NICKEL-ALLOYED TIN-LEAD EUTECTIC SOLDER FOR SURFACE MOUNT TECHNOLOGY

A new ternary solder alloy of tin-lead-nickel was developed for solder joints. It has an optimum composition range of (60-65)% Sn, (35-40)% Pb, (0.5-1.0)% Ni by weight. The alloy exhibits a higher strength and longer fatigue life than the pure 63% Sn-37% Pb eutectic. Melting point, wettability, and fatigue life were the key properties used to determine this composition. In particular, the melting point of the alloyed eutectic solder did not change for rapidly solidified samples when up to 2 wt. % Ni was added. Instead of modifying the solution melting temperature, the majority of the Ni precipitated out as a fine dispersion of Ni3Sn4. These Ni3Sn4 Particles help to strengthen the solder matrix. Reflow experiments on rapidly solidified material showed minimal coarsening and agglomeration of the Ni3Sn4 particles for up to 5 min of reflow cycles. The wettability of the solder, however, decreased significantly for contents of Ni above 1.5%. Low cycle fatigue tests show that for a solder with 0.5% Ni, the fatigue life was about twice as long as that of standard eutectic solder. Suggested processing for the alloy is rapid solidification to form the alloy powder which is incorporated into a reflowable paste.