Ensuring that products will work.

If all products were operated in controlled, benign conditions, there would be little need for environmental testing laboratories. When used in the real world, equipment is exposed to shock, vibration, temperature extremes, dust, humidity, and many more factors that affect performance. As part of a thorough design process, manufacturers subject products to simulated stresses to ensure that specifications will continue to be met in the actual use environments.

[FIGURE 1 OMITTED]

The nature of the product determines which tests are most important. For example, an engine air filter intended for rugged military applications would need to withstand severe sand and dust tests in addition to mechanical shock and vibration. If the filter were made of metal, it also might require salt spray and high humidity tests to prove it could endure these conditions for prolonged periods of time.

Because each kind of product is different and is used differently, many types of environmental tests are available. Several of these capabilities are demonstrated in real applications submitted by three accredited testing labs.

Military-Grade Vibration Testing

Standex Electronics, a global manufacturer of custom magnetics, reed switches, and sensors for the medical, military, industrial, and automotive markets, must meet stringent customer and regulatory requirements. Although it operates its own product-line testing equipment, the company also relies on expert, accredited testing services for time-sensitive, large-capacity, and unique scenarios.

To provide rapid, complete military-grade transformer vibration testing for an aerospace application, Standex turned to ESSC Test Laboratory. The lab has 20 years of experience and is ISO/IEC-compliant and accredited by The American Association for Laboratory Accreditation (A2LA).

The transformer test required precise equipment and customized fixturing. Successful testing and useful reporting hinged on how the parts were attached to the vibration table and how well the behavior was analyzed.

Each transformer weighed less than a pound, and 12 were tested at the same time (Figure 1). An LDS 750 System Shaker with a 6,000-lbf rating was used. It was coupled to a slip plate with a 19" x 21" interface plate to which were bolted two hollow aluminum cubes. These fixtures provided convenient mounting locations for six test boards, each with two transformers and related components.

ESSC performed the test in accordance with MIL-STD-202G, Method 204D Vibration, High Frequency. The test entailed 20-minute logarithmic cycling through the 10-Hz to 2,000-Hz frequency range, repeated 12 times over a 12-hour period, with the transformers oriented in each of three directions. These requirements correspond to Method 204D Test Condition E, which applies a peak-to-peak displacement of 0.06" from 10 Hz to about 127 Hz. For frequencies above 127 Hz, the displacement is reduced to maintain a peak 50g acceleration (Figure 2).

Method 204D supports testing with components de-energized or operating under specified load conditions. In this case, Standex elected to test the transformers together with related electrical components in a de-energized state.

For temperature and humidity testing, Method 106G was followed. It requires cycling five times among 25[degrees]C @ 95% humidity, 65[degrees]C @ 95% humidity, and -5[degrees]C, dwelling from three to eight hours at each temperature.

The customer specified three temperatures for thermal shock testing: 130[degrees]F, 155[degrees]F, and 170[degrees]F. The units were soaked at each temperature for 24 hours with inspection every eight hours.

According to Method 107G Thermal Shock, the transformer temperature was reduced to 25[degrees]C in less than five minutes after soaking at a high temperature. Similarly, the transformer temperature was increased from room temperature to the required maximum, also in less than five minutes. In contrast to these tests run in air at different temperatures, an almost instantaneous thermal shock can be created under Method 107G by using liquid immersion. The customer chose air in lieu of immersion.

Because ESSC is a division of Cincinnati Sub-Zero, the manufacturer of a long line of temperature-based products and thermal testing chambers, a great deal of practical environmental test experience is available. The lab delivers tests specially suited for design verification, product performance, regulatory compliance, failure analysis, life cycle, and environmental stress. In addition, ESSC uses the latest software to produce follow-up reports that document and analyze testing for customer and regulatory purposes.

[FIGURE 2 OMITTED]

Vibration Test Fixturing for Small Parts

Designing and building a product mounting fixture can be one of the most difficult aspects of a vibration testing project. Fixturing for vibration testing is intended to provide good transmissibility. This requirement may be compromised by the needs to secure the unit under test without damaging it and to access it easily for assembly and removal. This is especially important when high-volume testing is required. In addition, the fixturing has to be built quickly and inexpensively.

Chris Finch, technical sales manager at Trace Laboratories-Central, explained that traditional vibration fixtures are made by drilling and tapping aluminum cubes or plates and bolting down the product via its built-in flanges. If the unit doesn't have built-in mounting holes, more creative methods are necessary.

One method commonly used to secure test samples clamps the unit to an aluminum plate. Typically, the clamping fixture is built by drilling and taping holes in the plate and inserting threaded rods. An aluminum bar then is used to create the clamp, with protective foam between the samples and the fixture. Another method is to double-stick tape the product to the shaker table. Each method is application specific and has cost and efficiency advantages.

There are instances when traditional vibration fixtures are not an effective means of mounting a test unit. Fixturing issues can occur when conducting vibration testing on oscillators, transceivers, Ethernet adapters, flash drives, and similar components.

Often, these units are small, and many are tested at a time. They are round or have nonparallel sides that can be crushed if not handled properly. These components typically do not have mounting holes and cannot easily be attached to an aluminum plate. Using the sandwiching method will not work because the components can be damaged or slip out due to their nonstandard shape.

Instead, Trace Laboratories uses paraffin wax to rigidly mount these multiple, small, hermetically sealed units to the vibration equipment. Paraffin wax is pliable, inexpensive, and easy to use. It also is noncorrosive and an extremely good electrical insulator with an electrical resistivity of at least [10.sup.13] [ohm]-m.

Multiple test units can be placed into a block of paraffin wax, and the wax is placed onto the vibration adapter plate. The vibration inputs of the shaker system are directly tranferred to the test units. Paraffin wax allows the vibration testing to be conducted without the significant time and expense of complicated mounting fixtures or fear of damaging fragile components.

Although some laboratories still are using double-stick tape, paraffin wax is a better alternative. Double-stick tape can provide quick, inexpensive mounting of lightweight components; however, it cannot be used for all product geometries and is difficult to remove. And, while double-stick tape holds a unit by one surface, a unit can be embedded into the wax, allowing for better support and vibration transmission.

Upon completion of the vibration, the units can be manually removed and easily cleaned. The paraffin wax is removed by melting it with a heat gun.

Removal and cleaning can be conducted quickly, and additional samples can be under test in a few minutes. As long as it remains clean, the wax can be used repeatedly.

[FIGURE 3 OMITTED]

There are some considerations when working with paraffin wax. The testing should only be conducted at room temperatures because paraffin wax can begin melting at temperatures as low as 47[degrees]C. If the units are powered and monitored during the testing, be aware of their heat generation. In addition, the samples must be able to withstand 47[degrees]C during removal of the wax.

Paraffin wax should only be used on hermetically sealed units since the wax may be difficult to remove from part openings or crevices. Care must be taken when performing shock test fixturing because the wax may not properly hold the units during high impact.

Always ensure proper transmissability by mounting a response accelerometer to the test units. As a rule of thumb, only consider paraffin wax mounting for components that are too small or delicate to be mounted conventionally. Much of the experience at Trace Labs has been with very lightweight ICs, as shown mounted in paraffin wax in Figure 3.

Explosive Pressure Pulse Simulation

A customer recently asked Aero Nav Laboratories whether simulation tests could be performed to determine the survivability of a piece of equipment when subjected to explosive pressure pulse blasts. The equipment was designed to be robust and expected to survive moderate levels of pulse blasts like what would be experienced in the survivability zone of an explosion. This zone is defined as the area where the explosive effects are less severe than would be seen at the point of inception of a blast.