MEMS Packaging Reliability in Board-Level Drop Tests Under Severe Shock and Impact Loading Conditions—Part II: Fatigue Damage Modeling

Abstract

Damage in handheld electronic devices due to accidental drops is a critical reliability concern. This paper is the second of a two-part series and focuses on damage models for drop test durability of commercial off-the-shelf microelectromechanical systems (MEMS) components that are mounted on printed wiring boards. The modeling approach is based on the experimental results presented in the first part of this two-part series. In particular, the focus of this paper is on damage due to drop events under extremely high accelerations (20 $000g$ , where $g$ is the gravitational acceleration) achieved by a series of secondary impacts. Impacts with such high accelerations can occur in handheld electronic devices due to collisions between internal neighboring structures and can generate stress levels well beyond levels previously anticipated in typical use, or in conventional qualification tests. A calibrated dynamic multiscale finite element model is used to evaluate the stresses at the relevant failure sites. Utilizing the local stress at each failure site, a fatigue damage modeling approach is proposed to predict the interaction of the competing failure mechanisms in MEMS components. The proposed damage model is based on the deviatoric stress (or strain) at the failure site and uses a hydrostatic stress correction factor to address the influence of mean stress (depending on component orientation). The model estimates the damage accumulation rate for a given stress condition and integrates the accumulated damage over the entire response history. This approach makes it possible to address the influence of the post-impact transient response on fatigue damage accumulation. The damage-model constants are determined for each failure site of interest by relating the failure data to the corresponding stress/strain metrics. Damage modeling results are not only capable of matching the lifetimes of MEMS components in drop testing, but also provide an explanation of transitions in dominant failure sites observed in MEMS assemblies under different drop conditions.