1 Overview of Low Temperature Test
Low temperature testing is used to determine the usage, transportation, or storage capacity of components, equipment, or other products under low-temperature environmental conditions. With the continuous development of science and technology, the requirements for products are constantly increasing, and the quality and reliability indicators of electronic products working at low temperatures are receiving much attention.
2 The impact of low temperature
(1) The elasticity of flexible materials such as rubber decreases and cracks occur, such as low-temperature testing of heat shrink tubing;
(2) The brittleness of metals and plastics increases, leading to cracking or the formation of cracks, such as low-temperature testing of metal packaging shells;
(3) Due to the different shrinkage coefficients of materials, when the temperature change rate is large, it can cause the moving parts to get stuck or rotate improperly, such as in low-temperature testing of electrical connectors;
(4) Lubricant viscosity increases or solidifies, leading to increased friction between moving parts, causing slow movement, and even stopping work;
(5) Electronic component electrical parameter drift affects the electrical performance of products, such as low-temperature testing in integrated circuit three temperature testing;
(6) Ice or frost can cause structural damage or moisture to the product. The setting of low-temperature test conditions is determined by the environmental conditions of electronic component product design and use. When the product is required to be used at what ambient temperature, the laboratory should adopt corresponding temperature conditions to conduct adaptability test assessment. Therefore, the test conditions should be consistent with the environmental conditions without contradiction.
According to the standard regulations, the basic environmental testing standards for electronic components, instruments, and meters have lower temperature limits of 0 ℃, -10 ℃, -25 ℃, -40 ℃, and other levels. Based on investigations and collected data, the lowest temperature measured by meteorological stations in China is -51 ℃, and the lowest temperature measured in the Daxing’anling region was -62 ℃. Therefore, several levels such as -55 ℃ and -65 ℃ have been added. With the further development of electronic components, some connectors and cable assemblies need to withstand ultra-low temperature impact of -100 ℃.
Under specific conditions, ultra-low temperature test conditions of -100 ℃ or -110 ℃ need to be added. Under these harsh conditions, the assessment of electronic components is rigorous. For the duration of the experiment, international standards set the duration of low-temperature testing as 2 hours, 16 hours, 72 hours, and 96 hours. This is mainly considered from the perspective of the usage environment of electronic component products and is adopted by Western Europe, Japan, and other countries. But the US military standard generally adopts two types of maintenance: 24 hours or 72 hours.
Our military standard proposes to determine the testing time based on the weight of the product. The duration of the low-temperature test should depend on the product’s ability to ensure internal freezing under a certain low-temperature condition, that is, the internal temperature reaches equilibrium with the test conditions (deviation less than 3 ℃), and does not necessarily require an extension of the test time (some experimental verification has also been conducted on whether extending the test time has an impact), because the results at low temperatures do not have an “accumulation” meaning like the “thermal aging” effect at high temperatures. Therefore, as long as the sample is frozen through and there is enough time for the material to undergo shrinkage deformation due to temperature effects, the holding time is sufficient.
3 Low temperature testing techniques and methods
(1) Test purpose: Simulate a low-temperature environment and record changes in the product. It is commonly used for type testing and component screening testing during product development.
(2) The experimental conditions are specified according to a certain standard as follows:
Low temperature test Ab for non heat dissipation test samples: temperature gradient.
Low temperature test Ad for heat dissipation test sample: temperature gradient. Temperature: -65 ℃- 55℃；- 40℃；- 25℃；- 10℃；- 5℃；+ 5 ℃. Duration: 2 h; 16h； 72h； 96 h.
(3)Test procedure
Test Ab: Low temperature test of temperature gradient for non heat dissipating test samples, used to determine the adaptability of non heat dissipating electronic and electrical products (including components, equipment, or other products) for storage and use at low temperatures. The temperature change rate inside the box shall not exceed 1 ℃/min (average value over a period of no more than 5 minutes).
Experiment Ad: Low temperature test of temperature gradient of heat dissipation test samples to determine the adaptability of heat dissipation electronic and electrical products (including components, equipment or other products) for use under low temperature conditions.
(4) Technical and equipment requirements
Regarding technology.
If the purpose of the experiment is only to check whether the test sample can work normally at low temperatures, the time of the experiment should be limited to stabilizing the temperature of the test sample; If it is a test related to low-temperature durability or reliability, the required duration of the test shall be in accordance with relevant standards. During the experiment, it is important to distinguish between two types of test samples: those without artificial cooling and those with artificial cooling.
The test samples without artificial cooling are divided into tests without forced air circulation and tests with forced air circulation. The test without forced air circulation is a test method that simulates the influence of free air conditions. There are two methods for conducting forced air circulation experiments. Method A is suitable for test chambers that are large enough to meet the test requirements without forced air circulation, but only air circulation can be used to maintain the ambient temperature inside the chamber. Method B is used in situations where Method A cannot be applied, such as when the volume of the test chamber used for the experiment is too small and cannot meet the test requirements without forced air circulation.
The test samples with artificial cooling can generally be tested using the method of non forced air circulation. These three low-temperature tests are necessary for the performance testing of the test samples. The test samples must be energized or subjected to electrical loads according to relevant standards, and checked to determine whether they can achieve the specified functions. Before applying the specified test conditions as required, the sample should undergo an initial inspection of its appearance, electrical and mechanical properties.
Then, according to relevant standards, a load should be applied during or at the end of the test and an intermediate inspection should be conducted. During the inspection, the test sample should not be taken out of the test chamber. Restoration shall be carried out according to the prescribed requirements, and finally the appearance, electrical and mechanical properties of the sample shall be tested.
Requirements for experimental equipment.
The test chamber should be able to maintain the specified temperature conditions in the test workspace, and forced air circulation can be used to maintain uniform temperature. In order to limit the impact of radiation, the temperature difference between each part of the inner wall of the test chamber and the specified test temperature should not exceed 8% (according to Kelvin thermometer), and the test sample should not be directly irradiated by any heating and cooling components that do not meet the above requirements.