9 Common Sense You Should Know When Inspecting PCB

Label:PCB Inspection, Insulation Performance, Integrated Circuits

Oct 21, 20218680

9 Common Sense You Should Know When Inspecting PCB

When conducting PCB inspection, we must pay attention to some details in order to better ensure product quality. Today, engineers from JRPanel would like to share with you 9 common sense that you need to know about PCB inspection.


1. It is strictly forbidden to use grounded test equipment to contact live equipment to detect PCB boards 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 a grounded shell, although most radios have power transformers. When we come into contact with a more special TV or audio equipment, especially a larger output power, we must first find out whether the chassis of the machine is live, otherwise it is very easy to cause a short circuit of the power supply and cause further expansion of the fault.


2. Pay attention to the insulation performance of the soldering iron when testing PCB.


It is not allowed to use a soldering iron for soldering with power. Make sure that the soldering iron is not charged. It is best to ground the shell of the soldering iron. Be more careful with the MOS circuit. It is safer to use a low voltage circuit iron of 6~8V.

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


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


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


When measuring voltage or testing the waveform with an oscilloscope probe, do not cause a short circuit between the pins of the integrated circuit due to sliding of the test leads or probes. It is best to measure on the peripheral printed circuit that is directly connected to the pin. 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 testing instrument should be large.


When measuring the DC voltage of the IC pins, we should choose a multimeter with the internal resistance of the meter head greater than 20KΩ/V, otherwise there will be larger measurement errors for some pin voltages.

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


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


7. The PCB lead should be tested reasonably.


If it is necessary to add external components to replace the damaged parts of the integrated circuit, we should choose small components, and the wiring should be reasonable to avoid unnecessary parasitic coupling. In particular, we must handle the grounding terminal between the audio power amplifier integrated circuit and the preamplifier circuit.


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


When welding, make sure that the welding is firm. 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 judge the PCB as damage to the integrated circuit easily.


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 cause multiple voltage changes. These changes are not necessarily caused by damage to 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 changes in DC voltage.


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