There are generally several ways for PCB manufacturers to find PCB faults in SMT chip processing: the voltage measurement method first needs to confirm whether the voltage of each chip power pin on the PCB circuit board is normal, and then check whether various reference voltages are normal and whether the working voltage of each point is normal. For example, when a general silicon triode is turned on, the be junction voltage is about 0.7V, while the CE junction voltage is about 0.3V or less. If the be junction voltage of a triode is greater than 0.7V (except for special triodes, such as Darlington tube, etc.), it may be that the be junction is open.
Signal injection method of PCB board: add the signal source to the input end, and then successively measure the waveform of each point of PCB to see if it is normal, so as to find the fault point of PCB circuit board. Sometimes we also use some simple methods, such as holding a tweezer to touch the input terminals at all levels to see whether the output terminals respond. This is often used in audio, video and other amplification circuits. It should be noted that this method cannot be used for circuits with hot backplane or circuits with high voltage, otherwise it may lead to electric shock. If there is no response at the level before the collision, but there is a response at the level after the collision, it indicates that the problem is at the previous level, which should be checked emphatically.
Of course, there are many other ways to find PCB fault points, such as looking, listening, smelling, touching, etc. "looking" is to see whether the components on the PCB circuit board are mechanically damaged by SMT technology equipment, such as cracking, blackening, deformation, etc; "Listening" is to listen to whether the working sound is normal, such as something that should not ring, where it should ring, or the sound is abnormal; "Smell" is to check whether the PCB circuit board has peculiar smell, such as the smell of PCB circuit board being burnt, the smell of capacitor electrolyte, etc. "touch" is that SMT technicians use their hands to test whether the temperature of devices on the PCB circuit board is normal, such as too hot or too cold. Some power devices will generate heat when they work. If they feel cold, it can basically be judged that they do not work. But if the place that should not be hot is hot or the place that should be hot is too hot, this situation is abnormal. General power triode, voltage stabilizing chip, etc. usually in this case, the temperature of PCB board working below 70 ° C is no problem. What kind of concept is 70 degrees? Just try to press your hand up. If you can hold it for more than three seconds, it means that the temperature is about below 70 degrees. In this way, there is no problem.