Failure analysis and causes of electronic components

Date：2021-04-27 15:35:00 Views：2335

Circuit failure is a headache for every engineer. During the use of electronic components, failures and faults will also occur, which will affect the normal operation of the equipment. Let's learn about the failure analysis and failure causes of electronic components. If you are familiar with the fault types of components, sometimes you can quickly find the fault components through intuition, and sometimes you can find the fault through simple resistance and voltage measurement.

1. Resistor class

Resistor elements include resistance elements and variable resistance elements. Fixed resistance is usually called resistance and variable resistance is usually called potentiometer. Resistor components are used in a large number in electronic equipment and are power consuming components. The failure rate of electronic equipment caused by resistor failure is relatively high, accounting for about 15% according to statistics. The failure modes and causes of resistors are closely related to the product structure, process characteristics and service conditions. Resistor failure can be divided into two categories: fatal failure and parameter drift failure. Field application statistics show that 85% ~ 90% of resistor failures are fatal failures, such as open circuit, mechanical damage, contact damage, short circuit, insulation, breakdown, etc., and only about 10% 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, pollution between film and matrix, etc.

(2) Metal film resistor. Uneven resistance film, broken resistance film, loose lead, decomposition of resistance film, silver migration, oxide reduction of resistance film, electrostatic charge, lead fracture, corona discharge, etc.

(3) Wire wound resistor. Poor contact, current corrosion, loose lead, poor wire insulation, solder joint melting, etc.

(4) Variable resistor. Poor contact, poor welding, broken contact reed or lead falling off, impurity pollution, poor epoxy adhesive, shaft inclination, etc.

The resistance is prone to deterioration and open circuit faults. After the resistance is deteriorated, the resistance value tends to drift. Generally, the resistance is not repaired, but directly replaced with a new resistance. Wire wound resistance when the resistance wire is burnt out, it can be used after welding again in some cases.

Resistance deterioration is mostly caused by poor heat dissipation, excessive humidity or defects during manufacturing, while burnout is caused by abnormal circuit, such as short circuit, overload and so on. There are two common phenomena of resistance burning out. One is that the over current causes the resistance to heat, causing the resistance to burn out. At this time, a burnt paste can be seen on the surface of the resistance, which is easy to find. In another case, the resistance is open circuit or the resistance value becomes larger due to the instantaneous high voltage added to the resistance. In this case, the resistance surface generally does not change significantly. The resistance of this fault phenomenon can often be found in high-voltage circuits.

Variable resistors or potentiometers are mainly wired and non wired. Their common failure modes are: parameter drift, open circuit, short circuit, poor contact, large dynamic noise, mechanical damage and so on. However, the actual data show that the main failure modes are quite different between laboratory test and field use. The majority of laboratory faults are parameter drift, while the majority of field faults are poor contact and open circuit.

The failure of poor contact of potentiometer is common in field use. For example, it accounts for 90% in telecommunication equipment and about 87% in television, so poor contact is a fatal weak link for potentiometer. The main causes of poor contact are as follows:

(1) The contact pressure is too small, the spring stress is relaxed, the sliding contact deviates from the track or conductive layer, the mechanical assembly is improper, or the contact spring is deformed due to large mechanical load (such as collision, drop, etc.).

(2) The conductive layer or contact track forms various non-conductive films at the contact due to oxidation and pollution.

(3) The conductive layer or resistance alloy wire 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 conductive layer or resistance alloy wire of the potentiometer is oxidized, corroded, polluted or overloaded due to improper process (such as uneven winding, uneven thickness of the conductive film, etc.), resulting in local overheating, burning out the potentiometer and opening the circuit; If the sliding contact surface is not smooth and the contact pressure is too large, the winding will be seriously worn and disconnected, resulting in open circuit; Improper selection and use of potentiometer, or failure of electronic equipment endangers the potentiometer and makes it work under overload or large load. These will accelerate the damage of potentiometer.

2. Capacitors

The common fault phenomena of capacitors mainly include breakdown, open circuit, degradation of electrical parameters, 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 media materials; Electrochemical breakdown of dielectric; Inter electrode edge arcing in high humidity or low pressure environment; Dielectric transient short circuit under mechanical stress; Metal ions migrate to form conductive channel or edge flashover discharge; Dielectric breakdown caused by air gap breakdown in dielectric material; Mechanical damage of medium during manufacturing; The change of molecular structure of dielectric materials and the applied voltage higher than the rated value.

(2) Open circuit. Electrode and lead insulation caused by breakdown; The anode outgoing foil of electrolytic capacitor is corroded and broken (or mechanically broken); The low-level open circuit is caused by the oxide layer at the contact point between the outgoing line and the electrode; Poor contact or insulation between outgoing line and electrode; The metal foil from the anode of electrolytic capacitor leads to open circuit due to corrosion; Drying up or freezing of working electrolyte; Instantaneous open circuit between electrolyte and dielectric under mechanical stress.

(3) Electrical parameter degradation. Moisture and dielectric aging and thermal decomposition; Metal ion migration of electrode materials; Existence and change of residual stress; Surface contamination; Self healing effect of metallized electrode; Volatilization and thickening of working electrolyte; Electrolytic corrosion or chemical corrosion of electrodes; The contact resistance between lead and electrode increases; Effects of impurities and harmful ions.

Because the actual capacitor works under the comprehensive action of working stress and environmental stress, one or several failure modes and failure mechanisms will occur, and one failure mode will lead to the occurrence of other failure modes or failure mechanisms. For example, temperature stress can not only promote surface oxidation, accelerate the influence degree of aging and accelerate the degradation of electrical parameters, but also promote the decline of electric field strength and accelerate the early arrival of dielectric breakdown, and the influence degree of these stresses is a function of time. Therefore, the failure mechanism of capacitor is closely related to product type, material type, structural difference, manufacturing process, 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 component when there are multiple components in parallel. The determination of capacitor open circuit fault can be realized by paralleling the capacitor of the same model and capacity with the detected capacitor and observing whether the circuit function is restored. It is troublesome to check the change of capacitance electrical parameters, which can generally be carried out according to the following methods.

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

(1) Inspection of electrolytic capacitor. Place the multimeter in the resistance gear, and the measuring range depends on the capacity and withstand voltage of the measured electrolytic capacitor. Measure the electrolytic capacitor with small capacity and high withstand voltage, and the measuring range shall be at R * 10kW; The measuring range of electrolytic capacitor with large capacity and low withstand voltage shall be at R * 1 K W. Observe the charging current, the discharge time (the speed at which the gauge needle returns) and the last indicated resistance value of the gauge needle.

The identification methods of electrolytic capacitor quality are as follows:

① The charging current is large, the rising speed of the gauge needle is fast, the discharge time is long, and the return speed of the gauge needle is slow, indicating that the capacity is sufficient.

② The charging current is small, the rising speed of the gauge needle is slow, the discharge time is short, and the return speed of the gauge needle is fast, indicating small capacity and poor quality.

③ If the charging current is zero and the gauge needle does not move, it indicates that the electrolytic capacitor has failed.

④ At the end of the discharge, the resistance indicated by the meter needle when it returns to the end is large, indicating good insulation performance and low leakage.

⑤ At the end of the discharge, the resistance indicated at the end of the return of the meter needle is small, indicating poor insulation performance and serious leakage.

(2) General capacitor inspection with a capacity of more than 1 MF. The degree of leakage and breakdown can be checked by multiple measurements with the same polarity of multimeter resistance block (R & times; 10kW). Touch the two probes of the multimeter with the two leads of the measured capacitance and observe whether the probe swings slightly. For capacitors with large capacity, the watch needle swings obviously; For capacitors with small capacity, the needle swing is not obvious. Then touch the lead of the capacitor with the probe again, three times or four times (the probes are not aligned). Observe whether the needle swings slightly each time. If the needle swings every time it touches from the second time, it indicates that there is leakage in the capacitor. If the gauge needle does not move when touching several times in succession, it indicates that the capacitor is good. If the gauge needle swings to the end point at the first collision, it indicates that the capacitor has been broken down. In addition, for capacitors with a capacity of 1mf ~ 20mf, some digital multimeter can measure.

(3) Check capacitors with a capacity of less than 1 MF. The capacitance measuring gear of the digital multimeter can be used to measure the actual value of the capacitor more accurately. If there is no digital multimeter with capacitance measurement function, you can only use ohmic gear to check whether it has breakdown and short circuit. Use a good capacitor of the same capacity in parallel with the suspected capacitor and check whether it is open circuit.

(4) Accurate measurement of capacitor parameters. LCR bridge can be used to accurately measure the capacity of a single capacitor, and transistor characteristic tester can be used to measure the withstand voltage value.

The above is all the contents of "talking about the failure analysis and failure causes of electronic components". We can clearly know what the failure of electronic components is and what the causes of the failure are. It can not only have higher efficiency in circuit maintenance, but also improve the deficiencies of the circuit.