12 Fundamentals of Electromigration in Interconnects of 3D Packaging
361
(F BS ) and the driving force from thermal gradient (F TM ) in their tested 3D packaging
based on the simulated temperature gradient of 5345 °C/cm across the micro bumps
and the working temperature of about 134 °C. It is concluded that F BS is about 3–4
times larger than F TM , which explains why no Sn thermomigration happens in their
tested micro bumps. In other words, in order for Sn thermomigration to happen in the
micro bumps, the temperature gradient needs to be larger than about 20,000 °C/cm.
Compared the fact that 1000 °C/cm thermal gradient will induce thermomigration
in flip chip solder joint [38], micro bumps are more resistant to Sn thermomigration
due to the back stress effect. It can be seen that both thermomigration and EM risks
due to Sn flux divergence are low for micro bumps.
While thermomigration damage due to Sn diffusion can be minimal in micro
bumps due to back stress effect, metallization dissolution induced by thermal gradient
could be another concern that needs to be addressed. Chen et al. summarized the
thermomigration of Ni, Cu, Ag and Ti in solder joints [38]. It is noted that they all
migrate to the opposite direction of Sn atoms under thermal gradient. Sn migrates
from cold end to hot end, while Ni, Cu, Ag and Ti move from hot end to cold end. This
correlates to the positive or negative sign of heat of transport Q
* , which is defined by
the difference between heat carried by a moving atom per mole to the heat of atoms
per mole at the hot end. When Q
* is negative, the diffusion specie moves from cold to
hot (i.e. Sn), when Q
* is positive, atoms move from hot to cold (i.e. Ni, Cu, Ag, Ti).
Cu thermomigration in solder joint is much easier than Ni. Part of the reason can be
related to the low Ni solubility in solder alloy. It is reported that the solubility of Ni is
only 0.28 wt% in Pb-free solder at 250 °C, while the solubility of Cu is 1.54 wt% at
260 °C [41]. Cu migration was observed at thermal gradient of 1000 °C/cm in flip chip
solder joint [42], however, no Ni migration at thermal gradient of 1429 °C/cm [43].
When thermal gradient is 5345 °C/cm, Ni thermomigration was observed in micro
bumps as shown in Fig. 12.12 [39]. In a recent study, Ouyang et al. [44] reported
that Ag 3 Sn IMC particles or plates can suppress thermomigration of Cu. They can
inhibit the dissolution of Cu at the hot end and abnormal accumulation of IMCs at
the cold end. The ability of Ag 3 Sn to against thermomigration of Cu is speculated
to be related to the less susceptibility of Ag atoms to temperature gradient.
Thermomigration can induce the fast metallization dissolution and cause reliability concern. However, similar to the discussion in EM case, metallization dissolution will eventually transform the solder joint into full IMC joint when the volume
ratio of metallization to Sn is optimized. In this case, both thermomigration and EM
concerns should be minimal in micro bumps.
12.4 EM in TSV of 3D Packaging
In 3D packaging, micro bumps that we discussed above are interconnects between
Si dies. Inside a Si die, TSV, RDL and metal layers connect to TSV are also key 3D
packaging interconnects that may have EM reliability concerns due to their small
361
(F BS ) and the driving force from thermal gradient (F TM ) in their tested 3D packaging
based on the simulated temperature gradient of 5345 °C/cm across the micro bumps
and the working temperature of about 134 °C. It is concluded that F BS is about 3–4
times larger than F TM , which explains why no Sn thermomigration happens in their
tested micro bumps. In other words, in order for Sn thermomigration to happen in the
micro bumps, the temperature gradient needs to be larger than about 20,000 °C/cm.
Compared the fact that 1000 °C/cm thermal gradient will induce thermomigration
in flip chip solder joint [38], micro bumps are more resistant to Sn thermomigration
due to the back stress effect. It can be seen that both thermomigration and EM risks
due to Sn flux divergence are low for micro bumps.
While thermomigration damage due to Sn diffusion can be minimal in micro
bumps due to back stress effect, metallization dissolution induced by thermal gradient
could be another concern that needs to be addressed. Chen et al. summarized the
thermomigration of Ni, Cu, Ag and Ti in solder joints [38]. It is noted that they all
migrate to the opposite direction of Sn atoms under thermal gradient. Sn migrates
from cold end to hot end, while Ni, Cu, Ag and Ti move from hot end to cold end. This
correlates to the positive or negative sign of heat of transport Q
* , which is defined by
the difference between heat carried by a moving atom per mole to the heat of atoms
per mole at the hot end. When Q
* is negative, the diffusion specie moves from cold to
hot (i.e. Sn), when Q
* is positive, atoms move from hot to cold (i.e. Ni, Cu, Ag, Ti).
Cu thermomigration in solder joint is much easier than Ni. Part of the reason can be
related to the low Ni solubility in solder alloy. It is reported that the solubility of Ni is
only 0.28 wt% in Pb-free solder at 250 °C, while the solubility of Cu is 1.54 wt% at
260 °C [41]. Cu migration was observed at thermal gradient of 1000 °C/cm in flip chip
solder joint [42], however, no Ni migration at thermal gradient of 1429 °C/cm [43].
When thermal gradient is 5345 °C/cm, Ni thermomigration was observed in micro
bumps as shown in Fig. 12.12 [39]. In a recent study, Ouyang et al. [44] reported
that Ag 3 Sn IMC particles or plates can suppress thermomigration of Cu. They can
inhibit the dissolution of Cu at the hot end and abnormal accumulation of IMCs at
the cold end. The ability of Ag 3 Sn to against thermomigration of Cu is speculated
to be related to the less susceptibility of Ag atoms to temperature gradient.
Thermomigration can induce the fast metallization dissolution and cause reliability concern. However, similar to the discussion in EM case, metallization dissolution will eventually transform the solder joint into full IMC joint when the volume
ratio of metallization to Sn is optimized. In this case, both thermomigration and EM
concerns should be minimal in micro bumps.
12.4 EM in TSV of 3D Packaging
In 3D packaging, micro bumps that we discussed above are interconnects between
Si dies. Inside a Si die, TSV, RDL and metal layers connect to TSV are also key 3D
packaging interconnects that may have EM reliability concerns due to their small