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According to years of experience in the maintenance and analysis of defective PCBA, we will repair methods are grouped into seven categories, today here we do not do a detailed analysis of the PCBA circuit diagrams, but a general overview of our maintenance and analysis of the commonly used in several ways.
I.Visual method
Visual inspection is the basis of all maintenance. A piece of defective PCBA to hand, we need to do the first thing is to carry out a comprehensive PCBA appearance check to see if there is no broken line, Open, Short, cold welding, missing pieces, multiple pieces, wrong pieces, kneeling feet, and parts displacement, monument, anti-white, deformation or burned, etc., especially pay attention to the back of the defective PCBA is not connected to the tin? Also look at 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 step of maintenance analysis, but also a very important step.
Many people often ignore this point, a piece of bad PCBA to get their hands on, can't wait to start using a multimeter, oscilloscope and other inspection tools to check to check to wait until we spend a lot of time to check the problem, only to find that the defective points as long as a little visual can be easily solved, then regret has been too late? Most of the long-term maintenance and F/A personnel have these experiences and lessons. In fact, some pcba's have been repaired for days without a clue. In the end, usually unconsciously, the problem is solved visually. This is why it is important to pay special attention to this point in the actual maintenance analysis to avoid detours.
II.Comparison
As the saying goes, there is no difference without comparison. Comparison is also a maintenance tool we often use. We tend to measure some signal or value deviation in the maintenance process, but are not sure whether this is abnormal, then the best way is to take a good part of the control measurements against a piece,sometimes with samples or in order to get a stronger evidence, we tend to put a dozen to be compared.
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. Comparison of multiple and missing parts; B.Comparison of wrong parts; C.Comparison of the same parts from different manufacturers.
3.According to the function test can be divided into:
a. Comparison of function tests of different work stations b. Comparison of function tests of good and bad products c. Comparison of function tests of different fixture specifications; d. Comparison of function tests of different external equipment.
Comparison is a shortcut to help us find problems quickly. By comparing different repair methods, we can find the key to the problem in a timely and accurate manner.
III. Touch method
Strictly speaking, the touch method is also known as the temperature sensing method, that is, using the hand to directly feel the temperature of the chipset or other components on the PCBA, intuitively determine whether the PCBA is working properly.
This method can be used for actual maintenance analysis and function testing in the production line. In the PCBA production process, for those PCBAs with hot equipment but normal function tests, we have an example of letting operators touch the equipment to determine the quality of the PCBAs.
In the process of repairing and analysing bad PCBAs, we often have this experience. At present, we can not find the real cause of some problems, but when our hands sometimes unconsciously touch some BGAs or wafers, we will feel that they are very hot, and will soon warm up.
IV. Hand pressure method
In repairing some bad PCBAs, mainly unstable PCBAs, we may encounter some problems that cannot be measured when measuring signals. At this time, we can use our hands to press some chips on the PCBA, mainly BGA closed components. Let's see if it's okay!
If it is fine, we can say redokbga or try to replace the chip. Manual pressing is mainly used for bgaopen, cold soldering or tin cracks. It can be used for power-on testing and normal resistance testing. Its advantage is that it can reduce the tedious steps related to measurement and maintenance, and greatly shorten the maintenance time.
Manual pressing method is mainly applicable to bgaopen, cold soldering or tin cracks and other defective phenomena. It can be used for power-on test and normal resistance test. Its advantage is that it can reduce the tedious steps related to measurement and maintenance, and greatly shorten the maintenance time.
The manual press method also has a certain degree of stress. For a mature maintenance analyst, when pressing the PCBA by hand, the force should be appropriate. Don't use too much force, don't lift one end of the PCB and then press the BGA by hand to prevent the PCB from powering down or disconnecting.
When testing whether there is cold soldering when the BGA is powered up, the top of the 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 resistor value) is too high and there are no other components between the lines for us to open, how to determine which BGA is not soldered properly? In this case, we can first take a simple and effective method, that is to press the two related BGAs by hand. When we press the BGA, its resistance returns to normal, indicating that there is a bad soldering.
Repeatedly press in different directions to get the result. Once or twice may not be very effective, but don't be discouraged. Try again and again and press several times and you will soon find the answer.
It is worth remembering that we can determine open or cold soldering of some BGAs by manual pressure, but not all open or cold soldering can be determined by manual pressure.
V. Open Circuit Method
The open circuit method is a common way to measure the resistance value of PCBA components. The so-called open-circuit method refers to disconnecting the wires and checking them individually. Usually used to determine the cold welding, open circuit, some parts of the short circuit or a point of resistance value is too large or too small.
In the process of disconnecting the circuit, we sometimes need to remove some resistors, inductors, trimpots or other parts, and sometimes need to pick up some of the pins of the chip. Eliminating the drain resistance near the chip to measure the upper limit signal is a typical application of the breakout method.
It is important to stress here that when some signals are connected between BGAs and there is no small part in the middle of the circuit for us to disconnect, we must not cut the PCB wiring to find the unauthorised failure point.
Due to the company's strict appearance requirements for PCBA products, not only did we fail to troubleshoot the problem, but we also spent a lot of time, made a lot of detours, and sometimes even scrapped the PCBA because the BGA was reworked too many times.
It is not uncommon for the above examples to be prevalent in day-to-day maintenance operations.
As an excellent maintenance analyst, every step of the way, we must have strict logical thinking ability and accurate judgement, a clear head, clear thinking, orderly analysis, to achieve twice the result with half the effort.
VI.Dirt Removal Method
As the name suggests, is to remove the dirt on the PCBA?
PCBA on some bad phenomena, such as battery leakage current is too large or other external equipment, such as poor functionality test, we measure or replace parts before, you can visually look at the suspected place there is no tin slag and other debris, etc., if necessary, to clean it up, some of the bad phenomena of this trick will have a peculiar efficacy.
VII.Replacement method
We mentioned the replacement method refers to resistance, voltage, waveform measurement is ineffective when the maintenance method, because not every piece of defective PCBA can be used to detect tools intuitively detected.
Replacement method also has certain principles. We can decide which one to replace first and which one to replace later according to our own understanding of various pcba and maintenance experience. Secondly, the measurement of some key point signals can also be used as the basis of our replacement.
In actual maintenance, for defective PCBAs that are difficult to find, our replacement is a typical application of the replacement method.
As mentioned earlier, we only use the replacement method when we really cannot find the problem, but in actual maintenance, we should not replace BGAs or other wafers at any time out of laziness or temporary negligence without thoroughly checking the relevant wiring. This will usually affect its function.
In addition, excessive magnetic flux can cause excessive battery leakage current. Of course, some excessive flux can be detected with a multimeter. We should not forget this during maintenance.
Bad PCBA repair and analysis methods are many and varied. We not only need to master a variety of PCBA repair methods and analysis methods, but also flexible learning and use. For beginners, this is not an overnight thing. We have to be calm, to work hard, to guard against pride and impatience, to maintain a normal heart, in order to make a difference.