Development of lead-free solders with superior drop test reliability performance

The objective of this study is to investigate the alloying effects of Fe, Co, and Ni on the interfacial reactions between solder and Cu. Emphasis is placed on a systematic comparison study on the effect of Fe, Co, and Ni additions. Solders with simultaneous Fe and Ni addition as well as simultaneous Co and Ni addition are also prepared in order to investigate whether there is any interaction between the alloying elements. The results of this study can be summarized as below: (1) In multiple reflow study using the Sn_2.5Ag_0.8Cu, Sn_2.5Ag_0.8Cu_0.03Fe, Sn_2.5Ag_0.8Cu_0.03Co, Sn_2.5Ag_0.8Cu_0.03Ni, Sn_2.5Ag_0.8Cu_0.03Fe_0.03Ni, and Sn_2.5Ag_0.8Cu_0.03Co_0.03Ni solder over Cu substrate, Cu₆Sn₅ was the only reaction product for all the different solders used. (2) Reflows using the solder without doping produced a thin, dense layer of Cu₆Sn₅. The additions of Fe, Co, or Ni transformed the microstructure into a much thicker Cu₆Sn₅ with many small trapped solder regions between the grains. (3) The amount of Cu₆Sn₅ formed at the interface increased with the number of reflows. (4) In solid state aging study, both Cu₆Sn₅ and Cu₃Sn formed, but the additions of Fe, Co, or Ni produced a much thinner Cu₃Sn layer in all cases in this study. The growth of Cu₃Sn followed the parabolic kinetics. (5) The simultaneous addition of 0.03 wt.% of Fe and Ni was the most effective in reducing the Cu₃Sn thickness. The Cu₃Sn thickness was only one-third that of Sn_2.5Ag_0.8Cu. (6) The additions of Fe, Co, or Ni in an amount as small as 0.03 wt.% were effective in reducing the Cu₃Sn thickness at 160degC for at least 2000 hrs.