Development And Application Of Sn-Zn Based Hypoeutectic Solders For Soldering In Electronic Packaging

Sn-Zn based solders have drawn much attention and shown the potential to be one of thepromising lead-free solders due to their relatively low melting temperature, which is close tothat of the Sn-Pb eutectic solder, low cost and good mechanical properties. However, poorperformance in wettability and oxidation resistance has limited its applications. Aiming atovercoming the disadvantages of Sn-Zn based solder, Bi and La-Ce mix rare earth （MRE）were adopted to Sn-xZn （x=4⁶） hypoeutectic solders, and the melting characteristics,microstructural features, wettability, intermetallic compound （IMC） formation and themechanical properties were characterized with a comparison to Sn-9Zn eutectic solder. Basedon above, the composition optimization design of the solder was achieved; the influence ofadding La-Ce MRE to the solder on strength and fracture mode of solder joints was studied.The results on Sn-xZn-yBi （x=4⁶; y=1⁵） solder alloys show that:（1） Sn-Zn-Bi solderwith5wt%Zn and5wt%Bi exhibits the best wettability and low surface tension. Both themelting temperature and surface tension of Sn-5Zn-yBi solders decrease while the spreadingrate increases with increasing Bi content;（2） Microstructures of Sn-5Zn-yBi solders consist ofZn-rich phase and β-Sn phase, as well as Sn-Zn eutectic, while the Zn-rich phase shows finergrains with increasing Bi;（3） The planar intermetallic compound, Cu₅Zn₈, forms at theinterface between Sn-5Zn-yBi solder and Cu substrate, and the Cu₅Zn₈layer becomes thickerwith increasing the addition amount of Bi.The study on Sn-xZn-5Bi （x=4⁶） solder alloys indicates that:（1） The surface tensionand melting range of the solder increase with Bi content, while the onset melting temperaturesseem unchanged;（2） The wettability of Sn-xZn-Bi solders increases with increasing Zncontent and reaches the best at Zn content of5wt%.The results on Sn-5Zn-5Bi-zMRE （z=0,0.05,0.1,0.5） solder alloys exhibit that:（1）MRE has very slight influence on the melting temperature of Sn-5Zn-5Bi-zMRE solder alloys,while MRE can decrease the surface tension; The wettability of the Sn-5Zn-5Bi-zMREsolders increases obviously with increasing the addition amount of MRE up to0.05wt%and thereafter decreases with increasing MRE content, Sn-5Zn-5Bi-0.05MRE solder exhibitsbetter wettability than Sn-9Zn solder;（2） The intermetallic compound （Cu₅Zn₈） forms at theinterface between Sn-5Zn-5Bi-zMRE solder and Cu substrate, and the Cu₅Zn₈layer becomesthicker with addition of MRE;（3） After adding a small amount of MRE into Sn-5Zn-5Bibased solders, a finer and more uniform microstructure can be obtained, and the amount ofneedle-like Zn phases decreases;（4） The Sn-5Zn-5Bi-MRE/Cu joints exhibit higher tensilestrength then that of Sn-5Zn-5Bi/Cu and Sn-9Zn/Cu;（5） Ductile fracture takes place in theSn-5Zn-5Bi-MRE solder and brittle fracture occurs on the surface of Sn-5Zn-5Bi-MRE solderand IMC in Sn-5Zn-5Bi-MRE/Cu joints, while brittle fracture takes place in the IMCcompounds.