Detailed introduction of environmental test equipment

Environmental testing, a type of reliability testing, has evolved into a method of predicting how the environment in which a product is used affects the performance and functionality of the product. In other words, environmental tests are used to assess the degree of environmental impact on the product before it is put on the market; When the function of a product is affected, environmental testing is used to identify the cause and measures are taken to protect the product from environmental impacts to maintain product reliability. These tests have gone far beyond their original purpose and are now used in a wide range of applications, including the development of materials and products, various inspections during the production process, inspection before transportation and quality control after transportation, as well as to analyze defects during actual use of products and to improve new products. Environmental testing is very effective for testing methods and maintaining product reliability.

The environmental test we talk about here is a narrow sense, and actually refers to the environmental test that is artificially simulated (hereinafter referred to as environmental test). In a broad sense, environmental tests are basically divided into three categories: "natural exposure test", "artificial simulation test" and "field operation test". Natural exposure test is to test samples exposed to natural environmental conditions for a long time. Field operation test is to test the sample device in a variety of typical use sites and make it in normal operation. These two tests can directly reflect the performance and reliability of the product in actual use, and are also the basis for verifying the accuracy of the manual simulation test. However, the test cycle is long, the cost of manpower and material resources is large, and the former test conditions can not be controlled, affecting the reproducibility of the test, and sometimes can not keep up with the development of products; The latter data feedback is slow. Therefore, in order to identify the adaptability of products to the environment in a relatively short period of time, artificial simulated environmental tests are used in scientific research and production work, that is, the role of one or more environmental factors is simulated in the laboratory test equipment (box or room), and it is appropriately strengthened. The determination of the test conditions of the artificial simulation test requires that it can not only simulate the authenticity of the main factors in the environment, but also play a certain acceleration role in time, but the degree of acceleration should not change the law of the actual damage mechanism of the product. Therefore, the test conditions and methods of artificial simulation test must be organically linked with the grade and value of product environmental conditions.

As shown in Figure 1, the environmental test can be simply divided into "climatic environmental test", "mechanical environmental test" and "comprehensive environmental test". Climate-related environmental tests include environmental stress tests such as temperature, humidity and pressure, while mechanical environmental tests include environmental stress tests such as shock and vibration, and comprehensive environmental tests are stress tests that combine environmental factors such as climate and machinery.

Environmental test

Mechanical environment test

Comprehensive environmental test

Climatic environment test

Ambient stress: shock, vibration, collision, acceleration, high noise, high wind

Environmental stress: the combination of mechanical environment and climatic environment environmental factors

Ambient stress: temperature and humidity, gas, salt spray, wind and rain, pressure, solar radiation

Of course, the use of environmental testing equipment to conduct tests can not be completely accurate to reproduce the use of the product environment and simulate all environmental factors, it is important to understand the limitations of environmental testing. An environmental test consisting of a single factor (temperature, humidity, pressure, vibration, shock, or a substance such as salt) is called a simple environmental test. In practice, it is difficult to produce a completely single environment, and most test environments are very complex. Therefore, when designing the test conditions, the test personnel need to choose the most important environmental factors that have the greatest impact on the product, so the environmental test can only be a man-made environment that is very different from the real environment.

In general, product defects are caused by the following aspects:

a. The concentration and diversity of raw materials, friction, wear, stress, heat, current and electric field strength, which can affect the performance of certain aspects of the product;

b. Factors arising from product characteristics (raw materials, manufacturing processes, structural components, and mass production) during product design and manufacturing;

c. Stress caused by the external environment.

Therefore, the test conditions must be based on the specific product conditions, for different products are not the same. If the product being tested has changed, the corresponding environmental test should also be changed.

2. Temperature stress

Environmental stress conditions can cause product failure, and Hughes Aviation (USA) technical data clearly shows the relationship between failure and environmental stress (see Figure 2). It can be seen that the ground failure caused by temperature and humidity environmental stress accounts for about 60% of all environmental stress-induced failures, and there is a close relationship between temperature stress and failure.

At present, all over the world, no matter from the land to the sea or from the high altitude to the space, electronics and other fields of products are widely used. Because the temperature decreases with the increase of height or at high latitudes in winter, or some products are located close to refrigeration components, equipment or systems, or some products themselves include refrigeration components, equipment or systems, resulting in a low temperature environment. Low temperature will have different degrees of harmful effects on almost all materials, the physical properties, electrical properties of various materials that constitute the product will change, resulting in temporary or permanent performance decline, and even cause failure.

Similarly, the natural high temperature in the low-latitude tropical areas, the warming of solar radiation, the temperature rise caused by poor ventilation and the temperature rise caused by the heat dissipation sample in use, the high temperature will reduce the reliability of the electronic combination, and the sealing parts, rubber parts and plastic parts of the mechanical structure will be rapidly aging and deteriorating by high temperature and solar radiation. The structure, physical properties and electrical properties of various other materials will also change greatly, resulting in temporary or permanent damage and performance changes.

In addition, in the process of product storage, transportation, use and installation, in addition to the change of natural climate, it will also encounter changes in environmental temperature induced by human social practice. For example, the equipment is moved from the high temperature indoors to the relatively low temperature outdoors; Or move from outdoors where the temperature is relatively low to indoors where the temperature is higher; Or the equipment used outdoors is suddenly rained or soaked in cold water after intense solar radiation; Or the extreme rise in temperature leads to solder reflux phenomenon, or the temperature of the surrounding devices increases rapidly when the motor is started, and the temperature of the surrounding devices will plummet when the motor is turned off. Or the device may be connected to the power supply in a low-temperature environment, resulting in a steep temperature gradient inside the device. Cutting off the power supply in a low-temperature environment may result in a steep temperature gradient in the opposite direction. Or when the aircraft takes off or lands, the external equipment of the aircraft may have a sharp change in temperature and so on. Because the rapid temperature change will make the product subject to a certain thermal impact force, under the action of this thermal impact force, it will lead to the coating layer of electronic and electrical components falling off, the sealing material cracking or even breaking, the filling material leaking, etc., which will cause the electrical performance of electronic components to decline; For products composed of different materials, due to the uneven heating of the product when the temperature changes, the product deformation, cracking, crushing and so on. Due to the large temperature difference caused by the temperature change, the surface of the product will condense or frost at low temperature, and evaporate or melt at high temperature. The result of such high and low temperature repeated action leads to and accelerates the corrosion of the product.

The following table shows the main types of temperature stress induced failures.

Table 2 Main types of failure caused by temperature stress

Failure environment stress condition sensitive elements and materials

Classification (cause) failure mode in large classification

Temperature high temperature aging aging tensile strength aging

Insulation aging temperature + time plastic, resin

Chemical changes in thermal decomposition temperature of plastics, resins

Soften, melt, vaporize, sublimate and twist temperature metal, plastic, hot fuse

High temperature oxidation oxidation layer structure temperature + time connection point material

Thermal diffusion (metal compound structure) lead break temperature + time different metal connection site

Intermediate damage semiconductor hot spot temperature, voltage, electron energy heterogeneous materials

Heat accumulation combustion (residual heat combustion) Combustion heating + drying + time Plastics (e.g. wood chips with Vinylon and polyurethane paint)

Puncture the internal short circuit

Poor insulation high temperature (200~400℃) silver, gold, steel, magnesium, nickel, lead, palladium, platinum, tantalum, titanium, tungsten, aluminum

Non-intrinsic short circuit

Poor insulation high temperature (400~1000℃) halides of copper, silver, iron, nickel, cobalt, manganese, gold, platinum and palladium

Migration electromigration disconnect, lead break temperature (0.5Tm) + current (density 106A/cm2) e.g. Tungsten, copper, aluminum (especially aluminum leads in integrated circuits)

Spread metal fatigue, damage temperature + stress + time springs, structural elements

Plastic fatigue, damage temperature + stress + time springs, structural elements

Low-temperature brittle metals damage low-temperature body-centered cubic crystals (e.g., copper, molybdenum, tungsten) and close-packed cubic crystals (e.g., zinc, titanium, magnesium) and their alloys

Plastic damage Low temperature + low humidity high glass transition temperature (e.g. cellulose ethylene ammonia), low elasticity of amorphous (e.g. styrene, methyl acrylate)

Flux flow Flux flow sticking to cold metal surfaces noise, low temperature connections, especially to components connected to printed circuit boards (e.g. switches, connecting devices)

Therefore, when discussing the product life related to temperature, the expression of "θ℃ rule" is generally used. Specific applications can be expressed as the "10℃ rule", such as when the ambient temperature rises by 10℃, the product life will be reduced by half; When the ambient temperature rises by 20 ° C, the product life will be reduced to a quarter. This rule can explain how temperature affects product life (failure).

On the contrary, one can accelerate the failure of the product in a short period of time by increasing the ambient temperature or reducing the ambient temperature or using the alternations of ultra-high and ultra-low temperatures to determine the characteristics of the product and the fault problems caused by the different thermal expansion coefficients of the dissimilar materials that make up the components. It is widely used in development screening test, material characteristic test, limit test, evaluation test, quality confirmation test, accelerated life aging test and other occasions.

3. Temperature test equipment

Temperature test equipment is generally composed of studio, regulating equipment, auxiliary equipment and control system. The regulating system includes heater, evaporator and air supply device, auxiliary equipment main room refrigeration unit, control system includes temperature controller, program setter, safety alarm device and so on.

The heater uses the electric heating method of resistance wire or electric heat pipe; In addition to natural heating for the drying chamber of the non-cooling sample high temperature test, it is generally equipped with a air supply device for forced air circulation, and the air supply method is often sent from the upper side to the lower side or the whole face plate top to the lower side, which is very important for the uniform performance of the entire test equipment.

The refrigeration method is mostly used for mechanical refrigeration, the refrigerator has a semi-closed or fully closed form, the cooling circuit also has a water-cooled form or an air-cooled form, the refrigerant is now mostly environmentally friendly refrigerant, the throttling method uses capillary or electronic expansion valve, which can improve the accuracy and the life of the circuit, and can work continuously for a long time.

The control system now generally adopts PID mode, so that the heating power is automatically adjusted and controlled with the deviation of the actual temperature of the equipment and the set temperature, so as to achieve stepless adjustment and small temperature fluctuation; The display part has also been upgraded from the previous digital display mode to the LCD touch screen mode, such as the Pmeter installation of ESPEC Group is the use of 6.54-inch TFT color LCD display screen, clear and beautiful, more easy to operate.

In addition, the insulation layer of the equipment adopts glass wool or polyurethane foam layer, which has small thermal conductivity and good temperature resistance. Temperature detection using thermocouple, platinum resistor and thermistor, etc., high precision.

The temperature test needs to be considered thoroughly, so it is also crucial to consider the selection of appropriate test equipment.

1. The test range must be required to meet the product failure possibility test range, that is, whether it is a high temperature box or a low temperature box or a hot and cold shock box should be able to meet the extreme temperature conditions specified in the test requirements.

2. The test equipment must be selected in order to ensure that the volume of the test sample should not be greater than 1/5 of the working volume of the test equipment.

3. In order to ensure the uniformity of temperature in the test area, choose whether to use forced air circulation or no forced air circulation according to the heat dissipation of the sample. For example, the LG type drying box produced by Shanghai ESPEC uses hot air natural convection mode, which is especially suitable for powder drying, while the LC type drying box uses hot air forced circulation mode. The difference in the temperature distribution of the equipment will have a greater impact on the test results. When a larger sample is used, or the number of samples tested at the same time is large, the test results will vary greatly with different locations, so the equipment with the best temperature uniformity should be selected. For example, the Z series temperature control box produced by Shanghai ESPEC has a uniformity of 2.0 ° C.

4. Sample temperature can be measured in two ways: upwind and downwind sensor. The position and control method of the temperature control sensor of the equipment can be selected, and the appropriate equipment should be selected according to the test requirements.

5. To prevent the sample heat absorption or heat release caused by heat radiation or heat conduction in the test area, the heating or cooling system device of the equipment is required to have no effect on the sample.

6. Easy to record and display, the cycle test requires the installation of automatic counters, indicators, recording equipment and automatic closing instruments.

7. Easy sample placement can require a sample rack for placement or hanging, and the sample rack will not change its mechanical properties due to high and low temperature changes.

8. There should be a lead hole to introduce the test power supply into the equipment chamber.

9. There should be protective measures for both the damage of the sample and the safety of the test personnel. For example, there are observation Windows and lighting, and alarm devices such as power supply interruption, water shortage, overtemperature protection, and operator protection.

10. Whether remote monitoring function is required.

Precautions when using temperature test equipment:

1. When conducting bulk sample test, there will be temperature and humidity deviation between the updraft and downdraft, so the sample location should be carefully considered. Try to place in the center of the working space of the test equipment, the samples shall not contact and overlap each other, and a certain interval should be left to allow air circulation; It should also be ensured that the sample is easy to move during the test and easy to replace the sample during the test.

2. Check whether there are volatile substances such as oil and gas in the test area, check whether there are odor substances, and the test consequences of such substances must be confirmed in advance.

3. Pay attention to confirm the temperature of the test sample in the test area.

4. In order to keep the temperature in the temperature zone consistent, it is necessary to ensure the minimum fluctuation of the test environment temperature and equipment power supply as far as possible, to ensure that the test sample does not produce thermal radiation and does not absorb heat to ensure the stable temperature of the test zone.

5. After the temperature test equipment completes the test, the rapid removal of the test sample will cause unnecessary stress on the sample and may obtain unexpected results. Therefore, the sample must be removed after the test sample has cooled to the ambient temperature.