Effect Of Pr On The Microstructure And Properties Of Sn-9Zn Lead-free Solder

Due to the toxicity of Pb and the demand for high performance solders to meet the development of miniaturization and high-density of electronic product, it becomes urgent to develop some lead-free solders with excellent performance and low cost. Among all lead-free solders, Sn-Zn alloy has been recognized as the one of the most promising alloy since many advantages of the solder, such as low melting point close to the Sn-Pb alloy, excellent mechanical properties of solder joints, low cost and so on. But its deficiency in wettability, oxidation resistance and corrosion resistance limited its wide application in electronic product. So in this paper, we proposed adding some rare earth element Pr to the Sn-9Zn solder in the hope of optimizing the properties of the solder and studied the influence rules of Pr content on the properties and microstructures of the solder.Experimental results show that: Trace amount of Pr can greatly improve the wettability and the oxidation resistance of the solder because Pr accumulate on the surface of the liquid solder, this would not only lower the surface tension of the solder but also reduce the opportunity that Zn react with Oxygen, which are benefit to the wettability and the oxidation resistance of the solder. But Pr is easy to be oxidized in air,excessive Pr will cause mass oxidizing slag on the solder surface and worsen the wettability and accelate the oxidation of the solder. The optimal Pr content for wettability and oxidation resistance in this test is 0.08wt.%.With the increasing of the Pr content in the solder, the corrosion rate decreased and the corrosion potential increased, the corrosion mechanism for Sn-9Zn solder is a mixing of pitting and entire corrosion, and it became more uniform when little Pr was added, the corrosion pits became shallower and the corrosion product became more difficult to peel off, result in the improvement in corrosion resistance.Compared with Sn-9Zn, the shear force and pull strength all increased with Pr addition, the shear force gives an increase of 28.7wt.% when 0.04wt.% Pr was added and the pull strength gives an increase of 38wt.% when 0.08Pr was added. Large of Cu-Zn phase was found in the fracture of the QFP solder joints when exceesive Pr was added, which will degrade the mechanical properties of the solder joint.With the addition of Pr, the volume fraction and size of the needle-like phase tends to be decreased, the growth rate of the IMC at the interface was slower and the formation of Cu-Sn compounds between the IMC and Cu substrate was depressed, the microstructure of the interface was stabled and this is good for the reliability of the solder joints. But when 0.25wt.%Pr is added, bulk of Sn-Pr compounds formed in the solder near the interface, these Sn-Pr compounds were oxidized gradually after soldering and result in a large number of Sn whiskers growth in the solder. Over all, we suggest that the optimal Pr content in the Sn-9Zn solder is 0.08wt.%, the solder shows the best comprehensive properties at this time.