Transcript 2004 MAPLD The Negative Impact of Lead-free Products on Aerospace & Military Electronics Reliability Andy Kostic, Ph.D. Charlie Minter Fellow Northrop Grumman Electronic Systems Product Integrity Engineering Baltimore, MD [email protected] Technical Risk.

2004 MAPLD
The Negative Impact of Lead-free Products
on
Aerospace & Military Electronics Reliability
Andy Kostic, Ph.D.
Charlie Minter
Fellow
Northrop Grumman
Electronic Systems
Product Integrity Engineering
Baltimore, MD
[email protected]
Technical Risk Manager
Best Manufacturing Practices Center of Excellence
College Park, MD
[email protected]
2004 MAPLD
Some Facts About Lead Poisoning
• Harmful effects of lead (Pb) on the human body are well
documented
–
–
–
Acts as a neurotoxin
Inhibits hemoglobin production
Affects brain development
• Lead ingress to human body known to occur through inhalation
•
and ingestion
Dramatic reduction in human blood lead levels since 1972, the
beginning of transition to lead-free gasoline
–
78% reduction documented in 1991 by EPA
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Lead-Free Movement Background
•
In 1985, the Swedish government enacted the Chemical Products Act
based on the “Precautionary Principle”
– Precautionary Principle: “When an activity raises threats of harm to
human health or the environment, precautionary measures should be
taken even if some cause-and-effect relationships are not fully
established scientifically.”
•
There is no evidence linking the lead used in electronics
manufacturing and products to any harm to humans or the
environment
– Electronics industry uses less than 0.5% of world lead consumption
– No mechanism established for transfer of lead to blood through direct
contact or proximity to lead in electronics
– No evidence of elevated lead in blood of soldering personnel
There are a host of issues associated with lead-free, but danger
from landfill contamination is not one of them
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EU Legislation
• Fears over perceived harm from lead precipitated legislative action
in Europe
–
European Union (EU) drafted legislation related to recycling (Waste
Electrical and Electronic Equipment – WEEE) and prevention
(Restriction Of certain Hazardous Substances – ROHS) scheduled to
take effect in July 2006
• Pb (among other substances) used in electronic soldering to be
banned by pending EU legislation
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Japanese Initiatives
• Japan’s lead-free movement cites landfill space crisis and
•
potential for lead leaching into water supplies
Japan running out of landfill space; Government and industry
looking for ways to reduce amount of waste
–
–
–
Emphasis on costly and burdensome recycling
Some manufacturers looking to eliminate lead from products to
avoid recycling requirement
Ban on lead from electronic solder and component finishes will not
solve landfill problems
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Status of Legislation in the U.S.
• U.S. EPA considers both lead and silver toxic metals
–
Neither may exceed a maximum level of 5.0 mg/l in a landfill water
sample test
• Currently no Federal lead elimination legislation enacted /
pending in U.S.
–
–
Probable that some law will be enacted restricting lead use
January 2001, EPA published a rule that classified lead and lead
compounds as persistent, bioaccumulative, and toxic (PBT)
chemicals
• Tightened previous weight reporting thresholds requirements
on manufacturers by factor of ~ 250X
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Brief History of Lead-Free Movement
Industry Lead Consumption
Product
Consumption (%)
Storage Batteries
80.81
Paints, Ceramics, Pigments, Chemicals
4.78
Ammunition
4.69
Sheet Lead
1.79
Cable Covering
1.40
Casting Metals
1.13
Brass / Bronze Billets and Ingots
0.72
Pipes, Traps, Extruded Products
0.72
Solder (Excluding Electronic Solder)
0.70
Electronic Solder
0.49
Miscellaneous
2.77
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Source: Advancing Microelectronics, September/October 1999. p. 29
2004 MAPLD
Facts to Consider
• The mere presence of lead does not constitute hazard to humans
–
–
Miners working in Colorado lead mines and living on soil with 20,000 ppm
contamination (EPA accepted levels for most soil is 500 ppm) had lead
blood levels approximately 10 times lower than national standard set by
centers for disease control
People in downtown Cincinnati have lead blood levels about 5 times
higher than miners in Colorado
• Major contributors to landfill pollution
–
–
Lead-acid batteries (48.1%)
TV picture tubes and computer CRTS (35.8%)
• Both are exempt from current draft legislation or proposed industry
regulations
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Replacements More Toxic
• Proposed replacements for current tin-lead (SnPb) soldering
compound
–
–
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Tin-silver-copper (SnAgCu)
Tin-silver-bismuth (SnAgBi)
Both significantly more toxic than SnPb
• Silver has been cited as 70 to 90 times more toxic to humans and
dangerous to aquatic life than lead
Environmental justification for
lead-free electronics is unfounded
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Comparison of Human Toxicity Potential
(Extraction to Finished Product)
Source: Neil Warburg, IKP University of Stuttgart, Life-Cycle Study
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Comparison of Acidification Potential
(Extraction to Finished Product)
Source: Neil Warburg, IKP University of Stuttgart, Life-Cycle Study
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Comparison of Global Warming Potential
(Extraction to Finished Product)
Source: Neil Warburg, IKP University of Stuttgart, Life-Cycle Study
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Expected Lead Reduction
• Texas Instruments (TI) is a $9.83B electronics component
manufacturer that sells millions of devices all over the world.
They estimate that complete conversion to lead-free products
would result in an annual worldwide lead reduction equivalent
to only ten automobile batteries.
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Critical View of Motivating Factors
• Environmental rationale for lead-free conversion is not based on good
science – Why does this argument persist?
– Lack of knowledge
• Many acting on bad information that converting to lead-free will be
less toxic and help the environment
Fear of expected laws and regulations
–
– Marketing pressure
– Public likes “Green Products”
– “Lead-Free” sells
• “Silver-Free” will not sell
– Competitors changing, so they must too
• “Lemming Effect”
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Critical View of Motivating Factors
• Many are compromising principles for profit
– Lead-free products sell better
– Replacement materials are usually slightly cheaper
• Reliability of critical military / commercial aerospace systems will be
adversely impacted by lead-free conversion
– Increasingly dependent on commercial parts
– Exemption from EU legislation provides no benefit if reliable parts cannot
be obtained
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Potential Risks Associated with Lead-Free
• Entire manufacturing process must be revised to properly apply
the replacement materials
–
–
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•
•
•
•
Higher Reflow Soldering Temperatures
Circuit Board Glass Transition Temperatures
Changes in Part Moisture Sensitivity Levels (MSLs)
Unknown / untested life cycle reliability
Solderability
Use of lead-free solder may result in brittle solder joints
New solder fluxes needed
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Potential Risks Associated with Lead-Free
• Mixing technologies during assembly, repair, or upgrade
• Difficulty in tracking parts with differing finishes
• Inability of contractors to know what type parts they are getting;
•
plating may be performed by a third or fourth tier supplier
Occurrence of tin whiskers – reemergence of an insidious failure
mechanism
No standards or guidelines yet
developed to mitigate risks
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Tin Whiskers – One Critical Risk Issue
• Industry trend – commercial electronics moving to lead-free
components and solder
• Lead-free components vulnerable to tin whisker formation
• Military and aerospace industry, highly dependent on
commercial components, will be at higher risk
• Nationally important issue – impacts national security and
human safety
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What are Tin Whiskers?
• Tin whiskers are spontaneous, single crystal, hair-like growths from
surfaces that use lead-free Tin (Sn) as a final finish
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–
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•
Electrically conductive
May grow in days or years
Tin-plated electronic and mechanical parts
(e.g., nuts, bolts) grow whiskers
On hybrid
microcircuit lid
Whisker growth mechanism still not fully understood after
decades of study
–
Much conflicting experimental/documented evidence
•
No effective and accepted tests to determine the susceptibility
of platings to whisker
•
No mitigation technique guarantees protection against whisker
formation except the addition of 3% or more of lead to tin
Photo Courtesy of NASA Goddard Space Flight Center
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Tin Whiskers Background
• Phenomenon observed since 1940s
• Growth varies widely
–
–
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Within hours
After years of dormancy
Anytime in between
• Whisker shapes and forms vary from few microns to several
millimeters
• Up to 200 whiskers per square millimeter have been observed
• Whiskers can grow through thin conformal coating
• Major ad hoc government-industry group has formed to address tin
whisker problem – CALCE Tin Whiskers Group
http://www.calce.umd.edu/lead-free/
http://nepp.nasa.gov/whisker/
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What Causes Tin Whiskers?
Plating Chemistry
Pure Sn Most Prone
Some Alloys (Sn-Cu, Sn-Bi, rarely Sn-Pb)
Use of “Brighteners”
Incorporated Hydrogen
Codeposited Carbon
In General,
pH
Plating Process
Current Density
Bath Temperature
Bath Agitation
Deposit Characteristics
Grain Size/Shape
Crystal Orientation
Deposit Thickness
Sn Oxide Formation
Substrate
Material (Brass, Cu, Alloy 42, Steel, etc.)
Substrate Stress (Stamped, Etched, Annealed)
Intermetallic Compound Formation
Substrate Element Diffusivity into Sn
Factors that
Increase STRESS or
Promote DIFFUSION
Within the Deposit
GREATER
WHISKER
PROPENSITY
Environment
Temperature
Temperature Cycling (CTE Mismatch)
Humidity (Oxidation, Corrosion)
Applied External Stress
(Fasteners, bending, scratches)
Current Flow or Electric Potential???
HOWEVER….
Many Experiments Show Contradictory Results For These Factors
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Courtesy of Jay Brusse, NASA GSFC
2004 MAPLD
One Model for Whisker Growth Mechanism
1. Substrate elements (e.g., Cu, Zn) diffuse into Sn along grain boundaries.
2. Intermetallic Compound (IMC) may form preferentially in grain boundaries
3. As a result, stress builds in Sn layer
4. To relieve stress, whiskers EXTRUDE through ruptures in Sn oxide
Whisker
Sn Oxide
Sn Layer
IMC
(e.g., Cu6Sn5)
Substrate (e.g., Cu)
Sn Grain
Boundaries
Courtesy of Jay Brusse, NASA GSFC
Dormant missiles particularly vulnerable
2004 MAPLD
•
Why an Issue Now?
Smaller circuit geometries
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Matte Tin Plated 28pin SOIC
Stored at Ambient for 3 yrs
Lower voltages
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•
Whiskers can now easily
bridge between contacts
Adjacent whiskers can
touch each other
Broken off whiskers
can bridge board traces
and foul optics or jam MEMS
Whiskers can handle tens of milliamps without fusing
Manufacturers rapidly going to ‘green’ materials
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–
Pure tin plate included
Some changes made without notice
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Photo Courtesy Peter Bush, SUNY
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Tin Whisker Failure Mechanisms
• Stable short circuit in low voltage, high impedance
circuits where current insufficient to fuse whisker open
•
Transient short circuit until whisker fuses open
• Plasma arcing in vacuum potentially most destructive whisker can fuse open but the vaporized tin may initiate
a plasma that can conduct over 200 amps! Atmospheric
conditions with additional voltage/current may also
experience whisker induced arcs
•
Debris/Contamination: Whiskers or parts of whiskers
may break loose and bridge isolated conductors or
interfere with optical surfaces or microelectromechanical
systems (MEMS)
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Tin Whisker Example
Normally, whiskers are so thin that they
are difficult to see without a microscope
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Tin Whisker Example
Connector Pins
(Pure Tin-Plated)
~10 years old
Observed in 2000
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Courtesy of Jay Brusse, NASA GSFC
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Tin Whisker Examples
Whiskers penetrating acrylic conformal coating
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Courtesy of Tom Woodrow, Boeing
2004 MAPLD
Documented Failure: Tin Whisker Short
Microcircuit Leads
(“Matte” Tin-Plated)
Pin #6
Pin #7
Whiskers from this component caused a FAILURE in the Electric Power
Utility Industry > 20 YEARS!!! after fielding the system
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Courtesy of NASA GSFC
2004 MAPLD
TIN WHISKER FAILURE ON CRYSTAL OSCILLATOR
THRU HOLE OSCILLATOR.
EDGE OF
SOLDER DIP
LEAD DIAMETER 18 MILS.
BRITE TIN FINISH LEADS AND CASE.
SOLDER DIPPED WITHIN 50 MILS
OF GLASS SEAL AND HAND
SOLDERED TO PWB.
Courtesy RON FOOR, GDC4S, 27 FEB 2004
Kostic
TIN WHISKER GROWTH NOTED FROM SEAL TO ABOUT 20
MILS FROM EDGE OF SOLDER COAT. ELECTRICAL
FAILURE WAS TRACED TO A 60 MIL WHISKER THAT
SHORTED LEAD TO CASE.
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Stress Inputs vs. Time
Tin Whisker Growth on Component Leads
Plate
leadframe
and build
component
Build
board
and
system
deploy system to field
Goal “life” for commercial parts
Goal “life” for commercial hi-rel parts
Typical missile warranty
Typical missile service life
5yrs
10yrs 15yrs 20yrs
Plating
processes
25yrs
30yrs
TIME
Increasing compressive stress
from continual formation of
intermettallic compounds
Lead bending during board
assembly and installation
Exposure to temperature
cycling, vibration, humidity
........................................ applied stresses ..................................
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Courtesy Bill Rollins CALCE Tin Whisker Group
2004 MAPLD
Documented Tin Whisker Failure Experience
•
Weapon systems that were built between 1985 and
1992 have had documented tin whisker failures
– Failure rates varied from 1% to 10%
– Manufacturers of microcircuits/semiconductors BEGAN
shifting to pure tin in 1996-97
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•
6 Satellites: partial or complete loss (Galaxy – 3,
Solidaridad 1, Direct TV3, and HS 601) 1998-2002
Airborne radar systems
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Tin Whisker Mitigation Techniques
•
Matte tin (tin with a dull low gloss finish and larger grain size)
is more resistant to whiskering that bright tin
– It can still grow whiskers
•
Annealing tin can reduce the stresses in plating that contribute
to whisker growth
– The benefits are limited and only short term
•
Robotic solder dipping with tin-lead solder is a solution for
some, but not all, components.
– Components must be handled carefully to avoid damaging
them during the process
– Navy-Raytheon process verification project in progress
None of these are proven to provide the required degree
of protection for high reliability equipment.
Kostic
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Tin Whisker Mitigation Techniques
•
Conformal coatings can be applied, but their success is very
dependent on the coating material, thickness, and application
process
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•
This complex topic requires further investigation
Striping the finishes and replating with lead-tin solder is possible but
requires extra handling and exposure of finished parts to corrosive
materials
–
This sets the stage for corrosion related issues
None of these are proven to provide the required degree
of protection for high reliability equipment.
Kostic
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Timeline for “Tin Avoidance”
2003
2004
Number of Component
Suppliers that switched to
pure tin
2005
2006
2007
No lead allowed
in Europe & Japan
Suppliers offering parts
without pure tin
Window of opportunity to
buy tin-lead parts
Inadequate tin whisker
mitigation technology to
allow use of all pure tin
parts.
Need major conformal coating study
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Summary
Transition to Lead-Free Electronic Components
Poses Serious Risk to National Defense
• Environmental justification for conversion to lead-free is invalid
• Current replacements for lead-based solder compounds appear
•
•
•
to be more toxic and harmful to the environment
Reliability of products manufactured with lead-free components
will be lower
Dependence of military / aerospace industry applications on lower
reliability lead-free components will result in higher risk
Cost of parts manufacturing conversion to lead-free components
will be on the order of billions of dollars
Kostic
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