Sn-rich Corner Research Articles

Phase Equilibria in the Sn-Rich Corner of the Sn–Cu–Ni Ternary Alloy System at 240 °C

The interfacial reactions between solders and under bump metallization (UBM) have been of great interest recently in flip chip technology. Intermetallic compounds (IMCs), i.e., (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4, usually formed between solders and UBM. To fully understand the interfacial reactions and phase transformation phenomenon, a suitable phase diagram concerning solders, IMCs, and UBM materials is required. In particular, a Sn-rich phase region in the Sn–Cu–Ni ternary diagram is very critical in determining the concentration tendency of x and y values in (Ni1−x,Cux)3Sn4 and (Cu1−y,Niy)6Sn5 compounds. In this study, ternary Sn–Cu–Ni alloys were prepared and annealed at 240 °C. Three equilibrium phases, Sn, Ni3Sn4, and Cu6Sn5, were identified by x-ray diffraction analysis and also showed in backscattered electron imaging. Using electron probe microanalysis quantitative analysis, three acme compositions of the ternary region in the Sn–Cu–Ni isotherm near the Sn-rich corner were determined as 98.5 at.% Sn, (Ni0.80, Cu0.20)3Sn4 and (Cu0.59,Ni0.41)6Sn5. In addition, the solubility of Cu and Ni in (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5 compounds was evaluated. Finally, the isothermal section of the ternary Sn–Cu–Ni system at 240 °C was proposed on the basis of experimental results in this study.

Primary solidification phases of the Sn-rich Sn-Ag-Cu-Ni quaternary system

The eutectic and near-eutectic Sn-Ag-Cu solders are the most promising lead-free solders, and nickel is frequently used as the barrier layer material. Nickel dissolves into the molten Sn-Ag-Ni alloy during the soldering process, and the ternary solder becomes a Sn-Ag-Cu-Ni quaternary melt near the nickel substrate. Liquidus projection is the projection of the liquidus trough and it delineates the boundaries of various primary solidification phases. Information of liquidus projection is helpful for understanding the alloys’ solidification behavior. This study prepared the Sn-Ag-Cu-Ni alloys of various compositions at the Sn-rich corner. The alloys were melted at higher temperatures and solidified in air. The solidified alloys were metallographically examined to determine the phases formed, especially the primary solidification phases. No ternary or quaternary compounds were found. The knowledge of the primary solidification phases, phase formation sequences, and reaction temperatures determined in this study were put together with all of the available liquidus projections of the constituent ternary systems to determine the primary solidification phases of the quaternary Sn-Ag-Cu-Ni system at the Sn-rich corner.

Isoplethal Sections of the Liquidus Projection and the 250°C Phase Equilibria of the Sn–Ag–Cu–Ni Quaternary System at the Sn-Rich Corner

Isoplethal sections of the 250°C phase equilibria at 95 at%Sn and 97 at%Sn and that of the liquidus projection at 97 at%Sn of the Sn–Ag–Cu–Ni system were determined. Sn–Ag–Cu–Ni quaternary alloys at constant Sn contents were prepared. The alloys for the isothermal phase relationship determination were equilibrated at 250°C for various lengths of time, and then the equilibrium phases in the annealed alloys were analyzed. The alloys for the liquidus projection determination were melted at 1000°C, and were solidified in the furnace at a cooling rate approximately 10°C/min. The solidified alloys were metallographically examined and the primary solidification phases were determined. The isoplethal sections of the 250°C phase equilibria and the liquidus projection were constructed based on the quaternary experimental data and the 250°C isothermal sections and the liquidus projection of the constituent ternary systems, respectively. Neither ternary nor quaternary intermetallic compounds were found. Most of the binary compounds have significant ternary solubilities, but only very limited quaternary solubilities. Four different primary solidification phases are found of the Sn–Ag–Cu–Ni alloys with 97 at%Sn, and the compositional regime of the primary Ni3Sn2 phase is relatively much smaller.

Enthalpies of mixing of metallic systems relevant for lead-free soldering: Ag–Pd and Ag–Pd–Sn

The partial and integral enthalpies of mixing of binary liquid Ag–Pd alloys (with compositions up to about 55 at.% Pd) were determined at 1400 °C. A Calvet type microcalorimeter was used for the measurements employing a drop calorimetric technique. Additionally, eight sections in the Sn-rich corner of the ternary Ag–Pd–Sn system, which is relevant for lead-free soldering, were investigated at 900 °C. The ternary interaction parameters were fitted using the Redlich–Kister–Muggiano model for substitutional solutions. The isoenthalpy curves for Ag–Pd–Sn at 900 °C were constructed for the integral molar enthalpy of mixing.

Phase equilibria of the Sn-Ag-Cu-Ni quaternary system at the sn-rich corner

Knowledge of phase equilibria of the Sn-Ag-Cu-Ni quaternary system at the Sn-rich corner is important for the understanding of the interfacial reactions at the Sn-Ag-Cu/Ni contacts, which are frequently encountered in recent microelectronic products. Various Sn-Ag-Cu-Ni alloys were prepared and equilibrated at 250°C. The alloys were then quenched and analyzed. The phases were determined by metallography, compositional analysis, and x-ray diffraction (XRD) analysis. No quaternary phases were found. The isoplethal sections at 60at.%Sn, 70at.%Sn, 80at.%Sn, and 90at.%Sn at 250°C are determined. The phase equilibrium relationship was proposed based on the quaternary experimental results and the 250°C isothermal sections of the four constituent ternary systems, Sn-Ag-Cu, Sn-Ag-Ni, Sn-Cu-Ni, and Cu-Ag-Ni. Because there are no ternary phases in all these three systems, all the compounds are in fact binary compounds with various solubilities of the other two elements.

Au−Ni−Sn intermetallic phase relationships in eutectic Pb−Sn solder formed on Ni/Au metallization

Recent work has shown that a Au−Ni−Sn ternary compound with a nominal composition of Au0.5Ni0.5Sn4 redeposits and grows at the interface between eutectic Pb−Sn solder and Ni/Au metallization during aging at 150°C. The present work verifies the existence of the Au0.5Ni0.5Sn4 phase by examining the Sn-rich corner of the Au−Ni−Sn ternary phase diagram. The reconfiguration mechanism of the AuSn4 from the bulk solder is also discussed, with detailed observations of the Au0.5Ni0.5Sn4 microstructure. The results show that the Ni solubility limit in the AuSn4 phase is approximately 12 at.% at 150°C and thus, the Au0.5Ni0.5Sn4 phase is a ternary AuSn4-based compound with high Ni solubility. Due to the slight solubility and the fast diffusion of Au in the eutectic Pb−Sn at 150°C, the AuSn4 intermetallics in the bulk solder can reconfigure to form a AuxNi1−xSn4 compound at the interface where Ni is available. The AuxNi1−xSn4 compound layer consists of nanocrystals arranged in a larger grainlike morphology. It appears that the inherent lattice strain of the AuxNi1−xSn4 compound and the volume change due to its formation results in a nanocrystalline microstructure.

Experimental and thermodynamic assessment of Sn-Ag-Cu solder alloys

Sn-rich alloys in the Sn-Ag-Cu system are being studied for their potential as Pb-free solders. Thus, the location of the ternary eutectic involving L, (Sn), Ag3Sn and Cu6Sn5 phases is of critical interest. Phase diagram data in the Sn-rich corner of the Sn-Ag-Cu system are measured. The ternary eutectic is confirmed to be at a composition of 3.5 wt.% Ag, 0.9 wt.% Cu at a temperature of 217.2±0.2°C (2σ). A thermodynamic calculation of the Sn-rich part of the diagram from the three constituent binary systems and the available ternary data using the CALPHAD method is conducted. The best fit to the experimental data is 3.66 wt.% Ag and 0.91 wt.% Cu at a temperature of 216.3°C. Using the thermodynamic description to obtain the enthalpy- temperature relation, the DTA signal is simulated and used to explain the difficulty of liquidus measurements in these alloys.

Thermodynamic properties of ternary liquid zinc-alloys

The thermodynamic properties of zinc in the three ternary (Ag, Au, Cu)-Sn-Zn systems were measured with an emf method, and with the help of the Gibbs-Duhem equation the integral thermodynamic quantities of these three systems were calculated. In several binary systems (Sb-Zn, Au-Zn, Au-Sn) chemical short range order (CSRO) has been found in the melt, which was confirmed by different experiments such as emf, electrical conductivity, magnetic measurements, and calorimetry. It was therefore of interest to investigate the change of the anomalies in the thermodynamic properties by the addition of a third element. The exact determination of the thermodynamic functions of some of these alloys is also of some importance for industrial applications. Since lead in solder materials is a great environmental problem and a health concern it is safer and more economical to replace lead in the solder materials instead of cleaning up the waste. Some of these materials under consideration are ternary tin alloys. For the ternary (Ag, Au, Cu)-Sn-Zn systems no information is available in the literature on the thermodynamic properties and on the phase diagram in the Sn-rich corner.