12 Fundamentals of Electromigration in Interconnects of 3D Packaging
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substrate side during EM and cause the void nucleation and propagation along that
interface.
Solder systems are quite complex metallurgical systems. The dissolution of solutes
(Cu, Ni, Pd, Au) from the substrate surface finish and Cu bumps into the solder during
reflow makes the solder alloys extremely complex multi-component thermodynamic
system. The solute elements dissolved in the solder are interstitial in nature and hence
diffuse substantially fast in the solder matrix. Additionally, solders are multiphase
alloys and undergo several intermetallic reactions at the interfaces. This makes the
solder joint EM more complex than Al or Cu traces in electronic packaging.
Many factors can impact the EM failure induced by Sn diffusion, including solder
alloying, IMC formation, current crowding, metallization dissolution, and solder
joint height. All these factors are applicable to micro bumps in 3D packaging also.
1. The impact of IMC reaction and solder alloying
One unique feature for solder joint EM is its very dynamic nature under electron
wind and high temperature. Metallization from both cathode and anode will keep
dissolving into the solder alloy and react with Sn to form IMCs. This will either
coarsening the existing IMC particles in the solder matrix or increase the thickness
of the IMC on anode side. The IMC reaction between metallization (typically Cu
or Ni) and Sn cause volume shrinkage, which will generate Sn vacancies during the
reaction. This will increase the probability of solder voids nucleation and growth
at IMC/solder interface on cathode side, where Sn atoms flux divergence happens.
From this point of view, IMC reaction and the instability of the solder system can
reduce the EM capability. This IMC reaction impact on EM fail will be even more
dramatic in smaller 3D packaging micro bumps since the Sn volume is very limited
and easy to be exhausted.
Solder alloying can also impact the stability of the solder system. Seo et al. [12]
compared Sn-1.8Ag and Sn-0.5Cu with different joining path to control the Ni and
Cu dissolution in the solder system. They found Sn-1.8Ag shows much more stable
grain structure and stable Ag 3 Sn particles, which slow down Cu dissolution into
the solder. Stable microstructure can benefit both Sn diffusion and metallization
dissolution failure modes. Lu et al. [13] studied the effect of Zn doping on SnAg
solder microstructure and EM stability. They found that Zn reacts strongly with
alloying elements, such as Cu, Ag, and Ni. Zn-doped solders improve the stability of
the interface as well as the bulk microstructure. Stable microstructure in a Zn-doped
solder can effectively suppresses EM failure induced by Sn atom flux divergence.
Overall, it can be seen that factors that can slow down the IMC reactions or
metallization dissolution can increase the stability of the solder joint and improve
EM.
2. The impact of solder joint height
Based on Al EM experiments, Blech [14] observed a threshold current density under
which no EM damage happens. This threshold is inversely proportional to the length
of the strip. This so-called Blech effect has since been well studied in different
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substrate side during EM and cause the void nucleation and propagation along that
interface.
Solder systems are quite complex metallurgical systems. The dissolution of solutes
(Cu, Ni, Pd, Au) from the substrate surface finish and Cu bumps into the solder during
reflow makes the solder alloys extremely complex multi-component thermodynamic
system. The solute elements dissolved in the solder are interstitial in nature and hence
diffuse substantially fast in the solder matrix. Additionally, solders are multiphase
alloys and undergo several intermetallic reactions at the interfaces. This makes the
solder joint EM more complex than Al or Cu traces in electronic packaging.
Many factors can impact the EM failure induced by Sn diffusion, including solder
alloying, IMC formation, current crowding, metallization dissolution, and solder
joint height. All these factors are applicable to micro bumps in 3D packaging also.
1. The impact of IMC reaction and solder alloying
One unique feature for solder joint EM is its very dynamic nature under electron
wind and high temperature. Metallization from both cathode and anode will keep
dissolving into the solder alloy and react with Sn to form IMCs. This will either
coarsening the existing IMC particles in the solder matrix or increase the thickness
of the IMC on anode side. The IMC reaction between metallization (typically Cu
or Ni) and Sn cause volume shrinkage, which will generate Sn vacancies during the
reaction. This will increase the probability of solder voids nucleation and growth
at IMC/solder interface on cathode side, where Sn atoms flux divergence happens.
From this point of view, IMC reaction and the instability of the solder system can
reduce the EM capability. This IMC reaction impact on EM fail will be even more
dramatic in smaller 3D packaging micro bumps since the Sn volume is very limited
and easy to be exhausted.
Solder alloying can also impact the stability of the solder system. Seo et al. [12]
compared Sn-1.8Ag and Sn-0.5Cu with different joining path to control the Ni and
Cu dissolution in the solder system. They found Sn-1.8Ag shows much more stable
grain structure and stable Ag 3 Sn particles, which slow down Cu dissolution into
the solder. Stable microstructure can benefit both Sn diffusion and metallization
dissolution failure modes. Lu et al. [13] studied the effect of Zn doping on SnAg
solder microstructure and EM stability. They found that Zn reacts strongly with
alloying elements, such as Cu, Ag, and Ni. Zn-doped solders improve the stability of
the interface as well as the bulk microstructure. Stable microstructure in a Zn-doped
solder can effectively suppresses EM failure induced by Sn atom flux divergence.
Overall, it can be seen that factors that can slow down the IMC reactions or
metallization dissolution can increase the stability of the solder joint and improve
EM.
2. The impact of solder joint height
Based on Al EM experiments, Blech [14] observed a threshold current density under
which no EM damage happens. This threshold is inversely proportional to the length
of the strip. This so-called Blech effect has since been well studied in different