Chip detection knowledge

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Test knowledge

1. To understand the working principle of the integrated circuit and its related circuits before testing, to check and repair the integrated circuit, you must first be familiar with the functions of the used integrated circuit, the internal circuit, the main electrical parameters, the roles of the pins, and the normal voltage and waveform of the pins. The working principle of the circuit with the external components. If these conditions are met, analysis and inspection will be much easier.

2, the test should not cause short-circuit between the pins Voltage measurement or test waveforms with the oscilloscope probe, the probe or probe should not cause short circuit between IC pins due to sliding, it is best measured in a peripheral printed circuit directly connected to the pin. Any momentary short circuit can easily damage the integrated circuit, and more care must be taken when testing flat package CMOS integrated circuits.

3. It is strictly forbidden to use grounded test equipment to touch live TV, audio, video equipment, etc. in the absence of isolation transformers. It is strictly forbidden to directly test the TV, audio, video, etc equipment. Although the common tape recorders all have power transformers, when they come into contact with a particular television or audio equipment that has a large output power or a poor understanding of the nature of the power supply used, it is necessary to first understand whether the chassis is energized. Otherwise, it is very easy. TVs, audio equipment and other equipment that is charged with the backplane cause a short circuit in the power supply, which affects the integrated circuit and further increases the failure.

4, should pay attention to the insulating properties of electric iron does not allow live use of soldering iron welding, to confirm that the iron is not charged, it is best to the grounding of the iron housing, the MOS circuit should be more careful, can use 6 ~ 8V low-voltage circuit iron is more secure.

5, to ensure the quality of welding Welded indeed welding, solder accumulation, porosity is likely to cause Weld. The welding time is generally not more than 3 seconds. The power of the iron is about 25W. Soldered integrated circuits should be carefully examined. It is best to use an ohmmeter to measure if there is a short circuit between the pins, and confirm that there is no solder adhesion phenomenon and then turn on the power.

6. Don't easily determine the damage of the integrated circuit Do not judge easily that the integrated circuit is damaged. Because the vast majority of integrated circuits are directly coupled, once a circuit is not normal, it may result in multiple voltage changes. These changes are not necessarily caused by the damage of the integrated circuit. In addition, the measured voltage of each pin is normal under some circumstances. When the values match or come close, it doesn't necessarily mean that the integrated circuit is good. Because some soft faults will not cause changes in the DC voltage.

7, test instrument internal resistance to measure the integrated circuit pin DC voltage, should use the table head resistance greater than 20KΩ / V multimeter, otherwise some of the pin voltage will have a larger measurement error.

8. Pay attention to the heat dissipation of the power integrated circuit. The power integrated circuit should have good heat dissipation, and it is not allowed to work under high power without a heat sink.

9, the lead should be reasonable if you need to add external components to replace the damaged part of the integrated circuit, should use small components, and the wiring should be reasonable to avoid unnecessary parasitic coupling, especially to deal with audio power amplifier integrated circuit and preamplifier Ground between circuits.

Identification method

1, no road detection
This method is performed when the IC is not soldered into the circuit. In general, the multimeter can be used to measure the positive and negative resistance values of each pin corresponding to the ground pin, and it must be compared with the intact IC.

2, on the road detection
This is a method of detecting the DC resistance of the IC pins (IC in the circuit), the AC-DC voltage to ground, and the total operating current through the multimeter. This method overcomes the limitations of the substitute test method and the trouble of disassembling the IC. It is the most common and practical method for detecting ICs.

3, DC operating voltage measurement
This is a case in which the multimeter DC voltage block is used to measure the DC supply voltage and the operating voltage of the external components under the power-on condition. The DC voltage value of each pin of the IC is measured against the ground, and compared with the normal value, thereby compressing the fault range. Out of damaged components. There are eight points to note when measuring:

(1) The multimeter must have a sufficiently large internal resistance, and less than 10 times the resistance of the circuit under test, so as to avoid causing a large measurement error.

(2) The potentiometer is usually rotated to the middle position. If it is a TV, the signal source should use the standard color bar signal generator.

3) Anti-skid measures should be taken for the pens or probes. It is easy to damage the IC due to any short circuit. The following methods can be used to prevent the pens from sliding: Take a bicycle valve core and put it on the tip of the pen, and grow the pen tip about 0.5mm. This will not only make the tip of the pen well contact with the tested point, but also effectively prevent slippage. Even if it hits a nearby point, it will not short circuit.

(4) When the measured voltage of a certain pin is inconsistent with the normal value, it should be analyzed according to whether the voltage of the pin has important influence on the normal operation of the IC and the corresponding change of other pin voltage, and the quality of the IC can be judged.

(5) IC pin voltage is affected by peripheral components. When the external components are leaky, short-circuited, open-circuited, or changed in value, or the peripheral circuit is connected to a potentiometer with variable resistance, the position of the potentiometer slide arm will make the pin voltage change.

(6) If the IC pin voltage is normal, it is generally considered that the IC is normal; if the IC part of the pin voltage is abnormal, you should start from the maximum deviation from the normal value, check the external components for fault, if there is no fault, the IC is likely to be damaged .

(7) For dynamic receiving devices, such as TVs, the IC pin voltages are different when there is a signal. If it is found that the change of the pin voltage does not change, the IC damage can be determined by changing the signal size and the position of the adjustable component without change.

(8) For devices with multiple operating modes, such as video recorders, IC pin voltages are different under different operating modes.

4 . AC working voltage measurement
In order to grasp the variation of the IC's AC signal, a multimeter with a db jack can be used to approximate the AC operating voltage of the IC. When testing, the multimeter is placed in the AC voltage block, and the meter is inserted into the db jack. For a multimeter without a db jack, a 0.1-0.5 μf blocking capacitor must be connected in series with the front pen. This method is applicable to ICs with lower operating frequencies, such as video amplifiers and field scanning circuits of televisions. Since these circuits have different natural frequencies and different waveforms, the measured data are approximate and are for reference only.

5. Total current measurement
This method is a method of determining the quality of an IC by detecting the total current flowing into the IC power supply. Since most of the IC's internal components are directly coupled, when the IC is damaged (such as a pn junction breakdown or open circuit), it will cause the subsequent stage saturation and cut-off, causing the total current to change. Therefore, by measuring the total current method can determine the quality of the IC. It is also possible to measure the voltage drop in the resistor in the power path and use Ohm's law to calculate the total current.

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Chip detection knowledge

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