Sn/Cu solder joints serve as electrical pathways and mechanical supports in microelectronic packaging. This study demonstrates that the Cu plating formula has a strong effect on the incorporation of impurities in the Cu deposit and, consequently, affects the microstructural evolution of the Sn/Cu interface. An additive formula with polyethylene glycol (PEG) and chloride ions (Cl−) results in a high-level incorporation of impurities (carbon, oxygen, sulfur, and chlorine) in the Cu deposit. Segregation of impurities to the Sn/Cu interface occurs during thermal aging, which induces the formation of voids and Cu-impurity compounds (CuO, Cu2O, and CuS2) embedded in the Cu6Sn5 and Cu3Sn phase layers, forming an unusual alternating layer structure that is structurally loose and unstable. The addition of bis(3-sulfopropyl) disulfide (SPS) along with PEG and Cl− helps restore the microstructural integrity of the Sn/Cu interface by suppressing the incorporation of impurities. This suppression strongly depends upon the SPS concentration and the PEG molecular weight. The aging temperature also has a significant effect on the microstructural evolution of the Sn/Cu interface, which is determined by the change of the dominant diffusion species.

Impurity incorporation in the Cu electrodeposits as a result of the addition of organic additives in the Cu plating solution is investigated with four additive formulas. A common suppressor (polyethylene glycol, PEG) and chloride ions (Cl − ) are added in the plating solution as a control additive formula. Three organosulfides, 3-mercaptopropanesulfonsäure (MPS), bis(3-sulfopropyl) disulfide (SPS), and 3-(2-benzthiazolylthio)-1-propanesulfonsäure (ZPS), are used as accelerators and individually formulated with PEG and Cl − as the other three experimental formulas. The additive formulas of PEG + Cl − and PEG + Cl − + ZPS result in high-level impurity incorporation and cause the microstructural instability of the Sn/Cu joints during thermal aging. Voids and Cu-impurity compounds (CuO and CuS 2 ) are formed accompanying the growth of the intermetallic compounds (Cu 6 Sn 5 and Cu 3 Sn) which severely degrades the Sn/Cu joints mechanically. When the additive formulas are changed to PEG + Cl − + MPS and PEG + Cl − + SPS, the impurity incorporation is significantly suppressed and thereby inhibits the formation of voids and Cu-impurity compounds. The strong dependence of additive formulas on the impurity incorporation is attributable to the delicate interaction (adsorption competition) between suppressor (PEG) and various accelerators, in which the molecular structures of the organosulfides play a key role.