Electronic components often fail during use. Failure means that the circuit may malfunction, thus affecting the normal operation of the equipment. Here we analyze the failure causes and common faults of common components.
Most of the failures in electronic equipment are ultimately caused by the failure of electronic components. If you are familiar with the cause of component failure and locate the cause of component failure in time, you can eliminate the fault in time and allow the equipment to operate normally.
Temperature causes failure
One of the important factors for component failure is the impact of ambient temperature on components.
The impact of temperature changes on semiconductor devices
Since the forward voltage drop of P-N junction is greatly affected by temperature, the voltage transmission characteristics and anti-interference degree of bipolar semiconductor logic elements (TTL, HTL and other integrated circuits) composed of P-N as the basic unit are also related to Temperature is closely related.
When the temperature rises, the forward voltage drop of the P-N junction decreases, and its door-opening and closing levels will decrease. This makes the low-level anti-interference voltage tolerance of the component change as the temperature increases. Small; the high-level anti-interference voltage tolerance increases with temperature, causing output level deviation, waveform distortion, steady-state imbalance, and even thermal breakdown.
The P-N junction, the basic unit of a bipolar semiconductor device, is very sensitive to changes in temperature. When the P-N junction is reverse biased, the reverse leakage current formed by minority carriers is affected by changes in temperature. The relationship is:
In the formula:
ICQ: Reverse leakage current at temperature T0C
IICQ: Reverse leakage current at temperature TR℃
T-TR: Absolute value of temperature change
It can be seen from the above formula that for every 10°C increase in temperature, ICQ will double. This will cause the operating point of the transistor amplifier to drift, the transistor current amplification coefficient to change, the characteristic curve to change, and the dynamic range to become smaller.
The relationship between temperature and allowable power consumption is as follows:
In the formula:
Pcm: Maximum allowed power consumption
Ta: Operating ambient temperature
Tj: junction temperature of the transistor
Rja: Thermal resistance between junction and environment
It can be seen from the above formula that the increase in temperature will reduce the maximum allowable power consumption of the transistor.
The effect of temperature changes on resistance
The impact of temperature changes on resistors is mainly when the temperature rises. The increase in temperature will cause the thermal noise of the resistor to increase, the resistance value to deviate from the nominal value, and the allowable dissipation probability to decrease. For example, when the temperature of RXT Product's carbon film resistor rises to 100°C, the allowed dissipation probability is only 20% of the nominal value.
This property of resistors is not all bad. For example, specially designed resistors: PTC (positive temperature coefficient thermistor) and NTC (negative temperature coefficient thermistor), whose resistance is greatly affected by temperature, can be used as sensors. For PTC, when its temperature rises to a certain threshold, its Resistance will increase sharply.
Using this characteristic, it can be used in the overcurrent protection circuit of the circuit board - when the current through it increases to its threshold current due to some fault, the temperature of the PTC rises sharply, and at the same time , its Resistance becomes larger, limiting the current passing through it to protect the circuit. After the fault is eliminated, the current flowing through it decreases, the temperature of the PTC returns to normal, and its Resistance also returns to its normal value. For NTC, its characteristic is that its Resistance decreases as the temperature increases.
The effect of temperature changes on capacitance
Temperature changes will cause changes in the dielectric loss of the capacitor, thus affecting its service life. When the temperature rises by 10°C, the life of the capacitor is reduced by 50%. It also causes changes in the resistance-capacitance time constant, and even thermal breakdown due to excessive dielectric loss.
Humidity causes failure
One of the important factors for component failure is the impact of ambient humidity on components. If the humidity is too high, when dust containing acid and alkali falls on the circuit board, it will corrode the solder joints and wiring of the components, causing the solder joints to fall off and the joints to break. Excessive humidity is also the main cause of leakage coupling. Too low humidity can easily produce static electricity, so the humidity of the environment should be controlled at a reasonable level.
Excessive voltage causes failure
One of the important factors in component failure is the impact of excessive voltage on components. An important condition to ensure the normal operation of components is that the voltage applied to the components must ensure stability. Excessively high voltage may cause increased heat loss of components, or may cause electrical breakdown of components. Taking a capacitor as an example, its failure rate is proportional to the fifth power of the voltage applied across the capacitor. For integrated circuits, voltages exceeding their maximum allowable voltage will cause direct damage to the device.
Voltage breakdown refers to the maximum withstand voltage value that electronic devices can withstand. If the allowable value is exceeded, the device is at risk of failure. The manifestations of failure of active components and passive components are slightly different, but they also have upper voltage limits. Transistor components have a withstand voltage value. Exceeding the withstand voltage value will cause damage to the components. For example, exceeding the withstand voltage value of diodes, capacitors and other components will cause them to break down. If the energy is large, it will cause thermal breakdown and the components will be scrapped.
Vibration and impact lead to failure
One of the important factors in component failure is the impact of vibration and impact on components. Mechanical vibration and shock will accelerate the failure of some internally defective components, causing catastrophic failure. Mechanical vibration can also loosen solder joints and crimping points, leading to poor contact. If vibration causes undue contact between wires, some unexpected consequences will occur.
Possible failure modes and failure analysis:
Resistance failure analysis
The failure mechanisms of resistors and potentiometers vary depending on the type. The main failure modes of non-linear resistors and potentiometers are open circuit, resistance drift, mechanical damage to leads and contact damage; the main failure modes of wirewound resistors and potentiometers are open circuit, mechanical damage to leads and contact damage. There are mainly four categories:
Carbon film resistors. Lead breakage, substrate 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 substrate, etc.
Metal film resistors. Uneven resistive film, cracked resistive film, weak leads, decomposition of resistive film, silver migration, oxide reduction of resistive film, electrostatic charge, lead breakage, corona discharge, etc.
Wirewound resistors. Poor contact, current corrosion, weak leads, poor wire insulation, melted solder joints, etc.
Variable resistor. Poor contact, poor welding, broken contact or lead loss, impurity contamination, poor epoxy glue, axis tilt, etc.
Resistors are prone to deterioration and open-circuit faults. After the resistor deteriorates, the resistance often drifts to become larger. Resistors are generally not repaired, but replaced with new ones. When the resistance wire of a wirewound resistor burns out, in some cases the burnout can be re-welded before use.
Deterioration of resistors is mostly caused by poor heat dissipation, excessive moisture or manufacturing defects, while burnout is caused by abnormal circuits, such as short circuits, overloads, etc. There are two common phenomena of resistor burnout. One is that excessive current causes the resistor to heat up, causing the resistor to burn out. At this time, the surface of the resistor becomes scorched, which is easy to find. The other is that the instantaneous high voltage is applied to the resistor, causing the resistor to open circuit. Or the resistance value becomes larger. In this case, the resistor surface generally does not change significantly. Resistors with this fault phenomenon can often be found in high-voltage circuits.
There are two main types of variable resistors or potentiometers: wirewound and non-wirewound. Their common failure modes include: parameter drift, open circuit, short circuit, poor contact, loud dynamic noise, mechanical damage, etc. However, actual data shows that the main failure modes are quite different between laboratory tests and field use. Laboratory failures are mostly caused by parameter drift, while in the field they are mostly caused by poor contact and open circuits.
Poor potentiometer contact failure is common in field use. For example, it accounts for 90% of telecommunications equipment and about 87% of TV sets. Therefore, poor contact is a fatal weak link for potentiometers. The main reasons for poor contact are as follows:
Contact pressure is too small, stress relaxation, sliding contacts deviate from the track or conductive layer, improper mechanical assembly, or contact deformation due to large mechanical loads (such as collisions, drops, etc.).
The conductive layer or contact track forms various non-conductive film layers at the contact due to oxidation and contamination.
The conductive layer or resistance alloy wire is worn or burned, 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 or resistance alloy wire of the potentiometer is oxidized, corroded, contaminated, or overloaded due to improper processing (such as uneven winding, uneven thickness of the conductive film, etc.), resulting in local overheating and causing the potentiometer to burn out. 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; the potentiometer is improperly selected and used, or the failure of the electronic equipment endangers the potentiometer, causing it to be overloaded. Or work under a larger load. These will accelerate the damage of the potentiometer.
