TLP method was applied to the lead free soldering process; Sn-3.5Ag core solder (T-m of 221 degrees C)/Sn-Bi coating layer (T-m of 139 degrees C at eutectic) was reflowed on UBM pad. Sn-Bi was formed by electroplating method at current range of 2 similar to 6A/dm(2), 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 degrees 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 degrees 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 degrees C, Bi was resolved into Sn-3.5Ag core solder and completely homogenized, which is favorable from the viewpoint of long term fatigue reliability.