Common failure modes and mechanism analysis of resistors

Date：2022-05-30 14:31:20Views：791

As we all know, resistor is a very common electronic component, which plays a very important role in the protection of household appliances, industrial automation and other fields. However, in the process of using the resistor, you may have encountered that after the product has been used by the customer for a period of time, the circuit fails for no reason. The circuit may look intact or burn a large area. This phenomenon is mainly due to the failure of the resistor.

Failure mode: various failure phenomena and their manifestations.

Failure mechanism: refers to the physical, chemical, thermodynamic or other processes leading to failure.

The main failure modes and 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 matrix is broken, and the lead cap and the resistance body fall off.

2) Resistance drift exceeds the specification: the resistance film has defects or degradation, the matrix has movable sodium ions, and the protective coating is poor.

3) Lead fracture: welding process defect of resistance body, solder joint pollution, mechanical stress damage of lead.

4) Short circuit: migration of silver, corona discharge.

Failure mechanism analysis

The failure mechanism of resistors is multifaceted. Various physical and chemical processes under working or environmental conditions are the causes of aging of resistors.

(1) Structural changes of conductive materials

The conductive film of thin film resistor is usually obtained by vapor deposition, and there is amorphous structure to some extent. From the viewpoint of thermodynamics, the amorphous structure has the tendency of crystallization. Under working or environmental conditions, the amorphous structure in the conductive film tends to crystallize at a certain speed, that is, the internal structure of the conductive material tends to densify, which can often cause the decrease of the resistance value. The crystallization rate increases with the increase of temperature.

The resistance wire or film will bear mechanical stress during the preparation process, which will distort its internal structure. The smaller the wire diameter or the thinner the film, the more significant the stress effect. Generally, the internal stress can be eliminated by heat treatment, and the residual internal stress may be gradually eliminated during long-term use, and the resistance value of the resistor may change accordingly.

Both the crystallization process and the internal stress removal process slow down with time, but it is impossible to terminate during the service life of the resistor. It can be considered that these two processes are carried out at approximately constant speed during the working period of the resistor. The change of resistance value related to them accounts for a few thousandths of the original resistance value.

High temperature aging of electric load: in any case, the electric load will accelerate the aging process of the resistor, and the effect of the electric load on accelerating the aging of the resistor is more significant than that of increasing the temperature, because the temperature rise of the contact part between the resistor and the lead cap exceeds the average temperature rise of the resistor. Generally, the service life is shortened by half for every 10 ℃ increase in temperature. If the overload causes the temperature rise of the resistor to exceed 50 ℃ under the rated load, the service life of the resistor is only 1/32 of the service life under normal conditions. The working stability of the resistor during 10 years can be assessed through the accelerated life test of less than four months.

DC load - electrolysis: under DC load, electrolysis leads to aging of resistors. Electrolysis takes place 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 produce ion current. In the presence of moisture, the electrolysis process is more intense. If the resistance film is carbon film or metal film, it is mainly electrolytic oxidation; If the resistance 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 value, and film damage may occur along the spiral side of the groove. DC load test in hot and humid environment can comprehensively assess the oxidation resistance or reduction resistance of resistor matrix material and film, as well as the moisture resistance of protective layer.

(2) Vulcanization

After a batch of field instruments were used in a chemical plant for one year, the instruments broke down one after another. Through analysis, it is found that the resistance value of the thick film patch used in the instrument becomes larger, and even becomes an open circuit. Put the failed resistance under the microscope and observe it. It can be found that there are black crystalline substances at the edge of the resistance electrode. Further analysis of the composition shows that the black substances are silver sulfide crystals. The original resistance 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 on the conductive particles and binder. They form an intermediate layer between the grains and hinder the contact between the conductive particles, thus significantly affecting the resistance value.

The synthetic film resistor is made under normal pressure. When working in vacuum or low pressure, the desorption part will be attached with gas, which improves the contact between conductive particles and reduces the resistance. Similarly, when the thermal decomposition carbon film resistor made in vacuum works directly under normal environmental conditions, it will absorb part of the gas due to the increase of air pressure, so as to increase the resistance value. If the unfinished semi-finished product is preset at atmospheric pressure for a proper time, the resistance stability of the finished resistor will be improved.

Temperature and pressure are the main environmental factors affecting gas adsorption and desorption. For physical adsorption, cooling can increase the equilibrium adsorption capacity, and heating is the opposite. Gas adsorption and desorption occur on the surface of the resistor. Therefore, the influence on the film resistor is significant. The resistance value can change by 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 goes deep into the interior. Except for precious metal and alloy thin film resistors, the resistors of other materials will be affected by oxygen in the air. The result of oxidation is an increase in resistance. The thinner the resistance film, the more obvious the oxidation effect.

The fundamental measure to prevent oxidation is sealing (metal, ceramic, glass and other inorganic materials). Coating or potting with organic materials (plastics, resins, etc.) can not completely prevent the moisture permeability or air permeability of the protective layer. Although it can delay oxidation or adsorb gas, it will also bring some new aging factors related to the organic protective layer.

(5) Influence of organic protective layer

During the formation of the organic protective layer, volatiles or solvent vapors of polycondensation are released. During the heat treatment process, some volatiles diffuse into the resistor, causing the resistance value to rise. Although this process can last for 1~2 years, the time that significantly affects the resistance value is about 2~8 months. In order to ensure the stability of the resistance value of the finished product, it is appropriate to put the product in the warehouse for a period of time before leaving the factory.

(6) Mechanical damage

The reliability of the resistor largely depends on the mechanical properties of the resistor. The resistor body, lead cap and lead wire shall have sufficient mechanical strength, and the substrate defect, lead cap damage or lead fracture can lead to the failure of the resistor.

The above contents are related to the analysis of common failure modes and mechanisms of resistors compiled by the creative core detection team. I hope they can help you. Chuangxin testing is a professional testing organization for electronic components. At present, it mainly provides testing services for integrated circuits such as capacitors, resistors, connectors, MCU, CPLD, FPGA, DSP, etc. Specialized in functional testing of electronic components, incoming appearance testing of electronic components, anatomical testing of electronic components, acetone testing, X-ray scanning testing of electronic components, and RoHS component analysis testing. Welcome to call, we will serve you wholeheartedly!