Effects of Au and Pd in Pad Surface Finish on Electro-Migration of Flip Chip Interconnection Between Cu-Pillar and Sn-Bi Solder Alloy System

Recently, a low temperature interconnection technique has been required to solve various problems such as warpage reduction, SMT solder joint yield and energy consumption. Sn-Bi solders are strong candidate materials to resolve these difficult problems. However, their reliability and knowledge of microstructure and crystal orientation of the bump have been little investigated. In this study, electro-migration tests were performed with different Bi concentrations and pad surface finish. We find that Au and Pd element influence to inter-metallic compounds (IMCs) formation during electro-migration test. Mean Time To Failure (MTTF) of Sn57wt%Bi(Eutectic) /NiAu, Sn30wt%Bi(Sn30Bi)/NiAu and Sn30Bi/NiPdAu were 19, 141 and 600 hours, respectively. Electro-migration resistivity of using NiPdAu pad was higher than that of NiAu pad. Regarding to Cu-pillar / solder interface, thickness of (Cu, Au)6Sn5 or (Cu, Pd, Au)6Sn5 of Eutectic/NiAu, Sn30Bi/NiAu and Sn30Bi/NiPdAu were 5, 5 and 2 micro meters, respectively. And their concentration of Au or Pd were 15, 10 and 3 wt.%, respectively. In the case of Eutectic/NiAu, almost of Sn atoms was consumed to form inter-metallic compounds (IMCs) and Au atoms was consumed to form (Cu, Au)6Sn5 and (Cu, Ni, Au)6Sn5. On the other hand, in the case of using Sn30Bi, growth of IMCs were limited. In case of Sn30Bi/NiAu, More than 3 wt.% of Au atoms in (Cu, Au)6Sn5 and nearly empty of Au atoms (less than 2 wt.%) in Cu6Sn5 layer were observed at Cu-pillar / solder interface. And large number of unreacted Sn layer still existed under Cu6Sn5 layer. Au or Pd atoms accelerate to form (Cu, Au)6Sn5 IMCs. From results of EBSD analysis, in case of Eutectic/NiAu, randomly oriented (Cu, Au)6Sn5 was observed at Cu-pillar / solder interface. On the other hand, in case of Sn30Bi, [0001] oriented (Cu, Au)6Sn5 was observed in direction which Cu-pillar and solder were connected after flip chip bonding. And they also formed almost same direction after current stressing for 2800 hours. Initial stage of Au and Pd concentration in (Cu, Au)6Sn5 and (Cu, Pd, Au)6Sn5 at Cu-pillar / solder interface influenced to form crystal orientation of Cu6Sn5.