To form reliable Pb-free solder joints, minor alloying additions of Ni or Zn to Sn-rich solders have been recommended recently. Several beneficial effects of Ni or Zn minor alloying additions to Pb-free solders were reported to improve solder joint reliability. But the effects of Ni or Zn minor alloying additions on the bulk properties of solders are not systematically evaluated in light of understanding the electromigration or mechanical reliability of solder joints. Therefore, in this study, the minor alloying effects of Ni or Zn on the microstructure and microhardness in terms of Ni or Zn composition and cooling rate are investigated. The amounts of minor alloying elements investigated are in the range of 0.05- 0.15 wt% for Ni, and 0.2-0.6wt% for Zn, which cover the reported composition ranges to enhance solder/UBM joint reliability. Three cooling rates are employed during solidification; 0.02 °C/s (furnace-cooling), about 5 °C/s (aircooling), and 100 °C/s or higher (quenching). The microstructure of Ni or Zn doped solders is evaluated in terms of composition, undercooling during solidification, and cooling rate. The phase diagram analysis is conducted to explain the microstrctural variations. The microstructures of Ni or Zn doped solders are well correlated to their microhardness data. © 2011 IEEE.