Conducting Highly Accelerated Life Testing (HALT) in a Qualmark chamber is fairly straightforward; however, there are some unique characteristics of the process that can pose challenges along the way. To ensure the proper execution of the test—and the most accurate results possible—here are a few tips to keep in mind:
It could take months or even years of normal use for the failures of a given product to actually occur. Naturally, if you limit the stresses to the product’s specifications, you can expect the failure modes to take that much time to reveal themselves.
To conduct an effective HALT, you must stress the product beyond its specifications. This will achieve the dramatic fatigue acceleration needed to precipitate the failure modes in a matter of days.
Once the HALT is complete, you will likely face several failure modes. It may be tempting to ignore some of them—after all, you pushed the product past its limits, and you were expecting it to fail—but each warrants an analysis. Ignoring any of the issues that HALT reveals—even if they were anticipated—reduces the value of testing.
Start each analysis by asking, “Do I understand this failure mode enough to say if it could have a distribution that could cause it to show up at lower stress levels?” If you’re certain a specific failure will only ever occur in extreme circumstances that will never be experienced in normal usage, you may be able to ignore it. Otherwise, proceed with a comprehensive failure analysis. This will allow you to implement the proper corrective action before the product is released.
Highly accelerated life testing uses repetitive shock (RS) vibration to induce mechanical stress. Unlike electro-dynamic (ED) shaker systems, the RS table is not rigid. Instead, it flexes during operation. This requires a different approach to fixturing and to product placement on the table.
It’s important to take such differences into account, especially if you’re testing multiple units simultaneously. Configure them in such a way that they are subjected to similar levels of mechanical stress.
Fixturing products for HALT is fairly straightforward, but it’s easy to overlook essential elements that end up influencing the results of the test. In general, you want to clamp the product to rails, which are then fixed tightly to the table. The rails are necessary to ensure adequate airflow. Otherwise, the contact of the product directly on the table could affect thermal change rates. Strategies for fixturing the product before testing depend on its shape and dimensions.
Instrument the product itself when thermal and vibration stresses are applied, especially at points where failures are expected to occur. Monitoring the table has little benefit. By monitoring the product, however, you can gain a clear understanding of precisely how it responds, which is ultimately the key to making improvements.
It’s important to remember that successful HALT isn’t about passing or failing. It is not possible to pass HALT. The output of HALT is a list of failure modes rather than a pass or fail indication that you would get from traditional testing.
Instead of approaching HALT with such a black-and-white mentality, make it your goal to identify each product’s maximum capabilities. If product A experienced thermal failure below X degrees, for example, and product B did not experience any at that same threshold, that doesn’t mean you should stop testing product B. The goal is to find the failure modes, so in such a scenario, you should keep testing in order to identify where product B does experience thermal failure.
“Stimulate, not simulate,” is a common phrase among those who conduct HALT. It’s an easy reminder that the goal of HALT is not to simulate real-world conditions; rather, it’s to stimulate failure modes rapidly. This may mean modifying the product, such as removing the top cover or cutting holes in an enclosure. Even if such modifications don’t mimic the end-use configuration, they can be applied during HALT to improve the effectiveness of the test, as long as they don’t compromise the functionality of the product.
Unlike reliability testing, which is typically performed near the end of the development cycle, HALT is done much earlier. In fact, it should be performed as early as is feasibly possible. The insights revealed during HALT will influence the design and reliability of the final product as well as lend to more successful and efficient design verification testing. To get the most value from the HALT results, the engineers and other personnel working on the product are going to want those insights as soon as possible.
Implementing a thorough functional test routine is critical for ensuring the success of HALT. The goal of HALT is to induce failure, and if some failure modes are not detected, the value of the test will be reduced. Before every test, evaluate the functional blocks of the product and determine how failures in each of them will be detected and reported.
Without a comprehensive understanding of all that HALT entails, those who are involved in the testing process may not take the steps needed to garner accurate and insightful results. With that in mind, education is the foundation of ensuring successful HALT. Train all those who will be taking part in the process, so the tests will be as valuable as possible.
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