Basic knowledge of PCB testing

PCB (printed circuit board) testing is an important part of ensuring the quality of circuit boards during production and use. Effective testing can ensure the normal function of the circuit board, check manufacturing defects (such as short circuits, open circuits) and verify the quality of component welding.

The following are the basic points of PCB testing:

1. It is forbidden to directly test live equipment: In the absence of an isolation transformer, never use grounded test equipment to directly contact TV, audio or video equipment with live bottom plates. Especially for special equipment, be sure to confirm whether its chassis is live to avoid causing short circuits and damaging integrated circuits.

2. Electric soldering iron insulation: When using an electric soldering iron, make sure it is not live. It is best to ground the casing, especially when dealing with MOS circuits, it is safer to use a low-voltage soldering iron of 6~8V.

3. Understand the circuit principle: Before checking and repairing integrated circuits, you need to be familiar with their functions, internal structures and main electrical parameters, as well as the normal voltage and waveform of each pin. This will help to analyze and inspect more effectively.

4. Avoid pin short circuits: When measuring voltage or testing waveforms, be careful not to cause short circuits between integrated circuit pins due to probe sliding. It is best to measure on the peripheral circuit directly connected to the pin, as a momentary short circuit may damage the circuit.

5. Test instrument internal resistance requirements: When measuring the DC voltage of the integrated circuit pin, a multimeter with an internal resistance greater than 20KΩ/V should be selected to reduce measurement errors.

6. Pay attention to heat dissipation: Power integrated circuits should maintain good heat dissipation, and never operate at high power without a heat sink.

7. Reasonable leads: If you need to add peripheral components to replace the damaged integrated circuit part, you should use small components and wire them reasonably to avoid unnecessary parasitic coupling, especially pay attention to the grounding between the audio amplifier and the preamplifier circuit.

8. Ensure welding quality: When welding, ensure that the solder joints are firm to avoid solder accumulation and pores that cause cold soldering. The welding time should be controlled within 3 seconds, and the soldering iron power should be around 25W. After welding, be sure to check whether there is a short circuit between the pins, and power on after confirming that there is no solder adhesion.

9. Carefully judge whether the integrated circuit is damaged: Do not easily judge whether the integrated circuit is damaged. Because many integrated circuits are directly coupled, an abnormality in a circuit may cause voltage changes in multiple places, and these changes are not necessarily caused by damage to the integrated circuit. In addition, some soft faults will not cause DC voltage changes, so even if the measured voltage is normal, it cannot ensure that the circuit is good.