Generally speaking, tests conducted to evaluate and analyze the reliability of electronic products are called reliability tests. They are designed to predict the quality of products from the time they leave the factory to the end of their service life. After selecting environmental stresses that are highly similar to the market environment, the degree of environmental stress and the time of application are set. The main purpose is to correctly evaluate product reliability in the shortest possible time.
Reliability tests are designed to determine whether products that have passed reliability identification tests and entered mass production meet the specified reliability requirements under specified conditions, and to verify whether the reliability of products decreases with changes in process, tooling, work flow, and component quality during mass production. Only after these, can product performance be trusted and product quality be excellent.
Classification of reliability tests for electronic products
Some reliability monographs refer to tests in which samples are placed in natural or artificially simulated storage, transportation, and working environments as environmental tests. They are tests that assess the adaptability of products under various environmental conditions (vibration, impact, centrifugal, temperature, thermal shock, humid heat, salt spray, low pressure, etc.), and are one of the important test methods for evaluating product reliability. Generally, there are the following types:
(1) Stability baking, i.e. high temperature storage test
Test purpose: To evaluate the impact of high temperature storage on products without applying electrical stress. Products with serious defects are in a non-equilibrium state, which is an unstable state. The transition process from non-equilibrium state to equilibrium state is both a process that induces the failure of products with serious defects and a process that promotes the transition of products from non-stable state to stable state.
This transition is generally a physical and chemical change, and its rate follows the Arrhenius formula and increases exponentially with temperature. The purpose of high temperature stress is to shorten the time of this change. Therefore, this experiment can be regarded as a process to stabilize product performance.
Test conditions: Generally, a constant temperature stress and holding time are selected. The temperature stress range of microcircuit is 75℃ to 400℃, and the test time is more than 24h. Before and after the test, the test samples should be placed in a standard test environment, that is, a temperature of 25±10℃ and an air pressure of 86kPa~100kPa for a certain period of time. In most cases, it is required to complete the endpoint test within the specified time after the test.
Test purpose: To assess the product’s ability to withstand a certain temperature change rate and its ability to withstand extreme high and low temperature environments. It is set for the product’s thermal mechanical properties. When the thermal matching of the materials constituting the product components is poor, or the internal stress of the components is large, the temperature cycle test may cause the product to fail due to mechanical structural defects. Such as air leakage, internal lead breakage, chip cracks, etc.
Test conditions: Conducted in a gas environment. Mainly to control the temperature and time when the product is at high and low temperatures and the rate of conversion between high and low temperature states. The flow of gas in the test chamber, the position of the temperature sensor, and the heat capacity of the fixture are all important factors to ensure the test conditions.
The control principle is that the temperature, time and conversion rate required by the test refer to the test product, not the local environment of the test. The conversion time of the microcircuit is required to be no more than 1min, and the holding time in the high or low temperature state is required to be no less than 10min; the low temperature is -55℃ or -65-10℃, and the high temperature ranges from 85+10℃ to 300+10℃.
Test purpose: To assess the product’s ability to withstand drastic temperature changes, that is, to withstand a large temperature change rate. The test can cause product failure caused by mechanical structural defects. The purpose of the thermal shock test is basically the same as that of the temperature cycle test, but the conditions of the thermal shock test are much more severe than those of the temperature cycle test.
Test purpose: To assess the product’s ability to adapt to low pressure working environments (such as high altitude working environments). When the air pressure decreases, the insulation strength of the air or insulating material will weaken; corona discharge, increased dielectric loss, and ionization are likely to occur; reduced air pressure will worsen the heat dissipation conditions and increase the temperature of components. These factors will cause the test sample to lose its specified function under low pressure conditions, and sometimes cause permanent damage.
Test conditions: The test sample is placed in a sealed room and a specified voltage is applied. From 20 minutes before the air pressure in the sealed room is reduced until the end of the test, the sample temperature is required to remain in the range of 25+-1.0℃. The sealed chamber is reduced from normal pressure to the specified pressure and then restored to normal pressure, and the test sample is monitored during this process to see if it can work normally. The frequency of the voltage applied to the microcircuit test sample is within the range of DC to 20MHz, and the appearance of corona discharge at the voltage lead end is considered to be a failure. The low pressure value of the test corresponds to the altitude and is divided into several levels. For example, the A-level pressure value of the microcircuit low pressure test is 58kPa, corresponding to an altitude of 4572m, and the E-level pressure value is 1.1kPa, corresponding to an altitude of 30480m, etc.
Test purpose: To evaluate the ability of microcircuits to resist decay under humid and hot conditions by applying accelerated stress. It is designed for a typical tropical climate environment. The main mechanism of microcircuit decay under humid and hot conditions is corrosion caused by chemical processes and physical processes of microcrack enlargement caused by water vapor infiltration, condensation, and icing. The test also examines the possibility of electrolysis of the microcircuit materials under humid and hot conditions. Electrolysis will change the resistance of the insulating material and weaken the ability to resist dielectric breakdown.
Test conditions: There are two types of humid heat tests, namely, the alternating humid heat test and the constant humid heat test. The alternating humid heat test requires that the temperature of the test sample be raised from 25°C to 65°C for a certain period of time (generally 2.5 hours) in the relative humidity range of 90% to 100%, and maintained for more than 3 hours; then, in the relative humidity range of 80% to 100%, the temperature is lowered from 6s°C to 25°C for a certain period of time (generally 2.5 hours), and then the temperature is lowered to -10°C under any humidity and maintained for more than 3 hours, and then restored to a temperature of 25°C and a relative humidity equal to or greater than 80%. This completes a large cycle of alternating humid heat, which takes about 24 hours.
Generally, the above large cycle of alternating humid heat is carried out 10 times in a humidity resistance test. During the test, a certain voltage is applied to the test sample. The ventilation volume per minute in the test chamber is required to be greater than 5 times the volume of the test chamber. The test sample should be a sample that has undergone a non-destructive lead firmness test.
Test purpose: To evaluate the corrosion resistance of the exposed parts of components under salt spray, humidity and heat conditions by an accelerated method. It is designed for tropical seaside or offshore climate environments. Components with poor surface structure will corrode under salt spray, humidity and heat conditions.
Test conditions: The salt spray test requires that the exposed parts of the test sample in different directions must be under the same specified conditions in terms of temperature, humidity and salt deposition rate. This requirement is met by the minimum distance between the samples placed in the test chamber and the placement angle of the samples.
Test temperature: Generally required to be (35+-3)’C, the salt deposition rate within 24 hours is 2X104mg/m2~5X104mg/m2. The salt deposition rate and humidity are determined by the temperature and concentration of the salt solution that produces the salt mist and the airflow passing through it. The ratio of oxygen to nitrogen in the airflow should be the same as that of air.
Test time: generally divided into 24h, 48h, 96h and 240h.
Test purpose: to evaluate the working ability of microcircuits under high-energy particle irradiation environment. High-energy particles entering the microcircuit will cause microstructure changes and defects or generate additional charges or currents. This will lead to microcircuit parameter degradation, locking, circuit flipping or surge current causing burnout and failure. Irradiation exceeding a certain limit will cause permanent damage to the microcircuit.
Test conditions: Microcircuit irradiation tests mainly include neutron irradiation and gamma ray irradiation. It is further divided into total dose irradiation test and dose rate irradiation test. Dose rate irradiation test irradiates the test microcircuit in the form of pulses. In the test, the irradiation dose series and total dose should be strictly controlled according to different microcircuits and different test purposes. Otherwise, the sample will be damaged or the threshold value sought will not be obtained due to the irradiation exceeding the limit. The irradiation test must have safety measures to prevent human damage.
02. Life test
It is one of the most important and basic items in the reliability test. It is to put the product under specific test conditions to examine the law of its failure (damage) over time. Through the life test, we can understand the product’s life characteristics, failure laws, failure rates, average life and various failure modes that may occur during the life test. If combined with failure analysis, the main failure mechanism that causes product failure can be further clarified, as a basis for reliability design, reliability prediction, improving the quality of new products and determining reasonable screening, routine (batch assurance) test conditions, etc.
If in order to shorten the test time, the test can be carried out by increasing stress without changing the failure mechanism. This is an accelerated life test. The reliability level of the product can be evaluated through the life test, and the reliability level of the new product can be improved through quality feedback.
Purpose of life test: to assess the quality and reliability of the product under specified conditions and during the entire working time. In order to make the test results more representative, the number of samples participating in the test should be sufficient.
Test conditions: The life test of microcircuits is divided into steady-state life test, intermittent life test and simulated life test.
Steady-state life test is a test that must be carried out on microcircuits. During the test, the test sample is required to be applied with appropriate power to keep it in normal working condition. The steady-state life test environment temperature of the national military standard is 125℃ and the time is 1000h. Accelerated test can increase the temperature and shorten the time.
The temperature of the power microcircuit shell is generally greater than the ambient temperature. During the test, the ambient temperature can be kept below 125℃. The ambient temperature or shell temperature of the microcircuit steady-state life test should be adjusted based on the microcircuit junction temperature equal to the rated junction temperature (generally between 175℃℃-200℃).
Intermittent life test requires that the bias and signal be cut off or suddenly applied to the tested microcircuit at a certain frequency. Other test conditions are the same as steady-state life test.
Simulated life test is a combination test that simulates the application environment of microcircuits. Its combined stresses include mechanical, humidity and low pressure four stress tests: mechanical, temperature, humidity and electrical four stress tests, etc.
03. Screening test
Screening test is a non-destructive test that fully inspects the product. Its purpose is to select products with certain characteristics or eliminate products with early failures to improve the reliability of the product. During the manufacturing process, some products may have so-called early defects or failures due to material defects or process out of control. If these defects or failures can be eliminated early, the reliability level of the product in actual use can be guaranteed.
Characteristics of reliability screening test:
1. This test is not a sampling test, but a 100% test;
2. This test can improve the overall reliability level of qualified products, but it cannot improve the inherent reliability of the product, that is, it cannot increase the life of each product;
3. The screening effect cannot be simply evaluated by the screening elimination rate. A high elimination rate may be due to serious defects in the design, components, and processes of the product itself, but it may also be due to too high screening stress intensity. A low elimination rate may be due to fewer product defects, but it may also be due to insufficient screening stress intensity and test time. The screening elimination rate Q and the screening effect B value are usually used to evaluate the advantages and disadvantages of the screening method: a reasonable screening method should have a large B value and a moderate Q value.
04. On-site use test
All the above tests are conducted by simulating on-site conditions. Due to the limitation of equipment conditions, simulation tests can only apply single stress to products, and sometimes double stress can be applied, which is very different from the actual use environment conditions, and thus fails to expose the quality of products truthfully and comprehensively. On-site use tests are different, because they are conducted at the use site, so they can most truly reflect the reliability issues of products, and the data obtained are of high value for product reliability prediction, design and assurance. On-site use tests play a greater role in formulating feasibility test plans, verifying reliability test methods and evaluating test accuracy.
05. Identification test
Identification tests are tests conducted when evaluating the reliability level of products. It is a sampling plan formulated based on sampling theory. Identification tests are conducted under the condition that the producer does not cause products that meet the quality standards to be rejected.
Reliability identification tests are divided into two categories: one is product reliability identification test, and the other is process (including material) reliability identification test.
Product reliability identification tests are generally conducted when new product designs are finalized and production is finalized. The purpose is to assess whether the product indicators have fully met the design requirements and whether the product has met the predetermined reliability requirements. The content of the test is generally consistent with the quality consistency inspection, that is, all four groups of tests A, B, C, and D are performed, and products with radiation resistance strength requirements must also undergo group E tests. Reliability identification tests must also be performed when there are major changes in the design, structure, materials or processes of the product.
The reliability identification test of the process (including materials) is used to assess whether the production line’s selection and control capabilities of materials and processes can ensure the quality and reliability of the manufactured products and whether it can meet the requirements of a certain quality assurance level.
06. Others
(1) Constant acceleration test
The purpose of this test is to assess the ability of the circuit to withstand constant acceleration. It can expose failures caused by low microcircuit structural strength and mechanical defects. Such as chip detachment, open internal leads, tube shell deformation, air leakage, etc.
Test conditions: A constant acceleration greater than 1mm is applied in the direction of chip ejection, pressing direction and direction perpendicular to the direction. The acceleration value range is generally 49000m/s: -1225000m/s (5000~125000z). During the test, the shell of the microcircuit should be rigidly fixed on the constant accelerator.
(2) Mechanical shock test
The purpose of this test is to assess the ability of the microcircuit to withstand mechanical shock. That is, to assess the ability of the microcircuit to withstand sudden force. The microcircuit will be subjected to sudden force during loading and unloading, transportation and on-site work. For example, when falling or colliding, the microcircuit will be subjected to sudden mechanical stress, which may cause the chip to fall off, the inner lead to open, the tube shell to deform, and the air leakage to fail.
Test conditions: During the test, the shell of the microcircuit should be rigidly fixed on the test bench, and the outer lead should be protected. Five half-sine wave mechanical shock pulses are applied to the chip ejection direction, pressing direction and direction perpendicular to the direction of the microcircuit. The peak acceleration of the impact pulse is generally in the range of 4900m/s2~294 000m/s2 (500g~30000g) and the pulse duration is 0.1ms-1.0ms. The allowable distortion is no more than 20% of the peak acceleration.
(3) Mechanical vibration test
There are four main types of vibration tests, namely sweep frequency vibration test, vibration fatigue test, vibration noise test and random vibration test. The purpose is to assess the structural firmness and electrical stability of the microcircuit under different vibration conditions.
The sweep frequency vibration test makes the microcircuit perform equal amplitude resonant vibration. Its acceleration peak is generally divided into 196 m/s: (20e), 49The vibration fatigue test also requires the microcircuit to vibrate with equal amplitude, but its vibration frequency is fixed, generally tens to hundreds of Hz, and its peak acceleration is generally divided into three levels: 196m/s2 (20g), 4490m/s2 (50g) and 686ms2 (70g). It is performed once in each of the three perpendicular directions (one of which is perpendicular to the chip), and each time is about 32h. The test conditions of random vibration test are to simulate the vibrations that may be generated in various modern field environments. The amplitude of random vibration has a Gaussian distribution. The relationship between acceleration spectrum density and frequency is specific. The frequency range is from tens to 2000Hz.
The test conditions of the vibration noise test are basically the same as those of the sweep vibration test. The microcircuit is made to vibrate with equal amplitude resonance, and its peak acceleration is generally not less than 196m/s2 (20g). The vibration frequency changes logarithmically over time from 20Hz-2000Hz. The time required for the vibration frequency to return to 20Hz from 20Hz-2000Hz is not less than 4min, and it is performed once in three mutually perpendicular directions (one of which is perpendicular to the chip).
However, the microcircuit must be applied with the specified voltage and current. Measure whether the maximum noise output voltage on the specified load resistance during the test exceeds the specified value
