A Study Of The Creep Property Of Sn-9Zn Solder And Its Strengthen

The research of developing lead-free solders and investigating their reliability islaunched worldwide because of environment related regulations and the highintegration of microelectronics. Eutectic and near-eutectic Sn-Ag-Cu alloys have beenregarded as the most promising candidate of lead-free solders. However, eutecticSn-9Zn has also gained much attention because of its low cost, favorable meltingpoint and good mechanical properties.To meet the increasingly severe requirement for reliability of electronicsproducts in order to ensure their steady operating in service, new lead-free soldersshould have not only appropriate process window, but also better mechanicalproperties, especially fatigue and creep resistance. The study of creep properties oflead-free solder alloys is thus particularly important.This paper takes Sn-9Zn as the object of study. A bend-creep set-up wasestablished and its feasibility verified. Bend creep tests were performed to acquireparameters such as the creep stress exponent, activation energy and constants A, onwhich basis creep constitutive equations for Sn-9Zn which reflects the relationsbetween the steady creep rate and temperature and stress were established. Forcomparison, the creep resistance of Sn-9Zn, Sn-40Pb and Sn-3.5Ag-0.7Cu underdifferent cooling methods in solidification were studied. Shear strength and creepbehavior of Sn-9Zn/Cu solder joints prepared under new fluxes were investigated. Weproposed the idea and preparation method of in situ particles to reinforce the solderand carried out studies on it, including reaction kinetics between Cu particles andSn-9Zn melt, preparation of in situ reinforcing intermetallic compound particles andtheir influence on the microstructure, wettability, and tensile and creep properties ofSn-9Zn. The following results were obtained:1. The bend-creep tester is reliable and the data obtained reflect the creepbehavior. It can be employed to compare creep properties of different materials. The stress exponent and activation energy can be obtained from experiments using thistester, on which basis creep constitutive of materials can be established.2. The creep deformation behavior of Sn-9Zn is divided into two stress areaswith different creep characteristics: the low-stress area corresponds to low stressexponent, suggesting the grain boundary creep mechanism, while high-stresscorresponds to much higher stress exponent, which suggests a dislocation climbingmechanism.3. In Sn-40Pb, Sn-9Zn and Sn-3.5Ag-0.7Cu, the creep resistance ofSn-3.5Ag-0.7Cu is the highest, followed by that of Sn-9Zn. High cooling ratedrefines the microstructure of Sn-9Zn and Sn-3.5Ag-0.7Cu, resulting in higher creepresistance of Sn-3.5Ag-0.7Cu and lower creep resistance of Sn-9Zn.4. Sn-9Zn/Cu joints have maximum shear strength and shear creep strengthwhen～5%wt of SnCl₂ is contained in the solder flux, and the joints exhibittoughness fracture. It was showed that higher rosin concentration in the fluxincreases the creep strength of Sn-9Zn/Cu joints and does not affect the instantaneousstrength much.5. Formation kinetics of Cu₅Zn₈ on Sn-9Zn/Cu interface is characteristic ofparabolic growth, and is affected by the curvature of Cu particles, with largecurvature inhibiting its growth. At initial stage Cu₅Zn₈ grow more rapid which favorstheir being used as in situ particle reinforcing phases.6. In situ particle reinforced composite Sn-Zn based solders were obtained byadding Cu powders into Sn-9Zn melts. Cu₅Zn₈ particles are formed in the solders.Their strength and plasticity were improved and creep resistance were considerablyenhanced. Wettability of thesecomposite solders is better than that of Sn-9Zn. Thecomposite solders have optimum strength and plasticity when 0.5%wt of Cu powdersis added.