comparison of wrong parts;
C. Comparison of different manufacturers of the same part 3. According to the function test, it can be divided into:
A. Comparison of functional tests of different stations
B. Comparison of functional tests of good and defective products
C. Functional test comparison of fixtures with different specifications;
D. Comparison of different external device function tests Comparison is a shortcut that can help us quickly find problems. By comparing different maintenance methods, we can find the key to the problem in time and accurately.
3. Touch method
Strictly speaking, the touch method is also called the temperature sensing method, that is, to directly feel the temperature of the chipset or other components on the PCBA by hand, and intui judge whether the PCBA is working normally.
This method can be used for actual maintenance analysis and functional testing of the production line. In the PCBA production process, for those PCBAs whose equipment is hot but functionally tested, we have an example of letting the operator touch the equipment to determine the quality of the PCBA
In the process of repairing and analyzing bad PCBA, we often have such experience. Currently, we can't find the real cause of some problems, but when our hands touch some BGAs or chips unconsciously sometimes, we feel that they are hot and it heats up very quickly.
4.hand pressure method
When repairing some bad PCBAs, mainly unstable PCBAs, we may encounter some problems that cannot be measured when measuring signals. At this point, we can press some chips on the PCBA by hand, mainly BGA closure components. See if it's all right!
If it is good, we can say redokbga or try to replace this chip. The manual pressing method is mainly suitable for bad phenomena such as bgaopen, cold soldering or tin cracks. It can be used for power test and normal resistance value test. Its advantage is that it can reduce the cumbersome steps of related measurement and maintenance, and greatly shorten the maintenance time
The manual pressing method is mainly suitable for bad phenomena such as bgaopen, cold soldering or tin cracks. It can be used for power test and normal resistance value test. Its advantage is that it can reduce the cumbersome steps of related measurement and maintenance, and greatly shorten the maintenance time.
The manual pressing method also has a certain amount of 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 hands, in case the PCB is powered off or disconnected.
When testing whether there is cold welding when the BGA is powered on, you must press the top of the BGAJ by hand to achieve a better effect
When a signal is connected between two BGAs and we find 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 to determine which BGA is badly welded? At this time , we can first take a simple and effective method, which is to press two related BGAs by hand. When we pressed the BGA, its resistance returned to normal, indicating that there was a bad soldering.
Press repeatedly from different directions to obtain results. Once or twice may not have much effect, but don't be discouraged. With a lot of trial and error, you'll find out in no time.
It is worth reminding that we can judge the open welding or cold welding of some BGAs by manual pressure, but not all open welding or cold welding can be determined by manual pressure.
5.circuit breaking method
The open circuit method is a common method for measuring the resistance value of PCBA components. The so-called open circuit method refers to disconnecting the line and checking it separately. It is usually used to judge cold welding, open circuit, short circuit of some parts or too large or too small resistance value of a certain point.
In the process of breaking the circuit, we sometimes need to remove some resistors, inductors, transistors or other components, and sometimes we need to pick up the pins of some chips. Eliminating the drain resistance near the chip to measure the upper signal is a typical application of the open circuit method.
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, not only did not troubleshoot, but also spent a lot of time, took many detours, and sometimes even scrapped the PCBA because of too many BGA reworks.
It is not uncommon for the above example to be prevalent during routine maintenance operations.
As an excellent maintenance analyst, every step must have rigorous logical thinking ability and accurate judgment, clear mind, clear thinking, orderly analysis, so as to achieve twice the result with half the effort.
6.decontamination method
As the name suggests, it is to remove the dirt on the PCBA?
There are some bad phenomena on the PCBA, such as excessive battery leakage current or poor function test of other external devices. Cleaning it will have a strange effect on certain 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 detected with inspection tools.
The substitution method also has certain principles. We can decide which one to replace first and which one to replace later based on our own understanding and maintenance experience of various PCBAs. Second, the measurement of some keypoint signals can also serve as the basis for our replacement.
In actual maintenance, our replacement is a typical application of the replacement method for defective PCBAs that are difficult to find at present.
As mentioned before, we will only use the replacement method when we really can't find the problem point, but in actual maintenance, we should not check the relevant lines thoroughly because of laziness or temporary negligence. Replace BGA or other chips at any time. This usually affects its functionality.
In addition, excessive magnetic flux will also cause excessive battery leakage current. Of course, some excess flux can be detected with a multimeter. During maintenance, we should not forget this.
