466
L. Chen et al.
In Sect. 15.3, the effects of thermal stress at package level of 3D packaging are
discussed, with a focus on the reliability issue associated with warpage. Thermalstress induced warpage in multilayered structures can be evaluated by analytical
modeling. Two theories, the natural bending theory and the beam-like plate theory,
are introduced. To identify the warpage mechanism during assemble process, a flipchip package is studied, showing that warpage usually occurs after underfill is cured
followed by a sudden temperature change. The convectional control methods, such
as applying stiffer ring or a lid, are based on substrate control principle and thus may
introduce additional stress into the package. On contrary, the capped-die approach,
which is based on die control principle, is considered more efficient and is able
to achieve warpage-free design in 3-D packaging. The key control mechanism of
the capped-die approach is to reduce the mismatch of CTEs between the chip and
substrate. The capped-die concept proves more effective than the conventional control
methods through both experimental testing and numerical modeling. Ideally, it is
possible to achieve warpage-free packaging in the full range of temperature variation
with the approach. With warpage being controlled at a small and stable level, the
corresponding reliability issues caused by thermal stress at package level can be
effectively mitigated.
In Sect. 15.4, moisture-induced stresses, which includes both hygroscopic stress
and vapor pressure, have been considered together with thermal stress in order to
take account of the circumstance with both high temperature and high humidity (e.g.,
reflow process or HAST environment). To couple moisture and thermal effects, a
damage micromechanics continuum theory is presented, which allows the coupling
of moisture diffusion, heat conduction, matrix material deformation, as well as the
growth of void volume fraction. The total strain is formulated as the combination
of hygroscopic swelling, thermal expansion, and vapor-pressure induced strain. A
new constitutive law is then established according to the effective stress concept.
A complete set of governing equations are provided for integrated stress analysis,
which can be solved by thermal-moisture analogy and built-in user-defined swelling
functions in ANSYS. A bi-material assembly is used to illustrate the integrated
analysis of moisture diffusion, vapor pressure evolution and stress determinations.
The results show that the moisture- induced stresses play an important role in final
stress state during reflow, as the moisture-induced deformation (which includes both
hygroscopic swelling and vapor pressure induced strain) can be as significant as
thermal expansion. It is concluded that the theory for integrated stress analysis can
serve as a useful tool to analyze, understand, and improve the reliability of a 3D IC package in circumstance like reflow soldering or HAST conditions where
moisture-induced stress cannot be overlooked. With the normalized concentration
used in diffusion equation, the theory can be applied to any 3D assembly that has
many dissimilar materials. It is worth noting that there is still no consensus of vapor
pressure model to be used in microelectronics packaging. However, the theoretical
framework provided in this chapter does not depend on the vapor pressure model
adopted.
L. Chen et al.
In Sect. 15.3, the effects of thermal stress at package level of 3D packaging are
discussed, with a focus on the reliability issue associated with warpage. Thermalstress induced warpage in multilayered structures can be evaluated by analytical
modeling. Two theories, the natural bending theory and the beam-like plate theory,
are introduced. To identify the warpage mechanism during assemble process, a flipchip package is studied, showing that warpage usually occurs after underfill is cured
followed by a sudden temperature change. The convectional control methods, such
as applying stiffer ring or a lid, are based on substrate control principle and thus may
introduce additional stress into the package. On contrary, the capped-die approach,
which is based on die control principle, is considered more efficient and is able
to achieve warpage-free design in 3-D packaging. The key control mechanism of
the capped-die approach is to reduce the mismatch of CTEs between the chip and
substrate. The capped-die concept proves more effective than the conventional control
methods through both experimental testing and numerical modeling. Ideally, it is
possible to achieve warpage-free packaging in the full range of temperature variation
with the approach. With warpage being controlled at a small and stable level, the
corresponding reliability issues caused by thermal stress at package level can be
effectively mitigated.
In Sect. 15.4, moisture-induced stresses, which includes both hygroscopic stress
and vapor pressure, have been considered together with thermal stress in order to
take account of the circumstance with both high temperature and high humidity (e.g.,
reflow process or HAST environment). To couple moisture and thermal effects, a
damage micromechanics continuum theory is presented, which allows the coupling
of moisture diffusion, heat conduction, matrix material deformation, as well as the
growth of void volume fraction. The total strain is formulated as the combination
of hygroscopic swelling, thermal expansion, and vapor-pressure induced strain. A
new constitutive law is then established according to the effective stress concept.
A complete set of governing equations are provided for integrated stress analysis,
which can be solved by thermal-moisture analogy and built-in user-defined swelling
functions in ANSYS. A bi-material assembly is used to illustrate the integrated
analysis of moisture diffusion, vapor pressure evolution and stress determinations.
The results show that the moisture- induced stresses play an important role in final
stress state during reflow, as the moisture-induced deformation (which includes both
hygroscopic swelling and vapor pressure induced strain) can be as significant as
thermal expansion. It is concluded that the theory for integrated stress analysis can
serve as a useful tool to analyze, understand, and improve the reliability of a 3D IC package in circumstance like reflow soldering or HAST conditions where
moisture-induced stress cannot be overlooked. With the normalized concentration
used in diffusion equation, the theory can be applied to any 3D assembly that has
many dissimilar materials. It is worth noting that there is still no consensus of vapor
pressure model to be used in microelectronics packaging. However, the theoretical
framework provided in this chapter does not depend on the vapor pressure model
adopted.