Failure Modes: Various failure phenomena and their manifestations.
Failure mechanism: is the physical, chemical, thermodynamic or other process that leads to failure.
1. The main failure modes and failure mechanisms of resistors are:
1) Open circuit: The main failure mechanism is that the resistance film is burned or falls off in a large area, the substrate is broken, and the lead cap and the resistor body fall off.
2) The resistance drift exceeds the specification: the resistance film is defective or degraded, the matrix has movable sodium ions, and the protective coating is poor.
3) Lead breakage: defects in the welding process of the resistor body, contamination of the solder joints, and mechanical stress damage to the leads.
4) Short circuit: migration of silver, corona discharge.
2. Failure mode to total failure ratio table
3. Failure mechanism analysis
The failure mechanism of resistors is multi-faceted. Various physical and chemical processes that occur under working conditions or environmental conditions are the reasons for the aging of resistors.
(1) Structural changes of conductive materials
The conductive film layer of thin film resistors is generally obtained by vapor deposition, and has an amorphous structure to a certain extent. From a thermodynamic point of view, amorphous structures have 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 resistance value. The rate of crystallization increases with increasing temperature.
The resistance wire or the resistance film will be subjected to mechanical stress during the preparation process, which will distort its internal structure. The smaller the wire diameter or the thinner the film layer, the more significant the stress will be. Generally, the internal stress can be eliminated by heat treatment, and the residual internal stress may be gradually eliminated in the long-term use process, and the resistance value of the resistor may change accordingly.
Both the crystallization process and the internal stress relief process slow down over time, but are unlikely to terminate during the life of the resistor. These two processes can be considered to proceed at an approximately constant rate during the operation of the resistor. The resistance changes related to them account for about a thousandth of the original resistance.
High-temperature aging of electrical load: In any case, electrical load will accelerate the aging process of the resistor, and the effect of electrical load on accelerating the aging of the resistor is more significant than that of increasing the temperature. The reason is that the temperature of the contact part of the resistor body and the lead cap is The rise exceeds the average temperature rise of the resistor body. Usually, the life is shortened by half for every 10 ℃ increase in temperature. If the overload makes the temperature rise of the resistor exceeds the rated load by 50°C, the life of the resistor is only 1/32 of the life under normal conditions. The working stability of the resistors over a 10-year period can be assessed through an accelerated life test of less than four months.
DC load-electrolysis: Under the action of DC load, the electrolysis causes the aging of the resistor. Electrolysis occurs in the grooved resistor cell, and the alkali metal ions contained in the resistance matrix are displaced in the electric field between the cells to generate an ionic current. The electrolysis process is more severe in the presence of moisture. 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 result of electrolysis can increase the resistance value, and film damage may occur along one side of the groove spiral. The DC load test in the hot flash environment can comprehensively evaluate the anti-oxidation or anti-reduction performance of the resistor base material and film layer, as well as the moisture-proof performance of the protective layer.
(2), vulcanization
After a batch of field instruments have been used in a chemical plant for one year, the instruments have failed one after another. After analysis, it is found that the resistance value of the thick film chip resistor used in the instrument has become larger, and even becomes an open circuit. When the failed resistor was observed under a microscope, it was found that a black crystalline substance appeared on the edge of the resistor electrode. Further analysis of the composition found that the black substance was silver sulfide crystals. It turned out that the resistor was corroded by sulfur from the air.
(3) Gas adsorption and desorption
The resistance film of the film resistor may always adsorb a very small amount of gas on the grain boundary, or the conductive particles and the binder part, which constitute the intermediate layer between the grains and hinder the contact between the conductive particles. significantly affect the resistance value.
Synthetic film resistors are made under normal pressure. When working in vacuum or low pressure, part of the gas will be desorbed, which improves the contact between conductive particles and reduces the resistance value. Similarly, when the thermally decomposed carbon film resistor made in vacuum works directly under normal environmental conditions, part of the gas will be adsorbed due to the increase of air pressure, which will increase the resistance value. If the unengraved semi-finished product is preset under normal pressure for an appropriate time, the resistance stability of the finished resistor product will be improved.
Temperature and air pressure are the main environmental factors affecting gas adsorption and desorption. For physical adsorption, cooling can increase the equilibrium adsorption capacity, and heating can increase the amount of adsorption. Because gas adsorption and desorption occur on the surface of the resistor body. Therefore, the impact on film resistors is more significant. The resistance change can reach 1%~2%.
(4), oxidation
Oxidation is a long-term factor (different from adsorption), and the oxidation process starts from the surface of the resistor body and gradually goes deep inside. Except for 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 film layer, the more obvious the effect of oxidation.
The fundamental measure to prevent oxidation is sealing (inorganic materials such as metals, ceramics, glass, etc.). The use of organic materials (plastics, resins, etc.) for coating or potting cannot completely prevent the protective layer from being moisture-permeable or breathable. Although it can delay oxidation or adsorb gases, it will also bring some new innovations related to the organic protective layer. aging factor.
(5), the influence of organic protective layer
During the formation of the organic protective layer, volatiles or solvent vapors of polycondensation are released. The heat treatment process makes part of the volatiles diffuse into the resistor body, causing the resistance value to rise. Although this process can last for 1 to 2 years, the time to significantly affect the resistance value is about 2 to 8 months. In order to ensure the stability of the resistance value of the finished product, it is more appropriate to leave the product in the warehouse for a period of time before leaving the factory.
(6), mechanical damage
The reliability of a resistor depends largely on the mechanical properties of the resistor. The resistor body, lead cap and lead wire should have sufficient mechanical strength. Defects in the base body, damage to the lead cap or lead breakage can cause the resistor to fail.