Need an immediate increase in reliability? Is your product robust but you want complete confidence that everything has been done to minimize your field failures? Environmental Stress Screening (ESS) is the path for you. ESS is a series of intensified tests to identify potential manufacturing flaws. By utilizing temperature and vibration to accelerate failure at weak points, you can weed out short-lived units and ship only your highest quality product to your customers.
Over the last few decades, a shift in industrial trends has led to an ever-increasing global competition. Consumers today demand quality products, while manufacturers strive for the perfect balance of quality and cost. Cheaper products, though conveniently available, do not have a durable life. In addition, consumers today are far more conscious of quality and reliability than they were in the past. This trend has increased the desire for quality products that are reliable and can withstand the test of time and consumer expectations.
Today, top electronics manufacturers are achieving significant gains in reliability through Environmental Stress Screening (ESS). Using tools such as vibration, thermal cycling, and thermal shock, manufacturers now can run the final product through an accelerated profile to improve their confidence that a product will endure its intended life in the field.
Thermal shock and thermal cycling are used successfully to accelerate failures in products from many markets, including aerospace, automotive, military, and electronics, and are generally used in cases where the products encounter large temperature extremes. An example might be cable connectors in an automobile, space vehicle, or ruggedized computer. The temperature extremes will expand and contract elements of the connectors, making them fail completely, loosen and not make contact, or simply fail due to low quality materials.
The thermal shock chambers are generally dual zone chambers which have an elevator that transfers the samples being tested from the hot zone, at the top of the chamber, to the cold zone, at the bottom of the chamber. The cold zone is cooled with liquid nitrogen and the hot zone uses resistance heating elements to achieve the required temperatures. Temperatures typically range between -70 C and 180 C. The elevator within the chamber transfers the test samples in five seconds or less. A picture of a typical thermal shock chamber appears in Figure 1. Typical volume of the chamber is 1.8 cubic feet.
Thermal cycling chambers have a larger volume and are single zone. The temperature transition is achieved by turning off the heating elements and injecting liquid nitrogen. The transition is slightly slower than the thermal shock chamber, at roughly 60 C per minute. Temperature ranges are typically between -70 C and 150 C. A photo of a thermal cycling chamber appears in Figure 2. Typical volume of these chambers is 9 cubic feet.
Reliability engineers are able to use mathematical techniques, which are now available in user friendly software packages, to predict the actual life of a product in the real world, based on observations made during testing.
The ESS process identifies unanticipated flaws in design and discovers issues related to daily variations in the manufacturing process. Different environmental simulations bring to light specific failure modes in the product. Here are some common techniques and their associated failures when used with electronic packages:
Vibration is another useful tool on ESS. It has been used to identify resonances in a product that can cause the product to self-destruct. It can also be used to accelerate metal fatigue failures in products. Fatigue testing has traditionally been performed using servohydraulic systems, which can be expensive and take considerable time to perform. Certain products lend themselves to fatigue tests using electrodynamic vibration systems. The testing time can be reduced because high frequencies can be used and acceleration levels can be increased to shorten test times. Vibration data can be collected in real word situations and replayed through the vibration table. This can reduce the expense of field trials and predict the design and manufacturing defects on the unit before the unit ships and experiences warranty failures. Transportation testing can be performed on the unit to determine if failures will occur in transport.
Vibration testing is performed on an electrodynamic shaker system. The shaker can reproduce vibrations such as sine vibrations that occur on rotating machinery, or random vibration, such as that which occurs in an automobile driving down bumpy streets. The electrodynamic shaker operates like a giant loudspeaker system with frequencies limited between 5 and 3,000 Hertz (Hz). (Loudspeakers operate between 20 and 20,000 Hz, which is the range of human hearing.) There are numerous versions of vibration systems, but the most common is a system capable of performing vibration tests in three axes, one axis at a time. The vertical axis is done with the shaker in the horizontal position; the other two axes are done by rotating the vibration head 90 degrees and using a slip table. The photograph in Figure 3 shows the vibration table in the vertical position in the background. The slip table is in the foreground. It is a metal plate on a granite table which has lubricating oil continuously pumped between the plate and the granite to minimize friction.
Vibration testing is a versatile testing modality. Some specific applications include: ensuring that devices that are mounted to an engine will survive the engine vibration; ensuring that parking lot lamps will survive the vibration caused by the wind; ensuring that printed circuit boards in medical equipment reach their destination without cracking due to resonances encountered in shipping; and ensuring that components on a printed circuit board do not encounter metal fatigue and fail prematurely.
Highly Accelerated Life Testing (HALT) is another widely employed test. It is a technique that combines all of the above modalities, has been in use for the last 30 years, and has gained acceptance as a reliability improvement tool. It is used to expose design defects and constraints in a product by accelerating stress levels. HALT primarily uses a combination of thermal and vibratory step stresses to expose any latent weaknesses in a product. These primary environmental stresses may further be supplemented with additional stresses such as voltage and frequency. HALT stresses a product well beyond its design specifications, up to the destructive levels of the product or the fundamental limit of technology, and is a tool used to optimize product quality and reliability.
HALT testing does not have a specification, but instead uses guidelines. It is a five step process that starts with a cold step stress test, goes on to a hot step stress, followed by temperature cycling, then on to vibration, and finally a combination of temperature cycling and vibration. The test starts at room temperature and the temperature is lowered in 10 C increments until the product fails. Once the product fails, the temperature is raised in increments until it recovers. This establishes a lower operating limit for the product. Then the temperature is successively lowered and raised to find the temperature at which the product fails. This establishes the lower destructive limit. Cooled with liquid nitrogen, temperatures in the chamber can reach -100 C. The next step is to do the same with high temperature. The high temperature can reach up to 200 C. Then the temperature is cycled between the operating limits. The fourth step is a vibration step stress test in which the acceleration level is raised in 10G increments up to 60G. The vibration profile is pseudo-random in six degrees of freedom and is created using pneumatic hammers under the test table. Again the operating and destructive limits are established. Finally, all of the test modalities are combined.
In HALT testing the product usually, but not always, fails at each stage. It is up to the engineer to decide if changes should be made to correct the cause of the failure, depending on how the product will be used. The use of HALT testing can significantly reduce the random failure rate of a product during its useful life.
Another test that is an adjunct to HALT testing is Highly Accelerated Stress Screening (HASS) testing. Once the operating limits have been established in the HALT test, the HASS test can be used to screen products in which warranty returns increase after changes in components or production processes are made during the life of the product.
Testing can be performed on bare boards, populated subassemblies or full products, allowing focus on predetermined trouble areas. Depending on the level of reliability required, the number of test units can range from 100% to a few units from each vendor, manufacturing line or batch. Active monitoring can be used to track the shift in electrical properties as the circuits encounter the extremes of each test. If a measurement is found to be out of tolerance, a root cause analysis is conducted to determine the source of the problem and corrective action established.
The end product of all testing is a clearer understanding of how products will react in defined ranges of conditions. Once satisfactory results are obtained, manufacturers can confidently package these products and ship them to their customers.
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