Principles, Methods, Common Problems And Solutions Of Damp Heat Testing

Damp heat test is a commonly used experimental method with five major functions:
1. Evaluate the material’s resistance to moisture and heat
2. Verify the reliability of electronic products
3. Test the weather resistance of coating materials
4. Study the aging mechanism of materials
5. Evaluate product reliability and quality
Its functions are widely used in different fields;

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1. What is damp heat test?

Damp heat test technology is mainly used in:
1. Explore the impact of humid environment on products (research experiments in the development and design stages).
2. Identify the moisture-proof performance of the product (quality inspection or type test during the development and production stages).
3. Evaluate the safety and reliability of the product when used in a humid environment (safety or reliability test).

The main indicators determined after the test are generally to check the electrical and mechanical properties of the product, and also to check the corrosion of some samples.
There are generally three types of damp heat tests. Among them, the constant damp heat test is mainly suitable for general electrical and electronic products. The stress severity level is low and the test equipment requirements are not high.

The alternating heat and humidity test is suitable for products with harsh and complex environments. The humidity and heat test in military standards is actually alternating heat and humidity, and is suitable for military products or communication products in complex environments or that may be used in such environments. The alternating damp heat or damp heat test has stricter requirements on temperature, humidity, duration and cycle than the constant damp heat test, and the military standard damp heat test is even more stringent. Therefore, if a product has been subjected to alternating damp heat or the damp heat test required by military standards, there is no need to do a constant damp heat test. Generally, for important and critical products or military equipment, constant humidity and heat tests will not be selected when formulating reliability test plans or writing test outlines. The order of severity of the three damp heat tests, from low to high, is "constant damp heat", less than "alternating damp heat", less than "(military standard) damp heat". It should be noted that severity does not mean that more projects are better.

2. Physical phenomena of damp heat test conditions
In the hygrothermal test, temperature and humidity work together to form some physical phenomena and make the surface or interior of the sample damp.

1. Adsorption phenomenon:
Gas molecules (water vapor molecules in the hygrothermal test) may collide with the surface of a solid substance (sample) when moving in space. When a certain number of molecules continuously collide with the solid surface, before it returns to space, it must be in the solid substance (sample). The surface "stays" for a certain length of time. At this time, the concentration of gas on the surface is higher than its concentration in space, resulting in condensation. This phenomenon of gas "staying" on a solid surface is called adsorption. Therefore, adsorption can also be said to be an intermediate process between gas condensation and evaporation on a solid surface. According to the experimental results, the amount of gas adsorption is related to the properties of the solid material, the temperature and the pressure of the gas at equilibrium. The lower the temperature and the higher the pressure, the greater the adsorption capacity. (Interested students can study functional relationship expressions)
Physical adsorption is caused by van der Waals attraction, and the adsorption layer is generally a multi-molecule layer. The adsorption speed is fast, the energy required for adsorption is also small, and it can generally be carried out at low temperatures. In the moist heat test, physical adsorption is the most common phenomenon.

2. Condensation phenomenon:
Condensation is actually the adsorption phenomenon of water molecules on the sample, but it is generated when the test temperature rises. During the heating stage, when the sample surface temperature is lower than the dew point temperature of the surrounding air, water vapor will condense into liquid on the sample surface to form water droplets. During the heating stage of the alternating damp heat test, due to the thermal inertia of the sample, its temperature rise lags behind the temperature of the test chamber. Therefore, condensation occurs on the surface. The amount of surface condensation depends on the heat capacity of the sample itself, as well as the heating rate and relative humidity during the heating stage. During the cooling stage of the alternating heat and humidity test, condensation will also appear on the inner wall of the closed shell.

3. Diffusion phenomenon:
Diffusion is a physical phenomenon of molecular motion. In the process of diffusion, molecules always move from a place of high concentration to a place of low concentration. During the hygrothermal test, the rate at which water vapor in the air diffuses into materials with lower concentrations can be expressed by Fick's law. Therefore, the moisture intrusion caused by diffusion in the hygrothermal test depends not only on the absolute humidity and temperature in the test conditions, but also on the material of the sample.

4. Absorption phenomenon (also called circulation phenomenon).
Water vapor enters the material generally through voids. The speed at which water vapor passes through the gap depends on the size of the hole. If the size of the pores is smaller than the diameter of the water molecules, water vapor cannot enter. Since water vapor is mixed with air in space, its entry speed is also closely related to the mixing ratio of water vapor and air. When the ratio of water vapor to air is 1:1, the amount of water vapor equivalent to the saturated air at 80°C is taken as the limit. Anything above this limit is called high vapor pressure, and anything below this limit is called low vapor pressure. Then the mechanism of water vapor entering the gap will be discussed separately:
① Water vapor entry mechanism under low vapor pressure: When the temperature and water vapor pressure remain unchanged (equivalent to a constant humidity and heat test), water vapor enters the gap mainly due to diffusion, and its speed mainly depends on the air resistance in the gap ( permeability coefficient) and void size (the size of the voids also affects the entry rate, but not significantly). When the temperature changes (equivalent to the alternating heat and humidity test), the water vapor pressure difference on both sides of the gap forces air containing water vapor to pass through. At this time, the entry rate is not only related to the gap resistance and gap size, but also related to the water vapor pressure difference at both ends of the gap. It can be seen that the mechanisms of action of the constant damp and heat test and the alternating damp and heat test are different.
② Under high vapor pressure conditions, the entry speed of water vapor is related to the diameter of the gap. When the diameter of the gap is smaller than the average free path of water molecules, the entry of water vapor is a molecular flow; when the diameter of the gap is greater than the average free path, the entry velocity is a viscous flow. When the gap diameter is between the above two, it is transition flow. Under high vapor pressure, the entry speed of water vapor changes with the size of the gap, indicating that if the temperature is increased to accelerate the entry of moisture, there will be different rates for different gap sizes, and the acceleration multiples will be different.
To sum up, the entry of water vapor through absorption depends on the temperature and water vapor pressure (absolute humidity) and the material material.

5. Respiration:
We call the exchange of internal and external air caused by temperature changes in the cavity of the closed sample respiration. During the cooling stage of the alternating heat and humidity test, due to the sharp drop in temperature, the air temperature in the closed cavity drops or condensation on the inner wall of the cavity will reduce the pressure in the cavity, forming a suction phenomenon and sucking in humid air from the outside. Therefore, The amount of tidal volume inhaled during the cooling phase of respiration is related to the rate of temperature change and absolute humidity. This breathing phenomenon not only occurs when the test temperature alternates, but also occurs when a sample with a closed shell, such as a closed rotating motor, undergoes intermittent movement and the coils in the shell alternately heat or cool. It is not uncommon for motor products used under humid conditions to absorb moisture due to this respiration, and condense into water to accumulate in the shell for a long time.

3. Deterioration effects of moisture on different types of samples
There are generally two forms of sample moisture: one is surface moisture, which is usually caused by condensation and surface adsorption; the other is volumetric moisture, which is caused by water vapor diffusion and absorption. Sometimes the moisture adsorbed on the surface of the sample reaches a certain level, which will also speed up the volume of moisture. For closed type samples with cavities, although the interior is not directly exposed to high humidity conditions, the respiration caused by changes in test temperature will cause external moisture to enter the interior through gaps or cracks, causing internal moisture. At the same time, diffusion and absorption phenomena can also allow moisture to enter the closed shell through gaps. In addition, for some shells of organic materials, when the moisture absorption caused by the diffusion phenomenon reaches a stable level, the moisture can penetrate through the shell and enter the shell. The deterioration effect of the sample caused by moisture on the surface and volume refers to mechanical properties (size and strength) and non-mechanical properties (electrical properties and other properties); two changes.

4. The relationship between damp heat test conditions and actual humid environment
The temperature and humidity conditions of the hygrothermal test generally simulate the rarer conditions in the actual environment, and the duration of the effect is much longer than that in the actual environment. Therefore, in terms of simulation, it is harsher than natural conditions and has an acceleration effect on the sample. According to the moisture mechanism caused by several physical phenomena discussed above, it can be seen that the test results of samples of different materials and structures are not exactly the same. Therefore, it is difficult to obtain a unified acceleration coefficient for a universal artificial hygrothermal test method. Only for a sample with a specific or single property, a more appropriate acceleration coefficient can be determined after analysis and experimental comparison. The corresponding relationship between the classification of hot and humid environments and the severity of the test is a problem that has not been completely solved for many years. The severity level of the artificial damp heat test method is composed of the test conditions and the number of test cycles. The test conditions generally correspond to the actual environmental conditions of use of the sample, and the selection of the number of test cycles is more complicated. Usually, the number of test cycles is determined based on a comprehensive analysis of the characteristics of the sample and the influence of moisture and heat on its main mechanism. Generally, the appropriate number of cycles can be selected after comparing the results with the results of natural or field operation tests and finding out the relationship between them. However, so far, even internationally, a universally applicable mathematical model has not yet been developed to express the relationship between artificial hygrothermal tests and natural conditions. Therefore, although the preferred number of cycles is recommended in the test method standards, there are still many problems in practical applications.
The humidity and heat test period is the most reliable basis for the long-term storage period of the product. Current knowledge shows that the basic and most important factor affecting corrosion, especially in inventories, is the relative humidity in the warehouse. When the relative humidity is low, the corrosion rate does not increase rapidly as the temperature increases. They follow such an empirical relationship:

In the formula: A——rust degree
H——Relative humidity (%)
t——Atmospheric temperature (℃)
k——constant related to the type of metal material

According to this relationship, the corrosion degrees of different metal materials under different conditions can be obtained. According to this relationship, when the relative humidity (H) in the atmosphere is 65%, the corrosion degree A=0, which means that metal materials will not rust under these conditions. However, when the relative humidity is greater than 65%, the metal will rust, and as the humidity and temperature increase, the degree of rust increases sharply.

Whether it is long-term storage or accelerated corrosion testing, another common one is point matrix corrosion. Most of them are due to bumps in the process of dipping paint and packaging, "inclusions" in the melting process (mostly iron inclusions), and "dust inclusions" caused by bumps and scratches in the stamping process. Before surface treatment, No repair surface found. Therefore, point rust is also the most difficult source of corrosion to eliminate. The respiration in the cooling stage of the alternating damp heat test is more obvious for certain types of samples. Therefore, the cooling speed and humidity issues are particularly emphasized in the test method. Larger temperature changes in alternating damp heat, higher relative humidity during cooling, and long duration of high humidity will aggravate insulation dampness.

5. The significance of damp heat test
Constant humidity and heat avoid condensation by first raising the temperature and then raising the humidity (first dehumidifying and then cooling), which mainly causes product failure through the adsorption, absorption and diffusion of water vapor by the sample in a high temperature and high humidity environment.
Alternating moist heat uses the alternating process of condensation and drying caused by temperature cycles under high humidity conditions to cause the water vapor entering the interior of the sample to breathe, thereby accelerating the corrosion process.

6. Interruption processing of damp heat test
1. Constant humidity and heat test
When the test is forced to be interrupted due to special reasons such as a sudden power outage during the test, it is recommended to operate in the following manner:
1) If the environmental conditions in the box do not exceed the allowable error range during the interruption, the interruption time should be regarded as part of the total test time (generally, power is turned on in time to restore the environment in the box after an instantaneous power outage);
2) When the test conditions are lower than the lower limit of the allowable error during the interruption process, the required test environment should be reached again, and the test time outside the error range should be eliminated until the specified test time is completed;
3) If a test situation occurs, it is recommended to stop the test and re-test with a new sample. If it is judged by the relevant technical personnel that exceeding the required test conditions will not directly cause damage to the characteristics of the test sample, or the sample If the product is a repairable product, it can be processed according to Article 2. If the sample fails in subsequent tests, the test results should be considered invalid.

2. Alternating heat and humidity (humidity resistance test) test method
1) Equipment level damp heat test
When the test is interrupted due to special circumstances such as a sudden power outage during the test, it is recommended to operate in the following manner:
① If the environmental conditions in the box do not exceed the allowable error range during the interruption, the interruption time should be regarded as part of the total test time;
② When the environmental conditions in the box are lower than the lower limit of the allowable error during the interruption, the test should be restarted from the end point of the last valid cycle before the interruption (that is, the cycle where the interruption point is located is invalid);
③ If the test has occurred, it is recommended to stop the test and re-test with a new sample. If it is judged by the relevant technical personnel that exceeding the required test conditions will not directly cause damage to the characteristics of the test sample, or the sample is For repairable products, the environment in the box can be restored to the required environmental conditions and the test can be continued. If the sample fails in subsequent tests, the test results should be considered invalid.
2).Device level damp heat test
When the test is interrupted due to special circumstances such as a sudden power outage during the test, before completing the specified number of cycles (excluding the last cycle), if no more than one unexpected mid-test occurs, the cycle can be redone. If an unexpected test break occurs during the last cycle, an uninterrupted cycle will be required in addition to redoing the cycle. Any interruption of more than 24 hours requires redoing the test from start to finish.

7. Determination of effective working space for damp heat test
Damp heat test, including constant damp heat test, alternating damp heat test, and temperature/humidity combined cycle test.
GB/T 2423.3 constant heat and humidity test specifies a temperature tolerance of ±2°C.
The temperature tolerance specified in the four temperature levels of GB/T2423.9Cb constant heat and humidity test is ±2°C and the relative humidity tolerance is ±3%.
At the upper limit temperature specified in GB/T 2423.4 alternating heat and humidity test: the temperature tolerance is ±2% and the relative humidity tolerance is ±3%; at the lower limit temperature, the temperature tolerance is ±3°C; the relative humidity requirement is 95%.
At the upper limit temperature of the moisture exposure cycle in the temperature/humidity combined cycle test of GB/T 2423.34ZD, the temperature tolerance is ±2°C and the relative humidity tolerance is ±3%. Relative humidity is a parameter related to temperature. Different temperatures in the box will lead to different relative humidity. The difference in relative humidity is also related to its humidification method, wind speed, control accuracy, etc. Humidification methods and air circulation rates are generally fixed, and control accuracy can only be guaranteed through good maintenance, care and correct operating procedures. Its effective working space is generally smaller than that of high temperature testing, because only small temperature differences and small temperature fluctuations can ensure that the relative humidity difference remains at a small value.
GB/T 2423.3 points out: In order to keep the relative humidity tolerance specified in this standard within the required range, the temperature difference between any two points in the working space should not be greater than 1°C at any instant, and short-term temperature fluctuations must also be maintained within within a smaller scope. Determining the effective space for various heat and humidity tests must also be judged by measuring relative humidity. This is to ensure that the tested sample always remains within the specified tolerance range when conducting various heat and humidity tests.

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