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

Abstract

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 _2.5 Ag _0.8 Cu, Sn _2.5 Ag _0.8 Cu _0.03 Fe, Sn _2.5 Ag _0.8 Cu _0.03 Co, Sn _2.5 Ag _0.8 Cu _0.03 Ni, Sn _2.5 Ag _0.8 Cu _0.03 Fe _0.03 Ni, and Sn _2.5 Ag _0.8 Cu _0.03 Co _0.03 Ni solder over Cu substrate, Cu ₆ Sn ₅ was the only reaction product for all the different solders used. (2) Reflows using the solder without doping produced a thin, dense layer of Cu ₆ Sn ₅ . The additions of Fe, Co, or Ni transformed the microstructure into a much thicker Cu ₆ Sn ₅ with many small trapped solder regions between the grains. (3) The amount of Cu ₆ Sn ₅ formed at the interface increased with the number of reflows. (4) In solid state aging study, both Cu ₆ Sn ₅ and Cu ₃ Sn formed, but the additions of Fe, Co, or Ni produced a much thinner Cu ₃ Sn layer in all cases in this study. The growth of Cu ₃ Sn followed the parabolic kinetics. (5) The simultaneous addition of 0.03 wt.% of Fe and Ni was the most effective in reducing the Cu ₃ Sn thickness. The Cu ₃ Sn thickness was only one-third that of Sn _2.5 Ag _0.8 Cu. (6) The additions of Fe, Co, or Ni in an amount as small as 0.03 wt.% were effective in reducing the Cu ₃ Sn thickness at 160degC for at least 2000 hrs.