Int J Fract (2008) 152:37–49
DOI 10.1007/s10704-008-9264-9
ORIGINAL PAPER
Fatigue fracture of SnAgCu solder joints
by microstructural modeling
M. Erinc · T. M. Assman · P. J. G. Schreurs ·
M. G. D. Geers
Received: 15 February 2008 / Accepted: 1 October 2008 / Published online: 21 October 2008
© The Author(s) 2008. This article is published with open access at Springerlink.com
Abstract The ongoing miniaturization trend in the
microelectronic industry enforces component sizes to
approach the micron, or even the nano scale. At these
scales, the underlying microstructural sizes and the
geometrical dimensions are comparable. The increas-
ing influence of microscopic entities on the overall
mechanical properties makes conventional continuum
material models more and more questionable. In this
study, the thermomechanical reliability of lead-free
BGA solder balls is investigated by microstructural
modeling.Microstructural input is provided by orienta-
tion imagingmicroscopy (OIM), converted into a finite
element framework. Blowholes in BGA solder balls
are examined by optical microscopy and a statistical
analysis on their size, position and frequency is con-
ducted. Combining the microstructural data with the
appropriate material models, three dimensional local
models are created. The fatigue life of the package is
determined through a critical solder ball. The thermo-
mechanical reliability of the local models are predicted
using cohesive zone based fatigue damagemodels. The
simulation results are validated by statistical analyses
provided by the industry.
M. Erinc (
B
) · T. M. Assman · P. J. G. Schreurs ·
M. G. D. Geers
Department of Mechanical Engineering, Section of
Materials Technology, Eindhoven University of
Technology, 5600MB Eindhoven, The Netherlands
e-mail: muge@live.nl
Keywords Lead free · Cohesive zone modeling ·
Solder fatigue · Microstructural modeling
1 Introduction
In microelectronic packages, mechanical integrity and
electrical connection is provided by solder connections.
Themicroelectronics industry has switched to lead-free
solders in 2006 due to the toxicity of lead (Pb). Since
then, near-eutectic and eutectic compositions of SnAg-
Cu alloy are being extensively used as a replacement for
the traditional SnPb solder. Solder joints in microelec-
tronic devices are exposed to thermomechanical fatigue
loading caused by the repeated heating and cooling
of the device (Fig. 1). The different thermal expansion
coefficients (CTE) of packagematerials provoke cyclic
mechanical strains which results in fatigue crack ini-
tiation and propagation in solder joints. On top of the
CTE mismatch, Sn based solders are prone to thermal
fatigue deformation even without being mounted on
the package. The thermal anisotropy of the β-Sn phase
causes intergranular fatigue damage upon cyclic ther-
mal loading (Matin et al. 2006; Subramanian and Lee
2004; Telang et al. 2004; Vianco et al. 2004). Miniatur-
ization, i.e. decreasing solder dimensions, pronounces
this kind of failure since smaller jointswith a few grains
are more likely to fail by intergranular crack propaga-
tion.
Examples of both damage mechanisms are shown
in Figs. 2 and 3. In Fig. 2, cross-section of a BGA
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38 M. Erinc et al.
Fig. 1 Solder joints are exposed to cyclic thermomechanical
loading resulting from theCTEmismatch between package com-
ponents
package with eutectic SnAgCu solder balls subjected
to 2,000 thermal cycles between −40 and 125 ◦C is
shown. In Fig. 3, a micrograph of a bulk SnAgCu spec-
imen after 1,500 thermal cycles (between  T=−40
and 125 ◦C) is shown. Here it is important to note, the
bulk sample was only thermally cycled, it was mechan-
ically not constrained. Thus it is clear that there is an
intergranular damage mechanism, although not visible
in the presence of a joint. In a solder joint, cyclic ther-
momechanical loading causes highly localized defor-
mations at the bump/pad interface, generally leading
to overall failure. Thermal cycling of bulk specimens
clearly reveals that extensive deformation on Sn grain
boundaries also takes place. Both damage mechanisms
are clearly dependent on the microstructure due to their
interfacial nature.Motivated by this fact, cohesive zone
based interfacial fatigue damage models were charac-
terized by inverse modeling through dedicated fatigue
experiments (Erinc 2007; Erinc et al. 2004, 2005, 2007,
2008). Fatigue life predictions for BGAor flip-chip sol-
Fig. 2 SnAgCu BGA solder balls, N=2,000, T=−40 to
125 ◦C, courtesy of J. W. C. de Vries, Philips Applied Tech-
nologies, optical image
Fig. 3 Bulk SnAgCu, N=1,500, T=−40 to 125 ◦C, optical
image
der balls are generally conducted using Coffin-Man-
son, J-integral, power law or creep models (Tchankov
et al. 2008; Desai et al. 1998; Li and Wang 2007; Tee
et al. 2003; Zhang et al. 2008). In Towashiraporn et al.
(2005), the fatigue crack trajectory and fatigue life of a
solder joint is predicted through a coupled numerical-
experimental approach.A reviewon solder joint fatigue
models with respect to their applicability to chip-scale
packages is given in Lee et al. (2000).
Driven by the ongoing miniaturization trend in the
microelectronics industry, the underlying microstruc-
tural sizes and the geometrical dimensions of micro-
electronic components tend to be comparable. The
increasing influenceofmicroscopic entities on theover-
all mechanical properties makes continuum material
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