Common failure mechanism and fault analysis of electronic components

During the use of electronic components, there are often failures and faults, which affect the normal work of the equipment. The text analyzes the failure reasons and common faults of common components.

The vast majority of failures in electronic equipment are ultimately caused by the failure of electronic components. If you are familiar with the fault types of components, sometimes you can quickly find out the fault components by intuition, and sometimes you can find out the fault by simple resistance and voltage measurements.

1.Resistor class

Resistor class elements include resistance elements and variable resistance elements, fixed resistance is usually called resistance, variable resistance is usually called potentiometer. Resistor components are used in a large number of electronic equipment, and are a power consuming component. The rate of failure caused by resistor failure of electronic equipment is relatively high, accounting for about 15% according to statistics. The failure modes and reasons of resistors are closely related to the structure, process characteristics and service conditions of products. Resistor failures can be divided into two categories, namely, fatal failures and parameter drift failures. Field statistics show that 85%~90% of resistor failures are fatal failures, such as circuit opening, mechanical damage, contact damage, short circuit, insulation, breakdown, and so on. Only about 10% of the failures are caused by resistance drift.

The failure mechanism of resistor potentiometer varies according to the type. The main failure modes of non-linear resistors and potentiometers are open circuit, resistance drift, lead mechanical damage and contact damage. The main failure modes of wire-wound resistors and potentiometers are open circuit, lead mechanical damage and contact damage. There are four main categories:

(1) Carbon film resistor. Lead fracture, matrix defect, poor uniformity of film layer, groove defect of film layer, poor contact between film material and lead end, contamination of film and matrix, etc.

(2) metal film resistor. Uneven resistance film, resistance film rupture, weak lead, resistance film decomposition, silver migration, resistance film oxide reduction, electrostatic charge action, lead fracture, corona discharge, etc.

(3) wire wound resistor. Poor contact, current corrosion, lead is not strong, wire insulation is not good, solder joint melting, etc.

(4) Variable resistor. Poor contact, poor welding, contact reed broken or lead off, impurity contamination, epoxy glue is not good, shaft tilt, etc.

Resistance is prone to deterioration and open fault. After the resistance deteriorates, it is often the drift of the resistance value. Resistors are generally not repaired, but replaced directly with new resistors. Wire wound resistance When the resistance wire burned out, in some cases can be burned out after re-welding for use.

Resistance deterioration is caused by poor heat dissipation, excessive humidity or defects during manufacturing, while burning is caused by abnormal circuit, such as short circuit, overload and other reasons. Resistance burning common two phenomena, one is the current ambassador resistance fever caused by resistance burning, at this time the resistance surface can be seen in a scorch, it is easy to find. Another case is due to the instantaneous high voltage added to the resistance caused by open resistance or resistance value becomes large, this case, the resistance surface is generally not significantly changed, in the high voltage circuit can often be found this fault phenomenon of resistance.

Variable resistor or potentiometer mainly wire wound and non - wire wound two kinds. Their common failure modes are: parameter drift, open circuit, short circuit, poor contact, high dynamic noise, mechanical damage, etc. However, the actual data show that the main failure modes between laboratory test and field use are quite different, and the majority of laboratory failures are parameter drift, while the majority of field failures are poor contact and open circuit.

The failure of poor contact of potentiometer is common in field use. If be in telecommunication equipment amount to 90%, occupy about 87% in television set, reason contact bad is fatal weak link to potentiometer. The main causes of poor contact are as follows:

(1) Contact pressure is too small, reed stress relaxation, sliding contact deviates from track or conductive layer, improper mechanical assembly, or large mechanical load (such as collision, fall, etc.) leads to contact reed deformation, etc.

(2) Conductive layer or contact track due to oxidation, pollution, and the formation of a variety of non-conductive film at the contact.

(3) the conductive layer or resistance alloy line is worn or burned, resulting in poor contact of the sliding point.

The open circuit failure of potentiometer is mainly caused by local overheating or mechanical damage. For example, the potentiometer conductive layer or resistance alloy line oxidation, corrosion, pollution or due to improper process (such as winding uneven, conductive film layer thickness uneven, etc.) caused by overload, local overheating, so that the potentiometer burned out and open; The surface of the sliding contact is not smooth, and the contact pressure is too large, which will make the wound seriously wear and break, leading to open circuit; Potentiometer choice and use improper, or the fault of electronic equipment endangers potentiometer, make its be in over load or work below larger load. These will accelerate the damage of the potentiometer.

2. Capacitor Class

The common fault phenomena of capacitor mainly include breakdown, open circuit, electrical parameter degradation, electrolyte leakage and mechanical damage. The main causes of these faults are as follows:

(1) Breakdown. There are defects, defects, impurities or conductive ions in the medium; Aging of dielectric materials; Electrochemical breakdown of dielectric; In the environment of high humidity or low pressure interpolar edge arc; Transient short circuit of dielectric under mechanical stress; Metal ions migrate to form conductive channels or edge arc discharge; Dielectric material internal air gap breakdown resulting in dielectric electrical breakdown; Medium in the manufacturing process mechanical damage; The change of the molecular structure of the dielectric material and the applied voltage higher than the rated value.

(2) Open the road. The breakdown causes the electrode and lead to be insulated; The anode leading foil of the electrolytic capacitor is corroded and broken (or mechanically broken); The lead line and electrode contact point oxide layer resulting in low level open circuit; Poor contact or insulation between lead wire and electrode; The anode of the electrolytic capacitor leads to an open circuit due to corrosion of the metal foil; Drying up or freezing of working electrolytes; Instantaneous open circuit between electrolyte and dielectric under mechanical stress.

(3) Degradation of electrical parameters. Moisture and dielectric aging and thermal decomposition; Metal ion migration of electrode materials; Residual stress exists and changes; Surface contamination; Self-healing effect of metallized electrode of material; Volatilization and thickening of working electrolytes; Electrolysis or chemical corrosion of electrodes; Increased contact resistance between lead and electrode; Effects of impurities and harmful ions.

Because the actual capacitor is operated under the combined action of working stress and environmental stress, one or several failure modes and failure mechanisms will occur, and one failure mode will lead to another failure mode or failure mechanism. For example, temperature stress can not only promote surface oxidation, accelerate the impact of aging, accelerate the degradation of electrical parameters, but also promote the decline of electric field strength, accelerate the early arrival of dielectric breakdown, and the impact of these stresses is a function of time. Therefore, THE FAILURE mechanism of capacitors is closely related to the types of products, materials, structural differences, manufacturing technology, environmental conditions, working stress and other factors.

The breakdown fault of capacitor is very easy to find, but it is difficult to determine the specific fault element when there are multiple components in parallel. The fault can be determined by connecting the capacitor of the same type and capacity with the detected capacitor in parallel and observing whether the circuit function is restored. It is troublesome to check the change of capacitor electrical parameters. Generally, the following methods can be used.

First of all, one of the capacitor leads should be ironed off the circuit board to avoid the influence of surrounding components. Secondly, according to the different conditions of the capacitor, different methods are used to check.

(1) Inspection of electrolytic capacitors. The multimeter is placed in the electrical barrier, and the range depends on the capacity and voltage of the electrolytic capacitor being measured. Measuring capacity of small, high voltage electrolytic capacitance, measuring range should be located in R ×10kW block; When measuring electrolytic capacitance with large capacity and low voltage, the measuring range should be located in R × 1 k W block. Observe the size of the charging current, the length of the discharge time (the return speed of the hand) and the value of the resistance indicated by the hand.

The identification method of electrolytic capacitor quality is as follows:

① The charge current is large, the rise rate of the watch hand is fast, the discharge time is long, the return speed of the watch hand is slow, which indicates that the capacity is sufficient.

② The charge current is small, the rise rate of the needle is slow, the discharge time is short, the return speed of the needle is fast, which indicates that the capacity is small and the quality is poor.

(3) Charging current is zero, the needle does not move, indicating that the electrolytic capacitor has failed.

④ Discharge to the end, the needle back to the end of the indicated resistance value is large, indicating good insulation performance, small leakage.

⑤ discharge to the end, the needle back to the end of the indicated resistance value is small, indicating poor insulation performance, leakage is serious.

(2) Inspection of general capacitors with capacity above 1 mF. The multimeter can be used to block (R × 1 0 k W) the same polarity multiple measurement method to check the leakage degree and whether the breakdown. Touch the two pens of the multimeter with the two leads of the measured capacitor, and observe whether there is a slight swing of the needle. For large capacitance, the needle swing is obvious; For small capacitance, the needle swing is not obvious. Then use the pen again, three times, four times to touch the capacitor lead (the pen is not switched), each time to observe whether the needle has a slight swing. If from the second time every touch of the dial hand swing, it indicates that the capacitor has a leakage. Such as a few times when the hands are not moving, that the capacitor is good. If the first touch when the hand to the end of the pendulum, the capacitor has been broken down. In addition, for capacitors with capacity of 1mF~20mF, some digital multimeters can be measured.

(3) Capacitors with capacity below 1 mF shall be checked. The actual value of capacitor can be measured accurately using the capacitance measuring block of digital multimeter. If there is no digital multimeter with capacitance measurement function, only use the ohmic block to check whether it is a breakdown and short circuit. Use a good capacitor of the same capacity in parallel with the suspected capacitor and check that it is open.

(4) Accurate measurement of capacitor parameters. The capacity of a single capacitor can be accurately measured by LCR bridge, and the voltage value can be measured by transistor characteristic tester.

3. Inductors and transformers

Such components include inductors, transformers, oscillating coils, filter coils, etc. The fault is caused by external reasons, for example, when the load short circuit, because the current flowing through the coil exceeds the rated value, the temperature of the transformer rises, resulting in the coil short circuit, circuit opening or insulation breakdown. When the ventilation is poor, the temperature is too high or damp, it will also produce leakage or insulation breakdown phenomenon.

For the transformer fault phenomenon and reasons, common have the following: when the transformer is connected to the power supply, if the core buzzing sound, the fault may be the core is not clamped or the transformer load is too heavy; High heat, smoke, burnt smell or blown fuse may be coil short circuit or overload.

Generally, the following methods are used to check the faults of inductors and transformers:

(1) DC resistance measurement method. Use a multimeter to measure the electrical barrier inductance class of components. When measuring the antenna coil and oscillating coil, the measuring range should be placed in the minimum electrical barrier (such as R × 1 W block); When measuring the middle circumference and output input transformer, the measuring range should be placed in the low barrier (R × 10W or R × 10 0W), the measured resistance value and maintenance data or accumulated daily experience data are compared, if very close, it means the tested component is normal; If the resistance value is much smaller than the empirical data, it indicates that the coil has a local short circuit; If the value of the needle is zero, it indicates that the coil is short. It should be noted that the secondary resistance of the oscillation coil, the antenna coil and the middle circumference is very small, only a few tenths of an ohm. The reading should be especially careful, and it should not be misjudged as a short circuit. The resistance between the primary coil and the secondary coil measured with a high barrier (R ×10kW) should be infinite. If there is a certain resistance value between the primary and secondary, it means that there is leakage between the primary and secondary.

(2) power inspection method. The power transformer can be checked through the power, to see whether the secondary voltage is falling, if the secondary voltage is suspected of local short circuit (or primary). When the transformer is hot quickly or has burnt smell, smoke and other phenomena, it can be judged that the transformer must have a local short circuit.

(3) Instrument inspection method. The high frequency Q meter can be used to measure the inductance and its Q value, and the inductance short-circuit meter can be used to judge the local short-circuit phenomenon of the low frequency coil. With megaohm meter can measure the insulation resistance between the primary and secondary power transformer. If the transformer is found to have leakage phenomenon, it may be caused by poor insulation or moisture, at this time, the transformer can be removed to dry. In addition, the voltage regulating transformer of various carbon brush or copper brush, in the maintenance and improper use of the case is very easy to wear, its debris and carbon accumulation is often due to the short-circuit part of the coil burned and burned transformer, so usually pay attention to maintenance.

4. Integration block classes

The main causes of open or on-off electrode are metal migration, electric corrosion and process problems. The main causes of electrode short circuit are metal diffusion between electrodes, defects in metallization process or foreign bodies. The main causes of lead fracture are uneven wire diameter, insufficient lead strength, excessive hot spot stress and mechanical stress and electrical erosion. The main causes of electrical parameter drift are raw material defects and reactions caused by mobile ions. Mechanical wear and packaging cracks are mainly caused by packaging process defects and excessive environmental stress. Poor weldability is mainly caused by lead material defects, lead metal coating, lead surface contamination, corrosion and oxidation. The inability to work is usually caused by the work environment.

To sum up, we can know that in order to ensure that the equipment or system can work reliably, the reliability requirements for electronic components are very high. Reliability index has become one of the important quality indexes of components. Understanding the failure modes and mechanisms of components is very important for diagnosing device faults and maintaining device reliability