Printed circuit board

PCB board basic inspection of 9 small knowledge

PCB board testing is the time to pay attention to some details in order to be more prepared to ensure product quality, when testing PCB boards, we should pay attention to the following nine small common sense.

1, is strictly prohibited in the absence of isolation transformers, with grounded test equipment to touch the bottom plate charged TV, audio, video and other equipment to test the PCB board

Forbidden to use the shell has been grounded instruments and equipment directly test the TV, audio, video and other equipment without power isolation transformer. Although the general recorder has a power transformer, when contacting the more special, especially the output power or the nature of the power supply used is not well understood when the TV or audio equipment, the first thing to find out whether the machine chassis charged, otherwise it is very easy and the chassis charged TV, audio and other equipment caused by power short circuit, and integrated circuits, resulting in further expansion of the fault.

2, test the PCB board to pay attention to the insulation properties of the soldering iron

Do not allow the use of soldering iron soldering with electricity, to confirm that the iron is not charged, it is best to ground the iron’s shell, the MOS circuit should be more careful, can use 6 to 8V low-voltage circuit iron on the safer.

3, before testing the PCB board to understand the working principle of integrated circuits and their related circuits

Inspection and repair of integrated circuits before the first to be familiar with the function of the integrated circuit used, the internal circuit, the main electrical parameters, the role of the pins and the normal voltage of the pins, waveforms and peripheral components to form the working principle of the circuit. If you have the above conditions, then the analysis and inspection will be much easier.

4, test the PCB board do not cause a short circuit between the pins

Voltage measurement or waveform testing with an oscilloscope probe, the meter pen or probe do not cause a short circuit between the pins of the integrated circuit due to sliding, it is best to measure on the peripheral printed circuit directly connected to the pins. Any instantaneous short circuit can easily damage the integrated circuit, in the test flat type package CMOS integrated circuit should be doubly careful.

5, test PCB board test meter internal resistance to be large

Measurement of integrated circuit pins DC voltage, you should use the meter head resistance greater than 20KΩ / V multimeter, otherwise the voltage on some pins will have a large measurement error.

6, test PCB board to pay attention to the power integrated circuit heat dissipation

Power integrated circuit should be good heat dissipation, not allowed to work without a heat sink and in a high-power state.

7, the detection of PCB board leads to be reasonable

If you need to add peripheral components instead of IC internal damaged part, should choose small components, and wiring should be reasonable to avoid unnecessary parasitic coupling, especially to deal with the audio amplifier IC and pre-amplifier circuit between the ground terminal.

8, testing the PCB board to ensure the quality of welding

Welding does weld firmly, the buildup of solder, air holes are likely to cause false welding. Welding time is generally no more than 3 seconds, the power of the soldering iron application internal heating type 25W or so. Has been welded IC to look carefully, it is best to use the ohmmeter to measure whether there is a short circuit between the pins, to confirm that no solder adhesion phenomenon before turning on the power.

9, test the PCB board do not easily conclude that the integrated circuit damage

Do not easily judge the integrated circuit has been damaged. Because the vast majority of integrated circuits for direct coupling, once a circuit is not normal, may lead to multiple voltage changes, and these changes are not necessarily caused by damage to the integrated circuit, in addition, in some cases, the measured voltage of the pins and the normal value of the match or close, it does not necessarily mean that the integrated circuit is good. Because some soft faults do not cause changes in DC voltage.
PCB board debugging method
For the new PCB board just got back, we must first probably observe whether there are problems on the board, such as whether there are obvious cracks, there is no short circuit, open circuit and other phenomena. If necessary, you can check whether the resistance between the power supply and ground is large enough.

For a new design of the board, debugging often encounter some difficulties, especially when the board is relatively large, more components, often no way to start. But if you master a good 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 got back, we should first probably observe whether there are problems on the board, such as whether there are obvious cracks, there is no short circuit, open circuit and other phenomena. If necessary, you can check whether the resistance between the power supply and ground is large enough.

2、Then it is time to install the components. Independent of each other’s modules, if you are not sure to ensure that they work properly, it is best not to install all, but part of a part of the installation (for smaller circuits, you can install all at once), so that it is easy to determine the scope of the fault, so as not to encounter problems, no way to start.

In general, the power supply part can be installed first, and then on the power supply to test whether the output voltage is normal. If you are not too sure when powering up (even if there is a great deal of certainty, it is recommended that you add a fuse, just in case), consider using an adjustable regulated power supply with current limiting function.

First preset the overcurrent protection current, and then the voltage value of the voltage regulator power supply slowly upward, and monitor the input current, input voltage and output voltage. If the process of adjusting upward, there is no overcurrent protection and other problems, and the output voltage has reached normal, it means that the power supply part OK. conversely, disconnect the power supply, look for fault points, and repeat the above steps, until the power supply is normal.

3, the next gradual installation of other modules, each installed a module, power on the test, power on is also in accordance with the above steps to avoid overcurrent and burned components due to design errors or / and installation errors.
Ways to find the faulty PCB board
1, measuring voltage method to find the fault PCB board

The first thing to confirm is whether the voltage of the power supply pins of each chip is normal, followed by checking whether the various reference voltages are normal, in addition to the working voltage of each point is normal, etc.. For example, the general silicon transistor conduction, BE junction voltage at about 0.7V, while the CE junction voltage is at about 0.3V or less. If a transistor BE junction voltage is greater than 0.7V (except for special transistors, such as Darlington tubes, etc.), it may be BE junction on the open circuit.

2, signal injection method to find the fault PCB board

Add the signal source to the input, and then measure the waveform of each point in turn backward to see if it is normal to find the fault point. Sometimes we will also use a simpler way
method, such as holding a tweezer in your hand, to touch the input at all levels to see if the output reacts, which is often used in audio, video and other amplification circuits (but note that the circuit of the hot base board or high voltage circuit, you can not use this method, otherwise it may lead to electric shock). If you touch the previous level does not respond, and touch the next level of response, it means that the problem is in the previous level, should focus on checking.

3, other ways to find the fault PCB board

There are many other ways to find the point of failure, such as see, hear, smell, touch, etc..
“See” is to see whether the components have obvious mechanical damage, such as rupture, burn black, deformation, etc..
“listen” is to listen to the work of the sound is normal, such as something should not ring in the ring, the sound of the place does not ring or sound is not normal, etc..
“smell” is to check whether there is an odor, such as the smell of burning, the smell of capacitor electrolyte, etc., for an experienced electronic repair personnel, the smell is very sensitive to these odors.
“Touch” is to use your hand to test whether the temperature of the device is normal, such as too hot, or too cool.

Some power devices, work up will be hot, if the touch is cool, you can basically determine that it does not work up. But if the place where it should not be hot or the place where it should be too hot, it is not okay. The general power triode, voltage regulator chip, etc., work below 70 degrees is no problem at all. 70 degrees is about what a concept it is? If you press your hand up, you can hold on for more than three seconds, it means that the temperature is probably below 70 degrees (note that you should first try to touch, do not burn your hand).

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