In the working process of electronic products, in addition to electrical stress such as voltage and current of electrical load, environmental stress also includes high temperature and temperature cycle, mechanical vibration and shock, humidity and salt spray, electromagnetic field interference, etc. Under the action of the above environmental stress, the product may experience performance degradation, parameter drift, material corrosion, etc., or even fail.
After electronic products are manufactured, from screening, inventory, transportation to use and maintenance, they are all affected by environmental stress, resulting in continuous changes in the physical, chemical, mechanical and electrical properties of the products. Transient, it depends entirely on the type of environmental stress and the magnitude of the stress.
1. Temperature stress
Electronic products will be subjected to temperature stress in any environment. The magnitude of temperature stress depends on the type of environment, product structure and working state. Temperature stress includes steady-state temperature stress and changing temperature stress.
Steady-state temperature stress refers to the response temperature of electronic products when they work or are stored in a certain temperature environment. When the response temperature exceeds the limit that the product can withstand, the component product will not be able to work within the specified electrical parameter range, which may lead to softening and deformation of the product material or degradation of the insulation performance, or even overheating and burning. Overstress, high temperature overstress can lead to product failure in a very short action time; when the response temperature does not exceed the specified working temperature range of the product, the effect of steady-state temperature stress is manifested in the effect of long-term action. The long-term action causes the product materials to gradually age, the electrical performance parameters drift or exceed tolerances, and eventually lead to product failure. For the product, the temperature stress at this time is long-term temperature stress. The steady-state temperature stress experienced by electronic products comes from the ambient temperature load of the product and the heat generated by its own power consumption. For example, due to the failure of the cooling system and the leakage of high-temperature heat flow from the equipment, the temperature of the components will exceed the upper limit of the allowable temperature, and the components are subjected to high temperature. Overstress; in the long-term stable working state of the storage environment temperature, the product is subjected to long-term temperature stress. The high temperature resistance limit ability of electronic products can be determined by step high temperature baking test, and the service life of electronic products under long-term temperature can be evaluated by steady-state life test (high temperature acceleration).
The variable temperature stress refers to a thermal stress caused by the temperature change at the interface of the material due to the difference in the thermal expansion coefficients of the functional materials of the product when the electronic product is in a state of changing temperature. When the temperature changes drastically, it may cause the product to burst instantaneously at the material interface and fail. At this time, the product is subjected to temperature change overstress or temperature shock stress; when the temperature changes relatively slowly, the effect of changing temperature stress appears for a long time. The material interface is continuously subjected to thermal stress generated by temperature change, and micro-crack damage may occur in local micro-regions. This damage gradually accumulates and eventually leads to cracking or failure of the product material interface. At this time, the product is subjected to long-term temperature change. Stress or temperature cycle stress. The changing temperature stress of electronic products comes from the temperature changes in the environment where the products are located and their own switch working states. For example, in moving from a warm indoor to a cold outdoor, under strong solar radiation, sudden rain or immersion in water, rapid temperature change of aircraft from ground to high altitude, intermittent work in cold environment, rising and falling sun in space In the case of changes, reflow soldering and rework of microcircuit modules, etc., the product is subjected to temperature shock stress; the periodic change of natural climate temperature, intermittent working state, the working temperature change of the equipment system itself, and the change of communication equipment call volume cause equipment In the case of fluctuations in power consumption, the product is subjected to thermal cycling stress. The thermal shock test can be used to evaluate the resistance of electronic products when subjected to great changes in temperature, and the temperature cycle test can be used to evaluate the adaptability of electronic products to work for a long time under alternating high and low temperature conditions.
2. Mechanical stress
The mechanical stress of electronic products includes mechanical vibration, mechanical shock, and constant acceleration (centrifugal force).
Mechanical vibration stress refers to a mechanical stress generated by electronic products reciprocating around a certain equilibrium position under the action of environmental external forces. Mechanical vibration is classified into free vibration, forced vibration and self-excited vibration according to its causes; sinusoidal vibration and random vibration are classified according to the movement law of mechanical vibration. Therefore, most of the vibration test assessments use random vibration tests. The impact of mechanical vibration on electronic products includes deformation, bending, cracking, and fracture of products caused by vibration. Electronic products that are under the action of vibration stress for a long time will cause cracking of structural interface materials due to fatigue, resulting in mechanical fatigue failure. Resonance leads to overstress cracking failure, resulting in instantaneous structural damage to electronic products. The mechanical vibration stress of electronic products comes from the mechanical loads of the working environment, such as the rotation, pulsation, vibration and other environmental mechanical loads of aircraft, vehicles, ships, air vehicles and ground mechanical structures, especially in the transportation of products in non-working conditions And it is inevitable to bear mechanical vibration stress during the operation as a vehicle or airborne component under working conditions. The adaptability of electronic products to repetitive mechanical vibrations during operation can be evaluated through mechanical vibration tests (especially random vibration tests).
Mechanical shock stress refers to a kind of mechanical stress caused by a single direct interaction between electronic products and another object (or component) under the action of environmental external forces, resulting in sudden changes in force, displacement, speed or acceleration of the product in an instant. Stress, under the action of mechanical impact stress, the product can release and transfer a considerable amount of energy in a very short period of time, causing serious damage to the product, such as causing malfunction of electronic products, instantaneous open/short circuit, and cracking and breaking of the assembled packaging structure. Wait. Different from the cumulative damage caused by the long-term action of vibration, the damage of mechanical shock to the product is manifested as the concentrated release of energy, so the magnitude of the mechanical shock test is large and the duration of the shock pulse is short, and the peak value of the product damage is the main value. The duration of the pulse is only a few milliseconds to tens of milliseconds, and the vibration after the main pulse decays quickly. The magnitude of this mechanical shock stress is determined by the peak acceleration and the duration of the shock pulse. The magnitude of the peak acceleration reflects the magnitude of the shock force applied to the product, and the impact of the duration of the shock pulse on the product is related to the natural frequency of the product. related. The mechanical shock stress of electronic products comes from the drastic changes in the mechanical state of electronic equipment and equipment, such as emergency braking and impact of vehicles, air-dropping and falling of aircraft, launching of artillery fire, chemical energy explosions and nuclear explosions, missile explosions, etc. Strong mechanical impact, sudden force or sudden movement due to loading and unloading, transportation or field work will also subject the product to mechanical impact. The mechanical shock test can be used to evaluate the adaptability of electronic products (such as circuit structures) to non-repetitive mechanical shocks during use and transportation.
Constant acceleration (centrifugal force) stress refers to a centrifugal force generated by the continuous change of the moving direction of the carrier when the electronic product is working on the moving carrier. Centrifugal force is a virtual inertial force, which keeps the rotating object away from the center of rotation. Instead, it flew out in the tangential direction of the rotation trajectory at this moment, and the product was destroyed at this moment. The magnitude of the centrifugal force is related to the mass, speed and acceleration (rotation radius) of the moving object. For electronic components that are not firmly welded, under the action of centrifugal force, the components will fly away due to the separation of the solder joints. Product fails. The centrifugal force borne by electronic products comes from the running state of electronic equipment and equipment that continuously changes in the direction of motion, such as the direction change of running vehicles, aircraft, rockets and missiles, etc., so that electronic equipment and internal components are subject to centrifugal forces other than gravity. Its action time varies from a few seconds to a few minutes. Taking rockets and missiles as an example, once the direction change is completed, the centrifugal force disappears, and the centrifugal force acts again when the direction is changed again, which may form a long-term continuous centrifugal force. The fastness of the soldering structure of electronic products, especially large-volume surface mount components, can be evaluated by constant acceleration test (centrifugation test).
3. Moisture stress
Humidity stress refers to the humidity stress that electronic products endure when working in an atmospheric environment with a certain humidity. Electronic products are very sensitive to humidity. Once the relative humidity of the environment exceeds 30% RH, the metal materials of the products may be corroded, and the electrical performance parameters may drift or be out of tolerance. For example, under long-term high humidity conditions, the insulating properties of insulating materials will decrease after absorbing moisture, resulting in short circuits or high-voltage electric shocks; for contact electronic components, such as plugs, sockets, etc., the surface is prone to corrosion when moisture is attached to the surface and an oxide film is formed. , which increases the resistance of the contact device, which will cause the circuit to fail in severe cases; in a severe humid environment, fog or water vapor will cause sparks to appear when the relay contacts operate, making it impossible to operate; semiconductor chips are more sensitive to water vapor, once the surface of the chip is water vapor If it exceeds the standard, the corrosion of the wiring Al will become extremely rapid; in order to prevent the electronic components from being corroded by water vapor, encapsulation or hermetic packaging technology is adopted to isolate the components from the outside atmosphere and pollution. The moisture stress of electronic products comes from the water vapor on the surface of the attached materials and the water vapor infiltrated into the components in the working environment of electronic equipment and equipment. The magnitude of the moisture stress is related to the level of environmental humidity. The southeastern coastal area of my country is an area with high humidity, especially in spring and summer, the relative humidity is up to 90% RH or more, and the impact of humidity is an unavoidable problem. The adaptability of electronic products for use or storage under high humidity conditions can be evaluated through steady-state damp heat test and humidity resistance test.
4. Salt spray (salt fog) stress
Salt spray stress refers to the salt spray stress on the surface of electronic products when they work in an atmospheric dispersion environment composed of tiny droplets of salt. Salt spray generally comes from the marine climate environment and the inland salt lake climate environment. Its main components are NaCl and water vapor. The existence of Na+ and Cl- ions is the root cause of corrosion of metal materials. When the salt spray is attached to the surface of the insulator, its surface resistance will be reduced, and after the insulator absorbs the salt solution, its volume resistance will be reduced by 4 orders of magnitude; If the friction coefficient is too large, the moving parts may even be stuck; although the encapsulation and hermetic packaging technology is adopted to avoid the corrosion of the semiconductor chip, the outer pins of the electronic device are inevitably often lost due to salt spray corrosion; Corrosion from the printed circuit board (PCB) can short-circuit adjacent traces. The salt spray stress of electronic products comes from the salt spray in the atmospheric environment. In coastal areas or on ships and ships, the atmosphere contains a lot of salt, which has a serious impact on the packaging of electronic components. The suitability of electronic packaging to resist salt spray can be evaluated by means of accelerated corrosion by salt spray test.
5. Electromagnetic stress
Electromagnetic stress refers to the electromagnetic stress that electronic products are subjected to in the electromagnetic field where the electric and magnetic fields alternately change. The electromagnetic field includes two aspects: electric field and magnetic field, and its characteristics are represented by electric field strength E (or electric displacement D) and magnetic flux density B (or magnetic field strength H). In the electromagnetic field, the electric field and the magnetic field are closely related. The time-varying electric field will cause the magnetic field, and the time-varying magnetic field will cause the electric field. Electromagnetic waves can self-propagate in vacuum or matter. Electric and magnetic fields oscillate in phase and move perpendicular to each other in the form of waves in space. The moving electric field, magnetic field, and propagation direction are perpendicular to each other. The propagation speed of electromagnetic waves in vacuum is the speed of light ( 3×10^8m/s). Usually the electromagnetic waves concerned by electromagnetic interference are radio waves and microwaves. The higher the frequency of electromagnetic waves, the greater the electromagnetic radiation ability. For electronic component products, the electromagnetic interference (EMI) of the electromagnetic field is the main factor affecting the electromagnetic compatibility (EMC) of the component. This source of electromagnetic interference comes from the mutual interference between the internal components of the electronic component and the interference of external electronic equipment. There may be serious effects on the performance and functionality of electronic components. For example, if the internal magnetic components of the DC/DC power module generate electromagnetic interference to electronic devices, it will directly affect the output ripple voltage parameters; the impact of radio frequency radiation on electronic products will directly enter the internal circuit through the product shell, or be converted by the interface wire end into Conduct harassment and enter into the product. The anti-electromagnetic interference ability of electronic components can be evaluated through electromagnetic compatibility test and electromagnetic field near-field scanning detection.
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