Abstract:

The continuing increase of functionality, miniaturization, and affordability of handheld electronic devices has resulted in a decrease in the size and weight of the products. As a result, printed wiring assemblies (PWAs) have become thinner and more flexible, and clearances with surrounding structures have decreased. Therefore, new design rules are needed to minimize and survive possible secondary impacts between PWAs and surrounding structures because of the consequential amplification in acceleration and contact stress. This paper is the first of a two-part series and focuses on the drop test reliability of commercial off-the-shelf microelectromechanical systems (MEMS) components that are mounted on printed wiring boards (PWBs). Particularly in this paper, we are interested in gaining preliminary insights into the effects of secondary impacts (between internal structures) on failure sites in the MEMS assemblies. Drop tests are conducted under highly accelerated conditions of 20000 g (“g” is the gravitational acceleration). Under such high accelerations, the stress levels generated are well beyond those expected in conventional qualification tests. Furthermore, secondary impacts of varying intensities were allowed by changing the clearance between the PWB and the fixture. As a result, the stress and accelerations are further amplified, to mimic unexpected secondary impacts in a product if/when design rules fail to avoid such conditions. The amplification of the test severity is quantified by comparing the characteristic life (η in a Weibull distribution) of all the tested MEMS components at each clearance. Multiple failure sites from drop testing are identified, from packaging-level failures to MEMS device failures. The participation of competing failure sites is also demonstrated via characteristic life representations of each failure site at various clearances.

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