Environmental test chambers
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Environmental Test Chambers are pieces of equipment that perform scientific testing of a product under a variety of stresses and environmental conditions. These devices are able to simulate environments with extreme temperatures, humidity levels, altitude, radiation, wind, bacteria, dust and chemical exposure. Manufacturers from a variety of industries perform environmental testing including those in the automotive, aerospace, electronic and telecommunications industries.
Environmental testing is performed for a variety of reasons. From reducing product development time to reducing warranty costs, environmental testing is critical. For many manufacturers there is a concern about legal liability should user injury occur. Environmental testing allows the manufacturer to encounter and repair any potential issues before injuries occur. Another reason manufacturers participate in environmental testing is to make certain that their product will perform successfully during use. At some point in its life, almost all products are subject to the stresses of vibration and temperature changes and a well-design product needs to be able to operate appropriately when subjected to these stresses. However, the benefits of environmental testing are not limited to evaluating a products function and performance, but also to protect a company’s reputation. Environmental testing gives the manufacturer quality assurance that their product is ready to be put on the market. It also allows them to see how their product will withstand the storage and shipping conditions that it will encounter. Environmental testing allows manufacturers to ensure their product will be reliable under almost any condition it may be exposed to over its lifetime.
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[edit] Selecting an Environmental Test Chamber
Choosing an environmental test chamber for a test application requires great care and research. With many types and styles of environmental test chambers it is important to be able to define exactly what features and benefits are required for a successful project. Chamber manufactures and trade magazines offer great insight into how to choose the right environmental test chamber for the job. Evaluation Engineering magazine offers a good article on how to select the right chamber [1].
One of the most important considerations when choosing test equipment is the physical and thermal definition of the unit under test (UUT). Environmental test chamber manufacturers need this description in order to determine if the test requirements require a standard environmental test chamber or if custom designed equipment is required. This description includes the weight, dimensions, quantity, orientation, average specific heat, surface area and live load of the UUT during the testing procedure. The mounting of the UUT should also be defined.
Weight is necessary in order to determine the size of the equipment and to ensure the UUT and chamber temperature and time response are appropriate for test procedure. With the dimensions, quantity and orientation, an environmental test chamber designer can optimize the workspace. Specific heat of the material determines the amount of thermal energy that must be removed or added to during temperature transitions. Finally the live load of UUT provides the amount of steady-state power consumed and dissipated if the product is powered during the test.
Fixturing is a form of product mounting that is used in instances where the UUT requires power, bias, switching or monitoring during the test. Often, custom fixturing is needed when the product is connected to remote power supplies, monitors or other types of operations or diagnostic equipment. A fixture will change the properties of the UUT, therefore it is important to consider the weight, type of material and any possible blockages (such as air) when choosing an environmental test chamber. If no connections to a product are required, the product mounting can be as simple as stacking the UUT on trays or shelves.
Another important consideration is the type of environmental test to be performed. Most testing programs begin by creating a specification that identifies the procedures to be used for the stress-testing program. Usually engineers who are knowledgeable about the product will define the product test procedure and determine how the product will be tested. The test procedure focuses on insuring the functionality of the product and has a main goal of improving the product’s reliability. Sometimes, however, industry standards, such as MIL STD, IEC, JEDEC, Telcordia or SAE, will dictate the test procedures to be used. These organizations provide a benchmark of how a particular product should perform under certain circumstances.
Testing methods can range from as simple as temperature testing or can be extremely complex such as those with combined environmental testing applications. The following is a brief overview of only some of the most popular testing methods being used today.
[edit] Temperature
Temperature change is one of the most common stresses applied to a product. Temperature cycling is the process in which a product is subjected to multiple cycles of changing temperatures between predetermined extremes at relatively high rates of change fatiguing and causing inferior products to fail. It provides an excellent way to determine failure and analyze reliability of a product. Cycling will show at what temperature, both high and low, a product will cease to function properly. Most manufacturers have a standard high temperature range from +177°C to +190°C for reach-in chambers and welded walk-ins; however, it is possible for higher temperatures to be achieved with special construction and materials. To achieve low temperatures, manufacturers offer several refrigeration options. Liquid Nitrogen (LN2) is one popular refrigerant that allows temperatures to be as low as -184°C. Carbon Dioxide (CO2) is another popular refrigerant however it can only achieve a low temperature of -68°C. A final option is mechanical refrigeration. Mechanical refrigeration uses compressors to lower the temperature of the chamber and is available in two configurations: single stage and cascade. Single State uses only one compressor and is able to bring the temperature down to -40°C. Cascade uses two compressors in order to bring the chamber temperature down to -73°C or lower. A cascade refrigeration system consists of two interdependent refrigeration systems. The low-stage provides cooling to the chamber interior through a finned evaporator coil, which is located in the air plenum. The high-stage provides cooling to the cascade condenser. The cascade condenser is a heat exchanger that has one circuit which is the evaporator of the high-stage, and another circuit which is the condenser of the low-stage.
[edit] Humidity
Excess moisture is very damaging to a multitude of products especially electronics causing corrosion and oxidation. Low humidity levels can cause brittleness as well as interact with certain to cause high electrostatic discharge (ESD) conditions. Humidity testing allows a manufacturer to see how their product will perform in these conditions. Most manufacturers offer environmental test chambers ability to achieve 10% to 98% RH although, similarly to temperature, humidity levels can also be customized. Controlled humidity is usually limited to a maximum temperature of +85°C. The minimum humidity range is limited by a dewpoint condition of typically 5° to 8°C using refrigeration for dehumidification. For lower dewpont conditions, either Gaseous Nitrogen (GN2) or Dry Air (usually generated by a heatless desiccant air drier) is required.
[edit] Thermal Shock
Rapid temperature changes from extreme cold to hot environments thermally shocks and stresses a product. This causes permanent changes in electrical performance and can cause sudden overloading of materials. Thermal shock can be liquid-to-liquid, a product is immersed and transferred between hot and hold baths, or air-to-air, where the product is moved from a hot chamber to cold chamber. Thermal shock failures are due to thermal mismatches or materials with differences in rates of thermal expansion and contraction.
[edit] Altitude
Testing for altitude allows a manufacturer to simulate the effects of low pressure and high altitude on their product. Most environmental test chambers are able to simulate altitudes of up to 200,000 feet (0.169 Torr) above sea level. Altitude testing is important for manufacturers of products that might be shipped via air and is especially important for the defense and aerospace applications.
[edit] Vibration
At some point in its life a product will come in contact with vibration whether during production, transport or use by the end user. By performing vibration testing, manufacturers can determine if a product can withstand the rigors of its intended use environment. Most manufacturers offer shakers that are able to perform Sine, Random, and Shock vibration tests. Often, vibration is combined with another test method during testing.
[edit] Highly Accelerated Life Testing (HALT)
HALT uses step-by-step cycling of environmental variables such as temperature, shock and vibration in order to force products to fail. It performs vibration testing in all three axes using a random mode of frequencies. HALT testing can include the simultaneous cycling of multiple environmental variables (an example: temperature cycling and vibration testing) providing a closer approximation of real-world operating environments. Used as part of the product design process, HALT subjects a product to increasing stresses to ensure the basic design is reliable.
[edit] Highly Accelerated Stress Screening (HASS)
HASS testing applies stresses similar to those used in HALT and is an on-going screening test, performed on regular production units. However, this type of testing does not wish to damage the product instead it aims to verify that actual production units continue to operate properly. HASS uses environmental variables such as fast temperature cycling, shock and multi-axis vibration as a means of screening 100% of production units. It ensures that the entire final product will be reliable.
Other tests can be:
Cold Thermal Step Stress Test Hot Thermal Step Stress Test Rapid Thermal Cycling Test Combined Environment Test
[edit] Environmental Stress Screening (ESS)
Environmental stress screening is a means of screening products to expose detects (such as defective parts or process problems) that can't be detected by visual inspection or electrical testing. It was developed as a means of detecting product defects and production flaws in electronics. Typically ESS uses thermal cycling or vibration to expose failures. ESS can be performed as part of the manufacturing process, or it can be used in new product qualification testing.
[edit] AGREE
The Department of Defense Advisory Group on Reliability of Electronic Equipment or AGREE method of testing was originally designed to perform specific temperature, humidity and vibration tests in order to improve the quality and reliability of military hardware and to conform with MIL-STD 781 and 883. Today, AGREE test equipment is used to perform reliability, qualification and ESS for many industries including electronics, automotive and telecommunications equipment. AGREE chamber manufacturers usually offer chambers capable of temperatures ranging from -73°C +177°C and humidity levels ranging from 20% to 95% RH.
[edit] Types of Environmental Test Chambers
The environmental testing industry offers many types of test chambers ranging from small benchtop style chambers to chambers as large as a room. The test application and product loading requirements will primarily determine the type of chamber needed.
[edit] Benchtop Chambers
Benchtop chambers are the smallest size of environmental test chamber typically ranging from 0.5 cubic foot to 5.5 cubic feet. Designed to provide a compact chamber for testing small components and products, benchtop chambers are able to simulate a full range of temperature and sometimes humidity conditions. Benchtop chambers are great for testing small products, sub-assemblies and components in laboratories with limited space.
[edit] Upright or Reach-In Chambers
Upright or reach-in chambers are another popular chamber configuration on the market today. These chambers can range from 4 to 70 cubic feet or larger and are able to simulate almost any environment. Temperature, humidity, altitude, rain, solar and vibration are just some of the environments that can be produced by an upright or reach-in chamber.
[edit] Walk-In Chambers
Walk-in chambers are environmentally conditioned rooms ranging in size from those big enough to step into or those large enough to drive into. They can be used to test large components, assemblies and finished products, from computers and copiers to automobiles and satellites throughout a wide range of temperature and humidity environments. Walk-ins can also be used as a laboratory environment for conducting test procedures in food processing, pharmaceutical, and scientific applications. Walk-in chambers are able to be built to almost any size or configuration.
There are two major types of walk-in chambers constructions: modular and welded. A modular or panel walk-in is made from pre-fabricated panels. This type of walk-in is made for fast and simple installation. Modular walk-ins offer great flexibility in designing interior work space and can assembled to meet virtually any test or size requirement. However, due to this construction the temperature and humidity range is limited. Solid or welded walk-ins are made similarly to upright chambers. The chamber is welded together internally which makes this chamber more apt for high temperature testing. This style of walk-in chamber also may withstand altitude conditions. Welded walk-ins are often custom designed to best fit a clients testing requirements.
[edit] Thermal Shock Chambers
Thermal shock chambers have the ability to shock a product by moving it from a high temperature to a low temperature. With two or three separately controlled temperature zones between which products are automatically transferred, they thermally shock and stress a product quickly, exposing its reliability in the process. These chambers are available in different sizes and configurations depending on testing requirements.
Traditionally, temperature shock tests were performed in accordance with military test standards for components such as electronic chips, integrated circuits and printed circuit boards. However, today most commercial reliability and quality control programs require rapid and extreme temperature change testing.
Environmental test chambers and environmental testing allow organizations to design and produce products that are more reliable and safer for their intended end use. It also saves money and time as well as the organization's reputation through improved in customer satisfaction. While there are many things to consider when choosing an environmental test chamber and many different methods of tests that can be performed, the decision to perform testing is critical to the success of a product.
[edit] Bibliography
'The 10 Key Elements of Environmental Testing' by Patrick E. Collins. Chip Scale Review. September/October 2003.
'Why Test?' by Tom Lecklider. Evaluation Engineering. October 2001.
'Selecting an Environmental Test Chamber' by Eric Berchtold. Evaluation Engineering. October 2004.
