A highly stable electroplating bath for micro-scale Sn-Ag solder bumping

The non-cyanide and highly stable Sn-Ag alloy electroplating bath with three complexing agents of Ag
⁺
was investigated. In the electroplating bath, SnSO
4
and AgNO
3
were added as the main salt, and a large amount of sulfuric acid was added as the electrolyte. The complexing agents of Ag
⁺
were composed of the amine compound (complexing agent A), complexing agent B and complexing agent C, which effectively negatively shifted the deposition potential of Ag
⁺
while it has little effect on that of Sn
²⁺
, inhibiting the redox reaction between Sn
²⁺
and Ag
⁺
. The Sn-(1.5~2.0) wt. %Ag films with uniform, flat and superior adhesion were obtained with the current density of 4~8 ASD. With the increase of current density, Ag content in the Sn-Ag film decreased. Meanwhile, the top-view of the film became uneven due to the bigger grains formed under the larger current density. The Sn-Ag films with uniform distribution of Ag and Sn without voids were formed by using the current density of 4 ASD, Subsequently, the Sn-Ag micro bumps with the Within-Die < 8 % were obtained on the pattern wafer after reflowing at 260
^o
C for 60 s.