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

Abstract

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 /spl gamma/- Cu/sub 5/Zn/sub 8/, /spl beta/-CuZn and thin unknown Cu-Zn layer are formed in the Sn-Zn based solder but in case of Sn-Pb solder Cu/sub 6/Sn/sub 5/ IMC layer are formed at the interface of the Cu/solderjoint. Cu/sub 3/Sn 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 /spl beta/-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.