Environmental Stress Screening: TEMAK Temperature and Temperature-Humidity-Vibration Test Chambers

Constant Temperature, Temperature Cycling, and Vibration
Environmental Stress Screening (ESS) consists of three primary conditions: constant temperature, temperature cycling, and vibration. Since humidity's acceleration effect is relatively low, it is generally not included as a stress factor in most ESS processes. This article on the Application of Environmental Stress Screening Chamber discusses the failure modes or impact types of products caused by temperature cycling.

Constant Temperature Screening

Among the different ESS methods, constant temperature screening is the most widely used. To enhance screening efficiency, increasing the testing temperature can help reduce the overall testing time.

Environmental stress screening can be measured by screening degree. For constant-temperature environmental stress screening, the screening degree formula is as follows:

Where:

SS = Screening factor

e = The base of the natural logarithm

ΔT = Temperature difference between the test temperature and room temperature (room temperature is taken as 25°C)

t = Duration of the constant temperature screening (in hours)

Example: A manufacturer producing desktop computers plans to conduct constant-temperature screening on its products to eliminate early failures. Assuming the desktop computers have an upper operating temperature limit of 35°C and an aging time of 24 hours, the screening efficiency for this constant-temperature test is calculated as follows:

The factory's production volume is substantial, making standard temperature test chambers inadequate. The equipment must accommodate a large number of products simultaneously for testing, so walk-in temperature test chambers are predominantly used for screening tests in the factory.

A walk-in temperature test chamber refers to a chamber that allows personnel to enter directly. Such chambers feature larger dimensions, enabling the simultaneous testing of multiple products. A walk-in temperature test chamber refers to a chamber that allows personnel to enter directly. Such chambers feature larger dimensions, enabling the simultaneous testing of multiple products. The application of walk-in temperature test chamber in intelligent robot testing details the parameters and practical case studies of walk-in temperature and humidity test chambers.

If the budget is limited, existing factory buildings can be retrofitted by adding heating equipment, which is generally sufficient if the screening temperature is not too high. Many companies now utilize retrofitted existing facilities for constant-temperature screening. Compared to walk-in temperature test chambers, these offer greater space and can accommodate constant-temperature screening tests for extremely high production volumes.

Due to their large size, walk-in temperature test chambers cannot be transported after assembly. Therefore, all components are now manufactured at the supplier's facility, with final assembly taking place at the customer's site.

Since the equipment is assembled, relocating it later would cause most components to fail. Combined with the fact that the cost of disassembly and reassembly is nearly equivalent to purchasing a brand-new unit, it is essential to carefully consider the placement of walk-in temperature test chambers during procurement to avoid the need for future relocation.

Walk-in temperature test chambers are assembled using a modular construction method, which employs two primary techniques: adhesive bonding and welding.

Due to the inherent properties of adhesives, high temperatures cause them to soften, leading to joint cracking and abnormal temperature control. Generally, walk-in temperature test chambers bonded with adhesives have a maximum operating temperature of approximately 85℃.

If the screening temperature exceeds 85°C, a welded walk-in temperature test chamber must be selected. This is because welded structures exhibit relatively high thermal deformation temperatures. Opting for a welded temperature test chamber can generally meet all constant-temperature screening test requirements. However, welded assemblies come at a relatively higher cost.

Temperature Cycling Screening

The screening efficiency of constant-temperature screening is illustrated in the example above, where we observe relatively low screening efficiency. To enhance screening efficiency, the screening temperature must be increased. However, the screening temperature is constrained by the product's operating temperature range and cannot be raised indefinitely.

To enhance screening efficiency, temperature cycling was later introduced, significantly improving screening performance compared to constant-temperature screening. Temperature and humidity test types explain the effects of high temperature, low temperature, and thermal shock on products.

Below is the temperature cycling screening formula:

Where:

SS = Screening factor

e = The base of the natural logarithm

△T = The temperature range (difference between high and low temperatures)

V = Temperature change rate (°C/min)

N = Number of cycles

Example: A company employs high-low temperature cycling screening tests on its network switches to identify and eliminate products prone to early failure. The low temperature is -5°C, the high temperature is 55°C, the temperature change rate is 15°C/min, and each temperature extreme is maintained for 60 minutes. Five complete cycles are performed. The screening efficiency for this temperature cycling test is calculated as follows:

The critical factor in temperature cycling screening is the rate of temperature change, but this approach also has its drawbacks. Specifically, the rate of temperature change can significantly impact assembled products.

Since synchronizing internal temperature changes within the product cannot be achieved by altering the ambient temperature, thermal cycling screening is typically applied to printed circuit boards prior to assembly systems. Without enclosures, high airflow enables the completion of thermal cycling screening.

Achieving high temperature change rates necessitates specific PCB placement requirements: all PCBs must be arranged parallel without overlapping. This ensures each board surface receives sufficient airflow for effective temperature cycling. Correspondingly, fan airflow direction must align parallel to the PCBs. Additionally, a fan array is required to deliver adequate airflow to different positions, thereby guaranteeing the desired temperature change rate.

Horizontal airflow in existing temperature test chambers is optimal for thermal cycling and aging screening of factory electronic products. Its key feature involves simultaneous airflow and exhaust from both sides of the chamber, delivering sufficient air volume to horizontally or vertically mounted PCBs to achieve high temperature change rates.

If the test involves high temperature change rates, condensation may occur during temperature transitions. It is recommended to equip the device with a dry air purge system to control humidity within the chamber. This prevents condensation during temperature cycling tests, which could otherwise cause short circuits in electronic equipment.

Humidity Screening

Due to the low acceleration rate of humidity and the difficulty in achieving high humidity levels in large walk-in temperature and humidity test chambers, virtually no companies in the industry opt for humidity stress testing for screening purposes. Similarly, we do not recommend using humidity stress testing for screening here.

Random Vibration Screening

Typically, temperature and vibration account for approximately 80% and 20% of failures, respectively. The constant temperature and temperature cycling screening mentioned above can cover most failures, but they cannot cover all. Consequently, random vibration screening has emerged. The random vibration screening model is as follows:

Where:

SS = Screening factor

e = The base of the natural logarithm

G = Total random vibration energy (Grms)

t = Test duration (min)

Example: A company employs random vibration screening tests on its tablet computers to identify and eliminate products prone to early failure. The random vibration test operates within a frequency range of 5 to 500 Hz, with a vibration energy of 1.2 Grms and a screening duration of 30 minutes. The screening efficiency for this random test is calculated as follows:

Compared to other methods, random vibration screening has a lower degree of selectivity. Additionally, random vibration testing requires installation on a vibration table, which may cause surface scratches on products and necessitates manual intervention, resulting in lower efficiency. Most importantly, random vibration screening can only test a limited number of samples per cycle, failing to meet the production demands of large-scale manufacturing. A temperature humidity vibration test chamber—a device integrating temperature, humidity, and vibration—can be employed. This equipment satisfies the requirements for temperature/humidity cycling and vibration testing without damaging the samples.