342
K.-L. Lin
retarded the consumption of Cu pillar during current stressing, partly due to the slow
reaction rate of the Ni layer. The solder was completely converted to a mixed layer
of Cu 6 Sn 5 and (Cu, Ni) 6 Sn 5 . Meanwhile, the Cu 3 Sn layer grew rapidly at massive
consumption of the Cu trace layer and induced Kirkendall voids. The failure at long
time current stressing was mainly due to the consumption of most of the Cu trace
[12].
Kirdendall void is a typical defect formed at the interface when the diffusivities of
counter diffusing elements are different. The diffusivities of Cu and Sn are different
in Cu 3 Sn while similar in Cu 6 Sn 5 , Table 11.2. The formation rate of Cu 6 Sn 5 is larger
than Cu 3 Sn. Accordingly, in the case of microbump where Cu 6 Sn 5 and Cu 3 Sn are
formed sequentially, Kirkendall void will more likely form in the Cu 3 Sn layer or at
the interface between Cu 3 Sn and Cu. Void was generally not observed in the Cu 6 Sn 5
intermetallic compound. One of the potential methods of avoiding the formation
of Kirdendall void is by removing the voids from the diffusing media Cu 3 Sn. The
produce of nanotwinned Cu pillar by electroplating was able to sink the void formed
in the above mentioned intermetallic transformation reaction in a Cu pillar/Sn/Cu
pillar structure [43]. No void was observed in the structure during the transformation
of Cu 6 Sn 5 to Cu 3 Sn under long time thermal ageing. The nanotwinned Cu pillar was
produced by appropriate controlling the electroplating conditions [44].
Both Pd and Au form intermetallic compounds with Sn when adopted as metallization. PdSn 4 grows faster than AuSn 4 . A large columnar (Pd, Ni)Sn 4 may form
in the Cu/Ni/Pd/Au/Sn/Ni/Cu microbump structure, Fig. 11.8a [45]. The thermal
cycle test or electromigration test of such joint eventually convert the (Pd, Ni)Sn 4
to Ni 3 Sn 4 . The molar volume difference between the intermetallic compounds will
induce void or crack at the interface between the converting intermetallic compounds
[22], Fig. 11.8b–e [45] (readers are referred to Chap. 7 for the detail discussion
regarding electromigration). The molecular volumes are 71.13 cm
3 /mole for PdSn 4
and 75.25 cm
3 /mole for NiSn 4 . The compounds conversion actually involved (Pd,
Ni)Sn 4 and (Ni, Cu) 3 Sn 4 . Thus a volume difference of 9.65–22.3% is expected that
causes the volume shrinkage during the conversion [45]. The volume shrinkage will
result in formation of voids or cracks at the interface between the intermetallic [45].
11.6 Summary and Future Challenge
The industrial microbumping and bonding process has been mature and promising.
In light of the massive shrinkage in bump dimension comparing with conventional
flip chip solder bump, the solder materials compositions have been kept as simple
as possible. Sn or Sn–Ag have been the predominant solder constituents of the
microbump. Electroplating is the major deposition method for the solder bump
because of the small dimension of the microbump with Cu pillar design and the
productivity concern. The reduced volume of the solder joint results in high volume
fraction of intermetallic compound in the as reflow and prolonged thermal aged
K.-L. Lin
retarded the consumption of Cu pillar during current stressing, partly due to the slow
reaction rate of the Ni layer. The solder was completely converted to a mixed layer
of Cu 6 Sn 5 and (Cu, Ni) 6 Sn 5 . Meanwhile, the Cu 3 Sn layer grew rapidly at massive
consumption of the Cu trace layer and induced Kirkendall voids. The failure at long
time current stressing was mainly due to the consumption of most of the Cu trace
[12].
Kirdendall void is a typical defect formed at the interface when the diffusivities of
counter diffusing elements are different. The diffusivities of Cu and Sn are different
in Cu 3 Sn while similar in Cu 6 Sn 5 , Table 11.2. The formation rate of Cu 6 Sn 5 is larger
than Cu 3 Sn. Accordingly, in the case of microbump where Cu 6 Sn 5 and Cu 3 Sn are
formed sequentially, Kirkendall void will more likely form in the Cu 3 Sn layer or at
the interface between Cu 3 Sn and Cu. Void was generally not observed in the Cu 6 Sn 5
intermetallic compound. One of the potential methods of avoiding the formation
of Kirdendall void is by removing the voids from the diffusing media Cu 3 Sn. The
produce of nanotwinned Cu pillar by electroplating was able to sink the void formed
in the above mentioned intermetallic transformation reaction in a Cu pillar/Sn/Cu
pillar structure [43]. No void was observed in the structure during the transformation
of Cu 6 Sn 5 to Cu 3 Sn under long time thermal ageing. The nanotwinned Cu pillar was
produced by appropriate controlling the electroplating conditions [44].
Both Pd and Au form intermetallic compounds with Sn when adopted as metallization. PdSn 4 grows faster than AuSn 4 . A large columnar (Pd, Ni)Sn 4 may form
in the Cu/Ni/Pd/Au/Sn/Ni/Cu microbump structure, Fig. 11.8a [45]. The thermal
cycle test or electromigration test of such joint eventually convert the (Pd, Ni)Sn 4
to Ni 3 Sn 4 . The molar volume difference between the intermetallic compounds will
induce void or crack at the interface between the converting intermetallic compounds
[22], Fig. 11.8b–e [45] (readers are referred to Chap. 7 for the detail discussion
regarding electromigration). The molecular volumes are 71.13 cm
3 /mole for PdSn 4
and 75.25 cm
3 /mole for NiSn 4 . The compounds conversion actually involved (Pd,
Ni)Sn 4 and (Ni, Cu) 3 Sn 4 . Thus a volume difference of 9.65–22.3% is expected that
causes the volume shrinkage during the conversion [45]. The volume shrinkage will
result in formation of voids or cracks at the interface between the intermetallic [45].
11.6 Summary and Future Challenge
The industrial microbumping and bonding process has been mature and promising.
In light of the massive shrinkage in bump dimension comparing with conventional
flip chip solder bump, the solder materials compositions have been kept as simple
as possible. Sn or Sn–Ag have been the predominant solder constituents of the
microbump. Electroplating is the major deposition method for the solder bump
because of the small dimension of the microbump with Cu pillar design and the
productivity concern. The reduced volume of the solder joint results in high volume
fraction of intermetallic compound in the as reflow and prolonged thermal aged