Tin Whiskers: Attributes and Mitigation CARTS Europe October 2002 Nice, France Jay Brusse QSS Group, Inc. @ NASA Goddard Gary Ewell Jocelyn Siplon The Aerospace Corp. The Aerospace Corp.
Outline • Why ANOTHER Paper on Tin Whiskers? • What are Tin Whiskers? – Attributes – Examples – Failure Modes • Experience History • Tin Whiskers on Ceramic Capacitors (MLCCs) • Whisker Mitigation Strategies • Conclusions & Recommendations October 17, 2002 Tin Whiskers: Attributes and Mitigation 2
Why ANOTHER Paper on Tin Whiskers? • The PAST: – Tin Whiskers Known for ~60 Years – HUNDREDS of Independent Studies – Numerous Disparities Exist in Published Literature • The PRESENT: Combination of CONCERNING Factors Electronics Industry Conversion No Consensus Understanding of to Pure Tin Finishes Whisker Growth Mechanism(s) Due to Pending Pb-Free Legislation No “Accepted” Lower Application Voltages Accelerated Whisker Tests SMALLER Circuit Geometries “New” Discoveries of Whisker-Prone Items October 17, 2002 Tin Whiskers: Attributes and Mitigation 3
What are Tin Whiskers? • “Hair-Like” Single Crystal Structures that May Grow from Tin Finished Surfaces • LENGTH: Up to 10 mm (Typically < 1mm) DIAMETER: from 0.006 to 10 µ µ µ m µ • (Typical ~ 1 µ µ µ m) µ • Grow from the Base Not the Tip • Mechanical Stress Relief and Diffusion Processes in Tin Finish Drive Whisker “Extrusion” Whiskers are Fundamental Research NOT Dendrites is INCOMPLETE October 17, 2002 Tin Whiskers: Attributes and Mitigation 4
Factors that Influence Whisker Growth Plating Chemistry Substrate Pure Sn Most Prone Material (Brass, Cu, Alloy 42, Steel, etc.) Some Alloys (Sn-Cu, Sn-Bi, rarely Sn-Pb) Substrate Stress (Stamped, Etched, Annealed) Use of “Brighteners” Intermetallic Compound Formation Incorporated Hydrogen Substrate Element Diffusivity into Sn Codeposited Carbon In General, Factors that Plating Process Increase STRESS Environment High Current Density In Tin Deposit Bath Temperature Temperature Bath Agitation Temperature Cycling (CTE Mismatch) Humidity (Oxidation, Corrosion) GREATER Applied External Stress Deposit Characteristics (Fasteners, bending, scratches) WHISKER Grain Size/Shape Crystal Orientation PROPENSITY Deposit Thickness Sn Oxide Formation HOWEVER…. Many Experiments Show Contradictory Results For These Factors October 17, 2002 Tin Whiskers: Attributes and Mitigation 5
Sneaky Tin Whiskers!!! Incubation Period (DORMANCY) Initiation of growths may occur... – As Short as a Few Days after Plating, or – AS LONG AS MANY YEARS!!! AS LONG AS MANY YEARS!!! – Growth Rate – Up to 9 mm/yr – Typically Substantially SLOWER!!! These Attributes are UNPREDICTABLE thus Presenting a MAJOR Challenge October 17, 2002 Tin Whiskers: Attributes and Mitigation 6
Whisker Shapes & Features Nodules Solid/Striated Filaments Pyramids Kinks/Bends October 17, 2002 Tin Whiskers: Attributes and Mitigation 7
Examples of Components with Tin Whiskers 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 October 17, 2002 Tin Whiskers: Attributes and Mitigation 8
Examples of Components with Tin Whiskers Hybrid Microcircuit Package Lid Whiskers up to 2 mm Long Found Whiskers up to 2 mm Long Found Growing INSIDE Package Growing INSIDE Package Whiskers Like these Reportedly Have Broken Loose Inside Hybrids Creating Intermittent Shorts/Field Failures October 17, 2002 Tin Whiskers: Attributes and Mitigation 9
Examples of Components with Tin Whiskers Electromagnetic Relays (Tin-Plated Terminals, Case, Header) Whisker Shorts Between Terminal to Terminal, Terminal to Header, Case to Other Component, Whisker to Whisker!!! October 17, 2002 Tin Whiskers: Attributes and Mitigation 10
Examples of Components with Tin Whiskers Terminal Lugs (“AS-RECEIVED”) Whiskers up to 0.25 mm Long Inside Crimp Barrel October 17, 2002 Tin Whiskers: Attributes and Mitigation 11
Examples of Components with Tin Whiskers Test Points “Bright” Tin-Plated Phosphor-Bronze (Ambient Storage) October 17, 2002 Tin Whiskers: Attributes and Mitigation 12
Examples of Components with Tin Whiskers Fuse (Surface Mount Chip) “Matte” Tin-Plate Over Nickel Barrier Terminations-- Grew Whiskers after T-Cycle October 17, 2002 Tin Whiskers: Attributes and Mitigation 13
Examples of Components with Tin Whiskers Connector Pins October 17, 2002 Tin Whiskers: Attributes and Mitigation 14
Tin Whisker Failure Modes Electrical Short Circuits • Permanent (current < 10’s of mA) • Intermittent (current > 10’s of mA) Whisker Melts Whiskers BEND Due to Electrostatic Attraction Debris/Contamination Before After • Interfere with Sensitive Optics or MEMS 50 Volts • Shorts in Areas Remote From Whisker Origins METAL VAPOR ARC in VACUUM 0 Volts 0 Volts • Atmospheric Pressure < ~150 torr, Whisker Short can Vaporize into Highly Conductive PLASMA of Tin Ions if V > ~18 V and I > 10’s of Amps • Plasma can Form Arc Capable of Carrying HUNDREDS OF AMPERES • Arc Can Be Sustained by Tin Evaporated from Surrounding Areas October 17, 2002 Tin Whiskers: Attributes and Mitigation 15
“Reported” Tin Whisker-Induced Field Problems Space Application Medical Application Heart X Commercial Commercial Pacemaker Satellite #6 Commercial Satellite #4 RECALL Satellite #2 Commercial Satellite #5 X X Commercial Missile Commercial Satellite #3 Program #2 Missile Satellite #1 Program #1 Missile Military Program #4 Missile Aircraft Program #3 Tin Whiskers are NOT Just of Interest to Lab Researchers Defense Application October 17, 2002 Tin Whiskers: Attributes and Mitigation 16
Tin Whiskers and Multilayer Ceramic Capacitors (MLCCs) Past Research • Two Previous Papers by MLCC Manufacturers (1990 & 1997) Assert MLCCs Have Following Attributes that make them Highly Resistant to Whisker Ni-Underplate (> 2 µ µ m) µ µ “Large” Tin Grain, Well-Polygonized >5 µ µ µ m µ Reduces Diffusion that Causes Internal Stress Post-Plating Annealing “Thick” Matte Tin Plating Promotes Grain Growth & 5 - 10 µ µ m µ µ Reduces Residual Stress • ONE MLCC Mfr Experiment showed 18 Years WHISKER-FREE Observations for MLCCs Stored Continuously at 50°C HOWEVER ... HOWEVER ... October 17, 2002 Tin Whiskers: Attributes and Mitigation 17
PROFUSE DENSITY of Tin Whiskers Currently up to 240 µ µ µ µ m Long Found on Pure Tin-Plated MLCCs after T-Cycle + Additional Ambient Storage October 17, 2002 Tin Whiskers: Attributes and Mitigation 18
Example #1: Tin Whiskers and MLCCs MLCCs Inside Hybrid Microcircuit After T-Cycle User Application: Mfr • Hybrid Microcircuit (Herm-Sealed) “A” – Gold Plated Substrate Pads Substrate Line Spacing 125 µ m (min.) – • ORDERED Pd-Ag Terminated MLCC, but Supplier Shipped PURE TIN • Silver Epoxy Mounting Method Cross-Section of MLCC MLCC Construction (0805 size): • Barium Titanate Ceramic Body Silver Frit 17 µ µ m µ µ • Silver Frit Base Termination Ceramic (BaTiO3) Nickel 6.5 µ µ m µ µ • Nickel Barrier Layer Tin 6.5 µ µ µ m µ • Matte Tin-Plated Final Finish > 5 µ µ m µ µ • Average Grain Size October 17, 2002 Tin Whiskers: Attributes and Mitigation 19
Example #1: Tin Whiskers and MLCCs USER TEST ENVIRONMENT Test Condition : Temp Cycle: -40°C / +90°C (> 200 Cycles) T-Cycle Alone Produced Followed by “Ambient” Storage for 1 Year Whiskers ~ 100 µ µ µ m µ Density > 800/mm 2 Additional 1 Year Ambient Storage GROWTH Continues Currently 240 µ µ µ µ m max October 17, 2002 Tin Whiskers: Attributes and Mitigation 20
Example #2: Tin Whiskers and MLCCs Whiskers AFTER Vapor Phase Installation and T-Cycle Solder = Sn63 / Pb37 Pure Tin Plated Vapor Phase @ 217°C 2220 & 1812 Chip Caps Mfr “B” PCB = FR4 Whiskers up to 30 µ m Test Condition : Temp Cycle/Shock : -55°C / +100°C (50 - 400 Cycles) Soldering Operations DO NOT Always Reflow ALL Tin Surfaces Nor Mix them with the Mounting Solder October 17, 2002 Tin Whiskers: Attributes and Mitigation 21
User Whisker Mitigation Research on User-Mitigation Strategies is Limited • Most Approaches Come with Benefits & Limitations • Long-Term Effectiveness NOT Quantified Strategies to Consider (Complete Immunity NOT Guaranteed) • REDUCE STRESS in the Tin Plating Hot Oil Reflow / Hot Solder Dip (Preferably with Sn/Pb Solder) – – High Temp Anneal Substrate and Tin Finish – Underplate with Diffusion Resistant Barrier May Delay Onset • USE PHYSICAL BARRIERS to Insulate Against Potential Shorts – Conformal Coat or other Insulating Barriers – Increase Spacing of Surfaces of Opposite Polarity to > 0.5 inches • MINIMIZE REINTRODUCING STRESS thru Handling, Assembly & Application Combine MULTIPLE Mitigation Strategies to Increase Effectiveness AVOID PURE TIN, , if Possible AVOID PURE TIN if Possible October 17, 2002 Tin Whiskers: Attributes and Mitigation 22
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