Application of TLP (Transient Liquid Phase) Bonding Method to the High Tm Lead-Free Solder
TLP method was applied to the lead free soldering process; Sn-3.5Ag core solder (Tm of 221°C) / Sn-Bi coating layer (Tm of 139°C at eutectic) was reflowed on UBM pad. Sn-Bi was formed by electroplating method at current range of 2~6A/dm2, which had varied the coating layer composition from 96wt% to 28wt% Bi. Specimens were reflowed on copper pad at temperature range of 200 and 220°C, and then interface reaction was examined. It was claimed that high Bi adding in the plated layer improved the wetting performance on Cu. Even at the reflow temperature of 200°C, SEM observation revealed well-defined interface junctions between solder and copper pad, i.e. nucleation and growth of CuxSn intermetallic compound. After heating for 5min at 220°C, Bi was resolved into Sn-3.5Ag core solder and completely homogenized, which is favorable from the viewpoint of long term fatigue reliability.
Z.Y. Zhong, H. Saka, T.H. Kim, E.A. Holm, Y.F. Han and X.S. Xie
J. S. Lee et al., "Application of TLP (Transient Liquid Phase) Bonding Method to the High Tm Lead-Free Solder", Materials Science Forum, Vols. 475-479, pp. 1869-1872, 2005