Main Environmental Stresses Causing Electronic Product Failure

During the working process of electronic products, in addition to electrical stresses such as voltage and current of electrical loads, environmental stresses also include high temperature and temperature cycles, mechanical vibration and impact, moisture and salt spray, electromagnetic field interference, etc. Under the influence of the above environmental stress, products 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, causing the physical, chemical, mechanical and electrical properties of the product to continuously change. The change process can be slow or slow. Transient, it all depends on the type of environmental stress and the magnitude of the stress.

1. Temperature stress

Electronic products will withstand 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 are operated or 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 cause the product material to soften and deform, or the insulation performance to decrease, or even overheat and burn. The product is subjected to high temperatures at this time. Overstress and high-temperature overstress can cause product failure in a short period of time; when the response temperature does not exceed the specified operating temperature range of the product, the effect of steady-state temperature stress is manifested in the long-term effect, and the temperature Long-term effects will cause product materials to gradually age, and electrical performance parameters to drift or exceed tolerances, eventually leading to product failure. For the product, the temperature stress it endures 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 a failure of the cooling system or high-temperature heat flow leakage from the equipment, the temperature of the component will exceed the upper limit of the allowable temperature, and the component will withstand high temperatures. Over-stress; when the storage environment temperature is stable for a long time, the product is subjected to long-term temperature stress. The high temperature resistance limit capability of electronic products can be determined through the step high temperature baking test, and the life span of electronic products operating at long-term temperatures can be evaluated through the steady-state life test (high temperature acceleration).

Changing temperature stress refers to the thermal stress on the material interface caused by temperature changes when an electronic product is in a changing temperature state due to the difference in thermal expansion coefficient of each functional material of the product. When the temperature changes drastically, the product may burst 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 is manifested as a long-term The material interface continues to withstand the thermal stress generated under temperature changes, and micro-crack damage may occur in local micro-areas. This damage gradually accumulates, eventually leading to cracking or damage to the product material interface. At this time, the product is subjected to long-term temperature changes. Stress or temperature cycle stress. The changing temperature stress experienced by electronic products comes from the temperature changes of the environment in which the product is located and its own switching status. For example, when moving from a warm indoor to a cold outdoor, under strong solar radiation, sudden rainfall or immersion in water, rapid temperature changes of aircraft from the ground to high altitude, intermittent work in cold zone environments, and the sun-facing and back-sun changes in space. Changes, reflow soldering and rework of microcircuit modules, etc., the product is subjected to temperature shock stress; periodic changes in natural climate temperature, intermittent working conditions, changes in the operating temperature of the equipment system itself, and changes in the call volume of communication equipment cause equipment When power consumption fluctuates, the product is subjected to temperature cycle stress. The thermal shock test can be used to evaluate the resistance of electronic products to sudden changes in temperature, and the temperature cycle test can be used to evaluate the adaptability of electronic products to long-term operation under alternating high and low temperature conditions.

2. Mechanical stress

The mechanical stresses endured by electronic products include 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 external environmental forces. Mechanical vibration is classified according to the cause of its generation into free vibration, forced vibration and self-excited vibration; according to the movement rules of mechanical vibration, it is classified into sinusoidal vibration and random vibration. These two forms of vibration have different destructive powers on products. The latter is more destructive. Larger, so most vibration test assessments adopt random vibration tests. The impact of mechanical vibration on electronic products includes deformation, bending, cracks, fractures, etc. caused by vibration. Electronic products that have been under the action of vibration stress for a long time will cause the structural interface materials to crack due to fatigue and cause mechanical fatigue failure; if this occurs Resonance leads to over-stress cracking failure, causing instantaneous structural damage to electronic products. The mechanical vibration stress that electronic products bear comes from the mechanical loads of the working environment, such as rotation, pulsation, oscillation and other environmental mechanical loads of aircraft, vehicles, ships, aerial vehicles and ground mechanical structures, especially during transportation when the product is not in working condition. And as vehicle-mounted or airborne components, they are inevitably subjected to mechanical vibration stress during operation. The adaptability of electronic products to repetitive mechanical vibrations during operation can be evaluated through mechanical vibration tests (especially random vibration tests).

Mechanical impact stress refers to a mechanical stress caused by a single direct interaction between an electronic product and another object (or component) under the action of external environmental forces, resulting in a sudden change in force, displacement, speed or acceleration of the product in an instant. Stress. Under the action of mechanical impact stress, products can release and transfer considerable energy in a very short period of time, causing serious damage to the product, such as causing malfunction of electronic products, instant open/short circuit, and cracking and fracture of the assembly and packaging structure. wait. Different from the cumulative damage caused by long-term vibration, the damage to products caused by mechanical impact is a concentrated release of energy. Therefore, the magnitude of the mechanical impact test is large and the duration of the impact pulse is short. The peak value of product damage is the main 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 impact stress is determined by the peak acceleration and the duration of the impact pulse. The magnitude of the peak acceleration reflects the magnitude of the impact force applied to the product, while the impact of the duration of the impact pulse on the product is related to the natural frequency of the product. related. The mechanical impact stress endured by electronic products comes from drastic changes in the mechanical state of electronic equipment and equipment, such as emergency braking and impact of vehicles, airdrops and crashes of aircraft, the launch of artillery fire, chemical energy explosions and nuclear explosions, missile explosions, etc. Strong mechanical impact, sudden force or sudden movement due to loading, unloading, transportation or on-site work will also cause the product to withstand mechanical impact. Mechanical impact tests can be used to evaluate the adaptability of electronic products (such as circuit structures) to non-repetitive mechanical impacts during use and transportation.

Constant acceleration (centrifugal force) stress refers to a centrifugal force generated by the continuous change of the direction of motion of the carrier when electronic products are working on a moving carrier. Centrifugal force is a virtual inertial force that keeps a rotating object moving away from the center of rotation. The centrifugal force is equal in size and opposite in direction to the centripetal force. Once the centripetal force formed by the net external force and pointing to the center of the circle disappears, the rotating object will no longer rotate. Instead, it flies out along the tangent direction of the rotation trajectory at this moment, and the product is damaged at this moment. The size of the centrifugal force is related to the mass, speed and acceleration (radius of rotation) of the moving object. For electronic components that are not firmly welded, the components will fly away due to the detachment of the solder joints under the action of centrifugal force, causing the components to fly away. Product failure. The centrifugal force endured by electronic products comes from the continuously changing operating status of electronic equipment and equipment in the direction of movement, such as the direction changes of running vehicles, aircraft, rockets, and missiles, etc., which causes electronic equipment and internal components to withstand centrifugal forces other than gravity. Its action time ranges from a few seconds to a few minutes, taking rockets and missiles as examples. 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 firmness of the welding structure of electronic products, especially large-volume surface-mount components, can be evaluated through constant acceleration testing (centrifugal testing).

3. Moisture stress

Moisture stress refers to the moisture 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 exceed tolerances. For example, under long-term high-humidity conditions, the insulation performance of insulating materials will decrease after absorbing moisture, causing short circuits or high-voltage electric shocks; for contact electronic components, such as plugs, sockets, etc., when moisture is attached to the surface, corrosion will easily occur and an oxide film will form. , causing the resistance of the contact device to increase, and in severe cases, the circuit will be blocked; in a severely humid environment, mist or water vapor will cause sparks to appear when the relay contacts operate, and they will no longer be able to operate; semiconductor chips are more sensitive to water vapor, and once water vapor occurs on the surface of the chip If it exceeds the standard, the corrosion of wiring Al will become extremely rapid; in order to prevent electronic components from being corroded by water vapor, encapsulation or airtight packaging technology is used to isolate the components from the outside atmosphere and pollution. The moisture stress endured by electronic products comes from the water vapor attached to the surface of the materials in the working environment of electronic equipment and equipment and the water vapor that penetrates into the components. The magnitude of the moisture stress is related to the level of ambient humidity. The southeastern coastal areas of my country are areas with high humidity. Especially in spring and summer, the relative humidity reaches a maximum of more than 90%RH. The influence 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 tests and humidity resistance tests.

4. Salt spray stress

Salt spray stress refers to the salt spray stress that the material surface endures when electronic products work in an atmospheric dispersion environment composed of salt-containing tiny droplets. 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 presence of Na+ and Cl- ions is the fundamental cause of corrosion of metal materials. When salt spray adheres to the surface of an insulator, its surface resistance will be reduced. After the insulator absorbs the salt solution, its volume resistance will be reduced by 4 orders of magnitude. When salt spray adheres to the surface of moving mechanical parts, the production of corrosion products increases. If the friction coefficient is too large, moving parts may even get stuck; although encapsulation and airtight packaging technology are adopted to avoid corrosion of semiconductor chips, the external pins of electronic devices inevitably often lose their function due to salt spray corrosion; printing Corrosion on the PCB can short out adjacent wiring. The salt spray stress that electronic products bear comes from the salt-containing fog in the atmospheric environment. In coastal areas or on ships and warships, the atmosphere contains a lot of salt, which has a serious impact on the packaging of electronic components. The adaptability of electronic packages to salt spray can be evaluated by accelerating corrosion through a salt spray test.

5. Electromagnetic stress

Electromagnetic stress refers to the electromagnetic stress that electronic products bear in the electromagnetic field where the electric field and magnetic field change interactively. The electromagnetic field includes two aspects: electric field and magnetic field, whose characteristics are represented by electric field intensity E (or electric displacement D) and magnetic flux density B (or magnetic field intensity H) respectively. In the electromagnetic field, the electric field and the magnetic field are closely related. A time-varying electric field will cause a magnetic field, and a time-varying magnetic field will cause an electric field. The electric field and the magnetic field excite each other, causing the movement of the electromagnetic field to form electromagnetic waves. Electromagnetic waves can self-propagate in vacuum or matter. The electric field and magnetic field oscillate in phase and are perpendicular to each other. They move 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 a vacuum is the speed of light ( 3×10^8m/s). Usually the electromagnetic waves that electromagnetic interference focuses on are radio waves and microwaves. The higher the frequency of electromagnetic waves, the greater the electromagnetic radiation capability. 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 internal components of the electronic component and the interference from external electronic equipment. May have serious effects on the performance and functionality of electronic components. For example, if the magnetic components inside the DC/DC power module cause 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 into Conducted harassment enters the product. The anti-electromagnetic interference capability of electronic components can be evaluated through electromagnetic compatibility testing and electromagnetic field near-field scanning testing.