Comparative study of wetting behavior and mechanical properties (microhardness) of Sn-Zn and Sn-Pb solders

In this paper the solder balling, wetting, spreading, slumping and microhardness testing of the Sn-Zn based solders have been compared with the Sn-Pb solder. Two types of solders (Sn-9Zn and Sn-8Zn-3Bi) have been investigated along with Sn-37Pb solder for reference. The variation of these tests has been done as a function of reflow temperature from 220-250 °C. Solder balls of these three solder pastes after 15 min heating at 230 °C show no ball formation surrounding the central ball. Spread test shows that above 240 °C Sn-9Zn is very good and can be comparable to Sn-37Pb. The wetting angle of Sn-9Zn (39°) at 250 °C is even lower than the Sn-37Pb solder (41°). In case of Sn-8Zn-3Bi, the wetting angle is very high (77°) at 220 °C, which is unacceptable but it drops down to 48° at 250 °C. Line profiles of slump test show that after preheating at 160 °C, Sn-9Zn behaves similar to Sn-37Pb with better distinction in the finer pitch (120 μm). Microhardness shows two different characteristics for eutectic and non-eutectic solder pastes. Hardness of Sn-37Pb and Sn-9Zn (eutectic) decreases with increasing reflow temperature while the microhardness of Sn-8Zn-3Bi (non-eutectic) increases with increasing reflow temperature. Microstructural characterization at 220 and 250 °C shows grain coarsening in Sn-37Pb and Sn-9Zn solders, which cause the hardness to drop a little. For Sn-8Zn-3Bi, with increasing temperature the amount of hard Bi segregation increases which is the main cause of the rise in hardness. SEM images show the formation of Pb rich islands in Sn-37Pb, formation of Zn rod from spheroids in Sn-9Zn and precipitation of Bi-rich phase in Sn-8Zn-3Bi are the important features that contribute to different hardness nature.