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ARCHIVE >  March 2013 Issue >  Special Features: Test and Measurement > 

Highly Accelerated Stress Screenings
HASS proves effective in screening out failures.

Considering that electronics giants such as Apple® manage to introduce new and improved products every year, the need for faster and more accurate reliability checking comes into sharp focus. While traditional environmental stress screenings such as "burn-in" still have their place, sole reliance on this 70-year old technology can put manufacturers in last place.

Instead, quality and reliability engineers now seek cutting-edge testing systems and methodologies to supplement or even replace burn-in with advanced, more effective tests that yield better results in less time for less cost. This requires the compression of time and energy, literally, that only Highly Accelerated Stress Screening (HASS) and Audits (HASA) permit.

The move toward HASS and HASA improves reliability at less cost, allowing quality engineers to reconsider legacy-era environmental screenings that sacrifice time and money.

By fusing burn-in and accelerated stress testing into one powerful system, engineers can incorporate the best of traditional and accelerated production stress screening, enabling faster and more effective monitoring of high-volume production runs of electronic and electromechanical products.

"You save time because an ESS chamber might require 2-3 hours of testing, whereas a HASS chamber can do the test in half the time," notes Harry McLean, author of "HALT, HASS and HASA Explained/Accelerated Reliability Techniques."

Considered as one of the best experts on the real-world application of accelerated testing, McLean currently works as Director of Reliability Engineering for Redwood Shores, California-based Enecys Limited — premier supplier of smart, highly reliable grid-connected micro inverters and monitoring systems that convert DC power generated by photo-voltaic arrays into AC power for homes and businesses.

Back-Burnering Burn-in
Burn-in enjoyed widespread use in the early 50s and 60s, when the predominant failure mode of early solid state devices resulted from chemical reactions within the then new solid-state components. However, the technology used to manufacture integrated circuits has advanced to the point where chemical reactions are no longer a significant source of failures. Components, such as an entire IC can still pose a pre-existing reliability risk. But increasingly, the integration of hardware and software is becoming the precipitating cause of failures.

Take, for example, a case where the firmware has to perform temperature compensation. If the component is defective/marginal/out of tolerance, then the software value doesn't function properly any longer and the product shuts down, or even worse, provides erroneous data. Traditional burn-in may not detect such subtleties, particularly if the error occurs because of cold temperatures or changes in temperature. "With burn-in alone, you still could miss failure modes that are rate dependent, meaning change of temperature over time; and those do occur," says McLean. Another example is when component tolerances stack-up negatively to yield a marginal product. The complete product might pass a bench test, but under stress it will fail.

"By relying solely on burn-in and other environment stress screenings, you risk ending up with a product that will fail, and do so at the most inopportune times," cautions McLean.

The result is costly product recalls, ongoing warranty expenses and time-consuming support programs.

Using HASS & HASA
HASS proves effective in screening out failures that may have gone undetected during traditional burn-in testing processes by going beyond normal testing parameters to verify the integrity of mechanical interconnects, component tolerance compatibility and timing errors that only occur under high stress.
Components being cycled in test chamber.


With a HASS, a product is subjected to the outer margins of temperature, rapid thermal cycling, and vibration testing — simultaneously — to rapidly uncover any design weaknesses. For example, if the design calls out a maximum operating temperature of up to 50°C, HASS may push that product past the specification in order to uncover any weaknesses.

While a HASS that screens 100 percent of product runs prove invaluable in mission and life critical applications such as medical, avionics and defense, HASA (audits) save tremendous amounts of time for manufacturers who produce consumer goods such as cell phones, televisions, DVD players and laptops. So effective are accelerated audits that a sampling of just 15 percent or less may statistically validate the quality of a given product.

Joining the Old and New
New technology called QFusion® — a proprietary name of Denver, Colorado-based Qualmark Corporation — fuses traditional burn-in and HASS within a single system to provide faster and more effective detection of product flaws in the design and manufacturing phase. Catching flaws in the manufacturing phase is ideal, as these are 300 times less costly to repair than when a product is already in the marketplace, according to Frank Sun, Ph D, in his "An Introduction to Quantification of Burn-In and Environmental Stress Screening," presented at the 2012 Annual Reliability and Maintainability Symposium. Qualmark is a recognized leader in designing, marketing, and manufacturing accelerated life-testing systems. The company's authorized test centers have conducted over 5,000 HALT and HASS tests, within 30 different countries, since 1991.

By detecting marginal problems using a fusion of traditional burn-in, HASS and HASA, a manufacturer greatly reduces the odds of delivering a marginal product into the hands of the customer. Just as important, the rapid migration of the old toward the new stands to greatly reduce the cost of quality.

"On one product we conducted HASS on 100 percent of the units manufactured, but over time we could prove statistically that the units were not failing, so we dropped to auditing a small sample percentage," says McLean. "Even though these were very complex boards with 8 to 12 layers, double-sided, fine-pitch, we still weren't finding any defects, so we finally dropped the audits all together. After that, all we did was stuff the boards, test them, and ship the product. That's really the ultimate money-saver and where you want to go."

Other instances of positive outcomes accrued to a high-tech Fortune 100 company that realized reductions in warranty costs of $50 million in the first year of implementing HASS technology. Another involved a large contract manufacturer in China that reduced its testing costs by ten-fold utilizing Qualmark's HASS systems. Another manufacturer calculated that HASS was 18 to 36 times more effective than burn-in.

Optimized Results
Traditional burn-in chambers cannot provide the screening effectiveness of HASS, which employs hot and cold temperature, rapid thermal ramps and repetitive shock vibration. A HASS system must be utilized to reap the benefits of accelerated testing.

By convention, a HALT/HASS system is capable of going from -100 to +200°C, at rate of >60°C per minute as measured on the product. Unlike standard testing chambers, a HALT/HASS system must also produce vibration stresses such as pitch, roll and yaw around 3 axes (6 degrees of freedom) at the same time that hot and cold thermal stresses are applied. The latest Qualmark HASS systems have increased the level of energy in the lower frequency bands, which manifests the unwanted weaknesses in a product.

"In one case, a product failed just one parameter: vibration," McLean recalls. "We discovered it only because we stressed the product so hard that it changed the input impedance of the amplifier. At that point the software thinks that something is wrong in the front end and shuts the product down. We would have never uncovered this defect without putting it in one of Qualmark's chambers."


Contact: Qualmark, 10390 East 48th Avenue, Denver, CO 80238 888-425-8669 or 303-254-8800 fax: 303-254-8343 Web:
http://www.qualmark.com

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