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PCB inspection, you must know this 9 points

2019-06-12 14:17:38
Pay attention to the following 9 common senses when testing PCB boards to ensure the quality of the products.

It is strictly forbidden to use the grounded test equipment to contact the TV, audio, video and other equipment on the backplane to detect the PCB board without the isolation transformer.

It is strictly forbidden to directly test the TV, audio, video and other equipment without power isolation transformer with the equipment that is grounded. Although the general tape recorder has a power transformer, when it comes to a special TV or audio device that has a large output power or a poor understanding of the nature of the power supply used, it is first necessary to find out whether the chassis of the machine is powered or not. The TV, audio and other equipment with the bottom plate are short-circuited by the power supply, affecting the integrated circuit, causing further expansion of the fault.

Inspect the PCB board to pay attention to the insulation performance of the soldering iron

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

Before testing the PCB board, you need to understand how the integrated circuit and its related circuits work.

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

Test PCB board does not cause short circuit between pins

When measuring voltage or testing waveforms with an oscilloscope probe, the test leads or probes should not be short-circuited between the pins of the IC due to sliding. It is best to measure on a peripheral printed circuit that is in direct communication with the pins. Any short circuit in any moment can easily damage the integrated circuit, and more care should be taken when testing flat-packaged CMOS integrated circuits.

Detecting PCB board test instrument internal resistance is large

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

Pay attention to the heat dissipation of the power integrated circuit when detecting the PCB board

The power integrated circuit should have good heat dissipation, and it is not allowed to work under high power without a heat sink.

It is reasonable to detect PCB board leads

If it is necessary to add external components instead of the damaged parts inside the integrated circuit, small components should be selected, and the wiring should be reasonable to avoid unnecessary parasitic coupling, especially to deal with the grounding between the audio power amplifier integrated circuit and the preamplifier circuit. end.

Check PCB board to ensure welding quality

It is welded firmly during welding, and the accumulation of solder and the pores are likely to cause solder joints. The soldering time is generally less than 3 seconds, and the power of the soldering iron is applied to the internal heat type of about 25W. The soldered integrated circuit should be carefully viewed. It is best to use an ohmmeter to measure whether there is a short circuit between the pins, and confirm that there is no solder adhesion and then turn on the power.

Do not easily determine the damage of the integrated circuit by detecting the PCB board

Do not easily judge that the integrated circuit is damaged, because most of the integrated circuits are directly coupled. Once a circuit is abnormal, it may cause multiple voltage changes, and these changes are not necessarily caused by integrated circuit damage. In some cases, when the voltage of each pin is measured to be close to or close to the normal value, it may not be sufficient to indicate that the integrated circuit is good, because some soft faults will not cause a change in the DC voltage.