Impurity Incorporation in the Cu Electrodeposit and Its Effects on the Microstructural Evolution of the Sn/Cu Solder Joints

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 CuS2) are formed accompanying the growth of the intermetallic compounds (Cu6Sn5 and Cu3Sn) 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