Effects of under bump metallization and nickel alloying element on the undercooling behavior of Sn-based, Pb-free solders

A significant reduction of the undercooling of Sn-based solder alloys was previously reported when they were reacted with various under bump metallurgies (UBMs). In the present study, new experiments have been designed and carried out to understand the undercooling behavior of various Cu- and Ni-doped solders on Ni UBM. Two competing mechanisms were further investigated that include the formation of intermetallic compounds (IMCs) at solder/UBM interface and the change of solder composition because of the dissolution of Ni UBM into solder. Two types of IMCs, including both Ni₃Sn₄and Cu₆Sn₅that were formed at the interface, were correlated with the undercooling of Sn–0.2Cu and Sn–3.8Ag–0.2Cu solders. In addition, the compositional changes of various Sn-based solders after reactions with Ni UBM were analyzed. On the basis of the experimental results, it was found that the significant reduction in undercooling is primarily caused by dissolved Ni atoms from Ni UBM and the concurrent formation of Ni₃Sn₄IMC in the solder matrix. Finally, the beneficial effect of Ni dissolution is thermodynamically favorable as confirmed by the thermodynamic calculations and differential scanning calorimetry measurements with various Ni-doped solder alloys.