A Workshop sponsored by

National Electronics Manufacturing Initiative

National Institute of Standards and Technology

The Minerals, Metals & Materials Society

Organizers:

Ron Gedney, NEMI

Bill Boettinger & Carol Handwerker, NIST

NIST Research Team

C. E. Johnson, G. R. Stafford, M.E. Williams, K.-W. Moon

Tin Whiskers: Cause and Effect

N. Furuta & K. Hamamura, Jap. J.Appl.Phys. 8(1969) 1404

Sn Whiskers Grow from their Base, not the Tip

On inside of a drilled and polished hole in Sn-Al cast alloy

Variety of Whisker Types Observed

Bright Snwith1.5 wt% Cu

Non-filamentary Eruptionon Sn-1.5 wt % Cu electrodeposit

• Important scientific problem…… challenges materials science community

• Source of stress during crystal growth from aqueous solutions and from vapor phase

• Interaction of stress, solid diffusion and solid phase precipitation/coarsening

• Anamolous (fast) diffusion

• Stress relaxation (creep), recrystallization

• Important technology problem…challenges manufacturing community

• Why has Pb been an effective whisker retardant for 50 years?

• Bath chemistry impurities/additives/complexing agents

• Plating conditions

• Diffusion barriers

• Post plating processing (reflow/anneal)

Sn Whisker Growth is an:

Measurement of Stress Evolution in Bimetallic Cu-Sn PVD Thin Films:

L. Kabakian, E. Chason and K.S. KumarDivision of Engineering, Brown University, Providence, RI

02912 Real-time MOSS (Multi-beam Optical Stress Sensor) measurement for 4

day initial period

This workshop is intended to:

• Bring together key researchers on tin whiskers• Compare work that has been done• See if a consensus can be developed on a basic model

or

• Define the work that will help us reach such a consensus

• The end goal is to provide direction to the industry that will assure long-life cycle applications they will not be subject to tin whisker failures.

The Morning Session9:00 Introduction to the Workshop: W. J. Boettinger and C. A. Handwerker, Metallurgy Division, NIST (Gaithersburg, MD), R. W. Gedney, NEMI (Herndon, VA)

9:20 Stress in Electroplated Sn: Its Measurement and Implication in Spontaneous Whisker Growth: Chen Xu, PW Materials and Chemistry Group, Cookson Electronics (Jersey City, NJ)

10:00 Spontaneous Growth Of Tin Whiskers From Tin Electrodeposits on Phosphor Bronze Sheet: Dong Nyung Lee, School of Materials Science and Engineering, Seoul National University (Seoul, Korea)

10:40 Break

10:50 Tin-Whisker Microstructural Analysis using FIBs / RecrystallizationHypothesis: George T. Galyon, IBM Server Group (Poughkeepsie, NY)

11:30 Focused Ion Beam, Transmission Electron Microscopy, and Synchrotron Radiation Study of Sn Whiskers on Leadframe with Pb-free Surface Finish: K. N. Tu, Department of Materials Science and Engineering, UCLA.

12:10 Lunch (box lunches will be available for purchase)

The Afternoon Session

1:30 Panel Discussion: Questions posed by moderator and the audience.

Moderator: W. J. Boettinger, NIST

Panel members:

K. N. Tu, UCLA

D. N. Lee, Seoul National University

G. Galyon, IBM

Chen Xu, Cookson Electronics

B. Radhakrishnan, Oak Ridge Nat. Lab.

The Afternoon Session Topics~15 minutes each

• How do the following affect the WGT (Whisker growth Tendency)?

• Intrinsic plating stress

• Macroscopic stress relaxation /Recrystallization / Creep

• Alloying additions to Sn electrodeposit: e.g. Pb, Cu, ...

• Electrodeposit grain size & shape, texture, porosity/inclusions and precipitates

• Fast Diffusion of Cu, Ni in Sn ⇒ Stress

• Molar volume (density) of intermetallics compared to Sn & Cu ⇒ Stress

• Dislocation /Vacancy / Interstitial interaction

• Thermal Cycling

• Oxide

• Summarize

Intrinsic Plating Stress:Rumpled Bright Sn Plating on Amorphous Carbon

• Deposits > 1 µm thickdebond from substrate during plating.

• Buckling indicates large compressive stress.

• No Cu6Sn5 intermetallic.

Half of Glassy Carbon Disk

Half of Plating

Stress vs. time curves for 16.5 µm thick Sn & Sn-CuBright deposits on Phosphor Bronze

Grew whiskers!

No whiskers!(High Purity H2O)

1E+2 1E+3 1E+4 1E+5 1E+6 1E+7Time (s)

-30

-20

-10

0

Stre

ss (M

Pa)

219,16.5 µm221,16.5 µm

233,16.5 µm

234,16.5 µm

Pure Sn

Sn-CuSn-1.5 wt%Cu

No Whisker/Eruptions Observed in Bright Snusing 18 MΩ−cm water

wppm of Cu+2 in electrolyte

16000 1000150 400

0 0.4 0.8 1.2 1.6wt% Cu in deposit

0

400

800

1200

1600

Max

. Whi

sker

Len

gth

in 1

y. (

µm)

0 0.4 0.8 1.2 1.6wt% Cu in deposit

0

20

40

60

Whi

sker

& E

rupt

ion

Den

sity

(mm

-2)

Moon et al, (2001) NIST

Commercial Sn methanesulfonatebath + Cu methanesulfonate @ 60 mA/cm2

(ICP)

Addition of Cu produced whiskers

10Stress (MPa)

1E-13

1E-12

1E-11

1E-10

1E-9

1E-8St

ain

Rat

e (s

-1)#219 Pure Sn #245 Sn-Cu low Pb

#234 Sn-Cu

3 30206

#248 Sn-Cu high Pb

McCabe & Fine (Pure Sn): 10-7 at 3 MPa 10-2 at 12 MPan=6; σ > 8Mpan=8.6; σ < 8 Mpa

Knoop YSPure Sn

Knoop YSPure Sn alloys

n=11.7

n=5.0

n=12.3

n=9.5

Macroscopic Stress Relaxation by Power Law Creep

Bright Sn & Sn-Cu Deposits on Plated Cu

Pure Sn:No Intermetallicon Sn Grain boundaries (684 days)

Sn-1.42 wt% Cu:Intermetallic on Sn grain boundaries(219 days)

Grain size & shape, texture, IMC shape, location

1 day old Sn-1.5 wt%Cu plating on glassy carbon(953 ppm Cu+2 in Electrolyte)TEM by L. Bendersky, NIST

Precipitation of Fine Cu6Sn5 within the Sn grains for Sn-Cu Plating

• Plating alloyed Sn produces supersaturated solid solution• Ppt. of fine particles within the grain;• Sn- 1.5 wt% Cu alloy would have 4% Cu6Sn5 independent of Cu6Sn5 formed at substrate.

Coarser particles on grain boundaries

Fine particles within grains

Darkfield

Anomalous (Fast) Diffusion in Sn

DNia

DNic= 10-5 at 23 C

Warburton & Turnbull (1975) Interstitial/substitutionalinteraction

Sn-NiYeh and Huntington,Phys. Rev. Lett. (1984)

D. W. Stevens & G. W. Powell, Met. Trans 8A(1977) 1531

Stress if constrained to remain flat

CuSnUnconstrained

Stress Generation due to Unequal Diffusion Rates

CuSn

Like

0 20 40 60 80 100at % Cu

6

8

10

12

14

16

18

VM

(cm

3 /mol

e of

ato

ms)

Sn Cu

Cu6Sn5Cu3Sn

αβγ

Sn-Cu Molar Volume vs. Composition

• Normal reaction Sn + Cu → Cu6Sn5

decrease in molar volume

• ? Change in VM due to ppt. of Cu6Sn5 from supersaturated (Sn,Cu) solid solution formed by plating

• If DCu>> DSn ; i.e.,Sn atoms immobile and intermetallic forms by addition of Cu to Snalready present, molar volume increases

This workshop is intended to:

• Bring together key researchers on tin whiskers• Compare work that has been done• See if a consensus can be developed on a basic model

or

• Define the work that will help us reach such a consensus

• The end goal is to provide direction to the industry that will assure long-life cycle applications they will not be subject to tin whisker failures.