Effects of Zn additions on the structure of the soldered Sn-3.5Ag and Cu interfaces

Abstract

Purpose The interfacial structure is vitally important for achieving a good joint reliability during service. The purpose of this paper is to systematically explore the effects of Zn addition into the Sn-3.5Ag eutectic solder on the formation of intermetallic compound (IMC) layer at the interface between Sn-3.5Ag-xZn (x = 0, 0.9 and 3) solders and Cu pad. Design/methodology/approach To obtain useful information on the formation of interfacial structure and to determine an effective way to avoid the formation of brittle joints, a series of Sn-Ag lead-free solders with different Zn contents were prepared and soldered. To investigate the IMC layers between Sn-3.5Ag-xZn (x = 0, 0.9 and 3) lead-free solders and the Cu pads, three specimens of the Sn-3.5Ag-xZn/Cu were soldered at 250°C for one min. Findings It is found that the addition of Zn in the Sn-3.5Ag eutectic solder can prompt the formation of Cu5Zn8 IMCs, and restrain the formation of the Cu6Sn5 IMCs. Moreover, the addition of Zn in the Sn-3.5Ag eutectic solder will reduce the solubility of Cu in the liquid solder, which accelerates the growth of the formed IMCs. Consequently, the thickness of IMC layer increases with increasing the content of Zn. Originality/value This paper usefully demonstrates how the addition of Zn favoured the formation of the Cu5Zn8 phase and restrained the formation of the Cu6Sn5 phase. Moreover, the addition of Zn in the Sn-Ag eutectic solder would reduce the solubility of Cu in the liquid solder, which accelerates the growth of the formed IMCs. Consequently, the thickness of the IMC layer increased with increasing concentration of Zn.