Discussion on Failure Mode of Environmental Stress Test

High Temperature Stress


Basic parameters of high temperature stress

There are two basic parameters of high temperature stress: (1) upper limit temperature (TU); (2) Time (T). In addition, there is another parameter to be considered: ambient temperature (TE), because the variable that really affects the effect of constant high temperature is the difference between the upper limit temperature (TU) and the indoor ambient temperature (TE), that is, the temperature change amplitude (R) (r = Tu – TE)



High temperature stress characteristic analysis

High temperature test will cause high-temperature aging, heat accumulation, migration and spread of products. It is a static process or process. This method is to provide additional thermal action to make the products work continuously at the specified high temperature, make the heat spread, accelerate the potential defects of products into faults, and expose them in the form of faults.



High temperature stress-induced failure mechanism and sensitive elements

High temperature accelerates the oxidation of metal material surface, in which temperature and time will affect the size of fault defects, and its sensitive elements such as electroplated parts, alloys, etc;


High temperature changes the magnetic properties of the conducting magnet, and its sensitive elements, such as resistance (resulting in increased resistivity), etc;


High temperature deteriorates the tensile strength, damages the insulation performance of insulating materials and reduces the electrical strength, resulting in thermal breakdown, short circuit or open circuit of coils, and its sensitive elements such as plastic, resin, etc;


High temperature reduces the acid and alkali resistance of the product, reduces the mechanical strength of the material, and is easy to be damaged under stress;


High temperature causes electromigration, that is, temperature change will affect current, and its sensitive elements such as copper and aluminum (especially aluminum lead in integrated circuit), etc;


High temperature causes lubricant loss or reduced lubricity, resulting in increased mechanical wear, and its sensitive elements such as rotating parts (bearings and rotating shafts) of mechanical structure;


High temperature causes obvious changes in product characteristics and parameters, and its sensitive elements such as transistors, resistors, capacitors and transformers;


Under high temperature, the expansion of materials with different expansion coefficients is different, which will cause defects such as element looseness, size change, open welding, false welding, oxidation, softening, melting and seal failure, and sensitive elements such as plastics;


High temperature causes material to change color, turn yellow (white material), turn white (black material), turn white (black material), embrittlement and pulverization, and its sensitive components such as plastics.



Temperature Cyclic Stress


Basic parameters of temperature cyclic stress

There are six basic parameters of temperature cyclic stress: 1) upper limit temperature Tu; 2) lower limit temperature TL; 3) temperature change rate V; 4) Upper temperature holding time Tu; 5) Lower temperature holding time TL; 6) Number of cycles n.



Analysis of temperature cyclic stress characteristics

In the environmental reliability test, when the temperature cycle changes, the high temperature stress, low temperature stress and thermal fatigue interact on the product, and the material will expand and contract in varying degrees. The utility model is characterized in that increasing the temperature change range and temperature change rate can strengthen this process, thus enhancing the thermal stress and increasing the number of cycles, which will directly affect the excited stress.


In the temperature cycle test, the uniformity of air flow (velocity) in the test chamber is a very important parameter, which will affect the temperature change rate of the product. This requires that when multiple test products are tested at the same time, there should be appropriate spacing between the test products and between the test products and the test chamber wall, so that the air flow can circulate freely between the test products and between the test products and the chamber wall.


In the temperature cycle test, when the low temperature and high humidity condition changes to the high temperature and high humidity condition, because the air temperature rises faster than the product temperature, condensation will occur on the product when a certain temperature difference is reached. The greater the temperature difference, the more obvious the condensation phenomenon. If the condensed water cannot be discharged in time, it will increase the probability of product corrosion and reduce the insulation strength. For the whole equipment, it will lead to reduced sensitivity and frequency drift, which will seriously affect the product quality.



Failure mechanism and sensitive elements induced by temperature cyclic stress

Temperature cycle makes the expansion of different materials with different expansion coefficients different, resulting in peeling and cracking, and its sensitive elements such as paint coating;


The temperature cycle makes the joints with loose screw connection or riveting, and its sensitive elements such as screws, riveted parts, etc;


The temperature cycle relaxes the press fitting joint with insufficient mechanical tension;


Temperature cycle increases the brazing contact resistance of poor materials or induces open circuit, and its sensitive elements such as resistance elements;


Temperature cycle causes corrosion and pollution of contacts (solder joints), and its sensitive elements such as alloy materials.



Temperature Shock (Thermal Shock) Stress


Temperature impact stress, i.e. thermal shock, has six basic parameters: 1) upper temperature limit; 2) Lower temperature limit; 3) Residence time at the upper temperature limit; 4) Residence time at the lower temperature limit; 5) Temperature conversion time or temperature change rate; 6) Number of temperature shock cycles.


These parameters will determine the severity level of the impact of temperature impact test on the product. Generally speaking, the increase of temperature variation rate is conducive to the exposure of potential defects. In the environmental reliability test, the requirements of temperature impact stress test: the temperature change rate is greater than 15 ℃ / min, and the conversion time is <2-3min; <20-30s; <10s, which is different from the above temperature cyclic stress test. At present, in foreign countries, the temperature change rate adopted for high accelerated stress screening has reached 60 ℃ / min. in foreign temperature impact boxes, basket transfer time: <10s (transmitted through automatic transfer basket in the box).


Characteristic analysis of temperature impact force


The method of temperature shock can provide high temperature change rate and produce large thermal stress. It is an effective method to screen components, especially integrated circuit devices, but attention should be paid to the possible additional damage in the test; For power on and monitoring, the temperature shock method is inconvenient to use, or even impossible to achieve comprehensive monitoring

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