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2004 MAPLD. The Negative Impact of Lead-free Products on Aerospace & Military Electronics Reliability. Charlie Minter Technical Risk Manager Best Manufacturing Practices Center of Excellence College Park, MD Charlie@bmpcoe.org. Andy Kostic, Ph.D. Fellow Northrop Grumman

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slide1

2004 MAPLD

The Negative Impact of Lead-free Products

on

Aerospace & Military Electronics Reliability

Charlie Minter

Technical Risk Manager

Best Manufacturing Practices Center of Excellence

College Park, MD

Charlie@bmpcoe.org

Andy Kostic, Ph.D.

Fellow

Northrop Grumman

Electronic Systems

Product Integrity Engineering

Baltimore, MD

Andrew.Kostic@ngc.com

some facts about lead poisoning
Some Facts About Lead Poisoning
  • Harmful effects of lead (Pb) on thehuman 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

2

lead free movement background
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

3

eu legislation
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

4

japanese initiatives
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

5

status of legislation in the u s
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

6

brief history of lead free movement
Brief History of Lead-Free Movement

Industry Lead Consumption

7

Source: Advancing Microelectronics, September/October 1999. p. 29

facts to consider
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

8

replacements more toxic
Replacements More Toxic
  • Proposed replacements for current tin-lead (SnPb) soldering compound
    • 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

9

comparison of human toxicity potential extraction to finished product
Comparison of Human Toxicity Potential(Extraction to Finished Product)

Source: Neil Warburg, IKP University of Stuttgart, Life-Cycle Study

10

slide11

Comparison of Acidification Potential(Extraction to Finished Product)

Source: Neil Warburg, IKP University of Stuttgart, Life-Cycle Study

11

slide12

Comparison of Global Warming Potential(Extraction to Finished Product)

Source: Neil Warburg, IKP University of Stuttgart, Life-Cycle Study

12

expected lead reduction
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.

13

critical view of motivating factors
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”

14

critical view of motivating factors15
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

15

potential risks associated with lead free
Potential Risks Associated with Lead-Free
  • Entire manufacturing process must be revised to properly apply the replacement materials
    • 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

16

potential risks associated with lead free17
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

17

slide18

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

18

slide19

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
    • Electrically conductive
    • May grow in days or years
    • Tin-plated electronic and mechanical parts

(e.g., nuts, bolts) grow whiskers

  • 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

On hybrid

microcircuit lid

Photo Courtesy of NASA Goddard Space Flight Center

19

tin whiskers background
Tin Whiskers Background
  • Phenomenon observed since 1940s
  • Growth varies widely
    • 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/

20

what causes tin whiskers
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
    • pH
  • Substrate
  • Material (Brass, Cu, Alloy 42, Steel, etc.)
  • Substrate Stress (Stamped, Etched, Annealed)
  • Intermetallic Compound Formation
  • Substrate Element Diffusivity into Sn

In General,

Factors that

Increase STRESS or

Promote DIFFUSION

Within the Deposit

GREATER

WHISKER

PROPENSITY

  • Plating Process
    • Current Density
    • Bath Temperature
    • Bath Agitation
  • Environment
  • Temperature
  • Temperature Cycling (CTE Mismatch)
  • Humidity (Oxidation, Corrosion)
  • Applied External Stress (Fasteners, bending, scratches)
  • Current Flow or Electric Potential???
  • Deposit Characteristics
    • Grain Size/Shape
    • Crystal Orientation
    • Deposit Thickness
    • Sn Oxide Formation

HOWEVER….

Many Experiments Show Contradictory Results For These Factors

21

Courtesy of Jay Brusse, NASA GSFC

slide22

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

Substrate (e.g., Cu)

IMC

(e.g., Cu6Sn5)

Sn Grain

Boundaries

Courtesy of Jay Brusse, NASA GSFC

Dormant missiles particularly vulnerable

why an issue now
Why an Issue Now?
  • Smaller circuit geometries
    • 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

  • Lower voltages
    • Whiskers can handle tens of milliamps without fusing
  • Manufacturers rapidly going to ‘green’ materials
    • Pure tin plate included
    • Some changes made without notice

Matte Tin Plated 28pin SOIC Stored at Ambient for 3 yrs

Photo Courtesy Peter Bush, SUNY

23

tin whisker failure mechanisms
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)

24

tin whisker example
Tin Whisker Example

Normally, whiskers are so thin that they are difficult to see without a microscope

25

tin whisker example26
Tin Whisker Example

Connector Pins

(Pure Tin-Plated)

~10 years old

Observed in 2000

26

Courtesy of Jay Brusse, NASA GSFC

slide27

Tin Whisker Examples

Whiskers penetrating acrylic conformal coating

27

Courtesy of Tom Woodrow, Boeing

slide28

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

28

Courtesy of NASA GSFC

tin whisker failure on crystal oscillator
TIN WHISKER FAILURE ON CRYSTAL OSCILLATOR

THRU HOLE OSCILLATOR.

LEAD DIAMETER 18 MILS.

BRITE TIN FINISH LEADS AND CASE.

SOLDER DIPPED WITHIN 50 MILS OF GLASS SEAL AND HAND SOLDERED TO PWB.

EDGE OF SOLDER DIP

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.

Courtesy RON FOOR, GDC4S, 27 FEB 2004

29

stress inputs vs time tin whisker growth on component leads
Stress Inputs vs. TimeTin Whisker Growth on Component Leads

deploy system to field

Plate

leadframe

and build

component

Build

board

and

system

Goal “life” for commercial parts

Goal “life” for commercial hi-rel parts

Typical missile warranty

Typical missile service life

5yrs

10yrs

15yrs

20yrs

25yrs

TIME

30yrs

Increasing compressive stress

from continual formation of

intermettallic compounds

Plating

processes

Lead bending during board

assembly and installation

Exposure to temperature cycling, vibration, humidity

........................................ applied stresses ..................................

30

Courtesy Bill Rollins CALCE Tin Whisker Group

slide31

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
  • 6 Satellites: partial or complete loss (Galaxy – 3, Solidaridad 1, Direct TV3, and HS 601) 1998-2002
  • Airborne radar systems

31

tin whisker mitigation techniques
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.

33

tin whisker mitigation techniques34
Tin Whisker Mitigation Techniques
  • Conformal coatings can be applied, but their success is very dependent on the coating material, thickness, and application process
    • 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.

34

timeline for tin avoidance
Timeline for “Tin Avoidance”

2006

2007

2003

2004

2005

Number of Component Suppliers that switched to pure tin

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

35

summary
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

36