12 Fundamentals of Electromigration in Interconnects of 3D Packaging
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vacancy sink, it is very hard to cumulate enough vacancies for voids nucleation.
However, when impurities are present at the interface, they can pin the dislocations
at the interface and cause the interface less movable. In this case, the interface will
not be an effective vacancy sink anymore and voids will be formed due to vacancy
condensation. Only mono-layer of impurities at interface can trigger this mechanism
[37]. Actually, a more intuitive way to view this can be as following. Interfacial voids
can not be generate when the reaction interface is smoothly moveable and follows
well with the reaction front. However, if impurities pin the interface and make it less
moveable, the interface can not follow well with the reaction front and voids have to
be generated. The reason that the full IMC joint only failed at lower side regardless
of the EM polarity can be due to the Cu impurity difference between upper and lower
Cu bumps.
In general, EM resistance in micro bumps is more robust because of the transformation of full IMC joint. However, impurities from Cu plating should be avoided to
prevent the solder joint from Cu/Cu 3 Sn failure. While this is mainly a backing related
phenomenon, EM can definitely increase the failure rate due to its acceleration of
cathode side Cu dissolution.
12.3.3 Thermomigration Accompanied by EM
When current is applied to solder joints in electronic packaging, electron wind can
induce EM damages as discussed above. Meanwhile, Joule heating due to the high
current density can result in thermal gradient and induce thermomigration of the
solder joint [38–40]. The thermal gradient is closely related to the geometrical configurations of the electronic packaging. Typically, joule heating on the substrate side of
the solder joint is much less than that from the chip side where transistors and finer
interconnects will generate more heating. It is reported that a temperature difference of 10 °C across a 100 μm diameter solder joint (1000 °C/cm) is sufficient to
induce thermomigration and elements redistribution [38]. The large thermal gradient
is expected to be worse in micro bumps of 3D packaging since its diameter is more
than one order of magnitude smaller and stacked Si could generate more heat due to
bad thermal dissipation.
Similar to EM force, solder joint under thermal gradient force can also result in
two impacts on diffusion species. One is the migration of Sn atoms; the other is
metallization (Cu, Ni) dissolution.
Ouyang et al. [39] studied the thermomigration behaviors of micro bumps in 3D
packaging and compared them with flip chip solder joints. They observed that Sn
in solder alloy migrated to the hot side in the flip chip solder joint, but no Sn thermomigration was observed in micro bumps of 3D packaging, as shown in Figs. 12.11
and 12.12. They attribute this to the back stress gradient difference caused by solder
joint height. Just as mentioned in Blech effect, shorter solder joint height will result
in larger back stress gradient which can balance out the thermal migration force
on Sn atoms induced by thermal gradient. They further calculated the back stress
359
vacancy sink, it is very hard to cumulate enough vacancies for voids nucleation.
However, when impurities are present at the interface, they can pin the dislocations
at the interface and cause the interface less movable. In this case, the interface will
not be an effective vacancy sink anymore and voids will be formed due to vacancy
condensation. Only mono-layer of impurities at interface can trigger this mechanism
[37]. Actually, a more intuitive way to view this can be as following. Interfacial voids
can not be generate when the reaction interface is smoothly moveable and follows
well with the reaction front. However, if impurities pin the interface and make it less
moveable, the interface can not follow well with the reaction front and voids have to
be generated. The reason that the full IMC joint only failed at lower side regardless
of the EM polarity can be due to the Cu impurity difference between upper and lower
Cu bumps.
In general, EM resistance in micro bumps is more robust because of the transformation of full IMC joint. However, impurities from Cu plating should be avoided to
prevent the solder joint from Cu/Cu 3 Sn failure. While this is mainly a backing related
phenomenon, EM can definitely increase the failure rate due to its acceleration of
cathode side Cu dissolution.
12.3.3 Thermomigration Accompanied by EM
When current is applied to solder joints in electronic packaging, electron wind can
induce EM damages as discussed above. Meanwhile, Joule heating due to the high
current density can result in thermal gradient and induce thermomigration of the
solder joint [38–40]. The thermal gradient is closely related to the geometrical configurations of the electronic packaging. Typically, joule heating on the substrate side of
the solder joint is much less than that from the chip side where transistors and finer
interconnects will generate more heating. It is reported that a temperature difference of 10 °C across a 100 μm diameter solder joint (1000 °C/cm) is sufficient to
induce thermomigration and elements redistribution [38]. The large thermal gradient
is expected to be worse in micro bumps of 3D packaging since its diameter is more
than one order of magnitude smaller and stacked Si could generate more heat due to
bad thermal dissipation.
Similar to EM force, solder joint under thermal gradient force can also result in
two impacts on diffusion species. One is the migration of Sn atoms; the other is
metallization (Cu, Ni) dissolution.
Ouyang et al. [39] studied the thermomigration behaviors of micro bumps in 3D
packaging and compared them with flip chip solder joints. They observed that Sn
in solder alloy migrated to the hot side in the flip chip solder joint, but no Sn thermomigration was observed in micro bumps of 3D packaging, as shown in Figs. 12.11
and 12.12. They attribute this to the back stress gradient difference caused by solder
joint height. Just as mentioned in Blech effect, shorter solder joint height will result
in larger back stress gradient which can balance out the thermal migration force
on Sn atoms induced by thermal gradient. They further calculated the back stress