Investigations of interfacial reactions of Sn-Zn based and Sn-Ag-Cu lead-free solder alloys as replacement for Sn-Pb solder

摘要

The interfacial reactions of Sn-Zn based solders and a Sn-Ag-Cu solder have been compared with a eutectic Sn-Pb solder. During reflow soldering different types of intermetallic compounds (IMCs) are found at the interface. The morphologies of these IMCs are quite different for different solder compositions. As-reflowed, the growth rates of IMCs in the Sn-Zn based solder are higher than in the Sn-Ag-Cu and Sn-Pb solders. Different types of IMCs such as gamma-Cu_5Zn_8, beta-CuZn and a thin unknown Cu-Zn layer are formed in the Sn-Zn based solder but in the cases of Cu/Sn-Pb and Cu/Sn-Ag-Cu solder systems Cu_6Sn_5 IMC layers are formed at the interface. Cu_6Sn_5 and Cu_3Sn interfacial IMCs are formed in the early stages of 10min reflow due to the limited supply of Sn from the Sn-Pb solder. The spalling of Cu-Sn IMCs is observed only in the Sn-Ag-Cu solder. The size of Zn platelets is increased with an increase of reflow time for the Cu/Sn-Zn solder system. In the case of the Sn-Zn-Bi solder, there is no significant increase in the Zn-rich phases with extended reflow time. Also, Bi offers significant effects on the wetting, the growth rate of IMCs as well as on the size and distribution of Zn-rich phases in the beta-Sn matrix. No Cu-Sn IMCs are found in the Sn-Zn based solder during 20 min reflow. The consumption of Cu by the solders are ranked as Sn-Zn-Bi > Sn-Ag-Cu > Sn-Zn > Sn-Pb. Despite the higher Cu-consumption rate, Bi-containing solder may be a promising candidate for a lead-free solder in modem electronic packaging taking into account its lower soldering temperature and material costs.