PURPOSE: To improve the solderability of lead frame, by performing thin gold plating via Sn-Ni or Sn-Co alloy film.

CONSTITUTION: Sn-Ni or Sn-Co alloy plating layer is placed on the lead frame in place of conventional background plating layers such as copper. The constituent of alloy is 10 to 73 wt.% for Sn-Ni and 10 to 80 wt.% for Sn-Co. If Sn is less than 10%, the solderablity is not good and the plating layer is easily molten with heat with more than 80%. Optimum with about 33%. Next, the layer thickness is formed about 0.05 to 10μm and the gold layer is coated by about 0.5 to 3μm. With this constitution, even with thin gold plating film thikness, the solderability is excellent and it is economical.