Failure mode: various failure phenomena and their manifestations.
Failure mechanism: It 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 resistive film is burnt out 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 is beyond specification: the resistive film is defective or degraded, the substrate has movable sodium ions, and the protective coating is not good.
3) Lead wire breakage: welding process defects of resistor body, solder joint pollution, lead wire mechanical stress damage.
4) Short circuit: migration of silver, corona discharge.
2. Table of the proportion of failure modes in total failures
3. Failure mechanism analysis
The failure mechanism of resistors is multifaceted, and various physical and chemical processes that occur under working conditions or environmental conditions are the causes of resistor aging.
(1) Structural changes of conductive materials
The conductive film layer of the thin film resistor is generally obtained by vapor deposition, and there is 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 dense, which can often cause a decrease in the 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, and its internal structure will be distorted. The smaller the wire diameter or the thinner the film, the more significant the stress effect. Generally, heat treatment can be used to eliminate internal stress. 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 with the passage of time, but it is impossible to terminate during the use of the resistor. It can be considered that these two processes proceed at an approximately constant speed during the working period of the resistor. The resistance change associated with them accounts for about a few thousandths of the original resistance value.
High temperature aging of electrical load: In any case, electrical load will accelerate the aging process of resistors, and the effect of electrical load on accelerating the aging of resistors is more significant than that of increased temperature. The reason is the temperature of the contact part of the resistor body and the lead cap. The rise exceeds the average temperature rise of the resistor. Generally, the life span is shortened by half for every 10℃ increase in temperature. If the overload causes the temperature rise of the resistor to exceed the rated load by 50°C, the life of the resistor is only 1/32 of the life under normal conditions. It can pass the accelerated life test of less than four months to assess the working stability of the resistor during 10 years.
DC load-electrolysis: under DC load, electrolysis causes the resistor to age. Electrolysis occurs in the groove of the grooved resistor, and the alkali metal ions contained in the resistor matrix are displaced in the electric field between the grooves to generate ion current. When moisture is present, the electrolysis process becomes more severe. 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 the side of the groove spiral. Conducting a DC load test in a hot flash environment can comprehensively assess the resistance to oxidation or reduction of the resistor base material and film, 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 one after another. After analysis, it is found that the resistance value of the thick film chip resistor used in the meter has become larger, and even becomes an open circuit. When the failed resistor is observed under a microscope, it can be found that black crystalline material appears on the edge of the resistor electrode. Further analysis of the composition reveals that the black material is silver sulfide crystals. It turned out that the resistance was corroded by sulfur from the air.
(3) Gas adsorption and desorption
The resistive film of film resistors on the grain boundary, or the conductive particles and the binder part, may always adsorb a very small amount of gas. They form the intermediate layer between the crystal grains and hinder the contact between the conductive particles, thereby Obviously affect the resistance.
The synthetic film resistor is made under normal pressure. When working in vacuum or low pressure, the desorbed part is attached to gas, which improves the contact between conductive particles and reduces the resistance value. Similarly, when the thermally decomposable carbon film resistors made in vacuum work directly under normal environmental conditions, they will absorb some gas due to the increase in air pressure, increasing 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 will be improved.
Temperature and air pressure are the main environmental factors that affect gas adsorption and desorption. For physical adsorption, cooling can increase the equilibrium adsorption capacity, while heating is the opposite. As gas adsorption and desorption occur on the surface of the resistor. 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). The oxidation process starts from the surface of the resistor and gradually deepens into the interior. Except for precious metal and alloy film resistors, resistors of other materials are all affected by oxygen in the air. The result of oxidation is an increase in resistance. The thinner the resistive film, the more obvious the effect of oxidation.
The fundamental measure to prevent oxidation is to seal (metal, ceramic, glass and other inorganic materials). Coating or potting with organic materials (plastics, resins, etc.) cannot completely prevent the protective layer from permeating moisture or air. Although it can delay oxidation or adsorb gas, it will also bring some new ideas related to the organic protective layer. Aging factors.
(5) The influence of organic protective layer
During the formation of the organic protective layer, condensation polymerization volatiles or solvent vapors are released. The heat treatment process causes part of the volatiles to diffuse into the resistor, causing the resistance to rise. Although this process can last for 1 to 2 years, the time to significantly affect the resistance is about 2 to 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.
(6) Mechanical damage
The reliability of the resistance largely depends on the mechanical properties of the resistor. Resistor bodies, lead caps and lead wires should all have sufficient mechanical strength. Defects in the matrix, lead cap damage or lead breaks can all lead to resistor failure.