Inspection of PCB is the time to pay attention to some details in order to be more prepared to ensure the quality of product, when inspecting PCB , we should pay attention to the following 9 little common sense.

01 9 Common Senses for PCB Inspection

1. It is strictly forbidden to use grounded test equipment to touch TV, audio, video and other equipment that is live on the bottom plate without an isolation transformer To test the PCB board

It is strictly forbidden to directly test the TV, audio, video and other equipment without power supply isolation transformer with the equipment whose casing has been grounded. Although general tape recorders have power transformers, when you come into contact with special TVs or audio equipment, especially those with high output power or do not know the nature of the power supply used, you must first find out whether the chassis of the recorder is live, otherwise it will be very easy. The TV, audio and other equipment that are charged with the bottom board cause a short circuit of the power supply, which affects the integrated circuit, causing further expansion of the fault.

2. Pay attention to the insulation performance of the electric soldering iron when inspecting the PCB

It is not allowed to use a soldering iron for welding when it is charged. Make sure that the soldering iron is not charged. Be careful, it is safer to use a low-voltage soldering iron with 6~8V.

3. Before testing the PCB board, you must understand the working principle of the integrated circuit and its related circuits

Before checking and repairing the integrated circuit, you must first be familiar with the integrated circuit used Function, internal circuit, main electrical parameters, the function of each pin, the normal voltage of the pin, the waveform and the working principle of the circuit composed of peripheral components. If the above conditions are met, analysis and inspection will be much easier.

4. Do not cause a short circuit between pins when testing the PCB board

When measuring voltage or testing waveforms with oscilloscope probes, do not cause the test leads or probes to slip due to Short circuits between pins of integrated circuits, measured on peripheral printed circuits that are in direct communication with the pins. Any momentary short circuit can easily damage the integrated circuit, and extra care should be taken when testing flat packaged CMOS integrated circuits.

5. The internal resistance of the PCB board test instrument should be large

When measuring the DC voltage of the integrated circuit pin, you should choose a multimeter< with the internal resistance of the meter head greater than 20KΩ/V /u>, otherwise there will be a large measurement error for some pin voltages.

6. Pay attention to the heat dissipation of the power integrated circuit when testing the PCB board

The power integrated circuit should have good heat dissipation, and it is not allowed to work in a high-power state without a radiator.

7. The lead wires of the PCB board should be tested reasonably

If you need to add external components to replace the damaged parts inside the integrated circuit, you should choose small components, and The wiring should be reasonable to avoid unnecessary coupling, especially the ground terminal between the audio power amplifier integrated circuit and the preamplifier circuit.

8. Check the PCB board to ensure the welding quality

Make sure that the welding is firm during welding, and the accumulation of solder and air holes are likely to cause false welding. The welding time generally does not exceed 3 seconds, and the power of the soldering iron should be about 25W with internal heating. Check the soldered integrated circuit carefully, use an ohmmeter to measure whether there is a short circuit between the pins, and then turn on the power after confirming that there is no solder adhesion.

9. Do not easily judge that the integrated circuit is damaged when inspecting the PCB

Do not easily judge that the integrated circuit is damaged. Because the vast majority of integrated circuits are directly coupled, once a certain circuit is abnormal, it may cause multiple voltage changes, and these changes are not necessarily caused by damage to the integrated circuit. In addition, in some cases, the measured voltage of each pin is different from the normal When the values match or are close, it does not necessarily mean that the integrated circuit is good. Because some soft faults will not cause changes in DC voltage.

02 PCB board debugging method

For the new PCB board just brought back, we first have to roughly observe whether there are any problems on the board, such as whether there are obvious cracks, whether there are short circuit, open circuit, etc. If necessary, check that the resistance between the power supply and ground is large enough.

For a newly designed circuit board, some difficulties are often encountered in debugging, especially when the board is relatively large and there are many components, it is often impossible to start. But if you master a set of reasonable debugging methods, debugging will get twice the result with half the effort.

PCB board debugging steps:

1. For the new PCB board just brought back, we first have to roughly observe whether there are problems on the board, such as whether there are obvious cracks, Whether there is short circuit, open circuit and so on. If necessary, you can check whether the resistance between the power supply and the ground wire is large enough.

2. Then install the components. For independent modules, if you are not sure that they work normally, do not install all of them, but install them part by part (for relatively small circuits, you can install them all), so that it is easy to determine the fault range and avoid time-consuming When encountering a problem, there is no way to start.

Generally speaking, you can install the power supply first, and then power on to check whether the output voltage of the power supply is normal. If you are not very sure when powering on (even if you are sure, it is recommended that you add a fuse just in case), consider using an adjustable Regulated power supply.

First preset the overcurrent protection current, then slowly increase the voltage value of the regulated power supply, and monitor the input current, input voltage and output voltage. If there are no problems such as overcurrent protection during the upward adjustment process, and the output voltage has reached normal, it means that the power supply part is OK. On the contrary, it is necessary to disconnect the power supply, find the fault point, and repeat the above steps until the power supply is normal.

3. Next, gradually install other modules. After each module is installed, power it on and test it. When powering on, follow the above steps to avoid overheating due to design errors or/and installation errors. flow and burn components.

03 How to find PCB board faults

1. Measure the voltage method to find faulty PCB boards

The first thing to confirm is each chipWhether the voltage of the power supply pin is normal, and then check whether the various reference voltages are normal, and whether the working voltage of each point is normal, etc. For example, when a general silicon triode is turned on, the BE junction voltage is about 0.7V, while the CE junction voltage is about 0.3V or less. If the BE junction voltage of a triode is greater than 0.7V (except for special triodes, such as Darlington etc.), it may be that the BE junction is open.

2. Signal injection method to find the faulty PCB board

Add the signal source to the input terminal, and then measure the waveform of each point in turn to see if normal to find the point of failure. Sometimes we also use simpler methods, such as holding a pair of tweezers to touch the input terminals of each level to see if there is any response at the output terminal, which is often used in audio, video and other amplifier circuits (However, it should be noted that this method cannot be used for circuits with hot bottom plates or circuits with high voltage, otherwise it may cause electric shock). If there is no response before the touch, but there is a response after the touch, it means that the problem lies in the front, and you should focus on checking.

3. Other ways to find faulty PCB boards

There are many other methods to find fault points, such as seeing, listening, smelling, touching and so on.

"Looking" is to see if there is any obvious mechanical damage to the components, such as cracks, burnt black, deformation, etc.;

"Listening" is to listen to the working sound Whether it is normal, such as something that should not be ringing, the place that should be ringing is not ringing, or the sound is abnormal, etc.;

"Smell" is to check whether there is any peculiar smell, such as the smell of burning,  The smell of capacitor electrolyte, etc., for an experienced electronicmaintenance personnel, they are very sensitive to these smells;

"Touch" is to test whether the temperature of the device is normal by hand, for example, it is too hot or too cold.

Some power devices will heat up when they work. If they feel cool to the touch, it can basically be judged that they are not working. But if it's hot where it shouldn't be or if it's too hot where it should be, that won't work either. General power transistors, voltage regulator chips, etc., are completely fine when working below 70 degrees. What kind of concept is 70 degrees? If you press your hand up and hold it for more than three seconds, it means that the temperature is below 70 degrees (be careful to touch it tentatively first, and don't burn your hands)