Since its beginnings in the 1980s, the CALCE Electronic Products and
Systems Center (EPSC) has enjoyed unparalleled growth and has built an
international reputation as a leader in electronic systems design and
analysis. Today, the CALCE EPSC is a driving force behind the
development and implementation of physics-of-failure approaches to
reliability, as well as a world leader in accelerated testing, failure
analysis, and electronic parts selection and management.
The center currently works with over 100 international
organizations providing research support and information services.
CALCE EPSC collaborates on failure and reliability analysis with City
University of Hong Kong; on virtual qualification analysis and software
development with Hong Kong University of Science and Technology and
with Fudan University in Shanghai; on thermal management with ITRI in
Taiwan; and on the reliability of lead-free solders with Tatung in
Taiwan and RIST in Korea. In the past six months, 13 new organizations
have joined the CALCE Consortium, 5 of them from outside the U.S.A.
The CALCE EPSC represents a desirable model for research
centers in the way it incorporates industry and collaborative research.
Recently, the Secretary of the Ministry of Commerce, Industry and
Energy of Korea announced plans to build a professional research center
for excellence modeled after the CALCE EPSC. The center will be located
at the Han Yang University and its mission will be focused on improving
the reliability of Korean mechanical and electronic components, and
developing value-added products. Plans are also under way for
establishing a TWI/CALCE Electronics and Systems Reliability Center
based in the United Kingdom. This center will act as a central hub for
reliability issues in the UK and the rest of Europe.
CALCE EPSC faculty currently serve as editors for 6
internationally prestigious journals. They are also serving as authors,
keynote speakers and chairs of international conferences. These
contacts identify technology issues early and keep research efforts
focused on leading industry needs.
Recently, Prof. Y.C. Chan, EPA Center of City University of
Hong Kong and CALCE EPSC co-hosted the International IEEE Conference on
the Business of Electronic Product Reliability and Liability in Hong
Kong. In June of this year, they will cooperate on another conference
on Asian Green Electronics Manufacturing that will be held in Shenzhen,
China.
For more information on the Center’s expertise and resources, contact
Prof. Michael Pecht at 301-405-5323.
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Lead-free patents began to be issued in significant numbers in the
US by the late 1980s, mostly as plumbing alloys. However, the lead-free
movement in the electronics industry has the potential to add
significant value to intellectual property in this domain. This
justifies the exponential increase in the number of lead-free patents
filed and issued, as observed by CALCE EPSC researchers.
North American, Japanese and European legislation regarding
lead in electronic devices is vastly different. The US simply requires
that the release of lead or lead compounds be reported. In Japan,
household electronics must be recycled, but lead is largely being
eliminated voluntarily. Japanese manufacturers see an advantage in
lead-free technology for consumer preference. Europe has imposed a ban
on lead in electronics, effective in 2006, but European companies do
not publicize the progress of their lead-free programs as do the
Japanese. In fact, the Japanese zeal for lead-free products has carried
over into the domain of patents, where Japanese acquisitions of
lead-free intellectual property far exceed those of any other country
It appears from the work already performed that the future of
the National Electronics Manufacturing Initiative (NEMI) recommended
alloy may not be as bright as was originally thought. The patent
situation has changed since the NEMI- recommended composition was
decided, and continues to change rapidly. At present, a few patents in
the US and in Japan have been identified as potential obstacles to the
widespread use of Sn-Ag-Cu solders resembling the NEMI alloy. Yet what
may be more unsettling is if a number of Sn-Ag-Cu patent applications
currently pending in the US and in Europe are granted. There is one US
patent pending that lays claim to a composition that is practically
identical to the NEMI alloy. Moreover, several Japanese patent
applications in Europe are so close to the NEMI composition that
overlap may arise in processing.
The lack of an apparent “baseline?against which all
compositions can be easily compared results in potential overlap
between compositions. Claim breadth can also be a problem, given that
many patents cover not only a wide range of compositions, but also how
the compositions are made or used. For example, lead-free patents have
been found wherein applications of a solder as a joint or interface
between components, a PCB or electronic component joined with the
solder, and a joint made with the specified solder by every known
technique were included within the scope of a single patent.
In order to deal with increasing lead-free solder patents,
claims per patent and complexity of claims, CALCE EPSC is developing a
lead-free IP software tool that includes a lead-free patent database.
The database already contains the claims of over three hundred
lead-free patents worldwide. The software will identify if a given
solder formulation is covered under an existing patent.
For a short course at your organization on lead-free IP issues or on lead-free issues in general, contact Prof. Michael Pechtor Dr. Diganta Das.
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For the past four years, the CALCE EPSC has been conducting research
on health and life consumption monitoring of electronic systems based
on a physics-of-failure (PoF) approach.
Life consumption monitoring (LCM) is a method to assess a
product’s reliability based on its remaining life in a given life-cycle
environment. The process involves continuous or periodic measurement,
sensing, recording, and interpretation of physical parameters
associated with a product’s degradation. By determining the product’s
health based on actual life-cycle application conditions, procedures
can be developed to assess and maintain the product.
CALCE has demonstrated the life consumption methodology in an
automotive underhood environment through three case studies. FMEA was
used to recognize potential failure modes, determine the root causes of
the failure modes and then determine the relevant environmental
parameters. A data logger was used, along with environmental sensors,
to continuously monitor and record environmental loads on a circuit
card assembly. The recorded data was simplified and used with calcePWA
reliability analysis software to estimate the remaining life. CALCE was
able to characterize the impact of shock profiles resulting from a car
accident on the circuit card assembly. The performance of the circuit
card assembly was checked in real time through resistance monitoring.
The remaining life of the circuit card assembly was predicted at
different stages of the experiment with the available amount of
information. The most accurate prediction was obtained through the life
consumption monitoring approach, taking into account the shock caused
by the accident.
Estimating the impact of sudden changes in life cycle environment in
real time can have many advantages in making a risk-informed
maintenance decision. For example, the estimated remaining life after
an accident can be compared with the next mission requirement to ensure
that there is enough life left. If required, the mission requirement of
the product can be made less severe to get the intended life. This
concept is known as “extension of life.?
In-situ Semiconductor Health Monitors: Recently CALCE has been
working on applications of in-situ semiconductor health monitors for
health and life consumption monitoring. In-situ health monitors are
pre-calibrated cells (circuits) that are co-located with the actual
circuit on a semiconductor device. The in-situ health monitors thus
experience the same manufacturing process and environmental parameters
as the actual circuit. Selected failure mechanisms are accelerated in
the health monitors by changing the operational parameters. This can be
achieved by increasing the current density inside them. Because of
accelerated failure mechanisms, in-situ health monitors have higher
failure rates than actual circuits over the entire product life. The
failure distribution of the in-situ health monitors is statistically
calibrated at a point before the onset of the circuit failure
distribution, thereby predicting failure. The acceleration factors of
the health monitors can also be calibrated.
For more information on health and life consumption monitoring initiatives at the CALCE EPSC, contact Prof. Michael Pecht.
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Plastic encapsulated microcircuits (PEMs) still dominate the market
share of microcircuit sales worldwide due to their advantages in size,
weight, performance and cost. Despite these advantages, one important
disadvantage is that polymeric mold compounds absorb moisture, and thus
swell, when exposed to a humid environment. Hygroscopic stresses arise
in a PEM when the mold compound swells upon absorbing moisture and the
lead frame, die paddle and silicon die do not experience swelling.
Similar to the thermal stress produced by the mismatch in coefficient
of thermal expansion (CTE) between adjacent materials, the hygroscopic
stress increases as the hygroscopic swelling coefficient of the mold
compound increases. Accurate measurement of hygroscopic swelling is
essential in assessing the effect of hygroscopic stresses on package
reliability. However, only limited data on hygroscopic swelling can be
found in the literature and the results are somewhat inconsistent due
to the difficulty associated with ascertaining hygroscopic swelling
values.
In a recent study (C02-20), CALCE developed a novel procedure
to measure the coefficient of hygroscopic contraction (CHC) of mold
compounds. The procedure employs a whole-field in-plane displacement
measurement technique with submicron sensitivity and has numerous
advantages over the existing methods, which are essentially
point-measurement methods. The new procedure allows simultaneous
measurement of hygroscopic swelling in both in-plane directions (x and
y), while effectively canceling thermally induced deformations. A large
gage length nullifies the point-to-point variation within the sample
and the high sensitivity provides high measurement accuracy.
The procedure was used to analyze three commercially available
mold compounds (labeled A, B and C). Typical results are shown in
Figure 1, where hygroscopic strains obtained from three specimens of
mold compound sample “A?are plotted against moisture content (%). The
fringe patterns represent the deformations of a reference sample at a
time zero and a test coupon at four hundred hours of desorption after
the virtual saturation condition was achieved. It is evident that a
linear relationship exists between swelling and moisture content. The
constant of linearity is called the coefficient of hygroscopic contraction
(CHC). The CHC is a material property of the mold compound and, if it
is known, the hygroscopic strain can be determined by measuring the
moisture content in the mold compound, analogous to the
thermally-induced strain, which can be determined if a CTE and DT are
known.
The CHC at 85ºC for the three mold compounds was 0.26, 0.22 and 0.22 (%eh/%C).
The corresponding maximum moisture content in each mold compounds A, B
and C at the virtual equilibrium at 85ºC/85%RH was 0.50%, 0.54% and
0.34%, respectively. The leadframe absorbs no moisture and does not
undergo any hygroscopic strain. Therefore, the resulting hygroscopic
mismatch strains between the mold compound and a copper leadframe
should be identical to the swelling strain of the mold compound. The
hygroscopic swelling strains were determined by multiplying the CHC by
the moisture maximum content; they were 0.13%, 0.12% and 0.07% for mold
compounds A, B and C, respectively.
The coefficient of thermal expansion of mold compounds A, B
and C below their glass transition temperature are 13, 17 and 11
ppm/°C, respectively. The CTE of the copper leadframe is 17 ppm/°C. If
a change in temperature of 100ºC is considered, the thermal mismatch
strains at the mold compound/copper interface are 0.04%, 0% and 0.06%.
Therefore, the hygroscopic mismatch strains at 85ºC/85%RH of samples
“A?and “B?are over three times greater than the thermal mismatch
strains. Mold compound sample “C?shows hygroscopic and thermal mismatch
strains of the same magnitude.
The above results imply that hygroscopic swelling effects may
have a significant impact on PEM reliability. According to the SAE
document, Recommended Environmental Practices for Electronic Equipment Design,
electronic equipment is commonly subjected to a 38ºC/95%RH environment
throughout the automobile and environments of 66ºC/80%RH in multiple
locations in the automobile. In such environments, where packages are
subjected to both a temperature excursion and relative humidity change,
hygroscopic strains must be considered for reliability assessment.
Accelerated life testing conditions such as a HAST (Highly
Accelerated Stress Test) chamber, in which temperature, humidity and
pressure are used, may also present complications due to hygroscopic
swelling issues. The temperature range in a HAST chamber is typically
from 100ºC to 150ºC, the relative humidity is typically over 70
percent, and the pressure can be up to 50 psi. These conditions will
drastically increase the amount of moisture absorbed by the polymeric
materials in a package, and therefore greatly increase hygroscopic
swelling. The experimental results presented here imply that
hygroscopic swelling would play an important role in the
cycles-to-failure of the package being tested. For more information,
contact Dr. Bongtae Han.
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Many next generation electronic products will rely on programmable
solid-state modules for controlling and distributing power. These
products will include not only major defense systems such as the
electric tank and the next generation submarine, but also commercial
products such as motor drives, electric vehicles, robotics, and
telecommunications. The use of solid-state modules for power conversion
and distribution has the potential to significantly improve the
efficiency and performance of these electronic products. However, as
power supplies are often the heaviest and largest volume components of
electronic products, there is a need to develop more compact, modular,
lower cost power packaging structures that can ensure reliable
operation of the module under harsh environmental (high temperatures
and humidity, salt spray) and operational (high voltages, currents and
power dissipation levels) loading conditions. Module reliability is
essential to maximize performance, minimize life cycle cost, and ensure
safety.
Cost effective development, manufacture and use of power
modules will, therefore, require a fast and an inexpensive method of
evaluating reliability in the earliest stages of conceptual design. To
promote this development, CALCE EPSC, with the sponsorship of the
Office of Naval Research and Consortium members, has developed a set of
new physics-of-failure models to address the dominant failure
mechanisms in power electronic modules. Failure mechanisms addressed
include:
- fatigue of aluminum wedge wirebonds
- fatigue of high lead and lead-free solder die attach
- fracture of DBC (direct bonded copper) alumina substrates
- fatigue and stress relaxation of spring-loaded pressure contacts
Each model consists of a stress and damage model that uses the
environmental and operational loads, the system architecture and the
system materials as the inputs. The stress model describes the system’s
stress response to the applied loads; the damage model describes the
material response to the stress in terms of number of cycles or number
of hours to failure.
These models have been validated against accelerated thermal
cycling test results on sample coupons consisting of aluminum wires
bonded to DBC substrates, and against thermal shock test results on
actual ceramic hybrid power modules. The models are being used to
perform design reliability assessments on DC/DC converters this year as
part of CALCE core research project C03-11.
In addition, the models have been used to perform virtual qualification
and/or design assessments of ceramic hybrid power electronic modules
for a number of power electronics manufacturers and OEMs this year.
If you are interested in learning more about these efforts or
in having power electronic modules assessed for reliability, please
contact Dr. Patrick McCluskey at 301-405-0279.
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Electrochemical migration (ECM) is defined by the Institute for
Interconnecting and Packaging Electronic Circuits (IPC) as the growth
of conductive metal filaments on a printed circuit board (PCB) through
an electrolytic solution under the influence of a DC voltage bias
[IPC-TR-467A, 1997]. ECM can occur between leads and interconnects,
connector pins, electrodes in a capacitor, and traces on a PCB. ECM is
considered primarily a surface phenomenon to differentiate it from
conductive filament formation (CFF), which occurs internally in the
circuit board.
ECM can cause shorts due to the growth of metallic dendrites
and is often difficult to identify because the fragile dendrite
structure will burn, often leaving little trace it was there. These
failures are often intermittent and tend to be the primary cause of
failure in electronics that operate in benign environments, such as
telecommunications. As component pitch reduces and the use of leadless
packages increase, the amount of contamination trapped under packages
may increase unless new cleaning methodologies are used; hence, the
occurrence of ECM can be expected to rise.
An electrolytic solution, contamination, and a voltage bias are
necessary for ECM to occur, although some metals (e.g., silver) may
migrate with no contamination present. The solution is usually from
relative humidity that provides a water source. Water can then absorb
to the surface. Any contamination on the surface, such as halides or
weak organic acids, can increase the conductivity of the water. Once
this solution bridges two oppositely biased conductors a local
electrochemical cell is created. Metal atoms are dissolved into
solution at the anode, creating a metal ion. The metal ion then moves
through the solution and is deposited at the cathode. This process
continues, forming metal dendrites.
The primary factors affecting ECM are voltage bias, relative
humidity, temperature, contamination type and amount, conductor
metallization, and conductor spacing. As voltage bias, relative
humidity, temperature, and contamination concentrations increase and
conductor spacing decreases, ECM may become more prevalent.
Contamination is a preventable primary factor in printed
circuit boards. One common source of contamination is flux remaining on
the board due to insufficient cleaning. Other contamination can occur
during the assembly process, such as fingerprints. CALCE EPSC
recommends that statistical process control be used during the
manufacturing process to help identify potential contamination sources.
The use of conformal coatings is also recommended as a preventative
measure to protect against ECM. However, if the conformal coating is
applied to a contaminated printed wiring board, ECM may still occur.
Currently, CALCE EPSC is conducting experiments to determine
the correlation between chloride concentration and electrochemical
migration. CALCE EPSC is continuing ECM experiments in the coming year
with the use of fluxes to lay the foundation for developing a
time-to-failure model for ECM.
To obtain further information, contact Dr. Michael Osterman at 301-405-8023.
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Electrostatic discharge (ESD) is often a cause for device level
failures in electronic equipment but is often left uninvestigated and
accepted as inevitable. Semiconductor devices are susceptible to
direct, indirect and latent damage when subjected to ESD. Direct damage
results from physical destruction or degradation of a part of a device.
An indirect failure occurs when a device changes state due to conducted
or radiated electromagnetic interference (EMI) initiated by the
discharge. Latent failures are time-dependent phenomena. They occur
when a discharge makes a device susceptible to failures during
operation; although there is no apparent damage. ESD is often a common
scapegoat for unexplained failures. Further, there is a
misunderstanding regarding the differences between electrical
overstress (EOS) and ESD failures, and relationships between them.
For the past three years, the CALCE EPSC has worked with
companies including Samsung and Huawei, China to resolve suspected ESD
and EOS problems. The center conducted a physics-of-failure based root
cause analysis to identify the failure sites and mechanisms. The
analysis process starts with observation, recognition, recording, and
reporting of the discrepant condition, fact gathering through failure
mode and effect analysis, and part analysis, such as preliminary
analysis, external package analysis, physical dissection,
decapsulation, verification of failure after decapsulation and internal
visual inspection.
The CALCE EPSC has developed a team dedicated to ESD related
failure analysis to help companies identify whether a failure is caused
by EOS, ESD or other mechanisms and suggest best methods to replicate
failures; to test parts to the industry accepted standard human body
model (HBM), charged body model (CBM) and machine model (MM). The team
can also help develop an ESD control program. Benefits include not only
reduced ESD failures but also other possible reliability issues that
may have been masked by ESD. The team is headed by industry-experienced
professionals equipped with state-of-the-art instruments. Contact Dr. Osterman for further information.
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The CALCE EPSC and the nuclear reactor facility at the University of
Missouri-Rolla teamed up to explore lower cost alternatives to mitigate
the negative effects of ionizing radiation on COTS components. Testing
of sample parts under different shielding conditions revealed that the
polymer shielding process is a promising alternative to radiation
hardening. Further work is continuing for different part types and
opto-electronic components. Rather than develop radiation-hardened
electronics at high cost, the project assessed the potential for
shielding of radiation-resistant polymer/tungsten carbide composites
relative to a lead (Pb) reference. Malfunctions of components are
either due to a cumulative ionizing dose and/or a single event effect,
whereby a single, energetic dose or particle disables the part. Contact
Dr. Diganta Das or Dr. Akira Tokuhiro for further details.
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CALCE EPSC is collaborating with Merix, a printed circuit board
manufacturer, on a National Science Foundation grant focused on
decision-making for environmentally responsible product development.
The perspective of this program is that product development is an
information flow governed by those who make both design decisions and
development decisions under time and budget constraints, i.e., that
product development is treated as a decision production system.
At Merix, the focus will be on the generation of material
disclosure statements (MDS) that inventory all the materials present in
a finished product. Merix is being required to produce material
disclosure statements by many of its customers who are already
complying with (or preparing to comply with) various types of worldwide
environmental legislation. The information flow within Merix, in its
supply chain, and to its customers is being modeled to create a
representation that identifies the participants, the decision-making
and information-processing activities, and the nature of the
information flows.
The model will be used to determine the quality standards for
material disclosure statements, to identify mismatches between
available data and data needed to complete these statements, and to
identify other decision-making processes that use similar environmental
information.
The University of Maryland has also opened a dialogue with
Motorola to understand how a Merix customer is using MDS data and how
Motorola will articulate requirements for material disclosure
statements to their suppliers and qualify their suppliers to produce
MDSs in the future.
For more information, contact Dr. Peter Sandborn at 301- 405-3167.
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