Main environmental stress of electronic product failure caused by electronic reliability test

Date：2022-10-21 15:00:00Views：587

The purpose of environmental stress screening is to accelerate the internal potential defects into faults, find and eliminate them by applying reasonable environmental stress to the products. The purpose is to eliminate the early faults of the products. In the working process of electronic products, in addition to the electrical stresses such as voltage and current of electrical loads, environmental stresses also include high temperature and temperature cycle, mechanical vibration and impact, humidity and salt spray, electromagnetic interference, etc. Under the action of the above environmental stress, the product may have performance degradation, parameter drift, material corrosion, etc., or even failure. This paper collects and sorts out some materials, hoping to be of great reference value to all readers.

1、 Temperature stress

In any environment, electronic products will bear temperature stress. The size of temperature stress depends on the type of environment, product structure and working state. The temperature stress includes steady temperature stress and variable temperature stress. Steady state temperature stress refers to the response temperature of electronic products when working or storing under a certain temperature environment. When the response temperature exceeds the limit that the product can withstand, the component product will be unable to work within the specified electrical parameters, which may lead to the softening deformation of the product material or the reduction of the insulation performance, or even overheating burning. For the product, it is subject to high temperature overstress, which can lead to product failure in a very short time; When the response temperature does not exceed the specified operating temperature range of the product, the effect of the steady-state temperature stress is manifested in the effect of a long time action. The long time action of temperature causes the product material to gradually age, the electrical performance parameters to drift or exceed the tolerance, which ultimately leads to product failure. The temperature stress that the product bears at this time is a long-term temperature stress. The steady temperature stress borne by electronic products comes from the heat generated by the environmental temperature load in which the product is located and its own power consumption. For example, the component temperature will exceed the upper limit of the allowable temperature due to the failure of the cooling system and the leakage of high-temperature heat flow of the equipment; Under the long-term stable working condition of storage environment temperature, the product bears long-term temperature stress. The high temperature resistance limit 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).

Changing temperature stress refers to the thermal stress caused by temperature change on the material interface due to the difference of thermal expansion coefficients of functional materials when electronic products are in the changing temperature state. When the temperature changes violently, the product may burst at the material interface and become invalid. At this time, the product bears the temperature change overstress or temperature impact stress; When the temperature changes slowly, the effect of changing temperature stress is shown as a long-term effect. The material interface continuously bears the thermal stress generated under temperature change, and microcrack damage may occur in local micro areas. This damage gradually accumulates, eventually leading to cracking or damage failure of the product material interface. At this time, the product bears long-term temperature change stress or temperature cycle stress. The changing temperature stress borne by electronic products comes from the temperature change of the product's environment and its own switch working state. For example, when moving from warm indoor to cold outdoor, under strong solar radiation, sudden rainfall or immersion in water, rapid temperature change of aircraft from the ground to high altitude, intermittent work in cold zone environment, changes in sunrise and back sun in space, reflow soldering and rework of microcircuit modules, the products bear temperature impact stress; Under the condition that the periodic change of natural climate temperature, intermittent working state, the change of equipment system's working temperature, and the change of communication equipment's call volume lead to the fluctuation of equipment power consumption, the product bears the temperature cycle stress. Thermal shock test can be used to evaluate the resistance of electronic products to extreme temperature changes, and temperature cycle test can be used to evaluate the adaptability of electronic products to long-term work under the conditions of alternating high and low temperatures.

2、 Mechanical stress

Mechanical stress of electronic products includes mechanical vibration, mechanical impact and constant acceleration (centrifugal force). Mechanical vibration stress refers to a kind of mechanical stress produced by electronic products reciprocating around a balanced position under the action of external environmental forces. Mechanical vibration can be classified into free vibration, forced vibration and self-excited vibration according to its causes; According to the movement law of mechanical vibration, there are sinusoidal vibration and random vibration. These two types of vibration have different destructive forces on products, and the latter has greater destructive force. Therefore, random vibration test is mostly used for vibration test assessment.

The impact of mechanical vibration on electronic products includes the deformation, bending, crack, fracture, etc. of products caused by vibration. Electronic products under long-term vibration stress will crack the structural interface materials due to fatigue, resulting in mechanical fatigue failure; If resonance occurs, it will lead to overstress cracking failure, which will lead to instant structural damage of electronic products. The mechanical vibration stress borne by electronic products comes from the mechanical loads in the working environment, such as the rotating, pulsating, oscillating and other environmental mechanical loads of aircraft, vehicles, ships, air vehicles and ground mechanical structures, especially in the transportation of products under non working conditions and in the operation of on-board or airborne components under working conditions. Mechanical vibration test (especially random vibration test) can be used to evaluate the adaptability of electronic products to repetitive mechanical vibration in the working process. Mechanical impact stress refers to a kind of mechanical stress generated 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 of force, displacement, speed or acceleration of the product in a moment. Under the action of mechanical impact stress, the product can release and transfer considerable energy in a very short time, causing serious damage to the product, such as causing electronic product malfunction Instantaneous open/short circuit and cracking and fracture of assembly and packaging structure. Different from the cumulative damage caused by the long-term action of vibration, the damage of mechanical impact to products is the concentrated release of energy. Therefore, the magnitude of mechanical impact test is large and the duration of impact pulse is short. The peak value of product damage, that is, the duration of main pulse, is only a few milliseconds to tens of milliseconds. The vibration after the main pulse attenuates quickly.

The magnitude of the 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, and the impact of the duration of the impact pulse on the product is related to the natural frequency of the product. The mechanical impact stress borne by electronic products comes from the dramatic changes in the mechanical state of electronic equipment and equipment, such as the strong mechanical impact force caused by emergency braking and collision of vehicles, air dropping and falling of aircraft, firing of artillery, chemical energy explosion and nuclear explosion, missile explosion, etc. The sudden force or movement caused by loading, unloading, transportation or on-site work will also make the products bear mechanical impact force. Mechanical impact test can be used to evaluate the adaptability of electronic products (such as circuit structure) to non repetitive mechanical impact in use and transportation. Constant acceleration (centrifugal force) stress refers to a centrifugal force generated by continuous change of the moving direction of the carrier when electronic products are working on the moving carrier. Centrifugal force is a virtual inertial force, which keeps rotating objects away from the center of rotation all the time. Centrifugal force and centripetal force are equal in size and opposite in direction. Once the centripetal force formed by external force and pointing to the center of the circle disappears, the rotating object will no longer rotate but fly out along the tangent direction of the rotation track at this moment, which will damage the product at this moment. The size of centrifugal force is related to the mass, speed and acceleration (radius of rotation) of moving objects. For electronic components that are not firmly welded, under the effect of centrifugal force, the components will fly away due to the detachment of solder joints, which will lead to product failure. The centrifugal force borne by electronic products comes from the running state of electronic equipment and equipment that continuously changes in the direction of movement, such as the changing direction of running vehicles, aircraft, rockets and missiles, which makes electronic equipment and internal components subject to the centrifugal force other than gravity, and its operation time varies from a few seconds to a few minutes. Take rockets and missiles for example, once the changing direction is completed, the centrifugal force disappears, and the centrifugal force acts again when the changing direction is completed, It may form a centrifugal force that acts continuously for a long time. Constant acceleration test (centrifugal test) can be used to evaluate the fastness of electronic products, especially the welded structure of large volume surface mount components.

3、 Moist stress

Humidity stress refers to the humidity stress borne by electronic products 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 the standard. For example, under long-term and high humidity conditions, the insulation performance of insulating materials decreases after moisture absorption, resulting in short circuit or high-voltage electric shock; Contact electronic components, such as plugs and sockets, are prone to corrosion when their surfaces are attached with water, forming oxide films, which increase the resistance of contact devices, and even lead to circuit failure in serious cases; Under severe humid environment, the fog or water vapor will cause sparks when the relay contacts act, and the relay can no longer act; Semiconductor chips are more sensitive to water vapor. Once the water vapor on the chip surface exceeds the standard, the Al corrosion of its wiring will become extremely rapid; In order to avoid the corrosion of electronic components by water vapor, encapsulation or airtight packaging technology is adopted to isolate the components from the outside atmosphere and pollution. The moisture stress borne by electronic products comes from the moisture on the surface of the attached materials in the working environment of electronic equipment and equipment and the moisture penetrating into the interior of components. The magnitude of the moisture stress is related to the ambient humidity. The southeast coastal area of China is a region with high humidity, especially in spring and summer. The highest relative humidity is above 90% RH. The impact of humidity is an unavoidable problem. The suitability of electronic products for use or storage under high humidity conditions can be evaluated by steady-state damp heat test and humidity resistance test.

4、 Salt spray stress

Salt spray stress refers to the salt spray stress on the surface of materials when electronic products work in an atmospheric dispersion environment composed of small saline droplets. Salt fog 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 fundamental reason for the corrosion of metal materials. When the salt mist adheres 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; When salt mist adheres to the surface of moving mechanical parts, the friction coefficient is increased due to corrosion, and even the moving parts are stuck; Although encapsulation and airtight packaging technologies have been adopted to avoid corrosion of semiconductor chips, the external pins of electronic devices inevitably lose their functions due to salt spray corrosion; Corroders on printed circuit boards (PCBs) can short-circuit adjacent wiring. The salt fog stress borne by electronic products comes from the salt fog in the atmospheric environment. In coastal areas or ships, there is a lot of salt in the atmosphere, which has a serious impact on the packaging of electronic components. Salt spray test can be used to evaluate the adaptability of electronic packaging to salt spray.

5、 Electromagnetic stress

Electromagnetic stress refers to the electromagnetic stress borne by electronic products in the electromagnetic field with alternating changes of electric field and magnetic field. The electromagnetic field includes 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) respectively. In electromagnetic field, electric field and magnetic field are closely related. The time-varying electric field will cause magnetic field, and the time-varying magnetic field will cause electric field. The mutual excitation of electric field and magnetic field will cause the movement of electromagnetic field and form electromagnetic wave. Electromagnetic wave can self propagate in vacuum or substance. Electric field and magnetic field oscillate in phase and move vertically in space in the form of wave. The moving electric field, magnetic field and propagation direction are perpendicular to each other. The propagation rate of electromagnetic wave in vacuum is the speed of light (3 × 10 ^8m/s）。 Generally, the electromagnetic wave concerned by electromagnetic interference is radio wave and microwave. The higher the electromagnetic wave frequency is, the greater the electromagnetic radiation capacity is. For electronic component products, electromagnetic interference (EMI) of electromagnetic field is the main factor affecting electromagnetic compatibility (EMC) of components. This electromagnetic interference source comes from mutual interference between internal components of electronic components and interference of external electronic equipment, which may seriously affect the performance and function of electronic components. For example, if the magnetic components inside the DC/DC power module produce electromagnetic interference to electronic devices, it will directly affect the output ripple voltage parameters; The impact of RF radiation on electronic products will directly enter the internal circuit through the product shell, or be converted into conductive interference by the interface wire end, and enter the product interior. The anti electromagnetic interference ability of electronic components can be evaluated through electromagnetic compatibility test and electromagnetic field near-field scanning test.

The above is the main environmental stress related content that leads to the failure of electronic products, and I hope it will be helpful to you. Our company has a team of professional engineers and industry elites, and has built three standardized laboratories with an area of more than 1800 square meters, which can undertake a variety of test projects such as electronic component test verification, IC authenticity identification, product design material selection, failure analysis, functional testing, factory incoming material inspection and tape weaving.