Wetting and interfacial reactions of Sn-Zn based lead-free solder alloys as replacement of Sn-Pb solder

The world is moving toward ´green products´ for electronic devices and components due to the toxicity of Pb. In this paper two types of solder paste such as Sn-9Zn (eutectic) and Sn-8Zn-3Bi (noneutectic) have been investigated along with Sn-37Pb (eutectic) solder for reference. The variation of dissolution of Cu layer and IMC with reflow time shows different characteristics for eutectic and non-eutectic solder pastes. The morphologies of the IMC are quite different for different solder compositions. During as reflowed, the growth rate of IMC in the Sn-Zn based solder is higher than the SnPb solders. Different types of IMC such as γ- Cu₅Zn₈, β-CuZn and thin unknown Cu-Zn layer are formed in the Sn-Zn based solder but in case of Sn-Pb solder Cu₆Sn₅ IMC layer are formed at the interface of the Cu/solderjoint. Cu₃Sn IMC are formed after 10 minutes reflow due to the limited supply of Sn from the Sn-Pb solder. The wettability of Sn-Zn solder is found as least compared with the other solders. The size of Zn platelets is increased with an increase of reflow time for the Sn-Zn solder system. In the case of Sn-Zn-Bi solder, Bi offers significant effects on growth rate of IMC as well as size and distribution of Zn-rich phase in the β-Sn matrix. No Cu-Sn IMC are found in the Sn-Zn based solder during 20 minutes reflow. Cu-Zn IMC act as a good diffusion barrier for Sn. The consumption of Cu by all types of solders are ranked as Sn-Zn-Bi>Sn-Zn>Sn-Pb. Although Sn-Zn-Bi solder has higher dissolution rate of Cu layer and some voids are found at the interface, it can be used as a replacement of conventional Sn-Pb solder in the electronic industry.