Lead-Free Surface Finishes for 

Electronic Components:

Tin Whisker Growth

METALS

This project will provide data and materials measurements necessary to 

improve the reliability of solder interconnects degraded by the switch to leadfree technology. In particular, the state of compressive stress and the localized 

creep response (whisker growth) of tin-based lead-free electrodeposits are 

being measured. Molecular dynamics simulations aimed at understanding the 

conditions (stress, temperature, grain boundary diffusion, surface diffusion) at 

which the hillock/whisker growth processes can be initiated are being utilized 

to examine how stress and creep drive whisker growth. Industry will use these 

measurement methods and data to modify processing conditions to prevent the 

formation of tin-whiskers. 

Objective

Impact and Customers

• The 2007 International Electronics Manufacturing Initiative (iNEMI), and IPC technology roadmaps clearly 

articulate the U.S. microelectronics industry’s need for research and measurement to predict and prevent tin (Sn) 

whisker formation.

• There is a clear industrial consensus that more information is required regarding 

the influence of electroplating conditions, grain structure, compressive stresses, 

intermetallic compound (IMC) formation, diffusion of Sn, and thermal cycling 

effects on electroplated components.

• We maintain multilevel interactions with industry/government/university groups, 

including iNEMI, Aerospace Industries Association (AIA), Government Electronics 

and Information Technology Association (GEIA), and the UMD - Center for 

Advanced Life Cycle Engineering (CALCE). 

• iNEMI has asked the NIST Metallurgy Division to establish a standard technique 

for measuring the stress in electroplated Sn films, using the sin2y X-ray diffraction 

method.

Approach

Materials Science and Engineering Laboratory

Tin is widely used as a coating in the electronics industry because it provides excellent solderability, ductility, electrical 

conductivity, and corrosion resistance. Unfortunately, tin whiskers often grow spontaneously from tin electrodeposits and 

short-circuit finely pitched electrical components. Adding a few percent of lead inhibits whisker growth, but environmental 

concerns have resulted in a demand for lead-free (Pb-free) surface finishes, and consequently, a demand for whisker 

mitigation strategies.

Two conditions in the tin coating are necessary, but not sufficient, requirements 

for whisker growth. First, a compressive stress must exist in the film. Second, 

energy relaxation mechanisms such as grain growth and recrystallization must be 

impeded, e.g., by grain boundary pinning. We have focused on the measurement 

of stress using cantilever beam bending and X-ray diffraction methods, and the 

development of dynamic molecular models to understand initial hillock/whisker 

formation. We are also investigating pulse deposition techniques to eliminate the 

columnar grain shape of tin as a strategy to prevent tin whisker formation.

855-13-34b
Materials Science and Engineering Laboratory

NIST sponsored a workshop in April 

2008 to assess the state of the art residual 

stress measurement methods in Pb-free 

electrodeposits. Thirty researchers from 

industry, government labs and universities 

attended. The benefits and difficulties 

with both X-ray diffraction (sine squared 

psi) and wafer/beam curvature methods 

of stress measurement were discussed. 

Topics included: precision and accuracy 

of measurements; interpretation of 

measurements, e.g. spatial resolution, 

macro- and micro- stress; orientation (texture) 

effects; grain shape effects, stress gradients 

in the deposit; and Sn-Cu intermetallic (IMC) 

growth induced stress. Although it is easy to 

obtain stress values from these techniques, 

understanding the source of measurement 

errors and the interpretation of the results is 

underdeveloped.

We are measuring the stress of 

electrodeposited Sn as a function of time 

with two different techniques: sin2y X-ray 

diffraction, and cantilever beam deflection. 

The deflection of the cantilever beam is a 

measure of the average stress in the film. 

The X-ray diffraction method measures the 

lattice strain in a volume of Sn near the 

surface of the electrodeposit. The preferred 

orientation of the deposits and the low 

stress levels (~ 10 MPa) presented many 

challenges for this technique. A protocol for 

simultaneous stress measurements using 

both methods is being developed. A detailed 

analysis of errors in the measurement method 

as it applies to Sn has been performed. Also, 

a major summary of whisker/hillock growth 

in response to intrinsic electrodeposition 

stress and IMC generated stress has been 

published.

The mechanisms of hillock and whisker growth 

remain largely unknown. Molecular dynamic 

simulations are aimed at understanding 

the conditions (stress, temperature, grain 

boundary diffusion, surface diffusion) under 

which the hillock/whisker growth processes 

can be initiated. In stressed solids, surface 

evolution is often driven by grain boundary 

diffusion and can result in growth of hillocks 

and whiskers. The simulated geometries 

include a single boundary normal to the 

surface and a tricrystal with a wedge shaped 

surface grain, both under an applied stress 

parallel to the surface.

To mitigate the growth of whiskers we are 

working on grain structure modification. 

Results show that the columnar grain structure 

of Sn electrodeposits can be modified to an 

equiaxed structure by pulsed deposition 

combined with the addition of 3 at.% Bi. 

Pulsed deposition is used to selectively turn 

on and off the Sn deposition reaction. Bi is 

more noble than Sn; thus, during the off cycle 

a displacement reaction between metallic Sn 

on the electrode surface and Bi3+ in solution 

selectively dissolves Sn and deposits Bi, 

effectively terminating the growth from the 

previous cycle and forcing the Sn to nucleate 

a new grain on the Bi-enriched surface. The 

grain size is tuned by varying the pulsing 

conditions, and an equiaxed structure can 

be obtained. This surface enrichment of Bi 

by pulsed deposition is similar to that which 

occurs naturally during Sn-Pb deposition, 

and provides an avenue for breaking up the 

columnar grain structure inherent to pure Sn. 

An equiaxed grain structure enables uniform 

creep, and rapid stress relaxation, thus 

preventing whisker and hillock formation.

Accomplishments

William J. Boettinger

Kil-Won Moon

Gery Stafford

Mark Vaudin

Maureen Williams

(Metallurgy Division)

(301) 975-6170

maureen.williams@nist.gov

www.nist.gov/metallurgy

Learn More Publications

Boettinger WJ, Johnson CE, Bendersky LA, Moon K-W, Williams ME and Stafford GR 

Whisker and Hillock Formation on Sn, Sn-Cu and Sn-Pb Electrodeposits Acta Materialia, 

53: 5033 (2005)

Stafford GR, Williams ME, Johnson CE, Moon K-W, Bertocci U, Kongstein O and 

Boettinger WJ Whisker Formation in Pb-free Surface Finishes ECS Transactions, 1(13): 

71 (2006)

Williams ME, Moon K-W, Boettinger WJ, Josell D and Deal A Hillock and Whisker 

Growth on Sn and SnCu Electrodeposits on a Substrate Not Forming Interfacial 

Intermetallic Compounds J. Electr. Matls., 36: 214 (2007)

Sandnes E, Williams ME, Vaudin MD and Stafford GR Equiaxed Grain Formation in 

Electrodeposited Sn-Bi J. Electr. Matls., 37:490 (2008)

Tin whiskers

Effect of grain shape and mobility on whisker and 

hillock formation

A tin hillock