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ESS and HALT/HASS

The Problem

The short lifecycle of today’s electronic products creates pressures for rapid development and manufacturer of new products or product upgrades in order to stay ahead in the competitive market place.  Take for instance, the short model life of computers and printers.  A new model appears every few months.  Time-to-market is short, and customers expect the new models to work "out of the box" each and every time.

 These pressures can result in compromised product development and/or manufacturing problems caused by part, process, and workmanship defects.  The result is increased manufacturing costs, warranty costs, impacted profit margins, and worse, customer dissatisfaction with resulting loss of market share.

 

  The ESS Solution

Environmental Stress Screening (ESS), is a  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 of  product use.  The "bathtub curve" illustrates the life cycle of a product with early life and wearout defects.

ESS requires rapid product thermal cycling over a wide temperature range.  The objective is to stress the PCB components by means of differential expansion rates of the various mounted components, solder connections, and the PCB board 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 and different for each product configuration.

 
Note: The high levels of ESS stress are beyond the design margins of the product and can approach the destruct levels if care is not taken. 

  HALT / HASS

HALT/HASS are based on the ESS concept. 

The HALT test is utilized during product development to determine initial weakness in the product by increasing stress level in steps to the component destruct levels and replacing weak components with more rugged ones.  Obviously, engineering economies should govern the degree and level of replacement.  It still is a competitive world.

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. The HASS screen should be evaluated over time to assure it is doing its job.

Warning: Overstress can take unneeded life out of the product and can well cause new latent defects, which were not there to begin with.


      

  


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Sexton has worked and lectured worldwide in this field for some 25+ years, including representing the major HALT/HASS equipment manufactures. 

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