Electrodeposition Of Sn-Zn Lead-free Solders

Traditional Sn-Pb solders are widely used in the field of electronic packaging due to their excellent properties. However, lead is a toxic element, which is harmful to human health. Laws and legislations in many countries have been established to restrict the use of lead-containing materials. These days, the research of lead-free solders has become one of the attractive questions in materials science. More and more attention is paid to the Sn-Zn lead-free solders because of its similar melting point and good mechanical properties compared with Sn-Pb solders.In this paper, the Sn-Zn lead-free alloy solders were prepared by electrodeposition with sodium citrate and tartaric acid as complexing agent. The effects of electrolyte components and electrodeposition process on the deposit composition, phase structure and surface morphology were investigated. And the electrodeposition mechanism of Sn-Zn alloys was discussed by analysing cathode polarization curves. In addition, the wettability of Sn-Zn solders was also studied.The results show that both the electrolyte composition and electrodeposition process have a certain influence on the coating composition. The near eutectic solder Sn-9.28Zn was electrodeposited by adjusting the electrolyte components and process. The weight percentage of zinc in electrodeposition coatings increases with increasing the concentration of metal ions, conductive additives, current density and deposition time. The concentration of sodium citrate as a single complexing agent has little effect on the coating composition. While adding tartaric acid as an auxiliary complexing agent, the depostion of tin ion decreased and consequently the zinc content in electrodeposit increasd. When the pH value varies from 4 to 5, the electrolytic solution and coating composition tend to be stable. XRD phase analysis shows that the electrodeposited Sn-Zn solder mainly consists of the tetragonal tin phase and the hexagonal zinc phase. Moreover, the current density and deposition time have a great effect on the coating morphology. In the condition of low current density and short deposition time, the surface of electrodeposit is bright, flat and uniform, and also the fine microstruture is observed. As the current density and deposition time increase, the coating thickness increases and the grain becomes coarser. At the same time, the dendrites and spongy microstrutures appear.The cathodic polarization curves show that the addition of sodium citrate and tartaric acid causes the singnificant shift towards the negative direction in onset potential of Sn²⁺ and Zn²⁺and reduce the potential gap, which is beneficial to the co-deposition of Zn²⁺ and Sn²⁺.The wetting experiments of Sn-Zn solders show that oxidation of zinc has an important impact on the property of Sn-Zn lead-free solders. The wetting property can be improved using RA type rosin flux to remove the oxide film, but the surface need to clean since the inclusions and bubbles remained in the surface.