What is accelerated ageing testing, and why consider it?

Accelerated ageing testing uses a combination of accelerated stresses to expose product flaws in the design and manufacturing of a product. This serves to improve product reliability and reduce field failures and warranty expenses. Accelerated ageing tests are conducted in environmental chambers, with elevated temperature accelerating effective time often in combination with all shakers for creating all-axis vibration.

Accelerated age testing can be separated into highly accelerated life testing (HALT) and highly accelerated stress screening (HASS). Both techniques use stresses far beyond the normal operating condition of a product with the goal to identify problems and eliminate them and thus produce a more reliable product. Each category of testing artificially increases stress on the product or component to identify the impact that time, temperature, humidity, corrosion and vibration will have on the product or component.

HALT testing occurs first: it is used at the time of product development or when new suppliers, components or manufacturing processes are introduced. Typically, HALT is not a qualification test; the goal of HALT is to quickly promote failures and then determine their root causes. Once the causes of failure are identified, the failed components are repaired or replaced and the stress limits of the testing expanded. The product development team is searching for the weak link in the product design, the goal being to find the weak link in the product, eliminate it and then move on to the next weak link. HALT testing has, on many occasions, provided substantial (5–1000 times) MTBF gains and enabled the development of far more reliable products.

A typical HALT testing program would progress through the following steps:

• Cold thermal step stress
• Hot thermal step stress
• Rapid thermal shock stress
• Vibration step stress
• Combined thermal and vibration stress
   

Once product design has been completed, HASS testing can further be used to aggressively create stress on the product in order to detect product defects in manufacturing production screens. The accelerated stresses of the HASS program shorten the time to failure of defective units and therefore shorten the corrective action time and the number of units built with similar flaws.

The types of stresses used for both HALT and HASS are very similar, although the test procedures can vary. Both testing categories use rapid temperature cycling, often combined with all-axis vibration testing and electrical loading. Other tests performed may include accelerated corrosion testing, UV testing and humidity testing.

There are a number of mathematical models which can be used to predict the impact of lifetime under varying stress conditions, the most popular use the Arrhenius mathematical model. This mathematical model helps predict the amount of time required for tests to be performed at elevated temperatures in order to compress the amount of testing time necessary to produce a failure in a product or component. Testing at multiple temperatures can provide a quantifiable acceleration factor. The Arrhenius equation’s use is extensive in accelerated ageing tests and looks at the reaction rate of components.

Accelerated ageing can be used with a wide variety of products, from printed circuit boards to power supplies, medical devices, automotive parts, consumer electronics and a wide variety of other devices. The following are the main steps in the HALT/HASS process:

Precipitation creates a defect which was previously undetectable, for instance a poor solder joint. The stresses used may be vibration combined with thermal cycling and perhaps electrical overstress. Precipitation is usually achieved in HALT testing.

Detection involves actually determining that a fault exists. This often can be a challenge and various techniques are used to detect a detectable fault.

Failure analysis determines why the failure occurred. In the case of the solder joint, for example, why did it fail?

Corrective action requires changes to the design or manufacturing processes so that the failure will not occur again. If a manufacturer performs HALT testing and discovers weaknesses and then dismisses them as due to overstress conditions, they need to be sure that the faults would not have eventually occurred in the field at lower stress levels.

Verification of corrective action needs to be performed to determine that the fix to resolve the failure actually works. The fix could be ineffective or there could be other problems causing the anomaly that are not yet fixed or the fix could produce other faults that weren’t present in the initial testing.

Record keeping is the final step in the process, so that techniques learnt to resolve the fault are not forgotten and can be used in future product design.

Simultech distributes the Weiss Technik range of accelerated ageing test chambers, which enable the control of temperature and humidity and can be incorporated into ageing tests for performing failure-rate data and reliability analysis. The company provides sales, service, installation, commissioning, training, rental and basic on-site testing for customers across Australia and New Zealand.

Please follow us and share on Twitter and Facebook. You can also subscribe for FREE to our weekly newsletter and bimonthly magazine.