By David Askew, Mouser Electronics
When a design engineer considers the different harsh environmental conditions a product may encounter, vibration and shock may not immediately come to mind. However, vibration can be a major source of failure in many systems. Electronics are subjected to vibrational stresses in a wide spectrum of applications ranging from automobile, train, and aerospace systems to oil drilling equipment, power stations, and manufacturing plants. Even if intended for mild environments, most electronics are subject to some amount of vibration during their product lifecycle, whether from shipping and transportation or simple everyday use.
Vibration fatigue results from mechanical stress on a part. The stress on a part is a function of at least two major factors: the acceleration (along with its derivatives) due to the vibration, and the vibration frequencies as they relate to the resonant frequency of the part or PCB. If the vibration is excessively violent, then stresses on the PCB and its mounted parts will be larger, obviously, due to greater accelerations. However, even very small vibrations can cause damage if the frequencies cause a PCB to resonate.
The problem of vibration fatigue is, ironically, made worse in part by advances in IC fabrication and manufacturing. Although the adoption of chip-scale packages (CSP) has enabled greater performance and much higher packaging densities, it does come with a price. Chip miniaturization requires solder joints and electrical connections of both decreasing size and number. With smaller pads and fewer solder connections, design engineers face greater challenges in meeting shock and vibration requirements.
Design engineers often employ mechanisms to dampen vibration and reduce shock to an enclosure and sensitive electronics within it. Potting or underfilling the ICs can offer very effective support to solder joints, but often presents trade-offs with cost and thermal specifications.
Vibration may also cause mechanical failure in the housing itself. This is equally undesirable; a loosened screw, pin, or clip can lead to cascading failures that damage and even destroy the system. Elastomers and thermoplastic mounts can effectively reduce the likelihood of such an event by dampening these instead of transmitting the vibrations.
Mathematical models and finite element methods can be useful tools in predicting certain modes of failure, including many due to vibration. On the other hand, these methods are notoriously unreliable if they are not complemented with product stress testing. Stress testing can find defects before they become expensive problems. This testing requires time, of course, and tends to increase time-to-market. One solution is to use accelerated product reliability test methods, such as HALT (Highly Accelerated Life Test) and HASS (Highly Accelerated Stress Test). Unlike traditional single-axis testing, HALT/HASS exposes the product to random vibration in six (three linear and three rotation) axes, as well as high thermal change rates. The unit under test is subjected to progressively greater stresses and investigated for signs of failure. This methodology goes further than mere design verification testing because it stresses the product beyond its specifications to determine both operational and destruct limits. Done properly, accelerated vibration testing like HALT/HASS can increase product reliability while also reducing time-to-market.
These techniques are so effective, in fact, that many manufacturers employ them prior to military qualification testing. The MIL-STD-202 Test Method Standard specifies the testing procedure for vibration, which involves simple harmonic motion at 0.03 inches amplitude over a range of frequencies from 10 to 55Hz. Under electrical load conditions, the motion is applied for at least two hours along each axis (for a total of six hours), and the product is tested both during and after vibration.
References
Veprik, A.M. (2003). Vibration protection of critical components of electronic equipment in harsh environmental conditions, Journal of Sound and Vibration. 259 (1), pp.161-175
David Askew is a technical content specialist for Mouser Electronics, specializing in embedded systems and software. He holds a BSEE from the University of Texas at Arlington
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