PCB testing knowledge encyclopedia these nine points must be known

Date：2022-02-15 18:29:47Views：861

PCB board is an important part of modern electronic equipment. Due to the small volume and high installation density of patch components, the integration of PCB board is required to be further improved. Attention should also be paid to details during testing, so as to better prepare and ensure product quality. Before PCB testing, understand the working environment of the equipment, mainly considering the possible impact of external electrical parameters on the equipment; Ask what happens when the circuit board fails, and analyze the causes of the failure; Carefully check the components on the circuit board to find out which components play a key role in the circuit board; Take anti electromagnetic, static and other interference measures.

When testing PCB boards, we should pay attention to the following 9 points:

1. It is forbidden to use the grounded test equipment to contact the live TV, audio, video and other equipment on the bottom plate to detect the PCB without the isolation transformer

It is forbidden to directly test TV, audio, video and other equipment without power isolation transformer with instruments and equipment with grounded shell. Although general tape recorders have power transformers, when contacting special TV or audio equipment, especially those with large output power or little understanding of the nature of the power supply used, it is necessary to first find out whether the chassis of the machine is charged, otherwise it is very easy to cause power short circuit with TV, audio and other equipment charged on the bottom plate, which will affect the integrated circuit and further expand the fault.

2. When testing PCB board, pay attention to the insulation performance of electric soldering iron

It is not allowed to use soldering iron for welding with electricity. To confirm that the soldering iron is not charged, it is best to ground the shell of the soldering iron. Be more careful with MOS circuit. It is safer to use low-voltage circuit iron of 6 ~ 8V.

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

Before checking and repairing the integrated circuit, first be familiar with the 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, the analysis and inspection will be much easier.

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

When measuring the voltage or testing the waveform with an oscilloscope probe, the lead or probe shall not cause a short circuit between the pins of the integrated circuit due to sliding, and it is best to measure on the peripheral printed circuit directly connected with the pins. Any instantaneous short circuit is easy to damage the integrated circuit. Be more careful when testing CMOS integrated circuits in flat package.

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

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

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

The power integrated circuit shall have good heat dissipation, and it is not allowed to work at high power without radiator.

7. The lead of PCB board shall be tested reasonably

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

8. Welding quality shall be ensured when testing PCB

When welding, it is indeed firmly welded, and the accumulation of solder and pores are easy to cause false welding. The welding time is generally no more than 3 seconds, and the power of the soldering iron is about 25W. The welded integrated circuit 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. Don't judge the damage of integrated circuit easily when testing PCB

Do not easily judge that the integrated circuit is damaged. Because the vast majority of integrated circuits are directly coupled, once a circuit is abnormal, it may lead to multiple voltage changes, which are not necessarily caused by the damage of the integrated circuit. In addition, in some cases, when the measured voltage of each pin is consistent with or close to the normal value, it does not necessarily mean that the integrated circuit is good. Because some soft faults will not cause changes in DC voltage.

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