BMPCOE OverviewThe Best Manufacturing Practices Program

November 14, 2006

Brian Willoughby

www.bmpcoe.org

Reliability Impact of Current Tin Whiskers Assessments

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BACKGROUND

• 2001 Industry Migration to Lead(Pb)-free Electronics– Replace Pb With Pure Tin (Sn)– Use of Pure Sn Promotes Tin Whiskers Growth

• No Industry-Accepted Method for Fully Evaluating Predisposition to Form Tin Whiskers– Several Factors Contribute to Tin Whiskers Growth, but Study

Results Vary on Specific Impact of Each Factor– The National Electronics Manufacturing Initiative (NEMI) Credits

Stress as Cause for Tin Whiskers Growth, but Type and Degree of Stress Remain Debatable

Decades of Study Have Failed to Produce Conclusive Evidence for the Cause of This Growth Phenomenon

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Relay Leads

Photo courtesy of NASA Goddard Space Flight Center  

Whisker Growing Thru

~0.25 mil Thick Coating

Whiskers Growing Beneath

2 mil Thick Coating

Conformal Coat (Polyurethane)

Photos courtesy of NASA Goddard Space Flight Center 

TIN WHISKERS

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CAUSALITY FACTORS OF TIN WHISKERS

• Many Inconsistencies With Evaluation of Tin Whisker Causes:– Conflicting Results From Two Comparable Batches of Sn

• First Batch May Show High Degree of Tin Whisker Growth• Second Batch May Show Low Degree of Tin Whisker Growth

– Detection Models Measuring Likelihood of Whisker Growth Vary From Source to Source

– Lack of Validated Data Creates Reliability Concerns

• Industrywide Consensus of Four Basic Growth Factors: – Part Spacing (Less Than 5 mm Appears to be Critical)– Extended Time Periods in Service– Lack of Conformal Coating– Bright Gloss vs. Matte Finish

(Other Factors Such as Temperature and Stress Exist, but Degree of Impact and Direct Correlation to Tin Whisker Growth Is Debatable and Varies Among Studies)

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POTENTIAL RISKS OF Pb-FREE ELECTRONICS

• 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

• 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 (mechanical integrity)

– Use of lead-free solder may result in brittle solder joints– New solder fluxes needed

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EXPECTED LIFETIMES AND STRESSES

• Lifetime goals for products– Commercial – 5 years– Commercial High Reliability – 10 years– Missile Warranty – 15 years– Typical Missile Service Life -- > 25 years

• Stresses– Plating process – During Production– Lead bending (board assembly and installation)– During Production– Environmental – Primarily During Lifetime

• Temperature variations• Cycling• Vibration• Humidity

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MITIGATION• Studies Indicate Five Mitigation Options for Tin Whisker Growth:

– Universal Ban of all Sn. Use of Sn Alternatives is an Option, but Each Has a Disadvantage:

• Nickel Palladium Gold (NiPdAu) Lacks the Innate Corrosion Resistance of Sn• Palladium is Expensive (Approximately $1/kg)• Tin Silver Copper (SnAgCu) Susceptible Under Stress

– Use of Sn as Last Resort– Use of Sn With Conditions and Waivers

• Permitted or Restricted Under Preset, Predefined Conditions• Condition Could be Case-by-Case and/or Blanket Permission for Particular

Classification

– No Sn Restriction– Mixing Sn With Small Degree of Pb Contradicts Pb-Initiative

• Mix Tin With up to 3% Pb to Reduce Growth • Mix or Dip Sn With 5% (or More) Pb to Completely Eliminate Growth

Pb-Free Initiative Will Impact Reliability

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MITIGATIOND

egre

e of

Tin

Whi

sker

s Miti

gatio

n

Low High

Maximum

NoneProbability of a Reliable Tin Whiskers-Related System

Long-term Mitigation Effectiveness Not Fully Quantified as a Result of Varying Industry Opinions Regarding Sufficient Testing

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RECOMMENDATIONS

• Key Element of Tin Whisker Growth Reliability Analysis is Probability Calculation

• Most Apparent Risk-Reduction Practice Is Thorough Assessment of Sn Data vs. Criticality of System

• Trade Off: Excessive Mitigation vs. Excessive Cost– Huge Cost Risk in Qualifying and Sustaining a Multitude of

Pb-free Components

– Maintaining Separate Inventories, Processes, Pb and Pb-free Alloy Could Become a Process Management Nightmare

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BMPCOE RELIABILITY MODEL

MIL-HDBK-217F Reliability Prediction

Part MTBF Component A X hrs

Component B Y hrs

BMPCOE Tin Whiskers Reliability Model

% probability of Tin Whisker

ENHANCEDMIL-HDBK-217F Reliability Prediction

Part MTBF Component A X-new hrs

Component B Y-new hrs

MTBF = 2000 hrF/MH = 50

31% chance of Tin Whisker

Growth

MTBF = 1467 hr

F/MH = 68MTBF: Mean Time Between FailureF/MH: Failure per 1 Million Hours

BMPCOE Tin Whiskers Reliability Model Used to Factor MTBF Predictability

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BMPCOE RELIABILITY MODEL

• Estimates the Probability a System Failure is a Result of Tin Whisker Growth

• Factors MTBF Predictions from 217F

• Enhances Current Assessment of Risks

• Accepts Input Variables Regarding Sn Within a System– Gloss Finish vs. Matte Tin

– Spacing Greater Than 5 mm

– Conformal Coating

– Overall Time Period

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BMPCOE RELIABILITY MODEL

Tin Whiskers Growth Remains Full of Uncertainties

• Number of Sn Parts is Not a Factor of the Model– Studies Show That Quantity of Sn Has No Direct Correlation

With Tin Whiskers Growth

– A Batch of Five Parts Could Be More Susceptible to Tin Whiskers Growth Than a Batch of 1000 Sn Parts

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BMPCOE RELIABILITY MODEL

INPUT HERE:

800.00 Total Parts

INPUT HERE:

75.00% % of Total Parts w/ Tin

Area of Focus: 600.00 Actual Tin Part Totals

INPUT HERE:

1

Matte Tin, Yes or No1 = Yes0 = No

INPUT HERE:

0

Spacing of parts greater than 5 mm, Yes or No

1 = Yes0 = No

INPUT HERE:

1

Any type of Conformal Coating used, Yes or No

1 = Yes0 = No

INPUT HERE:

1200.00 Given Hours (hr) of MTBF

Updated MTBF: 940.72 New predicted MTBF as a result of the Tin Whisker Factor

Probability: 21.606%

LIkelihood Tin Parts is the Cause of Failure within given set of time, with

respect to conformal coating, spacing and brightness

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