[Repost] Nine things to note when testing PCB boards

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[Repost] Nine things to note when testing PCB boards [Copy link]

PCB board inspection is the time to pay attention to some details in order to better ensure product quality. When inspecting PCB boards, we should pay attention to the following 9 tips.

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

It is strictly forbidden to use instruments with grounded shell to directly test TV, audio, video and other equipment without power isolation transformer. Although general recorders have power transformers, when contacting more special TV or audio equipment, especially those with large output power or the nature of the power supply used is not well understood, first of all, it is necessary to find out whether the chassis of the machine is live, otherwise it is very easy to cause power short circuit with TV, audio and other equipment with live bottom plate, affecting the integrated circuit and causing the fault to expand further. 2. When testing PCB boards, pay attention to the insulation performance of the soldering iron. It is not allowed to use a soldering iron with power on. Make sure the soldering iron is not charged. It is best to ground the outer casing of the soldering iron. Be more careful with MOS circuits. It is safer to use a low-voltage circuit iron of 6~8V. 3. Before testing the PCB board, you must understand 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, internal circuit, main electrical parameters, the role 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, then analysis and inspection will be much easier. 4. Do not cause short circuits between pins when testing PCB boards When measuring voltage or testing waveforms with an oscilloscope probe, do not cause short circuits between integrated circuit pins due to sliding of the test lead or probe. 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. Be extra careful when testing CMOS integrated circuits in flat packages. 5. The internal resistance of the test instrument for testing PCB boards should be large. When measuring the DC voltage of integrated circuit pins, a multimeter with a head internal resistance greater than 20KΩ/V should be used, otherwise there will be a large measurement error for some pin voltages. 6. When testing PCB boards, pay attention to the heat dissipation of power integrated circuits. Power integrated circuits should have good heat dissipation and are not allowed to work in a high-power state without a heat sink. 7. The lead of the PCB board should be reasonable. If you need 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 ground terminal between the audio power amplifier integrated circuit and the preamplifier circuit. 8. Ensure the welding quality of the PCB board when testing. Make sure to weld firmly during welding. The accumulation of solder and pores can easily cause cold welding. The welding time is generally not more than 3 seconds, and the power of the soldering iron should be about 25W for internal heating. The soldered integrated circuit 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 before turning on the power.

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

Do not easily judge that the integrated circuit is damaged. Because most integrated circuits are directly coupled, once a 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, when the 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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