What are the differences between THB (Double 85), BHAST, and UHAST? HAST and THB can be selectively validated; If THB or HAST is done, UHAST can be omitted.

Let's first take a look at the differences in experimental conditions between THB, BHAST, and UHAST:

Whether it's THB BHAST、 Or UHAST, the purpose of which is to accelerate the simulation of product failure caused by material degradation due to environmental (temperature, humidity) and voltage stress during normal use under conditions such as high temperature, high humidity, high voltage, and bias voltage (if any). From its experimental conditions, it can be seen that the THB (Double 85 Experiment) testing conditions are closer to actual usage conditions compared to BHAST and UHAST, and the testing time is the longest (up to 1000 hours, or 41.7 days). Therefore, when the project cycle is not so tight and time is sufficient, THB is the most suitable validation condition for actual usage scenarios.
BHAST is equivalent to the acceleration model of THB, so theoretically it is equivalent, but it still needs to meet the following conditions:
For devices that can reach absorption equilibrium within 24 hours, the HAST test is equivalent to at least 1000 hours of the Double 85 test (note that at least 1000 hours are used here, so it can be concluded that the strength of the HAST test conditions is higher than that of the Double 85 test); For devices that require more than 24 hours to reach absorption equilibrium, it is necessary to extend the time to achieve absorption equilibrium. (The absorption balance here should be caused by water vapor absorption)
2. For molded devices, moisture can lower the effective glass transition temperature of the molding compound. Stress temperatures above the effective glass transition temperature may lead to failure mechanisms unrelated to the standard 85 º C/85% RH stress.
The testing conditions of HAST experiment are more stringent compared to Double 85; Moisture can lower the glass transition temperature (Tg) of the encapsulated material, at which many physical properties of the material (such as thermal expansion coefficient) undergo rapid changes, leading to failure mechanisms that are not consistent with practical applications, such as delamination caused by the rapid change in thermal expansion coefficient after reaching the glass transition temperature.
Therefore, the BHAST experiment provided two experimental schemes, namely the 130 ℃ and 110 ℃ schemes. If the Tg temperature of the packaging material is less than 130 ℃, the 110 ℃ scheme needs to be considered for alternative validation.
Scope of application of UHAST:
1. UHAST is suitable for moisture resistance evaluation and robustness testing; The robustness test here refers to the stability of product performance caused by material degradation due to temperature and humidity;
2. UHAST can be used as a substitute for unbiased high-pressure cooking experiments
3. Moisture can lower the glass transition temperature of the plastic enclosure, which may cause failure mechanisms unrelated to practical applications;
4. UHAST does not apply bias voltage to ensure that failure mechanisms that may be masked by bias voltage can be revealed;
Overall: 1 If conditions permit (mainly time), it is recommended to conduct THB experiments to approach practical applications as closely as possible; 2. BHAST is commonly used in the current larger consumer electronics market due to its high efficiency (only 96 hours required) and more stringent requirements compared to Double 85; However, it should be noted that when the Tg temperature of the encapsulated material is below 130 ℃, low-temperature (110 ℃) testing conditions or THB experiments should be used as alternative solutions; When the failure caused by BHAST cannot be determined whether it is related to the applied electric field stress, UHAST should be used for confirmation in order to facilitate targeted improvement in the future.

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