562
Y. Wang et al.
devices. This synergistic effect can couple with mechanical and other failure in the
future. Clearly, it needs our attention and requires more in-depth understanding.
17.5.6 Concluding Remarks
Electromigration and Joule heating are of concern for 3D IC packaging. In
microbumps, 100% IMC can help increase MTTF at higher current density. TSV
also has a higher resistance to EM failure. Nevertheless, EM failure can be transferred to RDL layer when it is poorly designed. In a complicated 3D IC system, Joule
heating must be considered during the design stage to avoid current crowding not
only in solder joints but also in redistribution metal layers. And EM failure always
occurs in the weakest link of the system. By including EM design rules upfront in
manufacturing may help predict or even prevent the weakest link in the system level
failure happen.
17.6 Thermomigration in 3D IC Packaging
17.6.1 Introduction
For the thermal management in 3D IC, heat generation and dissipation among the
stack of chips is the most critical reliability issue. When the Si chips are stacked
vertically with the dielectric underfill in between, the heat generation from Joule
heating of transistors in silicon chips is very large whereas the heat dissipation is very
bad due to the poor thermal conductivity of the underfill materials. Meanwhile, heat
dissipation depends on the temperature gradient according to Fourier’s law. While a
large thermal gradient is better for heat dissipation, it might lead to thermomigration
(TM), which is another reliability concern in this review [65].
In flip chip solder joints, a temperature gradient of 1000 °C/cm is sufficient to
induce TM according to many studies reported [66–68]. TM is a reliability concern
because it will lead to phase redistribution and void formation, which will cause
failure. For the eutectic Pb-Sn solder joint, Pb is found to be the dominant diffusing
species in TM, and it will diffuse from the hot end to cold end [69]. TM for Pb-free
solder joint is also well studied, and Sn is determined to move from the cold end to
hot end with a positive heat of transport [70]. If we assume for microbumps, TM will
also occur at the same temperature gradient, i.e., 1 °C across a microbump having a
diameter of 10 μm, it will induce a serious failure. It is hard to guarantee that we can
control a temperature difference of 1 °C. Hence, the reliability problem associated
with TM must be studied for microbumps. First, we shall discuss the fundamentals
of TM. Then, we will discuss the observation of TM in flip chip solder joints. Finally,
we will present the new challenges in TM for microbumps used in 3D IC packaging.
Y. Wang et al.
devices. This synergistic effect can couple with mechanical and other failure in the
future. Clearly, it needs our attention and requires more in-depth understanding.
17.5.6 Concluding Remarks
Electromigration and Joule heating are of concern for 3D IC packaging. In
microbumps, 100% IMC can help increase MTTF at higher current density. TSV
also has a higher resistance to EM failure. Nevertheless, EM failure can be transferred to RDL layer when it is poorly designed. In a complicated 3D IC system, Joule
heating must be considered during the design stage to avoid current crowding not
only in solder joints but also in redistribution metal layers. And EM failure always
occurs in the weakest link of the system. By including EM design rules upfront in
manufacturing may help predict or even prevent the weakest link in the system level
failure happen.
17.6 Thermomigration in 3D IC Packaging
17.6.1 Introduction
For the thermal management in 3D IC, heat generation and dissipation among the
stack of chips is the most critical reliability issue. When the Si chips are stacked
vertically with the dielectric underfill in between, the heat generation from Joule
heating of transistors in silicon chips is very large whereas the heat dissipation is very
bad due to the poor thermal conductivity of the underfill materials. Meanwhile, heat
dissipation depends on the temperature gradient according to Fourier’s law. While a
large thermal gradient is better for heat dissipation, it might lead to thermomigration
(TM), which is another reliability concern in this review [65].
In flip chip solder joints, a temperature gradient of 1000 °C/cm is sufficient to
induce TM according to many studies reported [66–68]. TM is a reliability concern
because it will lead to phase redistribution and void formation, which will cause
failure. For the eutectic Pb-Sn solder joint, Pb is found to be the dominant diffusing
species in TM, and it will diffuse from the hot end to cold end [69]. TM for Pb-free
solder joint is also well studied, and Sn is determined to move from the cold end to
hot end with a positive heat of transport [70]. If we assume for microbumps, TM will
also occur at the same temperature gradient, i.e., 1 °C across a microbump having a
diameter of 10 μm, it will induce a serious failure. It is hard to guarantee that we can
control a temperature difference of 1 °C. Hence, the reliability problem associated
with TM must be studied for microbumps. First, we shall discuss the fundamentals
of TM. Then, we will discuss the observation of TM in flip chip solder joints. Finally,
we will present the new challenges in TM for microbumps used in 3D IC packaging.