Role of Ni and Cu metallization dissolution in various Ni/solder/Cu jointing sequences

This study was conducted to the Ni and Cu dissolution behaviors, and the cross-interaction during Ni/Sn3.5Ag/Cu joints fabrication. To form such joint structure, two common soldering sequences were employed: an as-reflow Cu/Sn3.5Ag solder bump jointed to Ni (Seq. I), and an as-reflow Ni/Sn3.5Ag solder bump jointed to Cu (Seq. II). The research results revealed that a ternary compound, (Cu, Ni)₆Sn₅, would substitute for the binary Cu₆Sn₅ and Ni₃Sn₄ grew at both interfaces of Ni/solder/Cu. The morphology and thickness of (Cu, Ni)₆Sn₅ were strongly dependent on the soldering sequence, especially for the Cu-side interface. In Seq. I, the (Cu, Ni)₆Sn₅ (at the solder/Cu interface) was a dense layer with a typical scallop-like morphology. Interestingly, the compound layer became a dual-layer structure: a dense (Cu_0.93Ni_0.07)₆Sn₅ (near the Cu) with (Cu_0.81Ni_0.19)₆Sn₅ particles aggregated near the solder, when the Seq. II process was performed. The overall thickness of the (Cu, Ni)₆Sn₅ was about 2.5 times thicker than that grew via Seq. I even though the solder/Cu interface only experienced one reflow in Seq. I. Additionally, it was also found that the Cu concentrations in the Sn-Ag solder varied with soldering sequences. The variation in the Cu concentration and (Cu, Ni)₆Sn₅ micro structure correlated with the quantity of Ni introduced to the solder alloy. Dybkov´s dissolution kinetics for a solid into a liquid combined with Cu-Ni-Sn isotherm data will be utilized to describe the above Cu and Ni dissolution behaviors.