Investigations on microhardness of Sn-Zn based lead-free solder alloys as replacement of Sn-Pb solder

The microhardness of the Sn-Zn based solder pastes have been compared with the Sn-Pb solder paste. Two types of solder such as Sn-9Zn and Sn-8Zn-3Bi have been investigated along with Sn-37Pb solder for reference. The variation of microhardness with reflow temperature from 220 to 250 °C 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 the 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 the different hardness nature. Again the effect of cooling rate on hardness was also studied and compared among these three solder pastes. The result shows that the Sn-9Zn is the most sensitive to cooling rate. With the change in cooling rate, the hardness increase in Sn-9Zn is greater (58% after water cooling) than in Sn-37Pb (30%) and Sn-8Zn-3Bi (33%). The hardness profile along the distance from the centre shows that the hardness gradient is also found highest on Sn-9Zn solder paste whereas similar trendlines were found in Sn-37Pb and Sn-8Zn-3Bi solder. SEM images of air- and water-cooled microstructures show preferential Zn formed at the edge in Sn-9Zn solder while Pb rich islands at the centre (for Sn-37Pb) and precipitation of Bi at the edge (for Sn-8Zn-3Bi) was found along with grain refinement.