JEDEC JESD 22-A118
Accelerated Moisture Resistance - Unbiased HAST
|Publication Date:||1 March 2011|
This test method applies primarily to moisture resistance evaluations and robustness testing, and may be used as an alternative to unbiased autoclave. Samples are subjected to a non-condensing, humid atmosphere, similar to the JESD-22-A101, "Steady State Temperature, Humidity and Bias Life Test", but with a higher temperature. For the temperature limits defined by this procedure, the test will typically generate the same failure mechanisms as those in an unbiased "85 °C/85% RH" Steady State Humidity Life Test, but caution must be used if higher temperatures are considered since non-realistic failure modes can be generated. The use of a non-condensing environment avoids many irrelevant external failures, e.g., pin-to-pin leakage or lead corrosion. However, because absorbed moisture typically decreases glass transition temperature for most polymeric materials, the combination of high humidity and high temperature (> Tg) may produce unrealistic material failures. Thus, caution is needed if and when Unbiased HAST is required for reliability or qualification purposes.
The Unbiased HAST is performed for the purpose of evaluating the reliability of non-hermetic packaged solid-state devices in humid environments. It is a highly accelerated test which employs temperature and humidity under non-condensing conditions to accelerate the penetration of moisture through the external protective material (encapsulant or seal) or along the interface between the external protective material and the metallic conductors which pass through it. Bias is not applied in this test to ensure the failure mechanisms potentially overshadowed by bias can be uncovered (e.g., galvanic corrosion). This test is is used to identify failure mechanisms internal to the package and is destructive.