Microstructural development and mechanical properties of lead-free solder Sn-Cu alloys

Abstract

Several Sn-based solder alloys have been developed in order to replace the traditional Sn-Pb solder alloys. Some alloys can be highlighted such as Sn-Zn, Sn-Cu, Sn-Bi, Sn-In, etc. Lead (Pb) is considered hazardous to health and, as a consequence some commercial restrictions have been imposed by Europe and USA in the last years. Due to cost issues, Sn-Cu alloy is becoming interesting alternative. Also, it has been successfully transferred to practical production. The eutectic composition has a melting point equal to 227 °C. However, the effects of cooling rate on both the final as-cast microstructure and mechanical properties are barely known. Directional solidification systems may be very useful devices in order to examine the microstructure development as a function of the solidification thermal parameters like cooling rate, solidification velocity and temperature gradient. These thermal parameters can be determined and correlated with microstructure parameters after obtaining directionally solidified Sn-Cu alloy castings. The microstructure parameters are dendritic/cellular spacings and interphase spacings. This study aims to carry out directional solidification experiments with Sn-Cu alloys in order to determine the mentioned experimental correlations. Tensile tests and hardness results will permit to establish relationships between HV hardness, ultimate tensile strength, yield tensile strength, specific elongation and microstructure arrangement. The nature, size, morphology and distribution of Sn-Cu intermetallics may be examined.