Development Of New Flux Used For Sn-9Zn Lead Free Solder

Because of lead and lead contained compounds' toxicity to human body and environment, the development and practical application of suitable Pb-free solders has become a quite important issue for electronic packaging industry. Sn-9Zn solder is a promising candidate, which melting point (199℃) is very close to the melting point of commonly used Sn-Pb eutectic (183℃). When Sn-Zn eutectic solder is used, the soldering temperature is relatively low, the thermal damage does not take place easily.Secondly, the Sn-9Zn lead-free solder is low cost and the resource of Sn and Zn are huge on the earth. Sn-Zn solder also has excellent mechanical properties.But the disadvantage of Sn-Zn solder is poor wetting property. It impede the development of Sn-Zn solder. So designing a new kind of flux for Sn-Zn solder to improve its wetting property will be very meaningful.Enlightened by the research of alloying, dry-type flux and liquid flux are studied in thispaper. BiCl₃ and CuBr₂ were added to the flux as active separately.Rosin was necessary asfilm former. Ethyl alcohol is also used as solvent. Moerover, the mechanism of the propertyof the flux is also discussed in this article. The conclusions we can draw are:1,Compared with commercial RMA flux, the rosin based flux containing BiCl₃ and CuBr₂can improve the wetting property of Sn-9Zn solder remarkably. When the rosin content isfixed, the higher metal chloride content the better spreading ratio can be gotten. But whenthe metal chloride content is too high, moldability of the solder ball is bad. Meanwhile,the better spreading ratio can be gotten too with higher rosin content, when the metalchloride content is fixed.2,When the rosin based flux containing BiCl₃ is used in soldering process, BiCl₃ can react with Zn in the molten solder and the product were ZnCl₂ and Bi. Bi can enrich on the surface of molten solder.The content of Zn on the surface of molten solder is decreased. Then the surface tension is decreased and oxidation is avoided by enrichment of Bi from substitute reaction. So the wetting property of Sn-Zn solder can be improved remarkably. 3, When the rosin based flux containing CuBr₂ is used in soldering process, CuBr₂ can react with Zn in the molten solder and the product were ZnBr₂ and Cu. Be different to the BiCl₃, Cu can react with Zn sequentially, and the product is Cu-Zn compound. Then the surface tension is decreased and oxidization is avoided. So the wetting property of Sn-Zn solder can be improved remarkably.