Microstructural Formation Of Sn-Ag-Zn Solder And Its Jointed Interfaces

The formation and distribution of Intermetallic Compounds (IMCs) in the microstructure of lead-free solders directly affect the soldering performance during micro- electronic packaging process. And the IMCs formed at the interface between lead-free solders and Cu substrate will strongly affect the reliability of the soldering joints. So it is necessary to investigate the IMCs formed in the solder and at the interface. Firstly, this paper will systematically investigate the IMCs formed in the Sn-3.7Ag-xZn solder. The formation mechanism of bulk IMCs during solidification is clarified according to the competitive growth of two eutectic phases. Furthermore, the formation and evolution of IMCs formed at the interface between the Sn-Ag-xZn based solders and Cu substrate is investigated. Finally, the effects of Zn variation on the IMCs formation will be revealed.Firstly, the variation of Zn components on the microstructural formation of equilibrium Sn-3.7Ag-xZn solders was systematically investigated. It is found that the variation of Zn changed the microstructure of Sn-3.7Ag-xZn solders greatly. As the concentration of Zn increased, the formation of Ag-Zn compounds was promoted, but the formation of Ag3Sn was restrained. When the concentration of Zn reached 3wt. %, only AgZn phase and Cu₅Zn₈ phase were observed in the microstructure, and Ag3Sn had disappeared. During solidification of hypereutectic alloys, the leading IMCs phase could nucleate adhering to the primary IMCs phase due to their matching crystalline orientation relationships, leading to the formation of bulk Ag-Zn compounds or Ag3Sn IMCs at small melt undercooling.Then the IMCs formed at the interface between the Sn-3.7Ag-xZn solders and Cu substrate was investigated. The results suggested that the addition of Zn would change the IMCs formed at the interface totally. As the concentration of Zn increased, the interfacial structure turned into Cu₅Zn₈ and Cu₆Sn₅ two layers structure from the Cu₆Sn₅ one layer structure. When the concentration of Zn reached 3wt. %, only one Cu₅Zn₈ layer was observed at the interface. In a word, the addition of Zn improved the formation of Cu₅Zn₈, and restrained the formation of Cu₆Sn₅. Moreover, the addition of Zn would reduce the solubility of Cu in the liquid solder, which accelerated the growth rate of IMCs. Consequently, the thickness of IMCs layer would increase with increasing the concentration of Zn.Finally, the IMCs formed at the interface between the Sn-3.7Ag-0.9Zn-1In solder and Cu substrate was investigated. The research results suggested that the formation and evolution of IMCs could be divided into three stages. Firstly, because of the existence of Zn rich area, a thick Cu₅Zn₈ layer and a thin Cu₆Sn₅ IMCs layer, which were separated by an intermediate solder layer, were formed at the interface. As soldering time extended, the two IMCs layer both grew into the intermediate solder layer, especially the Cu₅Zn₈ layer. Finally, the Cu₅Zn₈ layer decomposed, and the Cu₆Sn₅ layer grew prominently. The intermediate solder layer disappeared simultaneously. The addition of In didn't participate in the interfacial reaction, but it made the Zn rich area far away from the interface, which lead to the formation of sandwich-structure at the early soldering stage, a Cu₅Zn₈ layer and a Cu₆Sn₅ layer with an intermediate solder layer inbetween.