Speaker � Kil-Won Moon
Correlation between Deposit Microstructure and Deposition Current Density
Kil-Won Moon, Soo-Kil Kim, Maureen Williams, and Bill Boettinger
NIST, Metallurgy Division 100 Bureau Drive, Gaithersburg, MD 20899-8555

The initial compressive stress of Sn electrodeposits is released by Coble creep, and this release process depends on its microstructure.  For example, an equiaxed fine grain deposit has a favorable deposit microstructure to release stress because it permits uniform swelling of the deposit by the transport Sn atoms along grain faces parallel to the surface.  We are investigating any correlations between Sn deposit microstructures and non-diffusive creep mechanisms.  Sn deposits, prepared as a function of deposition current density, will be characterized to understand surface structure and crystallographic information by SEM, FIB cross section, EBSD, and XRD.

Speaker - Bob Hilty
An Electrical Characterization of Tin Whiskers
Robert D. Hilty and Ned Corman, Tyco Electronics

Tin whiskers are one of the greatest reliability concerns with lead-free electronics.  The blueprint for failure is simple - a metallic filament of tin bridges between two conductors and leads to an electrical short.  While this makes intuitive sense, there are limits to when this failure mechanism will be active.  This research examines the role of tin oxides and surface contaminants on preventing electrical conduction between tin whiskers and an adjacent contact.  Contact resistance measurements were made to tin whiskers under various loading schema to evaluate the conditions under which whiskers would lead to electrical conduction.

Speaker � Greg Henshall
Impact of Second Level Board Assembly on Tin Whisker Growth for Alloy 42 TSOPs Subjected to Thermal Cycling
Greg Henshall, Hewlett-Packard

This presentation describes the results of tin whisker experiments aimed at increasing our understanding of the impact of board assembly (2nd level solder attach) on tin whisker behavior.  This study focused on matte Sn plated Alloy 42 TSOPs and modules assembled with these components using both eutectic Sn-Pb and Pb-free Sn-Ag-Cu processes.  The impact of the solder attach process on the physical characteristics and chemical composition of the finish were determined by SEM/EDX and then were correlated with the tin whisker response following 500 thermal cycles of -40 to +85 C.  For the set of materials and test conditions studied, we found board assembly to cause mixing of the matte Sn plating with the board assembly solder alloy.  This mixing occurred even outside the region where a macroscopic solder fillet could be detected.  Mixing of the plating and the solder often resulted in an overall thickening of the material covering the Alloy 42 leads.  These changes in the physical and chemical characteristics of the finish were correlated to a reduction in the length of tin whiskers following thermal cycling.  Overall, the results of this study suggest that 2nd level board assembly may help to mitigate tin whisker growth, at least for Alloy 42 components subjected to thermal cycling.  However, caution is warranted when considering the generality of these findings.  First, this study was limited to one substrate/finish material set and to short-leaded TSOPs components.  How board assembly will impact tin whisker growth on copper-based substrates under thermal cycle or isothermal conditions was not studied.  Second, TSOP leads are short, possibly making the coverage of the leads by solder more common than for other types of components with longer leads.  Finally, we have not studied the impact of board assembly on the characteristics of the finish and tin whisker behavior over a wide range of manufacturing materials and process conditions.

Speaker � Chen Xu
Whisker Mitigation Strategies
Chen Xu, Yun Zhang, Chonglun Fan and Joseph A. Abys, Enthone, Inc./Cookson Electronics, Jersey City, NJ and West Haven, CT

As the electronic industry shifts to lead-free manufacturing, Sn whisker remains a key reliability concern.  Several whisker mitigation strategies have been adopted by the component manufacturers, but not without controversy and heated debate.  In this paper, the strengths, weaknesses as well as rationales of various whisker mitigation methods are discussed in line of the current understanding of the mechanisms of the whisker formation. Experimental data of new whisker mitigation approaches will also be presented.

Speaker - Peng Su
A Finite Element Study of Strain Energy Density Distribution
Near A Triple Grain Junction and Its Implication on Whisker Growth
Peng Su and Min Ding, Freescale Semiconductor Inc., 6501 West William Cannon Drive, Austin, TX 78735, USA
Email:  Peng.Su@freescale.com

The phenomenon of whisker growth on electroplated pure matte Sn finish has been investigated extensively recently.  Important progress has also been made in the standardization of acceleration test methods.  One of the tests is an air-to-air thermal cycling (AATC) test between either -40�C or -55�C to 85�C.  During this test, whisker growth is driven by the thermal strain introduced into the Sn finish caused by the coefficient of thermal expansion (CTE) difference between Sn and leadframe materials.  Due to the anisotropic nature of tin�s mechanical properties, Young�s modulus (E) and CTE vary significantly along different lattice planes and lattice directions.  This makes the orientation of the Sn grains a particularly important material property.  In this work, a finite element model is established to investigate the strain energy density distribution near a triple grain junction within electroplate Sn finish.  The orientation and rotation of the grains are varied using real plating data, and the strain energy density distribution for each of the configurations is calculated with the finite element model.  If the thermal strain energy density can be considered a major driving force for whisker growth during the AATC test, rate of whisker growth for Sn finish with a particular grain orientation mix can then be extrapolated with the energy level data produced with this model.

Speaker � Wan Zhang
Effect of Lead on Tin Whisker Elimination
Wan Zhang and Neil Brown, Rohm Haas

Spontaneous tin whisker growth is one of the direct consequences of complete removal of Pb from tin electrodeposits.  To pursue a greater understanding of the whisker growth mechanism, the effects of Pb on the properties of matte tin electrodeposition at different alloy compositions were investigated with respect to cathodic polarization, crystallographic and microstructural characteristics of the deposits, and the formation of Cu-Sn intermetallic compounds (IMC) under ambient conditions at the interface between the Sn or SnPb films and the Cu substrate.  Furthermore, internal stress of pure Sn and 60Sn-40Pb deposits was measured as a function of storage time using the bent strip method.  The observations strongly suggest that Pb prevents tin whisker growth through lattice diffusion and grain boundary diffusion.  Other contributing factors such as predominant tin [220] textures and the equiaxed grain structure may also play supporting roles in the whisker retarding effect of Pb.

Speaker - Werner H�gel
Whisker growth under several conditions and corresponding test methods
Robert Bosch GmbH, T�binger Str. 123, D72762 Reutlingen