PCB Tech - PCB inspection, must know the points

Pay attention to the following 9 common sense points when inspecting PCB boards to ensure the quality of the product.

1. PCB inspection

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

It is strictly forbidden to directly test TV, audio, video and other equipment without a power isolation transformer with instruments and equipment with grounded enclosures. Although the general radio cassette recorder has a power transformer, when you come into contact with more special TV or audio equipment, especially the output power or the nature of the power supply used, you must first find out whether the chassis of the machine is charged, otherwise it will be very easy The TV, audio and other equipment that are charged with the backplane 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 soldering iron when testing the PCB board

It is not allowed to use a soldering iron when it is live. Make sure that the soldering iron is not live. It is best to ground the shell of the soldering iron. Be more careful with MOS circuits. It is safer to use 6~8V low-voltage circuit irons.

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

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

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

When measuring voltage or testing waveforms with oscilloscope probes, the test leads or probes should not cause short circuits between IC pins due to sliding. It is best to measure on the peripheral printed circuit directly connected to the pins. Any momentary short circuit can easily damage the integrated circuit. You must be more careful when testing flat-package CMOS integrated circuits.

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

When measuring the DC voltage of the IC pins, a multimeter with the internal resistance of the meter head greater than 20KΩ/V should be used, otherwise there will be a large measurement error for the voltage of some pins.

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

The heat dissipation of the power integrated circuit should be good, and it is not allowed to work in a high-power state without a heat sink.

7. The lead wire of the PCB board should be reasonable

If it is necessary to add external components to replace the damaged part of 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 end.

8. The quality of soldering must be ensured when testing PCB boards

When soldering, the solder is firm, and the accumulation of solder and pores are likely to cause false soldering. The soldering time is generally no more than 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, confirm that there is no solder adhesion, and then turn on the power.

9. Do not easily determine the damage of the integrated circuit when testing the PCB board

Do not judge that the integrated circuit is damaged easily, because most 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 the damage of the integrated circuit. In addition, in some cases, when the measured voltage of each pin matches or is close to the normal value, it may not always indicate that the integrated circuit is good, because some soft faults will not cause a change in the DC voltage.