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HASS (Highly Accelerated Stress Screen), also known as highly accelerated stress screening, is performed on the production line after the operating and destruct limits have been determined through HALT testing. Generally, 100% of products are required to participate in the screening. Its purpose is to ensure that manufactured products contain no latent defects, or at least to find and resolve these defects before the products leave the factory. HASS uses accelerated stress to find defective products within a short time, shorten the corrective action cycle, and identify other products with the same problems.
HASS includes the following:
* Pre-screening to remove latent defects that could develop into patent defects;
* Detection screening to find patent defects;
* Failure analysis;
* Corrective actions.
HASS testing includes three main phases:
HASS Development (HASS development phase)
Proof-of-Screen (screen verification phase)
Production HASS (HASS execution phase).
(1) HASS Development
The HASS profile must be based on the results obtained from the preceding HALT. Generally, the upper and lower operating limits in the combined temperature and vibration stress are reduced by 20%, while the vibration level is initially set at 50% of the destruct limit G value. Combined temperature and vibration stress testing is then performed under these conditions, and the specimen is observed for failures. If a failure occurs, it must first be determined whether it was caused by excessive environmental stress or by the quality of the specimen itself. In the former case, the temperature and vibration stress should be relaxed by 10% and the test repeated; in the latter case, the current test conditions are effective. If no failure occurs, the environmental stress should be tightened by 10% and the test repeated.
(2) Proof-of-Screen
When establishing the HASS profile, two principles must be observed: first, it must be able to detect latent defects that could cause equipment failure; second, the test must not damage the equipment or cause “internal injury”. To ensure that the results obtained in the HASS development phase meet these two principles, three test units must be prepared, and defects not manufactured or assembled according to standard workmanship, such as lifted components, cold solder joints and improper assembly, must be introduced into each unit. Each unit is then tested under the conditions obtained in the HASS development phase, and it is observed whether the artificial defects can be detected, in order to decide whether to tighten or relax the test conditions so that the HASS profile achieves the expected effect.
After the effectiveness verification is completed, new test units should be tested 30 to 50 times under the adjusted conditions. If no damage due to improper stress occurs, the HASS profile is considered to have passed the screen verification phase and can be used for Production HASS. Otherwise, the test conditions must be reviewed and adjusted to obtain the optimum combination.
(3) Production HASS
Any HASS profile that has passed the Proof-of-Screen is regarded as a fast and effective quality screening tool, but it still needs to be adjusted appropriately according to abnormalities fed back from products in customer use. In addition, when design changes occur, the test conditions need to be modified accordingly.
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