PCB News - Three testing methods for PCB boards

It is time to pay attention to some details in the detection of PCB board, so as to better prepare to ensure product quality. When detecting PCB board, we should pay attention to the following 9 common sense.

1. Detecting PCB 9 common sense

a. It is strictly forbidden to use the grounded test equipment to contact the live TV, audio, video and other equipment on the bottom plate to detect THE PCB board without the isolation transformer

Do not directly test TV, audio, and video devices without power isolation transformers with grounded devices. Although generally the recorder has power transformer, when exposed to a special especially or larger output power of the nature of the power supply do not know much about TV or stereo equipment, first make clear the machine chassis is charged, otherwise extremely easy and floor live TV, audio and other equipment power supply circuit, spread to the integrated circuit, further breakdowns.

b. Pay attention to the insulation performance of electric soldering iron when detecting PCB board

It is not allowed to use electric soldering iron, to make sure that the iron is not electric, the shell of the iron should be grounded, MOS circuit should be more careful, can use 6-8V low voltage electric soldering iron is safer.

c. Before detecting PCB board, we should understand the working principle of integrated circuit and related circuit

Before checking and repairing integrated circuits, we should first be familiar with the functions of integrated circuits, internal circuits, main electrical parameters, the role of each pin and the normal voltage, waveform of the pin and the working principle of the circuit composed of peripheral components. If these conditions are present, analysis and inspection are much easier.

d. test PCB board do not cause short circuit between pins

Voltage measurement or oscilloscope probe test waveform, pen or probe not due to sliding caused by short circuit between integrated circuit pins, in direct connection with the pins of the peripheral printing circuit for measurement. Any instantaneous short circuit can easily damage the integrated circuit, so extra care should be taken when testing flat packaged CMOS integrated circuits.

e. detection PCB board test instrument internal resistance should be large

When measuring the DC voltage of the pins of integrated circuits, use a multimeter whose internal resistance is greater than 20K ω /V; otherwise, there will be large measurement errors for some pin voltages.

f. Pay attention to the heat dissipation of power integrated circuit when detecting PCB board

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

g, PCB board lead detection should be reasonable

If it is necessary to add peripheral components to replace the damaged part 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 amplifier integrated circuit and the preamplifier circuit.

h. Ensure the welding quality of PCB board

When welding is really welded, the accumulation of solder and pores are easy to cause virtual welding. The welding time is generally not more than 3 seconds, and the power of the soldering iron is about 25W. The integrated circuit has been welded to carefully check, with ohmmeter to measure whether there is a short circuit between the pins, confirm no solder adhesion phenomenon before switching on the power supply.

i. detection of PCB board do not easily determine the damage of integrated circuit

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 lead to multiple voltage changes, and these changes are not necessarily caused by the damage of the integrated circuit, and in some cases, the measured pin voltage is consistent with or close to the normal value, it may not be able to show that the integrated circuit is good. Because some soft faults do not cause changes in DC voltage.

2. PCB board debugging method

For the new PCB board just brought back, we should first roughly observe whether there are problems on the board, such as whether there are obvious cracks, whether there is a short circuit, open circuit and other phenomena. If necessary, check that the resistance between the power supply and the ground is large enough.

For a newly designed circuit board, debugging often encounter some difficulties, especially when the board is larger, more components, often do not know how to start. But if you master a reasonable debugging method, debugging will get twice the result with half the effort.

PCB board debugging steps:

a. For the new PCB board just brought back, we should first roughly observe whether there are problems on the board, such as whether there are obvious cracks, whether there are short circuits, open circuits and other phenomena. If necessary, check that the resistance between the power supply and the ground is large enough.

b. and then the installation components. Independent modules, if you do not ensure that they work properly, do not install all of them, but part of the installation (for small circuits, can be installed), so that it is easy to determine the fault range, so that when you encounter problems, do not know how to start.

Generally speaking, you can install the power supply part first, and then check whether the power supply output voltage is normal. If you are not too sure when powering on (even if you are, you are advised to add a fuse, just in case), you can consider using an adjustable voltage regulator with current limiting function.

First preset the current protection, and then the voltage value of the regulator power supply slowly up, and monitor the input current, input voltage and output voltage. If no overcurrent protection occurs and the output voltage is normal, the power supply is OK. Otherwise, disconnect the power supply, find the fault, and repeat the above steps until the power supply is normal.

c. then gradually install other modules, each module installed, power on test, power on according to the above steps, in order to avoid overcurrent and burn out components due to design errors or installation errors.

3. Finding PCB board faults

a. Measure the voltage to find the faulty PCB board

The first thing to confirm is whether the voltage of the chip power pin is normal, and then check whether all kinds of reference voltage is normal, and whether the working voltage of each point is normal. A typical silicon triode, for example, will have a BE junction voltage of around 0.7V and a CE junction voltage of around 0.3V or less. If a triode has a BE junction voltage greater than 0.7V (except for special triodes such as Darlington tubes), the BE junction may BE open.

b. signal injection method to find the fault PCB board

Add the signal source to the input end, and then measure the waveform of each point in turn to see whether it is normal to find the fault point. Sometimes we will also use a simpler method, such as holding a tweezer to touch the input terminal at all levels to see if there is a reaction at the output terminal, which is often used in audio and video amplifier circuits (but it should be noted that this method cannot be used in circuits with hot base plates or high voltage circuits, otherwise it may lead to electric shock). If there is no reaction before the touch, and there is a reaction after the touch, it shows that the problem is in the front, should focus on inspection.

c. Other methods for finding faulty PCB boards

There are many other ways to find trouble spots, such as seeing, hearing, smelling, and touching.

"Look" is to see whether there is obvious mechanical damage to components, such as rupture, blackening, deformation, etc.;

"Listen" is to listen to whether the sound of work is normal, such as some things should not ring in the ring, the ring does not ring or the sound is not normal;

"Smell" is to check whether there is a smell, such as the smell of burning, capacitor electrolyte taste, for an experienced electronic maintenance personnel, is very sensitive to these smells;

"Touch" is to use the hand to test whether the temperature of the device is normal, such as too hot, or too cold.

Some power devices, when working, heat, if it is cold to the touch, you can basically judge that it does not work. But if it's hot where it shouldn't be or too hot where it should be, that's no good. General power triode, voltage regulator chip, etc., working in 70 degrees is completely no problem. What does 70 degrees mean? If you can hold your hand on it for more than three seconds, the temperature is probably below 70 degrees (be careful not to burn your hand).