Step Stress Testing

Step Stress Testing is a combination of traditional reliability testing and over-stress testing. The purpose of Step Stress Testing is to demonstrate one life of a product and then over-stress the product in incremental levels to find failure modes. The advantage of the test for many supplier-purchaser relationships is the ease with which the contracted specification required can be extended to make a step stress test.

The challenge of the test is in setting the amount by which the stress sources (in the example temperature, voltage, and vibration) will be increased. Since each stress source affects different failure modes and each failure mode is accelerated at different rates, setting a good proportional increase for each stress source is important.

A simple method that has some merit is to determine the destruct limit of the product for a given stress source (for example, 160 deg C) and divide the stress levels between the service level (80 deg C) to the destruct level (160 deg C) into ten even steps. This method has two advantages. It is simple to implement and it insures that the product will see an approximately even increase in damage from each stress source, provided that the product was experiencing an even amount of damage from each stress source at service conditions. Since a properly designed and optimized product will accumulate stress damage evenly throughout the product, this is a reasonable assumption.

The alternative is to determine the exponential relationship between increasing each stress and the effect on failure modes and determine a step level that keeps the stress damage accumulated from each stress source in the same proportion as seen during service conditions. This would require an extensive application of accelerated reliability tests for each stress source and the combinations.