Interfacial reactions and bump reliability of various Pb-free solder bumps on electroless Ni-P UBMs

Electroless Ni-P UBMs combined with screen-printing of solder pastes are one of the lowest cost flip-chip bumping techniques. Pb-free solder bumps on electroless Ni-P UBM can be easily fabricated as various Pb-free solder alloy pastes are available. Therefore, interfacial reactions between electroless Ni-P UBM and Pb-free solders should be investigated, because they are greatly affected by small amounts of alloying such as Ag, Cu, and Bi in Pb-free solders. Reliability of Pb-free solder bumps can be greatly affected by intermetallic growth and P-rich Ni layer at the interface. Recently, Pb-free solder alloys such as SnAg, SnAgCu, SnCu, and SnAgBi have been suggested as promising candidates for substituting Sn37Pb solder. In this study, these four alloys were selected as solder bump materials for electroless Ni-P UBM. The effects of Ag, Cu, and Bi in Pb-free alloys on interfacial reactions and bump reliability at electroless Ni-P/solder interfaces were investigated. It was found that the consumption rate of Ni-P UBM was much slower in SnAgCu and SnCu alloys than in SnAg and SnAgBi alloys during solder reflow. SnAgCu and SnCu solders also showed lower Ni-P UBM consumption rate than SnAg and SnAgBi during aging. In particular, more Cu-containing Sn0.7Cu solder showed lower Ni-P UBM consumption than SnAg0.5Cu solder for the same heat treatment conditions. Consumption of Ni-P UBM can be reduced by adding Cu, as Cu addition initially causes (Cu,Ni)₆Sn₅ phase rather than Ni₃Sn₄ phase. Bi addition in Pb-free solder alloys did not affect interfacial reaction with Ni-P UBMs. However, higher mechanical properties and lower melting point of Pb-free solder alloys can be obtained by Bi addition. Bump shear test results showed that all failure occurred inside soft solders, and shear strength was proportional to ultimate tensile strength of solder alloys. However, because P-rich Ni layer has been reported as a brittle failure site, it is suggested that Cu-containing Pb-free solder alloys such as SnAgCu and SnCu showing lower interfacial reaction rate with Ni-P UBMs are preferable.