The ESS Solution
Environmental Stress Screening (ESS), is the solution. It typically utilizes thermal cycling with or without vibration to precipitate latent defects, the so-called "early life defects" which appear during the early life stage ofproduct use. The "bathtub curve" illustrates the life cycle of a product with early life and wear out defects.
ESS requires rapid product thermal cycling over a wide temperature range. The objective is to stress components by means of differential expansion rates of the various mounted components, solder connections, and the component mounting assembly itself to precipitate the early life defects. Vibration is likewise utilized to further fatigue the product. Determination of the proper levels are typically determined by increasing environmental stress in step-by-step level. It is an empirical procedure which is different for each product configuration.
HALT/HASS are two applications of the ESS concept.
The HALT test is utilized during product development. It determine initial weakness in the product by increasing stress level in steps to the component destruct levels. Then it replaces weak components with more rugged ones, thereby increasing product reliability.
Resulting estimates of improved product life and warranty can be also be accomplished. For example, by applying HALT to a UUT (Pre Halt), we find destruct levels for a given stress-vs-time. By analizing these failures and then making corrections, we are then able to rechieve a more robust UUT (Post Halt). By relating Pre & Post Halt results found in actual use (Field Stress), an estimate of the improved life and subsequent warranty extension are possible.
Example of Warranty Improvement Using HALT
The HASS process is a production screen to assure that weak components are precipitated (failed). HALT experience and the step stress empirical evaluation is used to establish a good screen. It is important that the level of stress is tailored to the product. There is no one-stress-fits-all. Furthermore, the product must be functional tested while undergoing stress to detect failures and intermittents.
100% production screening is initially recommended. Subsequently, lot screen sampling may be possible. The HASS screen should be evaluated over time to assure it is doing its job.