Interface phenomena in lead-free soldering

There are many aspects of technological developments required for the establishment of lead-free microsoldering in the electronics industrial field. The structural integrity of solders and of soldered circuits is one of the great concerns. Most of Sn alloys involving pure Sn, Sn-Ag, Sn-Bi or their ternary alloys form two intermetallic compounds at the interfaces with Cu, i.e., Cu₆Sn₅ and Cu₃Sn. The former reaction layer is much thicker than the latter and the integrity of interface is strongly influenced by the presence of the Cu₆Sn₅ layer. The Sn-Zn alloy forms different Cu-Zn intermetallic compounds without Sn at the interface with Cu. These reaction layers degrade the heat resistance of the Sn-Zn/Cu interface beyond 130°C. The coating of Ni/Pd/Au layers on Cu. however, can raise the resistance up to 150°C. In all Sn-Bi alloys except for the eutectic alloy, lift-off was frequently observed. Lift-off occurs even at 2 wt% Bi content, which is closely related to the segregation of Bi to the solder/Cu interface. Rapid cooling from soldering temperature can stop the lift-off quite effectively