Tin-silver-copper eutectic temperature and composition
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I. INTRODUCTION
TIN-SILVER-COPPER eutectic solder is of interest for lead-free solder applications. Its advantages[1,2] over the binary tin-silver eutectic solder are a lower eutectic temperature (217 8C compared to 221 8C), slower growth of the intermetallic layer at the interface, increased strength, and a lower wetting angle. A wetting angle of 21 deg on Cu using a resin mildly activated (RMA) flux is reported.[2] The Sn-Ag-Cu ternary eutectic composition reported by Miller et al.[1] and Anderson et al.[2] is 4.7 wt pct Ag, 1.7 wt pct Cu, and the balance Sn. When solder of this composition was melted at a temperature close to 217 8C, primary crystallites were observed in the melt. This led us to a reevaluation of the eutectic condition in the Sn-Ag-Cu eutectic system. The composition found was 3.5 wt pct Ag, 0.9 wt pct Cu, and the balance Sn. In a binary system, over a range of compositions in which an a/b/liquid eutectic reaction is operational, where a is a nonfaceted phase and b is faceted, it is often the case that the nonfaceted phase grows much more easily than the faceted phase. As a result, “high” undercoolings can lead to the formation of a dendrites even in alloys with compositions at or to the b side of the eutectic. In order to produce a dendrite-free eutectic-like structure at such undercoolings, a starting composition even farther to the b side of the eutectic must be used.[3] Sn-Ag[4] and Sn-Cu[5] are two systems that behave like the generic a/b system described above: in both systems the microstructures of rapidly cooled eutectic alloys contain large volumes of Sn dendrites. In these systems, Sn is a nonfaceted phase and Ag3Sn and Cu6Sn5 are faceted. If, as one would expect, the Sn-Ag-Cu system behaves similarly, eutectic determinations based in part or in full on microstructural examination of as-solidified Sn-Ag-Cu alloys can lead easily to incorrect conclusions. For this reason, the present study relies on thermal analysis and microstructural examination of annealed, rather than as-solidified, alloys in its determination of the Sn-Ag-Cu eutectic composition.
II. EXPERIMENTAL PROCEDURE A. Alloy Preparation Samples of the Sn-Ag-Cu alloys listed in Table I were prepared in 17 g lots from 99.998 pct Sn rod, 99.999 pct Cu shot, and 99.99 pct Ag shot. The components were weighed out with an accuracy of 0.0003 g. The weighed lots were sealed in 10 mm i.d., 12 mm o.d. quartz tubes under a ,1 Pa atmosphere of high purity Ar. Alloys were synthesized in a furnace at 525 8C to 600 8C for 2 to 3.5 hours. The alloy samples were removed from the furnace six to eight times during this period, shaken vigorously, and returned to the furnace. Following synthesis, the samples were quenched in a 5 gallon container of cold tap water. In an attempt to limit segregation along the length of the solidified ingots, the quartz tubes were held in a horizontal position during quenching. The alloys remained stationary during solidification. After solidification, alloy ingots were removed from the quartz tubing, wiped clea
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