Despite RoHS, Products Must be Rugged and Reliable
Packaging Magazine, Volume 2, 2006
by John Starr, CirVibe
Contrary to some belief, designing a lead-free electronics
product that is rugged and reliable is easy—the product must meet or exceed
design service life exposure and it must be free of workmanship defects (infant
mortality failures eliminated).1
The age of RoHS is upon us. Even though the military and many other programs
are exempt from these regulations, the old laws of supply and demand are already
thinning out the availability of leaded components and it’s unlikely that
suppliers will continue to offer two versions of the same component. The
following article, part of PKG’s continuing coverage of critical RoHS issues,
looks at one mechanical aspect of RoHS implementation and how to test designs to
make them rugged and reliable despite some of the inherent weaknesses of
lead-free solder. –Ed.
All other factors equal, lead-free products are less rugged under vibration
than leaded. Because of a greater potential for solder joint problems, the need
to fully understand ESS (Environmental Stress Screening) is significantly
increased. Common lead-free solder joint failures include non-wetting, colder
solder joints, blow holes, solder balls, insufficient solder, icicle formation,
and more. ESS is critical to developing and maintaining a manufacturing process
that produces reliable products. Methods for rugged and reliable production must
include in-depth product understanding for design and ESS processes.
Lead-Free Solder Is Weaker—New Methods Needed
The defense and aerospace industries are exempt from lead-free restrictions,
however, it’s almost inevitable that they will need to change to lead-free parts
as manufacturers discontinue production of leaded products due to demand. A
joint DoD / NASA / Industry consortium, Joint Council on Aging Aircraft/Joint
Group on Pollution Prevention, has been evaluating the impact of lead-free
electronics on reliability (http://acqp2.nasa.gov/JTR.htm). The consortium’s
project is called the JCAA/JG-PP No-Lead Solder Project and boasts members from
all branches of the Armed Services, NASA, Boeing, Rockwell-Collins, Raytheon,
BAE Systems, ACI, Lockheed Martin, Texas Instruments, NCMS, Sandia National
Labs, and Marshall Space Flight Center among others.
The No-Lead Solder project tests included vibration. Vibration life
understanding is critical to product ruggedness and reliability.
The JCAA/JG-PP test items were specially constructed circuit cards, capable
of instant detection of failure of components. Circuit cards were populated with
CLCCs, PLCCs, TSOPs, TQFPs, BGAs and PDIPs. Sets of identical components were
used in different positions. Thirty circuit cards were vibration tested. A
picture of one of the cards is shown in
, with components labeled with their failure times. Step stress tests were
conducted to create failures. Step stress tests start with a fixed vibration
) . During each successive step, the input excitation was increased. Due to
the exponential relationship between stress and life-use rate, each successive
step accumulates damage at higher rates.
Vibration of electronics is complex. Time-to-failure differences are affected
by normal fatigue scatter, dimensional variations, multi-mode response
variations, test control, material properties control, production process, etc.
Each product is unique in design details and requirements. Each component
location experiences a different stress condition. Understanding a product’s
capabilities generally requires extensive testing or combined analysis and
testing due to product uniqueness. Products are unique at assembly level but are
not as unique at component level. For “test-only methods,” only identical
components in identical locations on identical test boards can be directly
compared. The JGAA/JC-PP tests compared failure times of components in identical
positions on the 30 boards. Lead-free test solder was used on one set of test
boards and the SnPb control solder on a second set of test boards.
The tests conducted on vibration showed a significant reduction in life
capabilities for lead-free designs. The preliminary report for the consortium
concluded that developers of electronics may need to develop new design
practices or methods to compensate for the strength and durability reductions.
Current Method Evaluation
Even though the development of electronic parts to military standards stopped
decades ago, manufacturers have made progress in component environmental life
capability. Commercial electronics also experience vibration, drop shocks and
hot and cold exposure. Since development of weak components could hurt
commercial sales, most components have some level of “mechanical loading”
capability. Circuit card assembly processes for leaded solder assemblies have
also improved over the years, greatly reducing the risk of flawed products due
to production process problems. Natural evolution has improved component quality
and production process methods.
There is an added advantage for designing for shock or vibration loading. A
circuit card’s natural shock or vibration response protects most components from
high stress, putting only a few parts at highest risk of failure. Even under
design development methods lacking an evaluation of shock or vibration
capabilities, there has been a reduced risk of that type of failure as component
capabilities increased and production processes improved.
Does lack of failure in a vibration test mean the product is fully
understood? Recently, a large government user of electronic products expressed
disappointment in the reliability of purchased products. Not only did some
products fail to meet reliability goals, but 30% of the time the requested
redesigns were less reliable than the original. A 30% value implies redesign
methods were not much better than “a best guess redesign,” not an in-depth
understanding of the product at point-of-failure level. This also implies that
the original reliability failures could have been due to inadequate development
and/or production processes.
Common Bad Practices
There are some development practices that can account for reliability
shortfalls. These “risky” practices used in the electronics industry
• Predefined ESS—Not customized to product
Frequently the ESS vibration spectrum is defined for a product before it is
designed. This was a lower risk situation for leaded electronics. If the
production process is reliable, the only real need for ESS is that it does no
harm. If the production process poses risk—ESS is far more complex and must be
customized to actual product details.
• Design for vibration by equation without design rules of limits of
Circuit card assemblies are extremely complex structures. A simple equation
is not capable of characterizing the fatigue life of a structure as complex as a
circuit card assembly.
• Reliability for vibration by parts count
Many reliability methods include evaluation of circuit card assemblies by
parts count, which is inherently inaccurate. Identical components on the same
assembly can experience damage rates that are many orders of magnitude apart. A
well advertised reliability database boasts 1012 years of field reliability, but
includes no vibration data. Reliability for electronics requires detailed
understanding of how it fails.
• HALT (or any test) without point-of-failure understanding
HALT (Highly Accelerated Life Testing) is a systematic method for finding
product fragility levels. Most documented HALT processes include the step
“understand the product at point-of-failure level whenever failure occurs.” This
step is often neglected, but it is one of the most important steps. Failures
provide an opportunity to learn about product design and should be fully
exploited. Comparing ruggedness of products with different failure levels can be
meaningless without product understanding.
Even with bad practices, many companies are still able to produce reliable
products with leaded solder electronic products. Inherent product ruggedness
from decades of experience helps reliability. However, RoHS is a new glitch in
An Answer to New Method Needs
One solution to the problem is offered by CirVibe Inc., which includes
software and provides training in methods of development of rugged and reliable
electronics that have worked for leaded products and meet the needs of the
“JGAA/JC-PP new method” required for lead-free. This approach uses
point-of-failure methods to define position-based damage rooted in the mechanics
of load transfer and physics of failure.
These methods were used to evaluate the JGAA/JC-PP vibration data. CirVibe
analysis verified that the lead-free products had lower vibration strength, and
using point-of failure methods numerically compared failures in different
locations in the design. By including failure data across positions with
point-of-failure analysis, the differences between solder types could be defined
to a higher level. The original consortium report and the CirVibe analysis
report are posted on NASA’s Web site.
In addition to obtaining better definition of strength differences of solder
types, position damage rates were used for predictions of failure time for a few
components for both leaded and lead-free designs.
shows one set of the predicted times compared to actual time to failure.
These are times-to-failure based on damage accumulation rates in the accelerated
step stress test. This set was for leaded BGAs. As can be seen, fatigue
predictions do not result in exact agreement with time to failure under test.
Fatigue failures have large scatter. Scatter also occurs in time-to-failure for
components in identical positions on different boards.
Rugged and Reliable Electronics
Obtaining reliable and rugged electronics with RoHS will be accomplished by
applying the methods that have worked to obtain increased ruggedness or
increased reliability of electronics in the past. This process can be
accelerated by eliminating poor practices. By elimination of poor practices,
greater ruggedness and higher reliability can be obtained at reduced cost. This
must include properly run Accelerated Life Tests (ALT) and Environmental Stress
Screening (ESS) processes supported by detailed understanding of the product.
Evaluating failures is required, numerically defining them in a meaningful way
at point-of-failure level. Failure data must be created in a “universal” format,
not a format meaningful to a single product.
Quick fixes without incorporating lessons learned may obscure the problem of
today and create a problem for tomorrow. ESS is at its best—effective and
non-damaging—when you know your product’s weakest parts under ESS and what in
the assembly is effectively screened.