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330 MEMS and Microstructures in Aerospace Applications

the POF approach is not a recent development, the Computer Aided Life Cycle
Engineering (CALCE) Electronic Products and Systems Center has become the
focal point for developing the knowledge base relative to microelectronics and
packaging 7–9 . In comparing the two approaches, there are problems with using
statistical field-failure models for the design, manufacture, and support of electronic
equipment. The U.S. Army began a transition from MIL-HDBK-217 to a more
scientific, POF approach to electronic equipment reliability. To facilitate the tran-
sition, an IEEE Reliability Program Standard is under development to incorporate
physics of failure concepts into reliability programs. 10 The POF approach has been
used quite successfully for decades in the design of mechanical, civil, and aerospace
structures. This approach is almost mandatory for buildings and bridges because the
sample size is usually one, affording little opportunity for testing the complete
product or for reliability growth. 10,11 POF is an engineering-based approach to
determining reliability. It uses modeling and simulation to eliminate failures early
in the design process by addressing root-cause failure mechanisms in a computer-
aided-engineering environment. The POF approach applies reliability models, built
from exhaustive failure analysis and analytical modeling, to environments in which
empirical models have long been the rule. 7,10 The central advantage of the POF in
spacecraft systems is that it provides a foundation upon which to predict how a new
design will behave under given conditions, an appealing feature for small spacecraft
engineers. This approach involves the following: 12

. Identifying potential failure mechanisms (chemical, electrical, physical,
mechanical, structural, or thermal processes leading to failure); failure sites;
and failure modes
. Identifying the appropriate failure models and their input parameters, includ-
ing those associated with material characteristics, damage properties, relevant
geometry at failure sites, manufacturing flaws and defects, and environmental
and operating loads
. Determining the variability for each design parameter when possible
. Computing the effective reliability function
. Accepting the design, if the estimated time-dependent reliability function
meets or exceeds the required value over the required time period.

A central feature of the POF approach is that reliability modeling, which is
used for the detailed design of electronic equipment, is based on root-cause
failure processes or mechanisms. These failure-mechanism models explicitly
address the design parameters which have been found to influence hardware
reliability strongly, including material properties, defects and electrical, chemical,
thermal, and mechanical stresses. The goal is to keep the modeling in a particular
application as simple as possible without losing the cause–effect relationships,
which benefits corrective action. Research into physical failure mechanisms is
subjected to scholarly peer review and published in the open literature. The failure
mechanism models are validated through experimentation and replication by mul-
tiple researchers. 12

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