The quasi-peritectic reaction, frequently observed in ternary alloy systems, takes the form L + alpha <-> beta + gamma, where L indicates the liquid phase, and alpha, beta and gamma indicate solid phases. The formation of microstructure resulting from the kinetics of this reaction is considered from a theoretical point of view and compared with experimental observations of microstructures formed by quasi-peritectic reactions in two ternary alloy systems, Bi-Pb-Sn and Cu-Ni-Sn. Based on these considerations, an explanation is proposed for experimental observations previously reported in literature concerning phase transformations from the liquid in multicomponent ferrous alloys.

The liquidus projection of the Bi-Cd-In ternary alloy system has been redetermined, using the conventional methods of thermal analysis and differential scanning calorimetry, as well as quenching experiments and a 'solidification path' technique. In the latter method, specimens were solidified unidirectionally at a very low speed in order to maintain a coherent solid/liquid interface while approximating freezing with complete mixing in the liquid. This produced macroscopic segregation that clearly revealed the phases formed on a scale suitable for straightforward chemical analysis in a scanning electron microscope. Two quasibinary sections were found, one of which (BiIn2 - Cd) divides the system into two independent regions. A newly discovered ternary phase, designated Z, as well as several previously unreported ternary four phase reactions of the quasiperitectic type, were observed. The Bi - Z join was found to be a quasibinary peritectic section. A number of solid state reactions of the eutectoid type were also noted. MST/4527.

In the expectation of a possible connection between Sn-Cu6Sn5 and Sn-Zn binary eutectics, several alloy compositions were selected along a possible joining path. Samples of each were solidified unidirectionally at a very slow rate, approximating freezing with complete mixing in the liquid (the 'solidification path' technique). This procedure allows the detection of reactions by the change in the phases frozen out with the change in composition of the liquid due to segregation. Three quasi-peritectic reactions (U1-U3) and a ternary eutectic reaction were found: U1, L+Cu6Sn5(sic)CuZn+Sn; U2, L+CuZn(sic)Cu5Zn8+Sn; U3, L+Cu5Zn8(sic)CuZn4+Sn; and E, L(sic)(Sn)+(Zn)+CuZn4.

The solid-liquid interfacial free energies of each of the individual phases comprising the eutectic system, Carbon Tetrabromide-Hexachloroethane, were measured as a function of composition using a “grain boundary groove” technique. Thermodynamic data were combined with groove shape measurements made from high resolution optical photomicrographs of the solid-liquid interfaces to give the interfacial free energy data. An interfacial free energy balance at the eutectic trijunction was performed to obtain all the forces acting on that point. The three interphase interfacial free energies at the eutectic trijunctions as well as a solid-solid phase boundary torque were evaluated. It was found that the solid-liquid interfacial free energies of the two phases of the eutectic could be evaluated from photomicrographs of growing or stationary eutectic interfaces. In addition, it was found that for a substantial range of freezing conditions the eutectic interface shape can be predicted from a knowledge of the interfacial free energies alone.

With the development and use of a variety of Pb free solders, it is probable that some solder joints in electronic assemblies may be made with solders of two different compositions. To investigate possible microstructures resulting from such procedure, samples were prepared using small balls of four different Sn–Ag–Cu (SAC) Pb free solders, as well as Sn–Zn–Al solder, melted together with eutectic Pb–Sn solder paste and also various SAC solder pastes, on a copper substrate. It was observed that using eutectic Pb–Sn solder paste with an SAC solder ball introduced some Pb–Sn eutectic microstructure and changed the ternary eutectic present from Ag 3 Sn–Cu 6 Sn 5 –Sn to Ag 3 Sn–Pb–Sn. Use of an SAC solder paste with Sn–Zn–Al solder introduced an apparent Ag–Cu–Zn ternary compound, replacing Zn lamellae of the Sn–Zn eutectic. With eutectic Pb–Sn solder paste, the Pb–Sn–Zn ternary eutectic was formed. It was noted that use of a high Sn solder results in rapid dissolution of the copper substrate.

The term “spalling” refers to the formation in a solder joint of a second layer of intermetallic compound, in addition to and separate from the usual single layer that normally forms on the substrate. It has been observed in Pb-free solders of the SAC type when used in conjunction with a nickel finish. Explanations offered to account for this phenomenon have been based on thermodynamic phase equilibria observed in isothermal sections of the Sn corner of the Cu–Ni–Sn system at temperatures 20°C or more above the solidification temperature of the solder, and without regard to reactions that occur at low temperatures, during freezing. Recent measurement of the liquidus projection of the Sn corner of the Cu–Ni–Sn system as shown the occurrence of a ternary quasiperitectic reaction in the composition range in which “spalling” is observed. Evaluation of the effect of this quasiperitectic reaction shows that its operation during freezing of the solder can account for the “spalling” phenomenon.

A new solder alloy, intended to replace eutectic Sn–Pb, consists of the near eutectic alloy 93 wt-%Sn plus 7 wt-%Zn, with approximately 60 ppm Al added to prevent excessive oxidation of the Zn component. On solidification, this alloy forms a broken lamellar microstructure of Zn in Sn. This paper reports a study of the interaction of this alloy with two common substrate materials, copper and electroless nickel immersion gold (ENIG). Initial contact of the liquid solder on copper for 60 s at 215-230°C formed on the copper a 2 μm thick layer of a Cu–Zn intermetallic compound. After aging for 24 h at 150°C, this compound layer thickened to approximately 3·5 μm. In addition, two layers, each containing particles of a different Cu–Zn compound replacing the Zn, formed between the interface layer and the original solder. After 150 h at 150°C, the interface layer reached 4·5 μm in thickness and the two compound containing layers a total of 65 μm. These rapid microstructural changes correspond to both Cu and Zn being rapid diffusers in solid Sn. On an ENIG substrate, contact with the liquid solder for 60 s at 230°C formed a thin (0·2 μm) and somewhat irregular reaction layer, with a composition consisting of Au, Sn and Zn. Aging for a few hours produced no appreciable change. After 265 hours at 150°C, a continuous layer 0·2 μm thick, with composition now including Ni, was formed on the ENIG interface. After 500 h at 150°C, this thin layer remained and a 10 μm thick layer denuded in Zn had developed in the adjacent solder with accompanying formation of particles having the composition 90 wt-%Zn with Ni and Au on the thin interface layer. Aging to 1150 h produced a denuded zone 17 μm thick and continued growth of the Zn-rich particles. These observed reaction rates appear to correspond to diffusion rates of the order of self-diffusion of Sn at 150°C.

A metallographic study is reported of the phases and reactions that occur in Ag–In–Sn (Pb-free) solder alloys containing approximately 3 wt-%Ag and up to 10 wt-%In. Specimens were prepared by very slow unidirectional solidification and as small castings. Three different intermetallic compound phases and two different matrix phases were observed, depending on the In content of the alloy. The probable reactions that produce these phases are discussed and compared with data from the published ternary liquidus projection. This study was carried out in the Department of Materials Science and Engineering, University of Toronto, as part of the CMAP program.

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 quasi-peritectic 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 similar to 5 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 Ag3Sn and Cu6Sn5. 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 microstructural stability of the Sn-3.8%Ag 0.7%Cu solder alloy was investigated by studying microstructural changes caused by heating small samples for various times, up to 1000 h. at 150 degrees C. The first change, evident at high magnification after heating for I h, occurred from the as cast lamellar plus fibrous form of the Ag3Sn and Cu6Sn5 interdendritic eutectic phases to a particulate form. With further heating, coarsening of the two compound phases occurred, gradually rendering the Sri dendrite pattern less distinct. Due to the very rapid diffusion of Cu in solid Sri, the Cu6Sn5 phase coarsened most rapidly, growing from its originally finely divided (200 nm) size in the ternary eutectic to form many particles up to 3 pin 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 Ag3Sn 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 Sri phase.

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 Ag3Sn, 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 techniques for quenching samples during freezing, slow unidirectional solidification and differential scanning calorimetry were used to evaluate the freezing characteristics of eutectic and near eutectic ternary Sn rich Ag – Cu – Sn alloys. The ternary eutectic temperature was verified as 217·2±0·4°C and composition as 3·5 wt-%Ag, 0·9 wt-%Cu, 96·5 wt-%Sn. The ternary eutectic was observed to be of the faceted (Ag3Sn) faceted (Cu6Sn5) nonfaceted (Sn) type. The Ag3Sn phase forms a broken lamellar microstructure while the Cu6Sn5 phase forms a fibrous microstructure in which the fibres become ‘hollow’ hexagonal prisms (filled by Sn) at very low growth rates. The volume fraction occupied by the two compound phases was measured as 5·5±0·5 vol.-% and calculated as 5·63 vol.-%. It was observed that the compound phases are poor nucleation catalysts for Sn. The presence of faceting phases results in the formation of a skewed coupled zone, causing formation of primary Sn dendrites on freezing of a melt even of eutectic composition. When fractured, both compound phases showed brittle behaviour.

Early studies of Ag-Sn and Cu-Sn binary alloys showed very low values, 0.04 wt.% for Ag and 0.0063 wt.% for Cu, for the solid solubility of these elements in Sn at the eutectic temperature. In recent work on “as-cast” Sn-Ag-Cu solder alloys, much higher values have been reported for the Ag and Cu content of the Sn phase. In the present study, wavelength dispersive x-ray microprobe measurements made on a near-equilibrium sample confirmed the earlier solubility values. It was concluded that higher values, some of which are reported in the current paper, represent nonequilibrium, supersaturated solid solutions.

The reaction of liquid tin with solid copper has been studied by heating small volumes ofpure tin on copper coupons at various temperatures and times, and evaluating the resulting reaction metallographically. Three reaction temperatures were used:260,400,and450 ° C. Specimen geometry was chosento 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 μm s-1 at260 ° C to 0.8 μm s-1 at 450 ° C, followed by a much slower stage. It was concluded that the first 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 atthe copper/liquid interface. Subsequent copper dissolution then occurs by solid state diffusion through the compound layer, a much slower process than direct dissolution.

A ternary eutectic alloy with a composition of 57.2 pct Bi, 24.8 pct In, and 18 pct Sn was continuously cast into wire of 2 mm diameter with casting speeds of 14 and 79 mm min−1 using the Ohno Continuous Casting (OCC) process. The microstructures obtained were compared with those of statically cast specimens. Extensive segregation of massive Bi blocks, Bi complex structures, and tinrich dendrites was found in specimens that were statically cast. Decomposition of γSn by a eutectoid reaction was confirmed based on microstructural evidence. Ternary eutectic alloy with a cooling rate of approximately 1 °C min−1 formed a double binary eutectic. The double binary eutectic consisted of regions of BiIn and decomposed γSn in the form of a dendrite cell structure and regions of Bi and decomposed γSn in the form of a complex-regular cell. The Bi complex-regular cells, which are a ternary eutectic constituent, existed either along the boundaries of the BiIn-decomposed γSn dendrite cells or at the front of elongated dendrite cell structures. In the continuously cast wires, primary Sn dendrites coupled with a small Bi phase were uniformly distributed within the Bi-In alloy matrix. Neither massive Bi phase, Bi complex-regular cells, nor BiIn eutectic dendrite cells were observed, resulting in a more uniform microstructure in contrast to the heavily segregated structures of the statically cast specimens.

The objective of the present study was to find an explanation for the observation made by earlier researchers that the distribution of silicon across dendrite branches in an aluminium-silicon casting alloy is sometimes anomalous, in that the concentration gradient is in the opposite direction to that predicted by solidification theory. Small specimens of aluminium alloy A356 were solidified to give a similar dendritic microstructure. One specimen was quenched from a temperature just above the eutectic temperature, giving the silicon distribution expected from theory, while a second specimen was cooled more slowly to give the anomalous silicon distribution, suggesting that it is caused by something occurring in the solid state. It was noted that there is a rapid decrease in solubility of silicon in aluminium with decreasing temperature below the aluminium-silicon eutectic temperature, so that a substantial amount of silicon is expected to come out of solution at temperatures above 500 degrees C on cooling. An estimate of the diffusion rate of silicon in aluminium showed that, for normal cooling rates, this can occur by diffusion of silicon to the interdendritic silicon particles formed during the final stages of freezing, thereby removing silicon directionally from the dendrite branches and producing the observed anomalous silicon distribution. MST/4400.

Microstructures of the two ternary eutectic alloys of the Bi–Cd–In system were studied using slow unidirectional solidification, followed by quenching to form a representative solid/liquid interface for subsequent observation. The eutectic reactions were found to take the form L↔BiIn+BiIn2+Cd at 77.5°C and L↔BiIn2+ɛ+Cd at 61.5°C. The 77.5°C eutectic was observed to be of the faceted (BiIn)–faceted (Cd)–non-faceted (BiIn2) type, while all three phases of the 61.5°C eutectic showed faceting. The BiIn and BiIn2 phases of the 77.5°C eutectic formed a quasiregular microstructure with the Cd phase growing relatively independently. The phases of the 61.5°C eutectic tended to form a lamellar microstructure with a BiIn2–ɛ–Cd–ɛ–BiIn2 phase sequence. Both eutectics were observed to obey the usual phase spacing law, λ2R=constant, where λ is the phase spacing and R is the growth rate.

To investigate the nature of the Bi-Pb-Sn ternary eutectic, specimens were solidified unidirectionally, at very low speeds and quenched to form a representative solid/liquid interface for subsequent study. Specimens made using the generally accepted composition, as reported by Ho et al., did not form all three solid phases from the start of freezing. Specimens produced using the composition reported by Sakurai, i.e., 54 wt-%Bi, 28 wt-%Pb, and 18 wt-%Sn, did give all three phases from the beginning of freezing, indicating that it is the correct eutectic composition. It was found that this eutectic is of the faceted (Bi) non-faceted (X phase) non-faceted (Sn) type. Under the freezing conditions used, a double binary microstructure was formed with one component consisting of Sn fibres in the X phase and the other of a Bi-Sn complex regular microstructure. While the occurrence of a double binary microstructure was predicted by C.S. Smith for a lamellar ternary eutectic, the current observation shows that it can also occur in a system with one fibrous phase. MST/4340.

Cast or solution treated specimens of a Bi-9.0Cd-26.7In (wt-%) alloy were observed to form a fine, three phase microstructure on aging at room temperature, replacing a single phase formed at a higher temperature. The three phases resulting from this solid state reaction were found to grow with a lamellar morphology into the high temperature phase, with a growth rate of 0.5-1.0 mu m h(-1) at room temperature. The equilibrium temperature for the transformation was found to be similar to 25 degrees C. Using a Hitachi S-4500 field emission SEM, the phase transformation was followed in progress at magnifications of 3000 and 10 000 times. It was noted that a volume change was associated with the transformation. It was concluded that the transformation is of the ternary eutectoid type.