January 12, 2021
Failure is directly affected by factors such as humidity, temperature, voltage, and machinery.
1. Temperature causes failure:
1.1 Ambient temperature is an important factor leading to component failure.
The effect of temperature changes on semiconductor devices: The basic unit PN junction of bipolar semiconductor devices is very sensitive to temperature changes. When the PN junction is reverse biased, the reverse leakage current formed by minority carriers is affected by temperature changes , Its relationship is:
Where: ICQ―――Reverse leakage current at temperature T0C
ICQR――Reverse leakage current at temperature TR℃
T－TR――The absolute value of temperature change
It can be seen from the above formula that ICQ will double for every 10°C increase in temperature. This will cause the operating point of the transistor amplifier to drift, the transistor current amplification factor 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:
Where: PCM——Maximum allowable power consumption
TjM―――Maximum allowable junction temperature
T――――Ambient temperature for use
It can be seen from the above formula that the increase in temperature will reduce the maximum allowable power consumption of the transistor.
Since the forward voltage drop of PN junction is greatly affected by temperature, the voltage transmission characteristics and anti-interference degree of bipolar semiconductor logic elements (integrated circuits such as TTL, HTL, etc.) composed of PN as the basic unit are also closely related to temperature. Relationship. When the temperature increases, the forward voltage drop of the PN junction decreases, and its opening and closing levels will decrease. This makes the low-level anti-interference voltage tolerance of the component smaller with the increase of temperature; The level of anti-interference voltage tolerance increases with the increase of temperature, causing output level shift, waveform distortion, steady-state imbalance, and even thermal breakdown.
2.1 The influence of temperature changes on resistance
The influence of temperature change on the resistance is mainly that when the temperature rises, the thermal noise of the resistance increases, the resistance value deviates from the nominal value, and the allowable dissipation probability decreases. For example, when the temperature of RXT series carbon film resistors rises to 100°C, the allowable dissipation probability is only 20% of the nominal value.
But we can also use this characteristic of resistors. For example, there are specially designed resistors: PTC (Positive Temperature Coefficient Thermistor) and NTC (Negative Temperature Coefficient Thermistor), whose resistance is affected by temperature. Big.
For PTC, when its temperature rises to a certain threshold, its resistance value will increase sharply. Using this feature, 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 a certain fault, the temperature of the PTC rises sharply, and at the same time, its resistance value becomes larger. , Limiting the current passing through it, to achieve the protection of the circuit. After the fault is eliminated, the current passing through it decreases, the temperature of the PTC returns to normal, and at the same time, its resistance value also returns to its normal value.
For NTC, its characteristic is that its resistance value decreases with increasing temperature.
2.2 The influence of temperature changes on capacitance
Temperature changes will cause the dielectric loss of the capacitor to change, thereby affecting its service life. When the temperature rises by 10°C, the life of the capacitor is reduced by 50%. At the same time, it also causes the change of the resistance-capacitance time constant, and even thermal breakdown due to excessive dielectric loss.
In addition, the increase in temperature will also reduce the insulation performance of inductors, transformers, chokes, etc.
3. Humidity causes failure
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.
The humidity is too low and it is easy to generate static electricity, so the humidity of the environment should be controlled at a reasonable level.
4. Excessive voltage causes device failure
The stability of the voltage applied to the components is an important condition to ensure the normal operation of the components. Excessive voltage will increase the heat loss of components and even cause electrical breakdown. For a capacitor, its failure rate is proportional to the 5th power of the capacitor voltage. For integrated circuits, a voltage exceeding its maximum allowable voltage value will cause direct damage to the device.
Voltage breakdown refers to the highest withstand voltage that electronic devices can withstand. If the allowable value is exceeded, there is a risk of failure of the device. The manifestation of the failure of active components and passive components is slightly different, but there are also allowable upper limits of voltage. Transistor components have a withstand voltage value. Exceeding the withstand voltage value will damage the components, such as exceeding the withstand voltage value of diodes, capacitors, etc., voltage exceeding the withstand voltage value of the component will cause them to breakdown. If the energy is large, it will cause thermal breakdown and the component will be scrapped. .
5. Vibration and impact:
Mechanical vibration and shock will accelerate the failure of some internally defective components and cause catastrophic failure. Mechanical vibration will also loosen solder joints and crimping points, resulting in poor contact; if vibration causes undesirable contact between wires, it will Produce some unintended consequences.
Possible failure modes and failure analysis.
Electrical OverStress (EOS) is a common way to damage electronic devices. It is a common cause of damage to components. Its manifestation is that overvoltage or overcurrent generates a large amount of heat energy, which makes the internal temperature of the components too high and damages them Components (commonly referred to as burnout) are a common way to damage electronic devices caused by pulses in the electrical system.