What is the failure mode of the resistor? In-depth analysis of failure mechanism must see

Failure mode: various failure phenomena and their manifestations.

Failure mechanism: is the physical, chemical, thermodynamic or other process that causes failure.

1. The main failure mode and failure mechanism of the resistor are

1) Open circuit: The main failure mechanism is that the resistive film burns out or the large area falls off, the matrix breaks, and the lead cap and the resistor body fall off.

2) Resistance drift is super-standard: the resistive film is defective or degraded, the substrate has movable sodium ions, and the protective coating is poor.

3) Lead breakage: resistance welding process defects, solder joint contamination, lead mechanical stress damage.

4) Short circuit: silver migration, corona discharge.

2, failure mode accounted for the total proportion of failures

(1), wirewound resistor

(2), non-wire wound resistor

3. Failure mechanism analysis

The failure mechanism of the resistor is multi-faceted, and various physical and chemical processes occurring under working conditions or environmental conditions are the causes of the aging of the resistor.

(1) Structural changes of conductive materials

The conductive film layer of the thin film resistor is generally obtained by a vapor deposition method, and an amorphous structure exists to some extent. According to the thermodynamics point of view, the amorphous structure has a tendency to crystallize. Under working conditions or environmental conditions, the amorphous structure in the conductive film layer tends to crystallize at a certain speed, that is, the internal structure of the conductive material tends to be densified, which can often cause a decrease in the resistance value. The rate of crystallization increases with increasing temperature.

The resistance wire or the resistive film is subjected to mechanical stress during the preparation process, and the internal structure is distorted. The smaller the wire diameter or the thinner the film layer, the more significant the stress influence. Generally, heat treatment can be used to eliminate internal stress, and residual internal stress may be gradually eliminated during long-term use, and the resistance of the resistor may change accordingly.

Both the crystallization process and the internal stress removal process slow down over time, but it is not possible to terminate during the use of the resistor. It can be considered that the two processes are performed at approximately constant speed during the operation of the resistor. The resistance changes associated with them account for about a few thousandth of the original resistance.

Electrical load high temperature aging: In any case, the electrical load will accelerate the aging process of the resistor, and the effect of the electrical load on the aging of the accelerating resistor is more significant than the accelerated aging of the elevated temperature, because the temperature of the contact portion between the resistor and the lead cap is The rise exceeds the average temperature rise of the resistor body. Generally, for every 10 °C increase in temperature, the life is reduced by half. If the temperature rises by 50 °C when the temperature rise of the resistor exceeds the rated load, the life of the resistor is only 1/32 of the life under normal conditions. The operating stability of the resistor over a 10-year period can be assessed by an accelerated life test of less than four months.

DC load—electrolysis: Under the action of DC load, electrolysis causes the resistor to age. Electrolysis occurs in the groove of the groove resistor, and the alkali metal ions contained in the resistor matrix are displaced in the electric field between the grooves to generate an ion current. The electrolysis process is more intense when moisture is present. If the resistive film is a carbon film or a metal film, it is mainly electrolytic oxidation; if the resistive film is a metal oxide film, it is mainly electrolytic reduction. For high-resistance thin-film resistors, the effect of electrolysis can increase the resistance, and film damage may occur along one side of the spiral of the groove. The DC load test in a hot flash environment can comprehensively evaluate the oxidation resistance or anti-reduction performance of the resistor base material and the film layer, as well as the moisture resistance of the protective layer.

(2), vulcanization

After a batch of field instruments were used in a chemical plant for one year, the instruments failed. After analysis, it was found that the resistance of the thick film chip used in the meter became larger, and even became an open circuit. When the failed resistor was placed under a microscope, it was found that black crystalline material appeared on the edge of the resistive electrode. Further analysis of the composition revealed that the black substance was silver sulfide crystal. The original resistance was corroded by sulfur from the air.

(3) Gas adsorption and desorption

The resistive film of the film resistor at the grain boundary, or the conductive particles and the binder portion, may always adsorb a very small amount of gas, which constitute an intermediate layer between the grains, hindering the contact between the conductive particles, thereby Significantly affects the resistance.

The synthetic membrane resistor is made under normal pressure. When working under vacuum or low pressure, the desorbing part is attached with gas, which improves the contact between the conductive particles and reduces the resistance. Similarly, when a thermal decomposition carbon film resistor fabricated in a vacuum is directly operated under normal environmental conditions, a part of the gas is adsorbed due to an increase in gas pressure, so that the resistance value is increased. If the uncut semi-finished product is preset at normal pressure for a suitable time, the resistance stability of the finished resistor product is improved.

Temperature and pressure are the main environmental factors that affect gas adsorption and desorption. For physical adsorption, the temperature drop can increase the equilibrium adsorption amount, and the temperature rise is the opposite. Gas adsorption and desorption occur on the surface of the resistor body. Therefore, the impact on the membrane resistor is more significant. Resistance changes can reach 1% to 2%.

( 4) oxidation

Oxidation is a long-term factor (unlike adsorption), and the oxidation process starts from the surface of the resistor and gradually goes deep into the interior. In addition to precious metal and alloy film resistors, resistors of other materials are affected by oxygen in the air. The result of oxidation is an increase in resistance. The thinner the resistive layer, the more pronounced the effect of oxidation.

The fundamental measure to prevent oxidation is sealing (inorganic materials such as metals, ceramics, and glass). Coating or potting with organic materials (plastics, resins, etc.) does not completely prevent the protective layer from being permeable or permeable. Although it can delay the oxidation or adsorption of gases, it also brings some new effects related to the organic protective layer. Aging factor.

(4), the impact of organic protective layer

During the formation of the organic protective layer, the polycondensation volatile matter or solvent vapor is released. The heat treatment process causes some of the volatiles to diffuse into the resistor, causing the resistance to rise. Although this process lasts for 1~2 years, the time that significantly affects the resistance is about 2~8 months. In order to ensure the stability of the resistance of the finished product, it is more appropriate to leave the product in the warehouse for a period of time before leaving the factory.

(5), mechanical damage

The reliability of the resistor is highly dependent on the mechanical properties of the resistor. The resistor body, lead cap, and lead wire should have sufficient mechanical strength, and the base defect, lead cap damage, or lead breakage can cause the resistor to fail.

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