Comparative study of wetting behavior and mechanical properties (microhardness) of SnZn and SnPb solders
In this paper the solder balling, wetting, spreading, slumping and microhardness testing of the SnZn based solders have been compared with the SnPb solder. Two types of solders (Sn9Zn and Sn8Zn3Bi) have been investigated along with Sn37Pb solder for reference. The variation of these tests has been done as a function of reflow temperature from 220250C. Solder balls of these three solder pastes after 15min heating at 230C show no ball formation surrounding the central ball. Spread test shows that above 240C Sn9Zn is very good and can be comparable to Sn37Pb. The wetting angle of Sn9Zn (39) at 250C is even lower than the Sn37Pb solder (41). In case of Sn8Zn3Bi, the wetting angle is very high (77) at 220C, which is unacceptable but it drops down to 48 at 250C. Line profiles of slump test show that after preheating at 160C, Sn9Zn behaves similar to Sn37Pb with better distinction in the finer pitch (120?m). Microhardness shows two different characteristics for eutectic and non-eutectic solder pastes. Hardness of Sn37Pb and Sn9Zn (eutectic) decreases with increasing reflow temperature while the microhardness of Sn8Zn3Bi (non-eutectic) increases with increasing reflow temperature. Microstructural characterization at 220 and 250C shows grain coarsening in Sn37Pb and Sn9Zn solders, which cause the hardness to drop a little. For Sn8Zn3Bi, 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 Sn37Pb, formation of Zn rod from spheroids in Sn9Zn and precipitation of Bi-rich phase in Sn8Zn3Bi are the important features that contribute to different hardness nature.
microhardness spreading slump eutectic contact angle