PCB News - 9 tips for basic PCB board testing

It is time for PCB board inspection to pay attention to some details in order to be more prepared to ensure product quality. When inspecting PCB board in PCB manufacturing, we should pay attention to the following 9 tips.

1. It is strictly forbidden to use grounded test equipment to touch the live TV, audio, video and other equipment of the bottom plate to test the PCB board without an isolation transformer.

It is strictly forbidden to directly test TV, audio, video and other equipment without a power isolation transformer with instruments and equipment with grounded shells. Although the general radio and cassette recorder has a power transformer, when you come into contact with more special TV or audio equipment, especially the output power or the nature of the power supply used, you must first find out whether the chassis of the machine is charged, otherwise it will be very easy The TV, audio and other equipment that are charged with the backplane cause a short circuit of the power supply, which affects the integrated PCB board, causing further expansion of the fault.

2. Pay attention to the insulation performance of the soldering iron when testing the PCB board

It is not allowed to use a soldering iron for soldering with power. Make sure that the soldering iron is not charged. It is best to ground the shell of the soldering iron. Be more careful with the MOS circuit. It is safer to use a low voltage circuit iron of 6~8V.

3. Before testing the PCB board, understand the working principle of integrated circuits and related circuits

Before inspecting and repairing the integrated circuit, you must first be familiar with the function of the integrated circuit used, the internal circuit, the main electrical parameters, the role of each pin, and the normal voltage of the pin, the waveform and the working principle of the circuit composed of peripheral components. If the above conditions are met, analysis and inspection will be much easier.

4. Do not cause short circuit between pins when testing the PCB board

When measuring voltage or testing the waveform with an oscilloscope probe, do not cause a short circuit between the pins of the integrated circuit due to sliding of the test leads or probes. It is best to measure on the peripheral printed circuit directly connected to the pins. Any momentary short circuit can easily damage the integrated circuit. You must be more careful when testing flat-package CMOS integrated circuits.

5. The internal resistance of the PCB board test instrument should be large

When measuring the DC voltage of the IC pins, a multimeter with the internal resistance of the meter head greater than 20KΩ/V should be used, otherwise there will be a large measurement error for the voltage of some pins.

6. Pay attention to the heat dissipation of the power integrated circuit when testing the PCB board

The power integrated circuit should have good heat dissipation, and it is not allowed to work in a high-power state without a heat sink.

7. The lead wire of the PCB board should be tested reasonably

If you need to add external components to replace the damaged parts of the integrated circuit, small components should be selected, and the wiring should be reasonable to avoid unnecessary parasitic coupling, especially the grounding between the audio power amplifier integrated circuit and the preamplifier circuit end.

8. To inspect the PCB board to ensure the welding quality

When soldering, the solder is firm, and the accumulation of solder and pores are likely to cause false soldering. The soldering time is generally no more than 3 seconds, and the power of the soldering iron should be about 25W with internal heating. The integrated circuit that has been soldered should be carefully checked. It is best to use an ohmmeter to measure whether there is a short circuit between the pins, confirm that there is no solder adhesion, and then turn on the power.

9. Do not judge the damage of the integrated circuit easily when testing the PCB board

Do not judge that the integrated circuit is damaged easily. Because most integrated circuits are directly coupled, once a circuit is abnormal, it may cause multiple voltage changes, and these changes are not necessarily caused by the damage of the integrated circuit. In addition, in some cases, the measured voltage of each pin is different from the normal voltage. When the values match or are close, it does not always mean that the integrated circuit is good. Because some soft faults will not cause changes in DC voltage.

PCB board debugging method

For a newly designed circuit board, debugging often encounters some difficulties, especially when the board is relatively large and there are many components, it is often impossible to start. But if you 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 that has just been taken back, we must first roughly observe whether there are any problems on the board, such as whether there are obvious cracks, short circuits, open circuits, etc. If necessary, check whether the resistance between the power supply and the ground wire is large enough.

2. Then the components are installed. Independent modules, if you are not sure that they are working properly, it is best not to install all of them, but to install part by part (for relatively small circuits, you can install them all at once), so that it is easy to determine the fault range. Avoid getting it when you encounter problems.

Generally speaking, you can install the power supply first, and then power on to check whether the output voltage of the power supply is normal. If you do not have much confidence when powering up (even if you are sure, it is recommended that you add a fuse, just in case), consider using an adjustable regulated power supply with current limiting function.

Preset the overcurrent protection current first, then slowly increase the voltage value of the regulated power supply, and monitor the input current, input voltage, and output voltage. If there is no overcurrent protection and other problems during the upward adjustment, and the output voltage has reached normal, the power supply is OK. Otherwise, disconnect the power supply, find the fault point, and repeat the above steps until the power supply is normal.

3. Next, install other modules gradually. After each module is installed, power on and test it. When powering on, follow the above steps to avoid over-current and burn out components due to design errors or/and installation errors.

The way to find the faulty PCB board

1. Find faulty PCB board by measuring voltage method

The first thing to confirm is whether the voltage of the power supply pins of each chip is normal, and then check whether the various reference voltages are normal, and whether the working voltage of each point is normal. For example, when a general silicon transistor 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, etc.), it may be that the BE junction is open.

2. Signal injection method to find faulty PCB board

Add the signal source to the input terminal, and then measure the waveform of each point in turn to see if it is normal to find the fault point. Sometimes we also use simpler methods, such as holding a tweezers with our hands, and touching the input terminals of all levels to see if the output terminals respond. This is often used in amplifying circuits such as audio and video (but note that the hot bottom plate For circuits with high voltage or high-voltage circuits, this method cannot be used, otherwise it may cause electric shock). If there is no response to the previous level, but there is a response to the next level, it means that the problem lies in the previous level and should be checked.

3. Other ways to find faulty PCB board

There are many other ways to find fault points, such as watching, listening, smelling, touching, etc.

"Seeing" is to see if there is any obvious mechanical damage to the component, such as cracking, burning, deformation, etc.;

"Listening" means listening to whether the working sound is normal, for example, something that shouldn't be ringing is ringing, the place that should be ringing is not ringing or the sound is abnormal, etc.;

"Smell" is to check whether there is any peculiar smell, such as the smell of burning, the smell of capacitor electrolyte, etc. For an experienced electronic maintenance personnel, they are very sensitive to these smells;

"Touching" is to test whether the temperature of the device is normal by hand, for example, it is too hot or too cold.

Some power devices will heat up when they work. If they are cold to the touch, it can basically be judged that they are not working. But if the place that shouldn't be hot is hot or the place that should be hot is too hot, that won't work either. For general power transistors, voltage regulator chips, etc., it is completely fine to work below 70 degrees. What is the concept of 70 degrees? If you press your hand up, you can hold it for more than three seconds, it means that the temperature is below 70 degrees (note that you must touch it tentatively first, and don't burn your hands).