Effect Of Co Content On The Microstructure And Brazing Property Of Sn-0.7Cu Lead-Free Solder

The green development of electronic products has promoted the lead-free process of solder in electronic assembly.Laws and regulations have been issued to restrict the use of lead-containing solders one after another.At present,Sn-0.7Cu solder has become one of the most promising lead-free solder with low cost among the lead-free solders developed all over the world.However,the tensile strength of Sn-0.7Cu solder is low,and the solderability of Sn-0.7Cu on Cu solder is worse than that of Sn-Pb solder.Therefore,it is important to improve the microstructure and brazing properties of Sn-0.7Cu solder to promote the development of lead-free packaging of electronic products.This study systemtically investigated the influence of Co alloying on the microstructure and mechanical properties of Sn-0.7Cu solder alloy.The effects of Co content on the microstructure formation of Sn-0.7Cu-xCo(x=0.5,1.0,1.5,2.0)(all mass percentages,wt.%)alloy at different cooling rates were analyzed.The effects of Co content on the mechanical properties and brazing properties of the alloy were also investigated.The main conclusions are as follows:(1)Intermetallic compound CoSn2 appeared after adding Co,and the volume fraction of CoSn2 increased with the increase of Co content.In addition,after adding a certain amount of Co,the granular Cu6Sn5 grains in the microstructure become striped.When the content of Co is 2.0 %,a small amount of CoSn formed by peritectic reaction in the microstructure.At low cooling rate(1 μm/s),the primary phase and the secondary phase alternately grow in the mushy zone,which formed banded structure.As the cooling rate increases(5-100 μm/s),the banded structure disappeared.The length of mushy zone are the same with each other at different cooling rates for the specimen with the same composition.(2)When the content of Co is 2.0 %,the tensile strength of the alloy is 69.18 % higher than that of Sn-0.7Cu,but the elongation of the alloy was decreased.The Vickers hardness of the alloy was increased with the addition of Co,and the Vickers hardness increased with the increase of Co content.(3)The melting point of Sn-0.7Cu-xCo alloy increased with the increase of Co content,and the maximum increase is about 5 ℃.The spreading area decreased first and then increased with the increase of Co content.When the content of Co is 1.5 %,the spreading area was the smallest,and the spreading property of Sn-0.7Cu was decreased by adding Co.The conductivity of Sn-0.7Cu-xCo decreased with the increase of Co content in the near equilibrium state.In addition,at the same cooling rate,the conductivity of Sn-0.7Cu-xCo alloy decreased with the increase of Co content.Due to the coupling of growth rate and component segregation,the electrical conductivity of the same solder alloy decreases with the increase of cooling rate.(4)Co improves the corrosion resistance of Sn-0.7Cu alloy,and with the increase of Co content,the corrosion resistance of the alloy increases gradually.