Physics of failure is a technique under the practice of
reliability design that leverages the knowledge and understanding of the processes and mechanisms that induce
failure
Failure is the state or condition of not meeting a desirable or intended objective, and may be viewed as the opposite of success. The criteria for failure depends on context, and may be relative to a particular observer or belief system. One ...
to predict reliability and improve product performance.
Other definitions of Physics of Failure include:
* A science-based approach to reliability that uses modeling and simulation to design-in reliability. It helps to understand system performance and reduce decision risk during design and after the equipment is fielded. This approach models the root causes of failure such as
fatigue,
fracture
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displ ...
,
wear, and
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
.
* An approach to the design and development of reliable product to prevent failure, based on the knowledge of root cause failure mechanisms. The Physics of Failure (PoF) concept is based on the understanding of the relationships between requirements and the physical characteristics of the product and their variation in the manufacturing processes, and the reaction of product elements and materials to loads (stressors) and interaction under loads and their influence on the fitness for use with respect to the use conditions and time.
Overview
The concept of Physics of Failure, also known as Reliability Physics, involves the use of degradation algorithms that describe how physical, chemical, mechanical, thermal, or electrical mechanisms evolve over time and eventually induce failure.
While the concept of Physics of Failure is common in many structural fields, the specific branding evolved from an attempt to better predict the reliability of early generation electronic parts and systems.
The beginning
Within the
electronics industry
The electronics industry is the economic sector that produces electronic devices. It emerged in the 20th century and is today one of the largest global industries. Contemporary society uses a vast array of electronic devices built-in automated or ...
, the major driver for the implementation of Physics of Failure was the poor performance of
military weapon systems during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
. During the subsequent decade, the
United States Department of Defense
The United States Department of Defense (DoD, USDOD or DOD) is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national sec ...
funded an extensive amount of effort to especially improve the reliability of electronics, with the initial efforts focused on after-the-fact or statistical methodology. Unfortunately, the rapid evolution of electronics, with new designs, new materials, and new manufacturing processes, tended to quickly negate approaches and predictions derived from older technology. In addition, the statistical approach tended to lead to expensive and time-consuming testing. The need for different approaches led to the birth of Physics of Failure at the
Rome Air Development Center (RADC). Under the auspices of the RADC, the first Physics of Failure in Electronics Symposium was held in September 1962. The goal of the program was to relate the fundamental physical and chemical behavior of materials to reliability parameters.
Early history – integrated circuits
The initial focus of physics of failure techniques tended to be limited to degradation mechanisms in
integrated circuits
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tin ...
. This was primarily because the rapid evolution of the technology created a need to capture and predict performance several generations ahead of existing product.
One of the first major successes under predictive physics of failure was a formula developed by
James Black of
Motorola
Motorola, Inc. () was an American multinational telecommunications company based in Schaumburg, Illinois, United States. After having lost $4.3 billion from 2007 to 2009, the company split into two independent public companies, Motorola ...
to describe the behavior of
electromigration
Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high dir ...
. Electromigration occurs when collisions of electrons cause metal atoms in a conductor to dislodge and move downstream of current flow (proportional to
current density
In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional a ...
). Black used this knowledge, in combination with experimental findings, to describe the failure rate due to electromigration as
:
where A is a constant based on the cross-sectional area of the interconnect, J is the
current density
In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional a ...
, Ea is the activation energy (e.g. 0.7 eV for
grain boundary diffusion in aluminum), k is the
Boltzmann constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constan ...
, T is the temperature and n is a
scaling factor (usually set to 2 according to Black).
Physics of failure is typically designed to predict wearout, or an increasing failure rate, but this initial success by Black focused on predicting behavior during operational life, or a constant failure rate. This is because
electromigration
Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high dir ...
in traces can be designed out by following design rules, while electromigration at vias are primarily interfacial effects, which tend to be defect or process-driven.
Leveraging this success, additional physics-of-failure based algorithms have been derived for the three other major degradation mechanisms (
time dependent dielectric breakdown DDB hot carrier injection
Hot carrier injection (HCI) is a phenomenon in solid-state electronic devices where an electron or a “ hole” gains sufficient kinetic energy to overcome a potential barrier necessary to break an interface state. The term "hot" refers to the ...
CI and
negative bias temperature instability BTI in modern integrated circuits (equations shown below). More recent work has attempted to aggregate these discrete algorithms into a system-level prediction.
TDDB: τ = τo(T) exp
G(T)/ εox''
where τo(T) = 5.4*10-7 exp(-Ea / kT), G(T) = 120 + 5.8/kT, and εox is the permittivity.
HCI: λHCI = A3 exp(-β/VD)exp(-Ea / kT)
where λHCI is the failure rate of HCI, A3 is an empirical fitting parameter, β is an empirical fitting parameter, VD is the drain voltage, Ea is the activation energy of HCI, typically −0.2 to −0.1eV, ''k'' is Boltzmann's constant, and ''T'' is temperature in
Kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ...
.
NBTI: λ = A εoxm VTμp exp( -Ea / kT)
where A is determined empirically by normalizing the above equation, m = 2.9, VT is the thermal voltage, μP is the surface mobility constant, Ea is the activation energy of NBTI, k i s Boltzmann's constant, and T is the temperature in Kelvin
Next stage – Electronic packaging
The resources and successes with integrated circuits, and a review of some of the drivers of field failures, subsequently motivated the reliability physics community to initiate physics of failure investigations into package-level degradation mechanisms. An extensive amount of work was performed to develop algorithms that could accurately predict the reliability of interconnects. Specific interconnects of interest resided at 1st level (wire bonds, solder bumps, die attach), 2nd level (solder joints), and 3rd level (plated through holes).
Just as integrated circuit community had four major successes with physics of failure at the die-level, the component packaging community had four major successes arise from their work in the 1970s and 1980s. These were
Peck: Predicts time to failure of wire bond / bond pad connections when exposed to elevated temperature /
humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
:
where ''A'' is a constant, ''RH'' is the relative humidity, ''f''(''V'') is a voltage function (often cited as voltage squared), ''E
a'' is the activation energy, ''K
B'' is Boltzmann's constant, and ''T'' is temperature in
Kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and ...
.
Engelmaier: Predicts time to failure of solder joints exposed to temperature cycling
: