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Resistance Basics and Failure Mechanisms
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1 Definition and explanation of resistance.
Resistance is one of the basic concepts in physics. From a physical point of view, the conductor resistance to current is called the resistance of that conductor, with the caveat that it is the current, not the voltage, that is impeded by the resistance.
The resistance of a conductor is also understood in the microscopic field. All conductors have an atomic structure inside, and the periphery contains free moving electrons and positive ions that form a lattice. In the process of electron movement, the free electrons collide with atoms and lattices, resulting in energy loss and impediment to electron movement.
2 The role of electrical resistance.
The actual characteristic of resistance is to prevent the current flowing through the conductor from changing, but only to limit, not to disallow sudden changes in current. Using this characteristic of resistance, resistance is generally used in current limiting, voltage dividing, impedance matching, signal detection, protection, heating, etc. What exactly is needed for resistance should be considered at the time of application.
3 The internal structure of solid resistors.
From the package type to distinguish, the resistor is divided into plug-in resistors and chip resistors, the production process of different types of plug-in resistors is basically the same, of course, chip resistors are also.
4 Equivalent model of the resistor.
Resistors have a corresponding equivalent model at both high and low frequencies.
In low frequency application conditions, the equivalent model of the resistor is its own characteristic resistance. Under high frequency application conditions, the resistor has parasitic parameters - parasitic inductance and parasitic capacitance, which will cause the actual high frequency impedance value of the resistor to vary with frequency.
5 the failure analysis of the resistor.
The failure modes of resistors are mainly divided into two types: ① open circuit (resistance value becomes large); ② resistance value overrun (resistance value becomes small).
The failure mechanism is as follows.
(1) temperature shock cracking: improper temperature control when the device is soldered leads to an overly strong temperature change above the resistor causing the resistor ends to fall off or the resistance value to change.
(2) mechanical stress cracks: the resistor is subject to external stress, resulting in cracks, commonly found in circuit board deformation, detection and gravity factors during the flow process, circuit board division, playing screws, etc.
(3) over-current or over-voltage: resistors in over-power use, whether the current exceeds the rated value, or surge voltage resulting in excessive power applied to the resistor, will lead to failure response.
(4) improper environmental storage: resistors are in harsh environments for a long time, such as high temperature, low temperature, high humidity, high salinity, etc., the resistors will be affected by the environment leading to electrode corrosion or oxidation, reducing the solderability of the resistors and causing failure response.
(5) ESD damage: actually similar to over voltage, ESD is a transient high voltage applied to both ends of the resistor, as most of the resistors are made using coating technology, ESD high voltage will produce damage to these coatings, resulting in resistor failure.
(6) Silver migration: the silver material used in the face electrode of the resistor has a high mobility, and after migration, the silver will undergo a chemical reaction and slowly be ionized, resulting in the failure of the resistor.
