10 Fundamentals of Bonding Technology and Process Materials …
305
studies have been focusing on the properties of grain structure and characteristics
of the grain boundaries due to recrystallization in Pb-free solder alloys joints [49–
52]. Of those studies, Wang et al. investigated the EM effect on a Sn-based Pb-free
solder joint that went through single or multiple reflow processes and derived that
having solder joints with a c-axis of a large β-Sn grain aligned to the current flow
direction could lead to premature EM failure, and multigrain structured β-Sn, i.e.
smaller grains, provides high-angle twin boundaries that increase the EM survival
time [51]. In addition to the solder joint structure, formation and growth of IMC,
i.e. the kinetics of IMC formation, are another factor that accounts for EM-induced
failure in Sn-based solder joints [53]. It is suggested from Bashir and coworkers that
a uniform IMC layer could delay EM-induced failure by slowing down the diffusion
of Cu atoms into the SAC-305 solder matrix [54].
Furthermore, more extensive studies have been explored, mainly focusing on
the influence of β-Sn grain orientation of Sn-based Pb-free solder joints, where the
thermomechanical fatigue response and EM behavior strongly depend on the β-Sn
grain orientation of the solder joints [46, 55–57]. In addition, the effect of the cooling
rate on the microstructure and mechanical properties of solder alloys has been studied
as well [58–60].
The Sn-based solder mainly consists of β-Sn crystals, widely known for its
anisotropicity in the crystal structure. The β-Sn poses a body-centered tetragonal
(BCT) structure and has the dimensions a = b = 5.83 Å and c = 3.18 Å. Due to this
innate structure of β-Sn, mechanical, thermal, and electrical anisotropic behaviors
are observed in widespread Sn network. The difference in lattice constants of Sn, as
previously stated, results in much faster interstitial diffusion of Cu and Ni along the
c axis than along the a and b axes [61, 62]. Furthermore, Lu et al. investigated the
effect of β-Sn grain orientation on EM degradation mechanism in Sn-based solders
and concluded that the failure of the solder joint was due to the highly anisotropic
diffusion behaviors of Cu and Ni along the c-axis of the β-Sn matrix that led to
a depletion of intermetallic compound (IMC) and under bump metallurgy (UBM)
[46]. They also suggested that better EM performance of SAC solders compared to
Sn–Cu (SC) solders is due to the presence of twin structures and stable Ag3Sn IMC
network [46].
Because the c-axis orientation in Sn-based solder matrix is a key determining
factor in the reliability of the flip-chip packages, the crystallographic orientations
of solder balls are of great importance. Based upon many studies discussed in
previous paragraphs, this paper explores the effect of different crystallographic
textures and microstructures of β-Sn and IMC structures obtained by two different
processes (reflow and TCB) on their reliability issues using EM degradation tests.
One noticeable difference and the factor that accounts for micro-structural change
between reflow and TCB process resides in their thermal profiles, such as cooling
rates and directional cooling for TCB. From our finite element analysis (FEA)
simulation, uniform temperature distribution within the reflow-processed solders
and anisotropic temperature distribution within the TCB-processed solders were
observed (Fig. 10.51).
305
studies have been focusing on the properties of grain structure and characteristics
of the grain boundaries due to recrystallization in Pb-free solder alloys joints [49–
52]. Of those studies, Wang et al. investigated the EM effect on a Sn-based Pb-free
solder joint that went through single or multiple reflow processes and derived that
having solder joints with a c-axis of a large β-Sn grain aligned to the current flow
direction could lead to premature EM failure, and multigrain structured β-Sn, i.e.
smaller grains, provides high-angle twin boundaries that increase the EM survival
time [51]. In addition to the solder joint structure, formation and growth of IMC,
i.e. the kinetics of IMC formation, are another factor that accounts for EM-induced
failure in Sn-based solder joints [53]. It is suggested from Bashir and coworkers that
a uniform IMC layer could delay EM-induced failure by slowing down the diffusion
of Cu atoms into the SAC-305 solder matrix [54].
Furthermore, more extensive studies have been explored, mainly focusing on
the influence of β-Sn grain orientation of Sn-based Pb-free solder joints, where the
thermomechanical fatigue response and EM behavior strongly depend on the β-Sn
grain orientation of the solder joints [46, 55–57]. In addition, the effect of the cooling
rate on the microstructure and mechanical properties of solder alloys has been studied
as well [58–60].
The Sn-based solder mainly consists of β-Sn crystals, widely known for its
anisotropicity in the crystal structure. The β-Sn poses a body-centered tetragonal
(BCT) structure and has the dimensions a = b = 5.83 Å and c = 3.18 Å. Due to this
innate structure of β-Sn, mechanical, thermal, and electrical anisotropic behaviors
are observed in widespread Sn network. The difference in lattice constants of Sn, as
previously stated, results in much faster interstitial diffusion of Cu and Ni along the
c axis than along the a and b axes [61, 62]. Furthermore, Lu et al. investigated the
effect of β-Sn grain orientation on EM degradation mechanism in Sn-based solders
and concluded that the failure of the solder joint was due to the highly anisotropic
diffusion behaviors of Cu and Ni along the c-axis of the β-Sn matrix that led to
a depletion of intermetallic compound (IMC) and under bump metallurgy (UBM)
[46]. They also suggested that better EM performance of SAC solders compared to
Sn–Cu (SC) solders is due to the presence of twin structures and stable Ag3Sn IMC
network [46].
Because the c-axis orientation in Sn-based solder matrix is a key determining
factor in the reliability of the flip-chip packages, the crystallographic orientations
of solder balls are of great importance. Based upon many studies discussed in
previous paragraphs, this paper explores the effect of different crystallographic
textures and microstructures of β-Sn and IMC structures obtained by two different
processes (reflow and TCB) on their reliability issues using EM degradation tests.
One noticeable difference and the factor that accounts for micro-structural change
between reflow and TCB process resides in their thermal profiles, such as cooling
rates and directional cooling for TCB. From our finite element analysis (FEA)
simulation, uniform temperature distribution within the reflow-processed solders
and anisotropic temperature distribution within the TCB-processed solders were
observed (Fig. 10.51).