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Observations on Whisker Formation on Electrodeposited Metallic Coatings used in

Electronics Manufacture

M.A.Ashworth, G.D.Wilcox, R.L.Higginson, R.J.Heath and Changqing Liu

Department of Materials

Wolfson School of Mechanical and Manufacturing Engineering Loughborough University

UK

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Co-investigators

Chris Stuttle, Roger Mortimer, Jing Wang, Han Jiang, Keming Chen, Andrew Ebbage and Liang Wu

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Presentation Contents

Introduction – what are tin (and zinc) whiskers?

An historical perspective on tin whisker investigations and current theories

Zinc whisker formation The WHISKERMIT research programme

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Nodule

Introduction to Whiskers

• “Hair-like” crystal structures that grow from surfaces coated by pure tin (or Zn or Cd) finishes

• Typically single crystals • Large variations in shapes and sizes - filaments (typically, 1-5µm diameter and <1mm in length) - nodules - odd-shaped eruptions (OSE)

Odd-shaped eruptions

Filaments

• Spontaneous growth in common service • Incubation period before growth • Growth mechanism is still not fully

elucidated • No useful models for predicting whisker

growth • Whiskers are not the same as dendrites

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Electrical Shorting Phenomena

Part A Part B

Fig.1 - Two mutually-isolated parts of one electrical circuit

Electroplated Tin coating

substrate

Short circuiting

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Issues Helping the Occurrence of Tin Whiskers

C.Minter, DMSMS Conf., Charlotte, USA, 10-13th July 2006.

• Miniaturisation in electronics – whiskers can bridge components and tracks

• Lower voltages – whiskers can sustain currents

• Environmental legislation – EU RoHS directive, pure tin often the first finish to replace Sn-Pb

Whiskers from an Historical Perspective

www.airforce.forces.gc.ca/.../liberator-eng.asp

S.C.Britton, ‘Spontaneous growth of whiskers on tin coatings: 20 years of observation’, Transactions of the Institute of Metal Finishing, 52, 95-102, 1974. • Electrodeposited tin coatings on brass should be applied over an undercoat of nickel or copper • Refrain from using bright tin •Tin electroplated coatings should be at least 8 μm if not flow melted • Heat treat tin coatings after electroplating • Avoid corrosion in storage and service environment • Hot dipped or flow melted coatings less problematic for whiskers • Sn-Pb coatings of over 8 μm are safer • Avoid local pressure • Immunity from whiskers from Sn(65%)-Ni alloy • Thick layers of resin effective as insulating barrier • Removal of whiskers by vacuum system

Cadmium electroplated leaves in capacitors in Consolidated Liberator aircraft radios. Whisker growth resulted in shorting between plates. Reported in H.L.Cobb, ‘Cadmium Whiskers’. Monthly Rev. Am. Electroplaters Soc., 33 (28), 28-30, Jan 1946.

Whisker growth on intermetallic phases in

Sn-3.8Ag-0.7Cu-1.0Ce/Er/Y solder joints. –

Yaowu Shi and Hu Hao, The Beijing University of

Technology

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The Driving Force for Whisker Growth

The driving force for the spontaneous growth of whiskers is generally accepted as resulting from the formation of compressive stress in the coating. The compressive stress may be generated by: Intermetallic formation (Cu6Sn5) CTE mismatch, e.g. Sn plated onto Alloy 42 Residual electroplating stress in deposit Externally applied mechanical stress

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Zinc Whiskers

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Other Examples of Zinc Whisker Growth

Studies of zinc whisker formation and growth from bright zinc electrodeposits; Sugiarto, Christie and Richards – Trans. Inst. Met. Fin 62, 92 (1984)

Zinc whisker growth on rotary switches; Downs and Francis –– Met. Fin. 23 (1994)

zinc whisker growth on hot-dip galvanised coatings; Lahtinen and Gustafsson –– J. Appl. Surf. Fin. 2, 15 (2007).

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Zinc Whiskers from Zn-Ni – Effect of Stress

Filamentary whisker growing from a Zn-Ni alloy electrodeposit

Whisker growing in the vicinity of a surface depression from the Zn-Ni alloy electrodeposits

Annealed at 150 C for 72 hrs

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WHISKERMIT Manufacturing and In-Service Tin

Whisker Mitigation Strategies for High Value Electronics

A project funded by the IeMRC December 2010-November 2013

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WHISKERMIT: Dual Mitigation Strategy

Modification of the electroplating process Process variables (e.g. current density) Bath constituents (e.g. additive concentration) Co-deposition of nanoparticles

Development of conformal coatings Polymeric coatings to retard whisker growth Optimisation of coating formulation

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Summary of Literature Survey

The role of Pb in mitigating whisker growth is not definitively established

Little information on the nature of the oxide film on the surface of Sn deposits

Few studies that look in depth at both the electrochemistry of the plating process, the characterisation of the deposit and the analysis of whisker growth

No single variable controls whisker formation and many parameters are interlinked

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Factors Affecting Whisker Growth

The characteristic structure of the tin deposit Preferred orientation (whisker prone textures) Grain morphology (equiaxed vs. columnar) Co-deposited inclusions (grain boundary pinning) Thickness of deposit

Bath composition and plating parameters: Additives and /or impurities Current density, electroplating profile (e.g. pulse plating) Temperature, agitation etc.

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Galvanostatic Electrodeposition of Tin in the Presence of Additives

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Effect of Pb Co-deposition on Deposit Grain Structure

5 μm

Pure Sn

96.5Sn-3.5Pb

92Sn-8Pb

5 μm

5 μm

Pb X-ray Map

Sn X-ray Map

A clear understanding the role of Pb in mitigating Sn

whisker growth will help guide future alloy modifications in

WHISKERMIT

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Effect of Pb Co-deposition on Electrodeposit Texture

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Coatings vs. Whiskers

Zinc whisker growing through a trivalent chromium conversion coating

Micrograph of acid zinc deposit with hexavalent chromium passivation film (heat treated at 150°C

for 24h)

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Conformal Coatings – Tin Whisker Mitigation?

Conformal (insulating) coatings - Uralane (polyurethane) - Urethane acrylic - Silicone - Acrylic - Parylene

Source : Lead-free from One NASA Perspective, Lead-free Summit, ACI Philadelphia, PA, July 13, 2005

‘Mole runs’

T.A.Woodrow and E.A.Ledbury, IPC/JEDEC 8th International Conference on Lead-Free Electronic Components and Assemblies, San Jose, CA, April 18-20, 2005

50µm

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Tin Whisker Growth on Sn-Mn Electrodeposits

0 days 17 hrs

7 days 28 days

10µm

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Preliminary Conformal Coating Data

Polymeric Coatings

• Alkyd : no whiskers were found on 100 µm coated areas 50 days after coating • Polyester Resin : whiskers began to penetrate 40 days after coating

• Polyurethane Lacquer : whiskers began to penetrate 15 days after coating

• Epoxy Resin : whiskers penetrated and covered half of coated area 40 days after coating

Coated and uncoated areas of polyurethane lacquer after 30 days

Uncoated Sn-Mn

Sn-Mn with polyurethane lacquer

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Tin and Zinc Whiskers – Summary and Possible Future Issues?

Problems accelerated by further ‘green’ initiatives ? Associating failures with whisker growth Are other metals capable of producing whiskers ? Electronics performing under harsher environments Will the whiskering mechanism be elucidated ? WHISKERMIT goals of reduction of tin whiskers

through modifying the tin electrodeposition process and improving conformal coating structures

Spreading the word!

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Spare Slides

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In-Service Whisker Formation

Whiskers

Whiskers

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Detail of Whisker Growth from Surface Eruptions

Striations perpendicular to whisker growth

direction

Whisker surface fluted parallel to

growth axis