12 Fundamentals of Electromigration in Interconnects of 3D Packaging
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time to grow grains along their favored directions. On the other hand, fast cooling
rate would make the grain orientation more random, which means more grains with
smaller angles between c-axis and substrate normal. This explains the cooling rate
impact on Sn grain orientation distributions.
2. Impact of alloying
While solder solidification rate is the key parameter to impact Sn grain orientations,
solder alloying is another factor that impacts the Sn grain size and orientation distribution. Sylvestre et al. [23] studied the impact of Ag content on the cyclic twin
formation for SAC solder alloys and showed that 2.2 wt% Ag in SAC alloy can
dramatically increase the chance of forming cyclic twins, which commonly occurs
during solidification of SnAg, SnCu and SAC alloys [24, 25]. This is a very unique
type of twinning and very less frequently observed in other metal systems. The formation of cyclic twins in Pb-free solder can possibly impact the distribution of c-axis
along with substrate normal direction. We know that cyclic twins share the [100] or
[010] common directions, but not in the direction of [001]. If one of the three cyclic
twins happens to have its c-axis in the direction of electron flow, the c-axis of the
other two grains cannot align with the electron flow because their common axis is not
of [001] orientation. In other words, for cyclic twinning structures, three orientations
are bundled together and they cannot have all their c-axis align with the electron
flow direction. This scenario of Sn crystalline structure will reduce the chance of
c-axis aligns with substrate normal. Therefore, adding Ag to the SnCu system can
improve the EM performance induced by metallization dissolution, which has been
experimentally observed [12, 15].
On the other hand, Cu atoms in the liquid solder can possibly play the same role as
Ag. It is reported that the formation of the cyclic twin structure initiates from a small
hexagonal cluster of Sn atoms centered on a Cu or Ag atom in the molten solder [25].
The quick dissolution of Cu from Cu column into the solder system during assembly
can reduce the possibility of c-axis of Sn grain align with substrate normal, which
can delay the metallization dissolution during EM [21].
3. Impacts of Solder Joint Height
Besides Sn grain orientation and solder alloying, solder joint height can also impact
metallization dissolution during EM. However, the effect of solder joint height on
metallization dissolution under EM has been seldom reported. Lu et al. [19] observed
that the resistance change vs stress time plot of SnAg solder joint during EM can
reach a plateau and they considered that as an evidence of Blech effect in SnAg
solder joint controlled by Sn diffusion failure mechanism. They further concluded
that solder joint height should not impact metallization dissolution since no such
plateau was observed in SnCu solder joints, which were mainly dominated by Ni
dissolution failure mode. However, no direct comparison has been done between
longer and shorter solder joints in their study.
Zhang et al. [26] studied the solder joint height impact on Cu dissolution of
Cu/SAC305/Cu joint with same volume but different height. They observed that
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time to grow grains along their favored directions. On the other hand, fast cooling
rate would make the grain orientation more random, which means more grains with
smaller angles between c-axis and substrate normal. This explains the cooling rate
impact on Sn grain orientation distributions.
2. Impact of alloying
While solder solidification rate is the key parameter to impact Sn grain orientations,
solder alloying is another factor that impacts the Sn grain size and orientation distribution. Sylvestre et al. [23] studied the impact of Ag content on the cyclic twin
formation for SAC solder alloys and showed that 2.2 wt% Ag in SAC alloy can
dramatically increase the chance of forming cyclic twins, which commonly occurs
during solidification of SnAg, SnCu and SAC alloys [24, 25]. This is a very unique
type of twinning and very less frequently observed in other metal systems. The formation of cyclic twins in Pb-free solder can possibly impact the distribution of c-axis
along with substrate normal direction. We know that cyclic twins share the [100] or
[010] common directions, but not in the direction of [001]. If one of the three cyclic
twins happens to have its c-axis in the direction of electron flow, the c-axis of the
other two grains cannot align with the electron flow because their common axis is not
of [001] orientation. In other words, for cyclic twinning structures, three orientations
are bundled together and they cannot have all their c-axis align with the electron
flow direction. This scenario of Sn crystalline structure will reduce the chance of
c-axis aligns with substrate normal. Therefore, adding Ag to the SnCu system can
improve the EM performance induced by metallization dissolution, which has been
experimentally observed [12, 15].
On the other hand, Cu atoms in the liquid solder can possibly play the same role as
Ag. It is reported that the formation of the cyclic twin structure initiates from a small
hexagonal cluster of Sn atoms centered on a Cu or Ag atom in the molten solder [25].
The quick dissolution of Cu from Cu column into the solder system during assembly
can reduce the possibility of c-axis of Sn grain align with substrate normal, which
can delay the metallization dissolution during EM [21].
3. Impacts of Solder Joint Height
Besides Sn grain orientation and solder alloying, solder joint height can also impact
metallization dissolution during EM. However, the effect of solder joint height on
metallization dissolution under EM has been seldom reported. Lu et al. [19] observed
that the resistance change vs stress time plot of SnAg solder joint during EM can
reach a plateau and they considered that as an evidence of Blech effect in SnAg
solder joint controlled by Sn diffusion failure mechanism. They further concluded
that solder joint height should not impact metallization dissolution since no such
plateau was observed in SnCu solder joints, which were mainly dominated by Ni
dissolution failure mode. However, no direct comparison has been done between
longer and shorter solder joints in their study.
Zhang et al. [26] studied the solder joint height impact on Cu dissolution of
Cu/SAC305/Cu joint with same volume but different height. They observed that