9 common senses for basic PCB inspection
When testing PCB board, we should pay attention to some details in order to ensure product quality more accurately.
When testing PCB board, we should pay attention to the following nine common sense.
1. It is strictly forbidden to use grounded testing equipment to contact the charged TV, audio and video equipment on the floor to detect PCB board without isolating transformer.
Although most recorders have power transformers.
When contacting special TV or audio equipments, especially those with high output power or little understanding of the nature of the power supply used, it is necessary to find out whether the chassis of the machine is charged or not. Otherwise, it is very easy to cause short circuit of the power supply with the charged TV and audio equipments on the chassis, which will affect the integrated circuit and cause further enlargement of the faults.
2. Attention should be paid to the insulation performance of electric soldering iron when testing PCB board.
It is not allowed to use soldering iron on live. To confirm that soldering iron is not electrified, it is better to ground the outer shell of soldering iron. More care should be taken for MOS circuits. It is safer to use 6-8V low-voltage circuit iron.
3. Understand the working principle of integrated circuits and related circuits before testing PCB boards
Before inspecting and repairing integrated circuits, we should be familiar with the functions, internal circuits, main electrical parameters, the roles of pins, the normal voltage, waveform of pins and the working principle of the circuit composed of peripheral components. If these conditions are met, then analysis and inspection will be much easier.
4. Test PCB board to avoid short circuit between pins
When measuring voltage or testing waveform with oscilloscope probe, the pen or probe should not be short-circuited between the pins of integrated circuits due to sliding. It is better to measure on the peripheral printed circuit directly connected with the pins. Any instantaneous short circuit is liable to damage the integrated circuit. We should be more careful when testing flat packaged CMOS integrated circuits.
5. The internal resistance of PCB board test instrument should be large.
When measuring DC voltage of IC pins, multimeter with internal resistance greater than 20K/V should be selected, otherwise there will be large measurement error for some pin voltages.
6. Pay attention to the heat dissipation of power integrated circuits when testing PCB boards
Power integrated circuits should dissipate heat well and not be allowed to work in high power without radiators.
7. Detection of PCB board lead should be reasonable
If it is necessary to add peripheral components to replace the damaged parts in the integrated circuit, small components should be selected, and the wiring should be reasonable to avoid unnecessary parasitic coupling, especially the grounding end between the audio power amplifier integrated circuit and the preamplifier circuit.
8. Inspecting PCB board to ensure welding quality
It is true that the solder is soldered firmly, and the accumulation of solder and pore are easy to cause virtual soldering. Welding time generally does not exceed 3 seconds. The power of soldering iron is about 25W of internal heat type. The welded integrated circuits should be carefully checked. It is better to measure the short circuit between the pins with an ohmmeter to confirm the solderless tin adhesion phenomenon and turn on the power supply.
9. Detecting PCB board should not easily determine the damage of integrated circuit
Don't easily judge that the integrated circuit is damaged. Because most integrated circuits are directly coupled, once a circuit is abnormal, it may lead to many voltage changes, which are not necessarily caused by IC damage. In addition, in some cases, when the measured pin voltages are in accordance with or close to the normal values, it may not be able to show that integrated circuits are good. Because some soft faults will not cause the change of DC voltage.
PCB board debugging method
For the new PCB board just brought back, we first need to roughly observe whether there are problems on the board, such as whether there are obvious cracks, whether there are short circuit, open circuit and other phenomena. If necessary, check that the resistance between the power supply and the ground wire is large enough.
For a newly designed circuit board, debugging often encounters some difficulties, especially when the board is large and there are many components, it is often impossible to start. But if we master a set of reasonable debugging methods, debugging will get twice the result with half the effort.
PCB board debugging steps
1. For the new PCB board just brought back, we first need to observe roughly whether there are problems on the board, such as whether there are obvious cracks, whether there are short circuit, open circuit and so on. If necessary, check that the resistance between the power supply and the ground wire is large enough.
2. Then there is the installation component. Independent modules, if you are not sure to ensure that they work properly, it is better not to install them all, but a part of them (for smaller circuits, you can install them all at once), so that it is easy to determine the scope of the fault, so as not to encounter problems when you can not start.
Generally speaking, the power supply can be installed first, and then power on to check whether the output voltage of the power supply is normal. If you are not sure when you turn on the power (even if you are sure, you are also advised to add a fuse, just in case), you may consider using an adjustable voltage regulator with current limiting function.
First, the overcurrent protection current is preset, then the voltage value of the regulated power supply is slowly raised, and the input current, input voltage and output voltage are monitored. If there is no over-current protection and the output voltage is normal in the process of up-regulation, then the power supply part OK. On the contrary, it is necessary to disconnect the power supply, find the fault point, and repeat the above steps until the power supply is normal.
3. The next step is to install other modules gradually. Every module is installed, it will be tested by power-on. When power-on, it will follow the above steps to avoid burning components due to design errors or/or installation errors.
Ways to Find Faulty PCB Board
1. Measuring Voltage Method to Find Fault PCB Board
First of all, it is necessary to confirm whether the voltage of each chip power pin is normal. Secondly, it is necessary to check whether the reference voltage is normal, and whether the working voltage of each point is normal. For example, when a 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 transistor is greater than 0.7V (except for special transistors, such as Darlington transistors), it may be that the BE junction opens.
2. Signal Injection Method for Finding Faulty PCB Board
The signal source is added to the input terminal, and then the waveform of each point is measured back to see if it is normal to find the fault point. Sometimes we also use simpler methods, such as holding a tweezers in our hands to touch the input terminals at all levels to see if the output terminals react, which is often used in audio and video amplifier circuits (but it should be noted that the circuit with hot backplane or high voltage can not use this method, otherwise it may lead to electric shock). If there is no reaction at the first level and there is reaction at the second level, it means that the problem lies at the first level and should be checked.
3. Other Methods of Finding Faulty PCB Board
There are many other ways to find fault points, such as watching, listening, smelling, touching and so on.
"Look" is to see whether the components have obvious mechanical damage, such as cracking, blackening, deformation, etc.
"Listening" is to listen to whether the sound of work is normal, such as something that should not be loud is ringing, where the sound is silent or abnormal, etc.
"Smell" is to check whether there are odors, such as burning odor, the smell of capacitor electrolyte, etc. For an experienced electronic maintenance personnel, these odors are very sensitive;
"Touch" means to test whether the temperature of the device is normal, such as too hot or too cold.
Some power devices will get hot when they work. If they feel cool, it can be basically judged that they are not working. But if it's not supposed to be hot or it's too hot, it's not going to work. General power transistors, voltage regulator chips and so on, working below 70 degrees is absolutely no problem. What is the concept of 70 degrees? If you press your hand up, you can hold it for more than three seconds, which means that the temperature is about 70 degrees below (pay attention to tentative touch first, don't burn your hand).
