PCB smt inspection nine basic common senses

It’s the time to pay attention to the details for PCB smt board inspection , in order to ensure the better products quality , we should pay attention to the following nine common sense during testing the PCB smt board.

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 smt board without the isolation transformer.

It is strictly forbidden to test the TV, audio, video and other equipment without power isolation transformer directly with the equipment that is grounded. Although the normal 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 figure out whether the chassis of the machine is powered or not. The TV, audio and other equipment with the bottom plate are power supply short-circuited , affecting the integrated circuit, causing further expansion of the fault.

2, inspection PCB smt board should pay attention to the insulation performance of the soldering iron

It is not allowed to use soldering iron for welding. It is better to ground the soldering iron shell to confirm that the soldering iron is not charged,. We should pay more attention to the MOS circuit. It is safer to use the 6~8V low voltage circuit iron .

3. Understand the working principle of integrated circuits and related circuits before testing PCB smt boards.

Before checking and repairing an integrated circuit, you must first familiarize 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.

4, Test the PCB smt board does not cause short circuit between the pins

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

5, Test PCB smt board test instrument internal resistance should be large

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

6, the detection of PCB smt board should pay attention to the heat dissipation of the power integrated circuit

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

7, the detection of PCB smt board leads should be reasonable

If it is necessary to add external components to take the place 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 end between the audio power amplifier integrated circuit and the preamplifier circuit.

8, inspection PCB smt board to ensure welding quality

It is welded firmly during welding, and the accumulation of solder and the pores are likely to cause virtual welding. The soldering time is generally no more 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 short circuit between the pins, and confirm that there is no solder adhesion and then turn on the power.

9, do not easily determine the damage of the integrated circuit PCB smt board

Do not judge that the integrated circuit is damaged easily. Because most of the integrated circuits are coupled directly, once a certain circuit is abnormal, it may cause multiple area voltage changes, which are certainly caused by integrated circuit damage. In some cases, the voltage of each pin is measured normal and the values are match or close, it does not actually mean that the integrated circuit is good. Because some soft faults do not cause a change in the DC voltage.