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84 The PCB Magazine • August 2016 in magnitude, dependent on the application or ultimate end use. From a simulation standpoint, mechanical shock testing would be the equiva- lent of knocking your widget off your desk, or possibly dropping your widget down the stairs. For vibration, the widget is mounted to a table that has the ability to move a relatively small distance laterally at a high frequency. This high-speed movement exposes the widget to a vibrational stress. In terms of simulation, this type of exposure is designed to determine how the widget might perform when bolted to something like a jet engine or a clothes dryer. The widget is typically operated during the ex- posure, so its performance can be monitored under the real world conditions. Climatic Exposures This category actually has some sub-cate- gories, as a spectrum of exposures are possible when delving into this realm. Of typical interest in the world of climatic exposure would be the following categories: temperature, humidity, and weathering. Temperature extreme testing is always of in- terest in respect to reliability. This testing can be done at a single extreme temperature (ele- vated or sub-ambient) or with both extremes, the latter of which being commonly referred to as thermal shock testing. For thermal shock, the testing exposes your widget to temperatures well above and well below normal atmospheric conditions in a cyclic process. The extremes in these temperatures can commonly vary from 150°C all the way down to -75°C with many other options in between and above and be- yond. As for the exposure itself, the cyclic na- ture of the testing—physically transferring the widget from hot to cold—results in a thermal stress being applied to the device under test. During the exposure, the widget can be monitored for functionality or it can be sim- ply tested pre- and post-exposure. In respect to simulation, cyclic thermal excursions are com- mon for many things in the real-world and could show how your widget survives a hike up a mountain, where the temperature at ele- vated altitude could be very different than on the ground where you started, or how it might survive in the engine compartment of an auto- mobile, where the temperature will cycle from ambient conditions up to the temperature of the engine itself. Humidity testing is usually performed in conjunction with elevated temperature testing and is thus commonly referred to as tempera- ture / humidity testing. Humidity levels of 60% to 95% RH are common for this type of test- ing, along with temperature levels from 30°C to 85°C. The environment is very harsh—hot and wet!—and is commonly used to explore the reli- ability of electronic-based widgets, such as what might happen if you took your GPS deep into the jungles of Brazil! Common tests like surface insulation resistance (SIR) and electrochemical migration (ECM) are performed in an attempt to promote an electrical issue for the widget that will cause it to malfunction. In terms of evaluation, pre- and post-exposure testing and/ or monitoring while at conditions are common ways to check reliability. Weathering type exposures are commonly centered around light effects. Commonly re- ferred to as UV or Xenon Arc exposures, these types of tests expose your widget to a variety of light spectra…as if your widget had been left out in the sun too long! The main variables in this type of testing are the irradiance level (which can be thought of as the light intensity) and the test duration. Pre- and post-exposure testing is very common to determine what ef- fect, if any, the light exposure has had on your widget. These tests could be related to visual characteristics, such as color and gloss, or me- chanical properties, such as flexibility and ten- sile strength. Ultimately, exposure of your widget to en- vironmental stress testing is a key step in the development process of any product and the es- tablishment of a reliability strategy to access the potential weaknesses, as a function of environ- ment, for your widget is critical to its long-term success. PCB Keith M. Sellers is operations man- ager with NTS in Baltimore, Mary- land. WELCOME YOUR PRODUCT TO THE REAL WORLD!

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