Seven methods commonly used in PCB maintenance are shared

According to years of experience in maintenance and analysis of PCB circuit boards, we summarize the maintenance methods into seven categories. Today, we will not do a detailed analysis of PCB circuit diagram, but a general overview of several methods we often use in maintenance analysis, which are as follows:

One, visual method

Sight is the foundation of all maintenance. A piece of bad PCB to hand, the first thing we need to do is to carry out a comprehensive PCB appearance inspection, to see whether there is broken line, Open, Short, cold welding, missing pieces, multiple pieces, wrong pieces, kneeling feet and parts displacement, monument, anti-white, deformation or burned and so on. In particular, we should pay attention to whether there is even tin on the back of PCB. In addition, we should also see whether the defective PCB is repaired, whether the BGA solder joint is regular, and whether there are traces of the movement of the soldering iron in other parts. Visual inspection is the first and very important step of maintenance analysis.

Many people often ignore this point, bad PCB get hand, can't wait to start with multimeter, oscilloscope and other detection tools to check to check, until we spend a lot of time to check the problem point, only to find that the bad point as long as a little visual can be easily solved, most long-term maintenance personnel have these experiences and lessons.

Two. Comparative Method

As the saying goes: no comparison, no difference. Comparison is also a means of maintenance that we often use. We tend to measure some signal or value deviation during maintenance, but are not sure if this is abnormal, then the best way is to take a good part of the measurement against the control, sometimes with samples or in order to get a stronger evidence, we tend to compare a dozen.

The comparison can be divided into the following aspects:

1. According to the measurement method, it can be divided into:

(1) Comparison of resistance measurement;

(2) Comparison of voltage and waveform measurement;

(3) Comparison of current measurement.

2. According to appearance, it can be divided into:

(1) Comparison of multiple pieces and missing pieces;

(2) comparison of wrong parts;

(3) Comparison of different manufacturers of the same part

3. Functional tests can be divided into:

(1) Comparison of functional tests at different stations;

(2) Comparison of functional tests of good and bad products;

(3) Functional test and comparison of fixtures of different specifications;

(4) Comparison of functional tests of different external devices

Comparison is a shortcut that can help us find problems quickly. By comparing different maintenance methods, we can find the key to the problem accurately and timely.

Three, touch method

Strictly speaking, touch method is also known as temperature sensing method, that is, hands directly feel the temperature of the chip set or other components on the PCB, visually judge whether the PCB is working properly.

This method can be used for maintenance analysis and function test of production line. During PCB production, we have an example of having the operator touch the device to determine PCB quality for those PCB where the equipment is hot but the functional test is normal

In the process of repairing and analyzing broken PCB, we often have this experience. At the moment, we can't find the real cause of some of the problems, but when our hands sometimes unconsciously touch some BGA or chip, we feel that they are very hot, and quickly heat up.

Four. hand pressure method

When fixing some bad PCB, mainly unstable ones, we may encounter some problems that cannot be measured when measuring the signal. At this point, we can press some chips onto the PCB by hand, mainly BGA closed components.

If it's good, we can say redokbga or try to replace this chip. Manual pressing method is mainly suitable for bgaopen, cold welding or tin crack and other bad phenomena. Can be used for power-on test and normal resistance value test. 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 suitable for bgaopen, cold welding or tin crack and other bad phenomena. Can be used for power-on test and normal resistance value test. Its advantage is that it can reduce the tedious steps related to measurement and maintenance, and greatly shorten the maintenance time.

The manual pressing method also has some stress. For a mature maintenance analyst, the force should be appropriate when pressing the PCB by hand. Do not apply too much force, do not lift one end of the PCB and then press the BGA with your hand in case the PCB is powered off or disconnected.

When testing for cold welding when the BGA is powered on, the top of the BGA must be pressed by hand for better results

When a signal is connected between two BGA's 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 do we determine which BGA is poorly soldered? At this point, we can first take a simple and effective method, which is to press the two related BGA. When we pressed the BGA, the resistance returned to normal, indicating the presence of a faulty weld.

Press repeatedly in different directions to get results. Once or twice may not work, but don't lose heart. Try and press a few times, and you'll soon find the answer.

It is worth reminding that some open or cold welds of BGA can be judged by manual pressure, but not all open or cold welds can be determined by manual pressure.

Five.circuit cutting method

Open-circuit method is a common method to measure the resistance value of PCB components. The open-circuit method means that the line is disconnected and inspected separately. Usually used to judge cold welding, open circuit, short circuit of some parts or a point resistance value is too large or too small.

In the process of disconnecting a circuit, we sometimes need to remove some resistors, inductors, audions or other components, and sometimes we need to pick up some pins of the chip. Eliminating the drain resistance near the chip to measure the upper limit signal is a typical application of the open circuit method.

It must be emphasized here that we must not cut the PCB wiring to find unauthorized points of failure when some signals are connected between BGA and there is no small part in the middle of the line for us to disconnect.

As the company has strict appearance requirements for PCB products, it not only failed to troubleshoot, but also spent a lot of time and took many detours, and sometimes even scrapped PCB because of too many BGA reworks.

It is not uncommon for the above example to be popular in daily maintenance operations.

As an excellent maintenance analyst, I must have rigorous logical thinking ability and accurate judgment in every step, have a clear mind, clear thinking, orderly analysis, and achieve twice the result with half the effort.

Six. Decontamination Law

As the name implies, it is to remove dirt from the PCB.

There are some undesirable phenomena on PCB, such as large battery leakage flow or poor functional testing of other external equipment. Before we measure or replace parts, we can have a visual look at the suspected place with wuxi slag and other sundries. When necessary, we should clean it.

Seven. Substitution method

The replacement method we mentioned refers to the maintenance method taken when the resistance, voltage and waveform measurements are invalid, because not every defective PCB can be visually detected by the detection tool.

The substitution method also has certain principles. We can decide which one to replace first and which one to replace later based on our understanding of various PCB and maintenance experience. Secondly, some key point signal measurement can also be used as the basis of our replacement.

In actual maintenance, our replacement is a typical application of the replacement method for the defective PCB which is difficult to find at present.

As mentioned before, we only use replacement methods when we really can't find the problem, but in actual maintenance, we should not be lazy or temporary negligence to replace BGA or other chips without thoroughly checking the relevant wiring. This will affect its function.

In addition, excessive magnetic flux will also cause excessive leakage current of the battery. Of course, some excess flux can be detected with a multimeter. We should not forget this during maintenance.

In short, PCB board maintenance and analysis methods are varied. We should not only master various maintenance methods and analysis methods, but also learn and use them flexibly. For starters, it's not an overnight thing. We need to be calm, we need to work hard, we need to guard against arrogance and impetuous, we need to keep a normal heart, in order to make a difference.