When testing printed circuit boards, it is time to pay attention to some details in order to ensure product quality. When doing the tests, there are 9 tips that you should know.

 

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

 

Although general tape recorders have power transformers, when you come into contact with special TVs or audio equipment, especially those with high output power, you must first find out whether the chassis of the radio is live. The TV, audio and other equipment charged with the bottom plate will cause a short circuit of the power supply, which affects the integrated circuit and causes further expansion of the fault.

 

2. Pay attention to the insulation performance of the electric soldering iron during tests.

 

It's not allowed to use a soldering iron to weld with electricity. Make sure that the soldering iron is not charged. It's best to ground the shell of the soldering iron. Be careful with the MOS circuit. It's safe to use a 6-8V low-voltage circuit iron.

 

3. Before testing the PCB, it is necessary to know 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 function of the integrated circuit, the internal circuit, the 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 are not problems.

 

4. Avoid short circuit between pins when testing PCB

 

When measuring voltage or testing waveforms with oscilloscope probes, the test leads or probes should not cause short circuits between the pins of the integrated circuit due to sliding. It is best to measure on the peripheral printed circuit that is directly connected to 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 test instrument should be large.

 

When measuring the DC voltage of the pins of an integrated circuit, choose the multimeter with an internal resistance greater than 20KΩ/V. Otherwise, there will be a large measurement error for some pin voltages.

 

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

 

The power integrated circuit should have good heat dissipation. It's not allowed to work in a high-power state without a radiator.

 

7. The test of PCB leads should be reasonable.

 

If it's necessary to add external components to replace the damaged parts inside the integrated circuit, small components should be selected, and the wiring should be reasonable to avoid unnecessary parasitic coupling, especially the grounding between the audio power amplifier integrated circuit and the preamplifier circuit.

 

8. Welding quality

 

Be sure to weld firmly. The accumulation of solder and pores 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. The integrated circuit that has been soldered should be carefully checked. It is best to 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 determine the damage of the integrated circuit.

 

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.

 

PCB debugging

 

For new PCB, we first need to roughly observe whether there are any problems on the board, such as whether there are obvious cracks, whether there are short circuits, open circuits, etc. If necessary, you can check whether the resistance between the power supply and the ground wire 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.

 

Debugging steps

 

1. For the new PCB, 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 circuits, open circuits, etc. If necessary, you can check whether the resistance between the power supply and the ground wire is large enough.

 

2. Then it's the installation of components. For independent modules, if you are not sure that they are working properly, it is best not to install all of them, but install them part by part (for a relatively small circuit, you can install them all at once), so that it is easy to determine the fault range. In order to avoid problems when you get it, you have 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 don't have much confidence when powering on (even if you have great confidence, it is recommended that you add a fuse just in case). You can consider using an adjustable regulated power supply with current limiting function.

 

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 installing everything, power on and test it. When powering on, follow the above steps to avoid overcurrent and burn out components due to design errors or/and installation errors.