Recent progress on the thermodynamic databases of calculated phase diagrams in microsolders and Cu-based alloys is presented. A thermodynamic tool, Alloy Database for Microsolders (ADAMIS), is based on comprehensive experimental and thermodynamic data accumulated with the calculation of phase diagrams (CALPHAD) method and contains eight elements, namely, Ag, Bi, Cu, In, Sb, Sn, Zn, and Pb. It can handle all combinations of these elements and all composition ranges. The elements of Al and Au have also been added to ADAMIS within a limited range of compositions. Furthermore, a database of Cu-based alloys, including binary (Cu-X), ternary (Cu-Fe-X, Cu-Ni-X, and Cu-Cr-X), and multicomponent (Cu-Ni-Cr-Sn-Zn-Fe-Si) systems, has also been developed. Typical examples of the calculation and application of these databases are presented. These databases are expected to be a powerful tool for the development of Pb-free solders and Cu substrate materials as well as for promoting the understanding of the interfacial phenomena between them in electronic packaging technology.

Key words: Pb-free solders, thermodynamic database, Cu-based alloys, calculation of phase diagram, phase-field method, interfacial reaction

INTRODUCTION

During the past decade, Pb-free solders, to replace conventional Pb-Sn alloys, have been designed and developed to meet the requirements arising from environmental and health issues concerning the toxicity of Pb.1-3 Many investigations have indicated that Pb-free solders are likely to be multicomponent alloys because the melting temperatures of binary candidates are either too high or too low, and their mechanical properties are not adequate. In view of the necessity to develop Pb-free solders with high efficiency, and the transition time involved in replacing Pb-bearing alloys, a thermodynamic database of microsoldering alloys for reliable predictions of liquidus, solidus, phase fractions and constitutions, equilibrium and nonequilibrium solidification behavior, etc., in a multicomponent system is required because it is difficult to understand these factors from available references. In addition, a thermodynamic database of the Cu-based alloys is also important for the design of substrate materials. The calculation of phase diagrams (CALPHAD) is an effective method for alloy design4 and has been widely used in development of new materials.5 Recently, the present authors have developed thermodynamic databases for microsoldering materials6-8 and Cu-based alloys9 within the framework of the CALPHAD method. This database is useful for the design of Pb-free solders and Cu-based substrate materials as well as for understanding the interfacial reaction between them. In the present paper, the validity of these two thermodynamic databases for calculation, alloy design, and applications is demonstrated.

THERMODYNAMIC DATABASE OF MICROSOLDERS

A thermodynamic tool of microsoldering materials named Alloy Database for Microsolders (ADAMIS) was developed by combining the thermodynamic database of microsolders6-8 with Pandat, (Madison, WI) a multicomponent, phase diagram-calculation software program.10 In this database, the phase equilibria in any system that includes the elements Ag, Bi, In, Cu, Sn, Sb, Zn, and Pb can be calculated in the whole composition range. Recently, two elements, namely, Al and Au, were added to this database, and calculations are available for a limited composition range of these elements.

In the database, the thermodynamic parameters for describing the Gibbs energy of each phase are evaluated by optimizing the experimental data pertaining to the phase boundary compositions and thermodynamic properties, such as activity, heat of mixing, and enthalpy of formation. Regarding the present database, there are some important alloy systems for which there has been little or no experimental data. Thus, experimental work to determine phase equilibria, such as liquidus, solidus, isothermal and vertical sections, etc., has been carried out, and a better estimation of the thermodynamic parameters with good agreement between the calculated and the observed phase equilibria has been obtained.11-21 The thermodynamic parameters were optimized and evaluated by Thermo-Calc (Stockholm, Sweden) software.4

ADAMIS is a user-friendly thermodynamic tool for the design of microsoldering materials. Beginners can easily manage it through the Windows interface. The calculated results are independent of the user's level of expertise because it has the ability to automatically find starting points and initial values for stable phase equilibria.

Figure 1 shows the main contents of ADAMIS, where much information, such as phase equilibria, projection of the liquidus surface, and simulation of solidification etc., can be obtained. In addition, the surface tension and viscosity in the liquid phase can also be calculated.

THERMODYNAMIC DATABASE OF Cu-BASED ALLOYS

A thermodynamic database of Cu-based alloys containing Cu-X binary, Cu-Fe-X, Cu-Cr-X, and CuNi-X ternary and multicomponent systems was also developed on the basis of the CALPHAD method. An outline of the assessed systems is shown in Table I. To obtain a good assessment for thermodynamic parameters, experimental investigations of phase equilibria, including not only first-order phase transformation but also order-disorder transition for some important systems with no or very little experimental data, were carried out on the basis of differential scanning calorimetry, energy dispersive x-ray spectroscopy, the diffusion couple method, etc.22-29 For example, the ordering of the bcc phase has been determined in the Cu-Al,26 Cu-Sn,27 and Cu-Fe-Al systems.28