Some aspects of nucleation and growth in Pb free Sn–Ag–Cu solder

Abstract

To investigate the minimum superheat necessary to solder components on printed circuit boards successfully using 95 . 5 wt-%Sn, 3 . 8 wt-%Ag, 0 . 7 wt-%Cu solder, experiments were carried out using separate solder balls of the type used in ball grid arrays. Significant differences in microstructure were observed depending on the peak temperature reached in the liquid on melting. On cooling, substantial undercooling was often observed, with values up to 18 K. Under some freezing conditions, the primary phase formed was Ag 3 Sn, while under other conditions the primary phase was Sn. The amount and type of eutectic microstructure formed was observed to vary with freezing conditions. The types of microstructure formed are illustrated. Nucleation phenomena and their effect on subsequent growth are discussed.

Microstructure Characterization and Creep Behavior of Pb-Free Sn-Rich Solder Alloys: Part I. Microstructure Characterization of Bulk Solder and Solder/Copper Joints

“…After primary dendrite formation, solidification of the eutectic follows from the remaining liquid phase. [45][46][47] Both water-quenched bulk specimens and the small solder joints underwent significant undercooling during solidification. This undercooling resulted in a similar dendrite to eutectic region ratio of~50/50 for both specimen types.…”

“…[1,5,6,[41][42][43][44][45][46][47][48] Microstructures for all three alloys and pure Sn in bulk form, cooled at 24°C/s and 0.1°C/s, are shown in Figure 2. In pure Sn, both water quenching (24°C/s) and furnace cooling (0.1°C/s) produced equiaxed Sn grains (Figure 2(a) and (e)), where the average grain sizes were 14 and 26 lm, respectively.…”

Formation Mechanism of Eutectic Cu<SUB>6</SUB>Sn<SUB>5</SUB> and Ag<SUB>3</SUB>Sn after Growth of Primary &beta;-Sn in Sn-Ag-Cu Alloy

“…4.3 Solidification process of Sn-Cu 6 Sn 5 and Sn-Ag 3 Sn binary eutectic structure Snugovsky et al 10) carried out experimental work on the solidification structure for Ball Grid Array condition using Sn-3.8Ag-0.7Cu alloy. They found that there were two binary eutectic structures in the specimen, i.e., Sn-Ag 3 Sn and SnCu 6 Sn 5 , which they named ''double binary''.…”

“…However, an unusual microstructure consisting of both Sn-Ag 3 Sn and Sn-Cu 6 Sn 5 binary eutectic structures is sometimes observed. 10) This unusual microstructure is known as a ''double binary eutectic'' structure. According to the phase diagram, these structures cannot be expected.…”

Thermophysical Properties of Sn–Ag–Cu Based Pb-Free Solders

“…Among them, the most promising one is undoubtedly the Sn-Ag-Cu ternary alloy, which has advantages of good wetting property, superior interfacial properties, high creep resistance, low coarsening rate, and so on [2]. Recently, much research about the effect of the Ag content on several properties except thermal properties for a fixed Cu content in the Sn-Ag-Cu alloy have been performed [3][4][5][6].…”