Measurements of Thermal Expansion Coefficients and Melting Ranges for Sn-9Zn-xAg Lead-free Solder Alloys

Abstract

To develop an Sn-Zn-xAg lead free solder alloy with optimal thermo-physical properties, this study is conducted to experimentally investigate the effects of silver content (0–3.5%) on the coefficient of thermal expansion (CTE) and melting range of this series of alloys.The CTE of Sn-Zn-xAg solder alloys is measured by using a dilatometer with a heating rate of 5 °C/min from 40 °C to 120 °C. Melting range is measured by using a differential scanning calorimetry (DSC) with heating rates of 5 °C/min and 0.5 °C/min.The results of dilatometer measurement show that for all the designed solder alloys, the CTE is increased with increasing temperature. When the temperature reaches 90 °C, the increase becomes less obvious. Furthermore, CTE increases linearly with increasing silver content. As the silver contents are over 2%, the values of CIE are greater than the conventional Sn-37Pb alloy, which may generate substantial thermal stress due to the difference in CTE between solder alloy and substrate. From the results of DSC measurement, the starting pint of the melting range is not substantially affected by the increased silver contents in the silver range under consideration. However, when the silver content exceeds 2%, a second peak can be observed from the heating curve, which results in a substantial increase in the end point of the melting range. These results are attributed to the appearance of Zn-Ag compounds, which can be observed in the microstructures. Based on the above observations, silver content should not exceed a maximum of 2% in order to develop an Sn-Zn-xAg lead free solder with proper thermo-physical properties.