Enhance mechanical property and electrical conductivity simultaneously of Sn–Cu–Co solder alloys by directional solidification

Abstract

Improving mechanical properties of solder alloy can result in a decrease of its conductivity. The decrease in conductivity of the solder increases the generation of Joule heat and reduces the reliability of the joint. This problem can be solved by adjusting the solidification parameters of solders. In this paper, the effect of the growth rate (V) on the microstructure evolution, microhardness and electrical conductivity of directionally solidified (DS) Sn-0.7Cu-xCo (x = 0.5, 1.0, 1.5, 2.0, mass %) alloys was investigated. The Sn-0.7Cu-xCo alloys consists of a Sn-based solid solution (Sn-ss), Cu6Sn5 phase, and CoSn2 phase. The volume fraction of the CoSn2 phase increases with increasing Co content. In the DS specimen, the CoSn2 phases change from block-shaped to a long strip shape with a long axis direction close to the heat flow direction. With increasing V, the growth direction tends to become disordered due to the weakening of the heat flow effect. The microhardness of DS Sn-0.7Cu-xCo alloys increases with increasing V due to an increase of solution strengthening effect. The microhardness of DS Sn-0.7Cu-xCo alloys increases with increasing Co content under the same V. The conductivity of the DS Sn-0.7Cu-xCo alloys decreases with increasing V. The conductivity of the DS Sn-0.7Cu-xCo alloy is even better than that of the as-cast Sn-0.7Cu alloy when V ≤ 20 μm/s. The findings in the research provide a new way to improve the strength of solder without reducing its conductivity.

Keywords

• Sn-Cu solder alloy
• Solidification parameters
• Conductivity
• Microstructure evolution
• Hardness