tin whiskers: cause and effectthor.inemi.org/webdownload/newsroom/boettinger.pdftin whiskers: cause...
TRANSCRIPT
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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
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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
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Variety of Whisker Types Observed
Bright Snwith1.5 wt% Cu
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Non-filamentary Eruptionon Sn-1.5 wt % Cu electrodeposit
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• 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:
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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
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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.
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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)
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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.
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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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.