Using the 'solidification path' technique, the liquidus projection of Sn rich corner of the Cu-Ni-Sn phase diagram was determined. Two quasi-peritectic reactions were found, which are related to the Cu-Sn and Ni-Sn binary eutectic reactions in this part of the system. The existence of a ternary Cu-Ni-Sn phase was confirmed. This phase was found to participate in both quasiperitectic reactions.

The effect of cooling rate on the formation of Sn dendrites during freezing was investigated for Pb free solder alloys having compositions near the Sn rich ternary eutectic of the Ag-Cu-Sn system. For Ag and Cu concentrations less than eutectic, the volume fraction of Sn dendrites tended to increase with cooling rate, while for Ag and Cu concentrations greater than eutectic a decrease was observed. For an alloy having the composition 3 . 8 wt-%Ag, 0 . 7 wt-%Cu and 95 . 5 wt-%Sn, the volume fraction of Sn dendrites was found to vary from y5 vol.-% at a cooling rate of 1 K s -1 to 65 vol.-% at 100 K s -1 , resulting in a corresponding enrichment of Ag and Cu in the eutectic regions, in the form of increased volume fractions of Ag 3 Sn and Cu 6 Sn 5 . The surface contour of frozen samples was observed to depend on the scale of interdendritic shrinkage during freezing, and was especially smooth for eutectic freezing.

The reaction of liquid tin with solid copper has been studied by heating small volumes of pure tin on copper coupons at various temperatures and times, and evaluating the resulting reaction metallographically. Three reaction temperatures were used: 260, 400, and 450°C. Specimen geometry was chosen to simulate a typical solder joint. The reaction was observed to occur in two stages: an initial fast stage with copper/liquid tin interface movement rates from 0 . 2 mm s 2 1 at 260°C to 0 . 8 mm s 2 1 at 450°C, followed by a much slower stage. It was concluded that the rst stage corresponds to direct dissolution of copper in liquid tin up to or beyond the liquidus concentration for the reaction temperature used. This is followed by the formation of an intermetallic compound layer at the copper/liquid interface. Subsequent copper dissolution then occurs by solid state diffusion through the compound layer, a much slower process than direct dissolution.MST/5510

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.

The microstructural stability of the 3 . 8Sn -0 . 7Ag -Cu solder alloy was investigated by studying microstructural changes caused by heating small samples for various times, up to 1000 h, at 150uC. The first change, evident at high magnification after heating for 1 h, occurred from the as-cast lamellar plus fibrous form of the Ag 3 Sn and Cu 6 Sn 5 interdendritic eutectic phases to a particulate form. With further heating, coarsening of the two compound phases occurred, gradually rendering the Sn dendrite pattern less distinct. Because of the very rapid diffusion of Cu in solid Sn, the Cu 6 Sn 5 phase coarsened most rapidly, growing from its originally finely divided (200 nm) size in the ternary eutectic to form many particles up to 3 mm or more in size in a time of 100 h. At that time, nearly 50% of the total Cu was contained in these particles. The Ag 3 Sn phase coarsened more slowly. Approximate measurements of average particle size as a function of time suggested that coarsening occurs by Ostwald ripening, controlled by diffusion in the Sn phase.MST/6075