What for thermal shock test chamber ?

Reliability environmental testing in addition to high temperature, low temperature, high temperature and high humidity, temperature and humidity combination of cycling, thermal shock test chamber is commonly used in the implementation of the temperature shock (thermal shock) of this common test items. Temperature shock test (Thermal Shock Testing, Temperature Shock Testing, referred to as: TST), the purpose of the temperature shock test is to exceed the natural environment through the drastic temperature changes [temperature change rate of more than 20 ℃ / min or even up to 30 ~ 40 ℃ / min], with the help of the thermal shock test chamber to quickly switch the Temperature zone function, to find out the product design and process defects, but there will often be temperature cycle and temperature shock confused together.

“Temperature cycling” refers to the process of high and low temperature changes, the rate of change of temperature can be specified and can be controlled; and “temperature shock” (thermal shock) of the rate of change of temperature is not specified (Ramp Time), the main requirements of the recovery time, according to the IEC specification of the temperature cycle of the test method there are three kinds of [Na, Nb, Nc]. Among them, the thermal shock test chamber is mainly used for the implementation of Na-type rapid temperature change test, the type of conversion time is short, the stress is large.

Thermal shock is Na test items defined in the rapid temperature conversion test (conversion medium for the air), temperature shock (cold and hot shock) of the main parameters are: high temperature and low temperature conditions, residence time, reversion time, the number of cycles, in recent years, the relevant test specifications are also further clarified, the use of thermal shock test chamber should be based on the surface temperature of the test article, rather than the air temperature in the test area of the test chamber.

Thermal shock test chamber is used to test the material structure or composite materials, in an instant by the very high and very low temperatures in a continuous environment, the degree of tolerance, so as to test in the shortest possible period of time due to thermal expansion and contraction of the chemical changes caused by the physical damage, the applicable object including metal, plastic, rubber, electronics .... Applicable objects include metal, plastic, rubber, electronic and other materials, which can be used as the basis or reference for the improvement of their products.

The following product defects can be identified during the thermal shock (temperature shock) testing process:

Peeling of connections due to different coefficients of expansion;

Moisture ingress after cracking due to different coefficients of expansion;

Moisture ingress leading to corrosion and short circuits.

According to the International Electrotechnical Commission (IEC), the following conditions are common temperature variations:

1. When the equipment is transferred from a warm indoor environment to a cold outdoor environment, or vice versa;

2. When the equipment is exposed to rain or soaked in cold water and suddenly cools down;

3. In on-board equipment installed outside (e.g., automobiles, 5G, outdoor surveillance systems, solar energy);

4. Under certain transportation [vehicle, ship, air] and storage conditions [unconditioned warehouses].

In order to meet the above challenges, the thermal shock test chamber is divided into “two box” and “three box” two kinds of structure.

Temperature shock is common [high temperature → low temperature, low temperature → high temperature] way, such a way is also known as [two box impact];

Another three-box thermal shock test chamber in the high temperature and low temperature between the increase in ambient temperature buffer, the process is [high temperature → ambient temperature → low temperature, low temperature → ambient temperature → high temperature], in the high temperature low temperature inserted in the middle of the action [ambient temperature], between the two extremes of temperature to increase the buffer. In the specification and test conditions, if there is ever a room temperature condition, usually its high and low temperatures will be extremely high and low, in the military specification and vehicle regulations will see that there is a temperature shock condition of experiencing room temperature.

IEC Temperature Shock Test Conditions:

High Temperature: 30, 40, 55, 70, 85, 100, 125, 155°C;

Low Temperature: 5, -5, -10, -25, -40, -55, -65°C;

Residence Time: 10min, 30min, 1hr, 2hr, 3hr (3hr if not specified).

When tested in a thermal shock test chamber, these temperature settings can be quickly realized to simulate actual harsh environmental conditions.

Dwell Time for Temperature Shock:

The dwell time for temperature shock (Dwell Time) is not only required by the specification, but also based on the weight of the test article and the surface temperature of the test article in two ways. In the operation of the thermal shock test chamber, these two control logic can realize automatic switching to ensure that the return temperature meets the standard.

Thermal shock residence time based on the weight of the specifications are:

GJB360A-96-107, MIL-202F-107, EIAJ ED4701/100, JASO-D001.... etc. When setting up the test program through the thermal shock test chamber, the residence time logic can be preset according to the specification to improve the test efficiency and consistency.

Thermal shock dwell time according to the table temperature control specifications: MIL-STD-883K, MIL-STD-202H (air above the test article). These standards place higher demands on the response of the sensing system of the thermal shock test chamber to ensure that the equipment can read the actual temperature of the test article or simulate the equivalent air temperature.

MIL883K-2016 specification requirements for [temperature shock]:

1. After the air temperature reaches the set value, the surface of the surrogate test article needs to be reached within 16 minutes (residence time of not less than 10min). Therefore, the temperature zone switching speed and air duct system of the thermal shock test chamber is particularly critical, directly affecting the validity of the test data.

2. High temperature and low temperature shock are more than the set value, but not more than 10 ℃. In the thermal shock test chamber temperature control system, can be overshooting the limit parameters to ensure that meet the requirements.

Residence time is based on the upwind air control diagram on the surface of the test article.

[Two box Thermal Shock Test Chamber] Thermal Shock Test Curve

Blue: Setting Temperature

Red：Actual Temperature

[ Three box thermal shock test chamber ] contains through the normal temperature shock actual curve

Blue: Set temperature

Red: Actual temperature

IEC Temperature Shock Test Follow-Up

Reason: The IEC temperature test methods are considered as part of a series of tests as much as possible, since some failures may not be apparent immediately after the test method is completed. After the initial shock test is completed using a thermal shock test chamber , the following follow-up verification is often required.

Subsequent tests to be performed:

IEC60068-2-17 Sealability Test

IEC60068-2-6 Sine Vibration

IEC60068-2-78 Steady State Damp Heat

IEC60068-2-30 Damp Heat Temperature Cycle

Prior to these tests, Environmental Stress Screening (ESS) using a thermal shock tect chamber helps to quickly expose potential failure points.

Tin whisker (whisker) temperature shock test conditions are organized as follows:

1. -55(+0/-10)°C ←→85(+10/-0)°C, 20min/1cycle (500 cycles check once);

 1000 cycles, 1500 cycles, 2000 cycles, 3000 cycles;

2. 85(±5)℃←→-40(+5/-15)℃, 20min/1cycle, 500 cycles;

3. -35±5℃←→125±5℃, 7min residence, 500±4 cycles;

4.-55(+0/-10)℃←→80(+10/-0)℃, stay 7min, 20min/1cycle, 1000 cycles.

These temperature shock cycles are mostly accomplished by using a thermal shock test chamber to ensure that the transfer time is less than 1 minute, to avoid incomplete stress relief effects.

Frequently Asked Questions about Temperature Shock Test:

1. Definition of Temperature Shock Test and Temperature Cycling Test

Temperature Shock Test: Simulate the drastic temperature transient encountered in the manufacture, transportation or use of the product (such as from a high temperature environment to a low-temperature environment all of a sudden), the use of thermal shock test chamber can quickly complete this process of alternating high and low temperatures. Its core feature is that the rate of temperature change is extremely fast (≥ 30℃/min), the conversion time is usually ≤ 1 minute, through the thermal shock test chamber (high temperature tank and low temperature tank) to achieve rapid conversion. Mainly assess the failure caused by creep and fatigue damage.

Temperature cycling test: simulate the adaptability of the product in the relatively slow alternating temperature changes in the environment. Temperature change rate is slow (usually ≤ 20 ℃ / min), generally do not use the thermal shock test chamber, but the use of temperature change rate can be controlled by the high and low temperature test chamber. The main assessment of failure caused by shear fatigue (stress caused by the difference in the coefficient of thermal expansion of the material).

2. How to distinguish temperature shock from temperature cycling from test conditions?

Comparison of key parameters:

Thermal shock test chambers are particularly suitable for high stress screening due to their high conversion efficiency and stable control accuracy.

3. Are the results of temperature shock and temperature cycling the same?

No, because they have different failure mechanisms:

Temperature Shock: Rapid thermal stresses realized by the thermal shock test chamber lead to creep fatigue within the material (e.g. cracking of welded joints, peeling of coatings).

Temperature cycling: Slow temperature changes cause shear fatigue (e.g. cumulative damage at the interface of different materials due to differences in thermal expansion).

Note: The same product may behave differently in the two tests,

e.g. :The temperature shock test is more likely to expose reflow defects, and the temperature cycling test is more likely to expose long-term thermal fatigue problems.

4. What is the effect of temperature difference between temperature shock and temperature cycling?

The larger the temperature difference, the more significant the failure. Thermal shock test chamber is usually set temperature difference ≥ 100 ℃, such as -40 ℃ to +125 ℃, simulating harsh scenes.

5. Does the shock temperature of temperature shock need to be overshoot (OVER) or below the set value?

Not required.

Standard requirements for temperature recovery time ≤ 5 minutes (air outlet) or ≤ 15 minutes (sample surface), but prohibit the temperature overshoot: thermal shock test chamber needs to be precise temperature control, the temperature should be quickly stabilized within the set range, to avoid overshooting to produce a non-real failure phenomenon.

6. Should the temperature shock start with high or low temperature?

It is recommended to start at low temperature and end at high temperature. Thermal shock test chamber program can be set according to this logic, starting from low temperature and ending at high temperature to reduce the risk of condensation.

7. Why do some temperature shock tests require “passing through ambient temperature”?

I. What is “pass through ambient”?

Temperature shock testing typically involves the rapid transfer of a product from one temperature extreme (e.g., high temperature) to another (e.g., low temperature) to evaluate its ability to withstand drastic temperature changes.

“Passing through ambient temperature” using a three-chamber thermal shock test chamber means that the product passes through the ambient temperature zone (usually referred to as 20~25℃) and then enters the target temperature zone when switching between high and low temperatures.

II.Why pass through the room temperature?

① Simulate the real use of the environment

Three box thermal shock test chamber simulation environment more closely match the actual situation. In many actual working conditions, the product from high temperature to low temperature or vice versa, often go through a “normal temperature transition” process.

For example, electronic products may be transported or power on from the ambient environment into the extreme temperature zone, “through the ambient” can be closer to the actual scene.

② Avoid thermal shock damage to the product structure

Certain materials (such as ceramics, glass, solder joints) are susceptible to thermal expansion and contraction stress concentrations under severe temperature differences, resulting in cracking, deformation or failure.

These products will need to use the thermal shock test chamber for normal temperature transition, slow down the stress concentration, reduce the risk of damage.

③ Improve the repeatability and safety of the thermal shock test

Two-box thermal shock test chamber directly from high temperature jump to low temperature (or vice versa) shock too quickly, which is likely to lead to uncontrollable risks, such as damage to the equipment, condensate generation and so on.

Three-box thermal shock test chamber “through the room temperature” to make the temperature change more controlled, easy to standardize the management.

8. Temperature shock cycle number definition and decision basis

Temperature shock test in the “cycle number” refers to the product in the high temperature and low temperature between the completion of a complete round-trip cycle times, that is, low temperature → high temperature → low temperature for 1 cycle (or high temperature → low temperature → high temperature for 1 cycle). Thermal shock test chamber with the cycle number of accurate control function, widely used in various types of standard certification. This parameter is a very critical indicator in the temperature shock test, because it is directly related to the degree of verification of the product resistance to thermal fatigue and durability.

9. How can the number of temperature shock cycles be reduced?

This can be achieved by increasing the stress intensity of a single cycle. Thermal shock test chambers can accelerate the rate of temperature rise and fall to increase the stress intensity by means of liquid nitrogen or high-efficiency ducting:

Extend the temperature range (e.g., from ΔT=80°C to 150°C);

Extend the extreme temperature hold time (e.g., from 15 to 30 minutes);

Reduce the changeover time (e.g., from 1 minute to 30 seconds);

Increase the rate of temperature change (with the support of equipment such as Increasing the rate of temperature change (requires equipment support, e.g. liquid nitrogen cooling).

Note: It is important to ensure that the accelerated stresses do not cause irrelevant failure mechanisms (e.g. exceeding material limits).

10. Can ovens and freezers be used as an alternative to thermal shock chambers?

No. The changeover time and temperature uniformity are not acceptable. Thermal shock test chambers have tight control capabilities and comply with standards such as IEC and others.