Environmental Test Labs Provide Services Plus GuidanceEnvironmental Test Labs Provide Services Plus Guidance

Most design engineers and test technicians are specialists themselves. But when it comes to verifying their designs under expected environmental conditions or conducting life-cycle performance evaluations, supplementary expertise usually is required.

Of course, many of the largest electronic and most avionics companies have sufficient environmental test needs to require a staff of in-house environmental specialists and temperature chambers, shakers, and related control and data-acquisition equipment. But for most other companies, it is more cost-effective to work with experts at environmental test labs.

Many independent environmental test labs, such as Wyle Laboratories, National Technical Systems, QualMark, Quest, and Trace Laboratories, have established facilities in several locations. Each facility has a full range of state-of-the-art environmental test equipment and staff to provide test and consulting services. Others, such as Anco Engineers, develop and manufacture special environmental test equipment in addition to furnishing engineering and lab services.

A third group consists of in-house environmental labs that take on assignments from other companies to supplement their workload. L3 Communications at Lockheed Martin Wideband Systems and the Tandem Product Evaluation Center at Tandem Computers are examples of this category.

Services Provided

Cognizant that few engineers have environmental expertise, test labs generally do not require you to present them with a completely defined test plan that includes numerical test targets. Instead, many work with you to establish a plan that meets your specifications and quality goals.

“In most cases, we help establish tests and test limits,” said John Alba, environmental lab manager at Quest. “We also work with some customers to help them design a product that meets certain vibration and temperature requirements.”

A few labs concentrate efforts on particular environmental test issues, such as transportation-related or stress testing. Not only do the labs perform tests, but they also provide targeted consulting and design services.

Lansmont, for instance, operates three technical service centers that address transit-environment-related problems. “Our testing and design efforts primarily focus on the hazards of shock, vibration, compression, and drop and the interactions and relationships of products and packages involved in withstanding those,” said Bill Kipp, the company’s marketing director. “By performing hazards measurements, product testing and improvements, material testing, and package design and testing, we find the proper balance between environmental impact, performance, and cost.”

QualMark provides special services pertaining to highly accelerated life testing (HALT) and highly accelerated stress testing (HASS). “HALT is performed as part of a prototype evaluation to identity the weak links in a new design and discover the operating and destruct limits of a product,” explained Neill Doertenbach, technical product manager of OVS Systems at QualMark. “HASS is a production test to identify latent process or component problems.

“We provide HALT and HASS services using specially designed chambers located in our Accelerated Reliability Test Centers,” Mr. Doertenbach continued. “Our engineers work closely with the customer’s design engineers to devise tests that will stress the product to quickly expose design flaws. Stresses are applied and test methods are defined by our engineers. Functional tests to be performed during HALT are defined by the customer.”

Wyle Laboratories also uses special facilities to perform HALT and HASS as well as ESS. “We combine temperature cycling with broadband random vibration,” explained Bob Porter, manager environmental testing at Wyle Laboratories. “Wyle also operates electrodynamic shakers and temperature-cycling chambers up to drive-in size.”

Most environmental test labs perform a wide range of tests and related activities. For electronic equipment, these fall into three categories, each with a different set of goals, application requirements, and limits:

Prototype Evaluation—to determine whether the basic design can withstand the environmental extremes under which the equipment is expected to operate.

Production Sampling—to ensure that the product quality continually meets standards.

Environmental Stress Screening (ESS) or HASS—to establish appropriate procedures for ESS or HASS and subsequent application to production units to weed out latent or infant-mortality failures.

Specifying Meaningful Limits

Test and stress-application limits may vary widely since they always are a function of the environment in which particular equipment is expected to be transported, stored, and operated. “For instance, for automotive electronics, it is not uncommon to conduct temperature cycling from -40°C to +125°C,” said Jeffry Schutt, general manager of Trace Laboratories-Central. “Certain areas of the vehicle even might experience more adverse environments.

“Oftentimes, customers are not prepared to establish test limits for their products,” Mr. Schutt continued. “In this case, we provide guidance because, by working with diverse manufacturers, we have acquired a broad knowledge base of typical operating environments and appropriate test limits for various types of products and applications.”

For most electronic equipment used indoors, operating temperatures range from 0°C to 40°C, while typical storage temperatures are -40°C to +65°C, said Mr. Alba of Quest. “Vibration and shock limits depend on the operating environment. Loose cargo and drop tests often are performed according to standards of the American Society for Testing and Materials.

“Anco generally performs vibration tests in accordance with IEEE 344, Bellcore TR-NWT-000063, and MIL-STD-167 specifications,” said Dr. Paul Ibanez of Anco. “Test limits usually are defined by these specificationsor are the result of structural dynamic calculations carried out by the customer. The typical frequency range of interest is 0.5 Hz to 100 Hz, acceleration levels typically are between 0.1 g and 20 g, and displacements may be up to ±5 inches.

“Occasionally we suggest test levels—usually for consumer products. Companies testing military products typically know what they want,” Dr. Ibanez concluded.

ESS, HALT and HASS

The purpose of ESS is to deliver quality products that experience minimal failure rates during normal use which leads to high customer satisfaction and low warranty costs. ESS programs usually are initiated because of unsatisfactory field failure rates or as a preventive measure. Regardless of the initial motivation, the products are subjected to stresses in excess of those they are likely to experience during normal use but not extreme enough to deteriorate their performance or significantly shorten their life.

Devising efficient ESS procedures and environmental application limits requires a cooperative effort of design, quality, product-test, and environmental-test personnel. Actual or potential failure modes must be identified, environmental stress levels established, results analyzed, ESS levels refined, and possibly product design or manufacturing processes altered.

“To assist in establishing appropriate ESS procedures and levels, we typically carry on a dialog with our customers to learn about the product, such as its potential weak links, fatigue levels, wear-out mechanisms, and expected life,” explained Mr. Schutt of Trace. “Once these are known, we work with them to develop effective screens that do not significantly reduce the life of the product but yet help to precipitate any latent manufacturing defects or design marginalities.

“Developing an effective ESS program is an evolutionary process involving testing, analyzing, and modifying the product,” Mr. Schutt continued. “While the ESS program is being conducted, it is critical to analyze the nature of the failures that are being precipitated and to assess whether they are related to design, manufacturing, wear-out, or fatigue. The results of the failure analysis must be fed back to both the test and screen-development personnel so that the design, or the screen, can be modified accordingly.”

“Mathematical models help determine screen strength to precipitate latent defects,” commented Quest’s Mr. Alba. “However, each situation is unique, and the optimum ESS implementation not only depends on the type of product, but also on production volumes, product costs, and desired quality parameters.

“If defects are happening in the field, a stronger screen may be required. But overstressing also is costly since it may damage good units. As a result, you also must examine the cost of the screen vs the cost of field failures,” Mr. Alba concluded.

HALT and HASS differ from conventional ESS. With HALT, the product is tested far beyond its in-use specifications to identify weak links in the design.

“By applying HALT, you identify product weaknesses that would not show up in typical design-verification testing but that would be a significant warranty issue six months or a year later,” said Mr. Doertenbach of QualMark. “During HALT, the actual operation and destruct limits of the product are found and pushed as far as possible. Once determined, they are used to define the stresses and limits applied in HASS. HALT results are necessary to implement HASS.”

Data obtained during HALT also can be useful for establishing ESS limits. “We suggest that a HALT evaluation be performed at the prototype or preproduction stage to discern design or manufacturing flaws that could precipitate failures,” said Mr. Porter of Wyle Laboratories. “Later, for production units, ESS should be performed at levels based on earlier HALT findings.”

ESS Throughput

For large production runs, the time required to perform ESS becomes significant since stresses must be applied and functionality verified on each unit before it can be shipped. Throughput can be improved by applying ESS in parallel to several units at a time. But this is not always practical since additional or larger environmental chambers and shakers may be required—which increases expenditures. A better solution is to find ways to decrease the required screen application time by using ingenuity, more capable equipment, or better instrumentation.

“Many times, we suggest that customers try our tree-axis pneumatic shaker for ESS instead of doing one axis at a time on electrodynamic shakers,” said Maynard B. Morris, senior staff engineer at L3 Communications of Lockheed Martin Wideband Systems. “The profiles on the pneumatic shaker are not as rigidly controllable as on the electrodynamic shaker, but in many cases, the results are the same or better.

“For some in-house tests, we have reduced vibration test time from several hours to 15 minutes. For thermal ESS, we attach sensors to the test items to determine when they have reached thermal stability. Once this happens there is little reason to maintain the test items at the same temperature,” he continued.

In some cases, it still is advantageous to perform ESS in parallel. “We currently are setting up to do ESS (vibration and temperature) on eight items at the same time for one of our customers,” Mr. Morris said. “Initial setup costs are a little higher but the recurring cost per item is lower.”

Acknowledgments

These companies provided information for this feature:

Anco Engineers (303) 443-7580

Lansmont (408) 655-6600

L3 Communications, Lockheed Martin (801) 594-2349

National Technical Systems (714) 879-6110

QualMark (303) 254-8800

Quest Engineering Solutions (508) 667-7000

Tandem Product Evaluation Center (800) 538-3107

Trace Laboratories (708) 867-0400

October 1997