The melting-point increase of Sn-Bi solder joint by Cu particles addition

Abstract

Solder is one of the main materials for interconnects in microelectronic packaging and Sn-Pb solder alloys have been the most common solders for a long time. However, the use of Pb has been very limited because of the negative effects of Pb on the human health and the environment. Therefore, Sn-Bi based solders were recommended as a low temperature solder to replace Sn-Pb solders because of their low temperature melting point. Low temperature melting point of Sn-Bi solders reduces the damage due to the thermal effect of the high temperature on the mechanical properties of the joints. However, its low melting point allows Sn-Bi solders to be used in a limited range of applications. The objective of this research is to enable the Sn-Bi based solders to be used in higher temperature applications. Therefore, this research represents how to increase the melting point of the solder joint after the reflow while keeping the advantage of low melting point before the reflow. The method which was tried in this research in order to increase the melting point of the Sn-Bi solder after the reflow is to convert the whole Sn phase of the solder to Cu6Sn5 and Cu3Sn intermetallics by adding Cu particles to the solder before the reflow. The binary Sn-Bi and Sn-Bi + 30 wt. %Cu particles were investigated in this research. Scanning electron microscopy (SEM) and electron probe micro-analyzer (EPMA) were used to examine the microstructure of the solder parts after the reflow. Various reflow profiles were tried to enable the whole Sn phase react with the added Cu particles. Differential scanning calorimetry (DSC) was also used in this research to investigate the melting point of the solder parts. The results after the reflow at 230°C for 50 min show that almost all Sn phase was successfully converted to the Cu6Sn5 and Cu3Sn phase.