Failure Mechanism and Fault Analysis of Common Electronic Components
In the course of using electronic components, failures and failures often occur, which affects the normal operation of the equipment. The text analyzes the causes of failures and common faults of common components.
Most of the faults in electronic equipment are ultimately caused by electronic components failure. If you are familiar with the type of fault of the component, sometimes you can quickly find the faulty component through intuition. Sometimes you can find the fault by simple resistance and voltage measurement.
1 resistor class
Resistor-like components include resistive components and variable resistive components. Fixed resistors are commonly referred to as resistors, and variable resistors are commonly referred to as potentiometers. Resistor-like components are used in electronic devices in a large number and are power-consuming components. The failure rate of electronic devices caused by resistor failure is relatively high, accounting for about 15%. The failure mode and cause of the resistor are closely related to the structure, process characteristics and use conditions of the product. Resistor failure can be divided into two categories, namely fatal failure and parameter drift failure. Field usage statistics show that 85%~90% of resistor failures are fatal failures, such as open circuit, mechanical damage, contact damage, short circuit, insulation, breakdown, etc. Only about 10% are caused by resistance drift.
The failure mechanism of the resistor potentiometer varies depending on 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 wirewound resistors and potentiometers are open circuit, lead mechanical damage and contact damage. There are four main categories:
(1) Carbon film resistors. Lead breakage, matrix defects, poor uniformity of the film layer, groove defects in the film layer, poor contact between the film material and the lead end, contamination of the film and the substrate, and the like.
(2) Metal film resistors. The resistive film is uneven, the resistive film is broken, the lead is not strong, the resistive film is decomposed, the silver migration, the resistive film oxide reduction, the electrostatic charge action, the lead breakage, the corona discharge, and the like.
(3) Wirewound resistors. Poor contact, current corrosion, poor lead wires, poor wire insulation, solder joint melting, etc.
(4) Variable resistors. Poor contact, poor soldering, contact reed rupture or lead drop, impurity contamination, poor epoxy glue, shaft tilt, etc.
Resistance is prone to deterioration and open circuit failure. After the resistance is deteriorated, the resistance is often increased. The resistor is generally not repaired and the new resistor is replaced directly. Wirewound Resistor When the wire is blown, in some cases the blown process can be re-welded and used.
Resistance deterioration is mostly caused by poor heat dissipation, excessive moisture or defects during manufacturing, and burnout is caused by abnormal circuits such as short circuit and overload. There are two common phenomena of resistor burnout. One is that the current is too large, causing the resistor to burn out due to resistance heating. At this time, the surface of the resistor is visible in the form of a burnt paste, which is easy to find. In another case, since the instantaneous high voltage is applied to the resistor to cause the open circuit or the resistance to become large, in this case, the surface of the resistor generally does not change significantly, and the resistance of the fault phenomenon can often be found in the high voltage circuit.
Variable resistors or potentiometers are mainly wired and non-wire wound. Their common failure modes are: parameter drift, open circuit, short circuit, poor contact, large dynamic noise, and mechanical damage. However, the actual data shows that the main failure modes between laboratory tests and field use are quite different. The laboratory failures are mostly caused by parameter drift, while the site is mostly poorly contacted and open.
Poor contact with potentiometers is common in field use. For example, in telecom equipment, up to 90%, and in TV sets, about 87%, so poor contact is a fatal link to potentiometers. The main causes of poor contact are as follows:
(1) The contact pressure is too small, the reed stress is relaxed, the sliding contact is off track or conductive layer, the mechanical assembly is improper, or a large mechanical load (such as collision, drop, etc.) causes deformation of the contact spring.
(2) The conductive layer or the contact track forms various non-conductive film layers at the contact due to oxidation and contamination.
(3) The conductive layer or the resistive alloy wire is worn or burnt, resulting in poor contact at the sliding point.
Potentiometer open circuit failure is mainly caused by local overheating or mechanical damage. For example, the conductive layer of the potentiometer or the resistance alloy wire is oxidized, corroded, contaminated, or overloaded due to improper process (such as uneven winding, uneven thickness of the conductive film layer, etc.), causing local overheating and causing burnout of the potentiometer The open circuit; the surface of the sliding contact is not smooth, and the contact pressure is too large, which will cause the winding to be severely worn and disconnected, resulting in an open circuit; improper selection and use of the potentiometer, or malfunction of the electronic device jeopardizing the potentiometer, making it overloaded Or work under a large load. These will accelerate the damage of the potentiometer.
2 capacitors
Common fault phenomena of capacitors mainly include breakdown, open circuit, degradation of electrical parameters, electrolyte leakage and mechanical damage. The main causes of these failures are as follows:
(1) Breakdown. Defects, defects, impurities or conductive ions in the medium; aging of the dielectric material; electrochemical breakdown of the dielectric; arcing of the inter-electrode edge under high humidity or low pressure; instantaneous short circuit of the dielectric under mechanical stress; migration of metal ions Forming a conductive channel or edge arcing discharge; dielectric air gap breakdown inside the dielectric material causes electrical breakdown of the medium; mechanical damage of the medium during the manufacturing process; changes in the molecular structure of the dielectric material and the applied voltage is higher than the rated value.
(2) Open the road. Breakdown causes electrode and lead insulation; electrolytic capacitor anode lead foil is corroded (or mechanically broken); lead wire and electrode contact point oxide layer to cause low level open circuit; lead wire and electrode contact poor or insulated; electrolytic capacitor anode lead The metal foil is opened due to corrosion; the dry or frozen working electrolyte; the instantaneous open circuit between the electrolyte and the dielectric under mechanical stress.
(3) The electrical parameters are degraded. 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 of material; volatilization and thickening of working electrolyte; electrolytic corrosion or chemical corrosion of electrode; Lead and electrode contact resistance increases; impurities and harmful ions.
Since the actual capacitor works under the combined action of working stress and environmental stress, one or several failure modes and failure mechanisms are generated, and there is also a failure mode that causes another failure mode or failure mechanism to occur. For example, temperature stress can promote surface oxidation, accelerate the degree of aging, accelerate the degradation of electrical parameters, and promote the decline of electric field strength, accelerate the early arrival of dielectric breakdown, and the degree of influence of these stresses is a function of time. Therefore, the failure mechanism of the capacitor is closely related to the type of product, the type of material, the difference in structure, the manufacturing process and environmental conditions, and the working stress.
Capacitor breakdown faults are very easy to find, but in the case of multiple components connected in parallel, it is more difficult to determine the specific faulty component. Capacitor open circuit fault can be determined by connecting the same type and capacity of the capacitor in parallel with the detected capacitor to observe whether the circuit function is restored. The inspection of the change of the electrical parameters of the capacitor is troublesome, and generally can be carried out according to the following method.
First, one of the capacitor leads should be burned from the board to avoid the effects of surrounding components. Secondly, different methods are used to check according to the different conditions of the capacitor.
(1) Inspection of electrolytic capacitors. The multimeter is placed in an electrical barrier, and the range depends on the capacity of the electrolytic capacitor to be tested and the magnitude of the withstand voltage. Electrolytic capacitors with small measuring capacity and high withstand voltage should be located in R &TImes; 10kW block; electrolytic capacitors with large measuring capacity and low withstand voltage should be located in R &TImes; 1 k W block. Observe the magnitude of the charging current, the length of the discharge time (the speed at which the hands are retracted), and the resistance value indicated by the last indication of the hands.
The identification method of the quality of electrolytic capacitors is as follows:
1 The charging current is large, the needle rises fast, the discharge time is long, and the retraction speed of the hands is slow, indicating that the capacity is sufficient.
2 The charging current is small, the needle rises slowly, the discharge time is short, and the retraction speed of the hands is fast, indicating that the capacity is small and the quality is poor.
3 The charging current is zero, the needle does not move, indicating that the electrolytic capacitor has failed.
4 When the discharge is finished, the resistance value indicated by the return of the needle to the end is large, indicating that the insulation performance is good and the leakage is small.
5 Discharge to the end, the indication value indicated when the needle is returned to the end is small, indicating poor insulation performance and serious leakage.
(2) General capacitor inspection with a capacity of 1 mF or more. The multimeter can be used to check the degree of leakage and whether it is broken down by multimeter electric resistance (R & TImes; 1 0 k W). Touch the two test leads of the multimeter against the two leads of the capacitor under test to see if the hands are slightly swung. For a capacitor with a large capacity, the hands are swung significantly; for a capacitor with a small capacity, the hands are not oscillating. Then use the test lead to touch the lead of the capacitor again, three times, four times (the test pen is not adjusted), and observe the needle for a slight swing every time it is touched. If the hand is swung once every time it is touched from the second time, it indicates that the capacitor has leakage. If the hands do not move when touched several times, the capacitor is good. If the hands are at the end of the first touch, the capacitor has been broken down. In addition, for capacitors with a capacity of 1mF to 20mF, some digital multimeters can be measured.
(3) Capacitor inspection with a capacity of 1 mF or less. The actual value of the capacitor can be measured more accurately using the capacitance measurement of the digital multimeter. If you do not have a digital multimeter with capacitance measurement, you can only use an ohmic block to check if it breaks through the short circuit. Use a capacitor of the same capacity in parallel with the suspected capacitor to check if it is open.
(4) Accurate measurement of capacitor parameters. An accurate measurement of the capacity of a single capacitor can use an LCR bridge, and the value of the withstand voltage can be measured using a transistor characteristic tester.
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