Comparison of Empirical and Physics-of-Failure based reliability prediction/assessment methods

1. Role Of The Physics-of-Failure (PoF) Approach
PoF is an approach to aid in the design, manufacture, and application of a product by
assessing the possible failure mechanisms due to expected life-cycle stresses. PoF is used
for reliability assessment, not reliability prediction. Reliability assessment involves the
evaluation of the package’s potential to survive for the mission life in the application
environment. Relevant attributes for this assessment include dominant failure
mechanisms, the stress drivers for failure, and a pareto ranking of the time-to-failure due
to the dominant failure mechanisms.
How do microelectronic device failures manifest themselves ? Failures can be broadly
categorised by the nature of the loads mechanical, thermal, electrical, radiation, or
chemical that trigger or accelerate the mechanism. The PoF approach aids in determining
potential causes, locating failures, and developing effective tests and screens. This
approach supports good engineering judgement in evaluating the impact of stresses on the
product or its elements
PoF Process
Inputs
Outputs
Operational
Ranked list of
Loads
expected failure
including power
Life Cycle Sensitivity Analysis mechanisms
dissipation, voltage,
Stress and sites
current, and frequency
Evaluate sensitivity
Profiles
of the product life
to the application
-
Environmental Stress Analysis
Stress-margins
Evaluate the
Loads Thermal
safe operating region
on products including Thermo-
for the desired life
temperature, relative mechanical
cycle profile
humidity, pressure, Radiation Design
-
shock and their cyclic Hygro- trade-offs
Evaluate potential
ranges, rate of change mechanical
screening and
and time Electromagnetic
accelerated test
The life cycle includes Vibration shock
transportation, storage, conditions
Diffusion Screening
handling and conditions
application
environments
Reliability Assessment
Accelerated
Products Determine appropriate test conditions
materials, geometry, failure mechanism models
architecture and and calculate time-to-failure
defectives for each failure mechanism
Ref: CALC E EPRC
Hilaire Perera, hilaireperera@rogers.com Long Term Quality Assurance (LTQA)

2. Purposes of Reliability Assessment
The two primary purposes for quantitative reliability assessment of systems are to :
1) Assess the capability of the parts and design to operate reliably in a given
application (robustness, durability)
2) Estimate the number of field failures or the probability of mission success
Purpose # 1 does not require statistically based data or models, but rather, component
part-selection/qualification and design techniques. It is for this purpose that the Physics-
of-Failure (PoF) approaches have merit.
Purpose # 2 requires empirical data, and models (MIL-HDBK-217, etc) derived from
those data. This because component field-failures are predominantly caused by
component and manufacturing defects which can be quantified only through the
statistical analysis of empirical data. PoF approaches address only wearout phenomena,
which from the industry viewpoint, constitute an irrelevant portion of all electronic
component failures
Comparison of Empirical and PoF
Prediction/Assessment Methods
Empirically based models: PRISM; Physics-of-Failure
MIL-HDBK-217; CNET, Bellcore,
British Telecom; etc
• •
Reflects actual field failure rates and Models specific failure mechanisms
defect densities
• Valuable for estimating end-of-life for
• Can be a good indicator of actual field- known failure mechanisms
reliability
• Highly complex and costly to apply
• Difficult to collect good quality field
•
data Cannot be used to:
1) estimate field reliability
• 2) model defect-driven failure
Difficult to keep up-to-date
mechanisms
• Difficult to distinguish correlated
•
variables (e.g., quality and Not practical to use for the reliability
environment) assessment of an entire system
Hilaire Perera, hilaireperera@rogers.com Long Term Quality Assurance (LTQA)