Microstructure development in Tin-Silver-Copper
Bulk Solder Material
Sean Seekins, B. Talebanpour, U. Sahaym, I. Dutta
Research Experience for Undergraduates in Materials Science and Engineering
Department of Mechanical and Materials Engineering, Washington State University, Pullman, Washington
Isothermally Aged 110 Hrs
Methods
Introduction
In recent years lead-based solders for microelectronics
applications have fallen out of use due to government
constraints restricting the use of lead due to the hazardous
nature of the metal. For this reason new solders comprised of
Tin, Indium, and Bismuth have been developed as possible
replacements. These solder materials will be used in a variety
of applications including flip chip electronic packages (Fig 1).
Sn-Ag-Cu (SAC) solders have a complex microstructure
consisting of a proeutectic region containing β-Sn grains and a
eutectic region containing Ag3Sn and Cu6Sn5 particles located
in the β-Sn matrix [1] (Fig 2). Previous research on SAC
solders has shown that when subjected to isothermal aging
(IA) and thermal mechanical cycling (TMC) the intermetallic
particles distributed throughout the matrix grow due to
Ostwald ripening [2]. A study conducted at WSU showed
recrystallization of β-Sn grains in samples subjected to IA [3].
Other studies have shown that SAC solder samples generally
consist of only a few orientations of β-Sn throughout the
sample [4]. The goal of this summer REU project was to
observe the coarsening of particles and determine the β-Sn
grain orientation in samples subjected to IA.
A scanning electron microscope with a backscattered electron
(BSE) detector was used to obtain images of as-reflowed
solder samples as well as samples subjected to 110 hours of
isothermal aging at 145oC. Contrast in the BSE images comes
from the difference in atomic number at the sample surface
as well as the difference in grain orientation. Electron
backscatter diffraction (EBSD) was used to obtain information
about the orientation of β-Sn grains within the proeutectic
region.
Figure 3: Schematic
diagram of EBSD
setup. The sample is
tilted at 70 degrees
and an electron beam
is fired at the sample.
The lattice structure
of the sample causes
diffraction bands to
be generated and
reflected onto a
phosphor screen.
Orientation
information can be
gathered from the
bands
In order to obtain good orientation information from an EBSD
scan the sample must be prepared carefully. 5 mm diameter
cylinders of as reflowed SAC solder were obtained and ground
in steps using silicon carbide paper. After grinding the
samples were polished with diamond suspension and 0.02
micron colloidal silica for the final step.
Conclusions
Particle coarsening was observed after the samples were
aged for 110 hours at 145oC

There are many different orientations of β-Sn grains present
throughout the sample as opposed to only one or a few
distinct orientations

The boundaries between adjacent β-Sn grains are generally
low-angle boundaries but a few high angle boundaries are
also present within the samples

Recrystallized β-Sn grains appear in the isothermally aged
samples after aging

References
Results
Figure 1: Flip Chip Electronic Package
Figure 5: EBSD Orientation Image Map of SAC Solder Aged 110
Hours with Misorientation Angles and Planes and
Corresponding Backscattered SEM Image
As-Reflowed
1. P. Kumar, Z. Huang, I. Dutta, G. Subbarayan, R. Mahajan. “Influence of
Microstructure on Creep and High Strain Rate Fracture of Sn-Ag Based Solder
Joints”,Lead-free Solders: Materials Reliability for Electronics, edited by K. N.
Subramanian, Wiley series in Electronic and Optoelectronic Materials, John
Wiley, 2010.
2. I. Dutta, P. Kumar, G. Subbarayan. "Microstructural Coarsening in Sn-Ag-based
Solders and Its Effects on Mechanical Properties", The Minerals, Metals, and
Materials Society, JOM Vol. 61 No. 6 June 2009.
3. U. Sahaym, B. Talebanpour, I. Dutta, P. Kumar, P. Borgesen. "Recrystallization
and Ag Sn Particle Redistribution during Thermo-mechanical Treatment of Bulk
Sn-Ag-Cu Solder Alloys", Submitted to Scripta Materialia, June 2012
3
Figure 2: Microstructure of SAC solder
Figure 4: EBSD Orientation Image Map of As-Reflowed
SAC Solder with Misorientation Angles and Planes and
Corresponding Backscattered SEM Image
4. A. LaLonde, D. Emelander, J. Jeannette, C. Larson, W. Rietz, D. Swenson, and
D. W. Henderson, “Quantitative metallography of beta-Sn dendrites in Sn-3.8Ag0.7Cu ball grid array solder balls via electron backscatter diffraction and
polarized light microscopy,” J. Electron. Mater., vol. 33, p. 1545, 2004.
This work was supported by the National Science
Foundation’s REU program under grant number
DMR-1062898