Based on years of experience in PCBA maintenance analysis, we divide maintenance methods into seven categories. Today, we will not do a detailed analysis of the PCBA circuit diagram, but rather a general overview of the methods we often use in maintenance analysis. Methods as below:

1. Visual method

Vision is the foundation of all maintenance. When a bad PCBA arrives, the first thing we need to do is to conduct a comprehensive visual inspection of the PCBA to see if there are any broken wires, open circuits, short circuits, cold welding, missing parts, multiple parts, wrong parts, kneeling feet and parts. Displacement, erection, anti-whitening, deformation or burning, etc., pay special attention to whether there is continuous 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, and whether there are traces of the soldering iron moving in other parts. Visual inspection is the first and very important step in maintenance analysis.

Many people tend to ignore this. After getting a piece of bad PCBA, they can't wait to check and check constantly with test tools like multimeter, oscilloscope, etc. Only when we spend a lot of time looking for the problem, we find that this dead pixel can be easily solved with a little visual inspection. It's 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 unknowingly, the problem is resolved visually. Therefore, special attention should be paid to this point in the actual maintenance analysis to avoid detours.

2. Comparison Methods

As the saying goes: there is no difference without comparison. Comparison is also a maintenance tool that we often use. We tend to measure some signal or value bias during maintenance, but not sure if this is abnormal, then the best approach is to have good measurements against controls, sometimes using samples or to get stronger evidence, we tend to put a dozen Compare left and right.

The comparison can be divided into the following aspects:

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

A. Comparison of multiple parts 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:

One. Comparison of functional tests of different stations b. Function test comparison of good and bad products C. Function test comparison of different specifications of fixtures; D. Function test comparison of different external devices

Comparison is a shortcut that can help us find problems quickly. By comparing different fixes, we can pinpoint the crux of the problem in a timely and accurate manner.

3. Touch method

Strictly speaking, the touch method is also called the temperature sensing method, that is, the temperature of the chipset or other components on the PCBA is directly felt by hand, and it is intuitively judged whether the PCBA is working normally.

This method can be used for actual maintenance analysis and functional testing of production lines. In the PCBA production process, for those PCBAs where the device is hot but functionally tested, we have an example where the operator touches the device to determine the quality of the PCBA

We often have this experience in the process of repairing and analyzing bad PCBAs. At the moment we can't find the real cause of some problems, but when our hands sometimes inadvertently touch some BGAs or chips, we can feel them hot and get hot very quickly.

4. Hand pressing method

When repairing some bad PCBAs, mainly unstable ones, we may encounter some unmeasurable problems when measuring signals. At this point, we can press some chips on the PCBA by hand, mainly BGA closed components. See if it will be fine!

There is also a certain emphasis on manual pressing. For an experienced maintenance analyst, the pressure when pressing the PCBA by hand should be appropriate. Do not use too much force, do not lift one end of the PCB, and press the BGA with your hand to prevent the PCB from being powered off or disconnected.

When testing whether there is cold soldering when the BGA is powered on, you must press the top of the BGA 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 wires for us to open, how can we determine which BGA is badly soldered? At this point, we can first take a simple and effective method, which is to press the two related BGAs by hand. When we pressed the BGA, its resistance returned to normal, indicating poor soldering.

Press repeatedly from different directions to obtain results. A time or two may not have much effect, but don't be discouraged. Try and press multiple times and you'll find out in no time.

It is worth reminding that we can judge the open or cold soldering of some BGAs by manual pressure, but not all open or cold soldering can be determined by manual pressure.

5. Circuit break 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 at a certain point.

In the process of breaking the circuit, we sometimes need to remove some resistors, inductors, transistors or other parts, and sometimes we need to connect some pins of the chip. 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 the BGAs, and there is a small part in the middle of the line for us to disconnect, we must not cut the PCB traces to find unauthorized fault points.

Because the company has strict requirements on the appearance of PCBA products, it not only failed to eliminate faults, but also spent a lot of time, took a lot of detours, and sometimes even scrapped the PCBA due to excessive BGA rework.

It is not uncommon for the above example to be popular in 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, and more results with less effort.

6. Decontamination method

As the name suggests, is to remove the dirt on the PCBA?

There are some undesirable phenomena on the PCBA, such as excessive battery leakage current or poor functional testing of other external devices. Before measuring or replacing parts, we can visually check whether there are tin slag and other debris in the suspicious place, and clean it if necessary. This trick will have a unique effect on some undesirable phenomena.

7. Substitution method

The replacement method we are talking about refers to the maintenance method taken when the resistance, voltage, and waveform measurements are invalid, because not every defective PCBA can be detected intuitively with inspection tools.

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

In actual repair, our replacement is a typical application of defective PCBA replacement methods that are hard to find at present.

As mentioned earlier, we only use the replacement method when we really can't find the problem, but in actual maintenance, we can't be lazy or temporarily negligent without thoroughly checking the relevant wiring and replacing BGA or other chips at any time. This usually affects its functionality.

In addition, excessive magnetic flux can also lead to excessive battery leakage current. Of course, some excess flux can be detected with a multimeter. During the repair process, we should not forget this.

In conclusion, the maintenance and analysis methods of PCBA are varied. We must not only master various maintenance methods and analysis methods, but also learn to use them flexibly. For starters, this doesn't happen overnight. We need to be calm, work hard, guard against arrogance and rashness, and maintain a normal mind in order to make a difference.