Investigation On The Preparation Method And Properties Of SN-58BI Lead-free Solder With Carbon Nanotubes

Sn-58Bi solder have potential to be one of the promising lead-free low temperature solders for its low melting point, high wettability and etc. However, its low plasticity hinders its applications as electronic packaging materials. Carbon nanotubes (CNTs) have great potential to be used as reinforcement material in the fabrication of the composite because of its excellent mechanical properties. In this work, varying weight of the multi-walled carbon nanotubes (MWCNTs) was selected as the reinforcement for Sn-58Bi-CNTs solder. The solder was prepared through the ball-milling method and then followed with low temperature melting process. The morphologies of the carbon nanotubes in the mixture and the Sn-58Bi-CNTs solder were investigated; the influences of carbon nanotubes on the mechnical properties of solder such as the bending strength, tensile strength, and thoughness and solder joint quality were also studied.The solder was prepared through the ball-milling method and low temperature melting process. The ball-milling method was beneficial for the uniform distribution of CNTs in the mixture. During the ball-milling process, the grinding balls rotated and compressed the solder powders to deform. Some individual CNTs were involved in the deformation and embedded in the deformed solder powders mechanically. Some of CNTs attached on the surface of the Sn-58Bi powder. During the process of low temperature melting process, a large portion of CNTs floated on the surface of the liquid flux or wiped off from the melted alloy. Though a large portion of CNTs escaped, there were a few CNTs stayed in the grain boundary of as-casted solder alloy. When the Sn-58Bi-CNTs solder was re-melting again, the flotation phenomenon of the CNTs disappeared. The CNTs distributed uniformly in the solder matrix. Also the agglomerate structures of CNTs were formed in some areas. The presence of CNTs can reduce the dendrites tissue and refine the microstructurere of Sn-58Bi solder alloy.The bending test result showed that the solder had good ductivity as well as strength. When the percentage of CNTs was 0.03wt%, the flexural strength had increased by 10.5% compared with Sn-58Bi solder. The elongation of the solder had increased obviously without sacrifice the strengh. The elongation of Sn-58Bi-0.03wt%CNTs was 48.90% higher compared to that of the Sn-58Bi solder. The strengthing mechanisms can be attributied as the refinement strengthening, dislocations strengthening, and second-phase strengthening. A fracture surface comparison with Sn-58Bi-0.03wt%CNTs composite solder and monolithic Sn-58Bi alloy was made to identify the reinforced effect of CNTs on Sn-58Bi-CNTs solder.The Sn-58Bi-0.03wt%CNTs had better wetting property than Sn-58Bi solder, the influence of carbon nanotubes on the melting point did not change obviously. The tests of solder joint strength was conducted on Sn-58Bi and Sn-58Bi-0.03wt%CNTs solder. The pads were coated with solder alloy, which were reflowed after placed with the components. Pull tests results showed that carbon nanotubes can improve the pulling-off force of solder joints. The strengthing of solder joint could be attributed as the refinement strength and influenced on the fracture mode caused by carbon nanotubes.