Study of Sn-Bi-Cu Lead-free Solder

Electronic packaging is a technology of manufacturing electronic products that are composed of IC chips and electric devices. In the practice of package system, solder plays a crucial role in the assembly and interconnection of silicon die. It provides the electrical and mechanical connection between the silicon die and the bonding pad. It also serves as a path for dissipation of the heat generated when devices working. During soldering process, solders will be melted and form a joint between the substrate and the IC chips. The solder joint formed provides important mechanical and electrical connections. The Sn-37Pb alloy, a eutectic composition, or Sn-40Pb alloy, a near eutectic composition, is primarily used as a solder material. However, in recent years, development of alternative lead-free solders has drawn tremendous attention due to the concern of human health and environment. A great deal of effort have been made on solder alloys based on Sn being the primary or major constituent, due to the inexpensive, lower melting point and excellent wetting properties with copper and brass. The new alternative solder must also be tin-containing binary, ternary and even quaternary alloys. Studies on lead free solder joint were widely investigated, such as Sn-Ag, Sn-Bi, Sn-Cu, Sn-Zn, Sn- Ag-Cu, etc. The excellent mechanical property makes Sn-Ag- Cu a pretty good alternative material and investigated widely in recently years. However, that the melting temperature of eutectic 217degC is higher than the commonly used solder systems and the high cost due to the Ag element which considered as strategic resource have to be make it turn to selecting other components. The Sn -Bi-Cu system has lower melting temperature due to addition of Bi and lower cost because of non-silver. Besides, Bi has the unusual thermal behavior of expanding when cold and shrinking when hot, making the Sn-Cu-Bi solders have better thermal resistance and therefore good reliability. However, at room temperature, only a little Bi could be solid soluble in Sn. The excess Bi would precipitate in the Sn phase, which results in poor mechanical property. So, research on the Sn -Bi-Cu solders seems very important. But seldom reports can be seen, especially the system solders with high content Bi. However, it is not no way to resolve such a difficulty, this paper reports a new lead-free alloy, developed by nitrogen protected condition melt, Sn-17Bi-0.5Cu, which has great potential as a lead-free solder system. And its powders are prepared by rapid solidification atomization, which can restrain the solute segregation. Initial trials on the manufacture of solder joints with this alloy solder paste revealed that a high quality bond with copper could be formed. The shear strength of the solder joint is higher than that of Sn-37Pb, Sn-58Bi or Sn-0.7Cu, and equivalent to Sn-3.0Ag-0.5Cu. Its melting range of 190degC to 200degC is slightly higher than that of eutectic tin-lead solder, which makes it possible to surface mounted under the same condition as Sn-37Pb solder. And the spreading area shows that its wettability is better than the Sn- Bi, Sn-Cu binary alloy and Sn-Ag-Cu ternary alloy solder paste, and average value is slightly higher than that primarily used Sn-37Pb solder paste. Examination of the micro structure of the powders revealed that it mainly consists of small rich bismuth (1 to 2 um) and the primary phase beta-Sn (about 1 to 2 um also) particles finely dispersed in a nearly pure tin matrix with a small amount of Cu
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Sn
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particles.