Overview Phase Diagrams Phase Diagrams for Lead-Free Solder Alloys Ursula R. Kattner Author’s Note: The identification of any commercial product or possible contamination from other and flux compatibility) were tin-based. trade name does not imply endorsement or recommendation by the National Institute of Standards and Technology. solder materials such as the formation Common alloying additions are low- of low-melting eutectics, and reactions melting metals, such as bismuth, The need for new, improved solder with various substrates. A recent study antimony, and indium, or metals forming alloys and a better understanding of a eutectic reaction with (Sn), such as reactions during the soldering process Knowledge of the silver and copper. (The elemental symbol grows steadily as the need for smaller in parenthesis is used to distinguish and more reliable electronic products phase equilibria the disordered solid solution based on increases. Information obtained from of solder-alloy and this element from the pure element.) phase equilibria data and thermody- Substrate materials may consist of namic calculations has proven to be solder/substrate copper, copper that has been coated an important tool in the design and systems provides the or plated with tin-lead or tin-bismuth understanding of new lead-free solder solders, nickel-tin, nickel-gold, or alloys. A wide range of candidate basic roadmap for nickel-platinum alloys. alloys can be rapidly evaluated for the initial selection Knowledge of the phase equilibria proper freezing ranges, susceptibility of candidate solders of solder/alloy and solder/substrate to contamination effects, and reactions systems provides the basic roadmap for with substrate materials before the and contributes to the initial selection of candidate solders expensive process of preparing and the understanding and contributes to the understanding testing candidate alloys is initiated. of solder wetting and spreading. Phase of solder wetting equilibria data provide not only informa- INTRODUCTION and spreading. tion about the liquidus and solidus Intense international competition temperatures of a candidate solder alloy, makes it necessary for microelectronics of high-temperature lead-free candidate but also information about possible and supporting industries to design solder alloys1 showed that the only intermetallic phase formation, either and produce smaller, more functional, lead-free alloys that fulfilled the initial within the solder during solidification and reliable electronic products more selection criteria (toxicity, cost, oxida- or in reaction with the substrate mate- economically. At the same time, there is tion resistance, solidus temperature, rial by a combination of isothermal growing global interest in eliminating solidification and solid-state reaction. toxic elements, such as lead, from Reaction of substrate material that electronic products. In recent years, has been pre-tinned with a tin-lead or substantial efforts were made to develop tin-bismuth alloy with a solder of a lead-free solders that are suitable different composition may result substitutes for classic tin-lead eutectic in the formation of a low-melting solders. Simultaneously, there is a higher-component eutectic. In this growing need for solders that can case, the multi-component phase be used for applications with more diagram can be used to evaluate demanding service conditions such as the possible effects resulting from such in automotive, avionic, military, or a contamination. oil-exploration industries. For these Traditionally, phase diagrams are purposes, it is necessary to evaluate the determined from thermal analysis, properties of candidate solder alloys examination of microstructure, and other that are related to manufacturing and experimental methods. However, the reliability. Of particular interest are Figure 1. The calculated phase diagram experimental determination of phase of the Sn-Pb system.16 properties such as freezing range diagrams is a time-consuming, costly (liquidus and solidus), the effects from task since the number of possible systems 2002 December • JOM 45 for solder, the magnitude of the binary THE CALPHAD METHOD excess Gibbs energies is fairly small,10 The CALPHAD method uses a series indicating that ternary interactions of models to describe the concentration, may not be significant. Therefore, the temperature dependence, and, if neces- accuracy of the calculated constituent sary, pressure dependence of the Gibbs binary systems is of crucial importance energy functions of each individual for obtaining quality calculations phase in a system.2,3 Commercial and of ternary and higher-component public domain software packages are systems.11 available for the calculation of phase The efforts in the development of equilibria from these Gibbs energy thermodynamic descriptions for systems functions.2,3 One of these, Thermo- that are relevant to solders have resulted Calc,8 was used for the calculation of in several thermodynamic databases the phase diagrams shown in Figures that are either available in the public Figure 2. The calculated phase diagram 1 to 10. The most common models for domain4,12 or are commercial.5 of the Sn-Bi system.19 the description of the concentration ACCURACY REQUIREMENT dependence are the regular solution model for disordered solution phases The knowledge of temperatures and increases drastically as the number of and the sublattice model for ordered compositions of the invariant equilibria elements increases. For example, compounds. The choice of the sublattice and regimes of primary phase solidifica- increasing the number of elements under model description for solid phases tion is of significant importance for consideration from six to seven increases depends on their crystal structure.9 If the design of new solder alloys and the number of binary systems from 15 to the homogeneity range of a phase is for the specification of manufacturing 21, the number of ternary systems from narrow, this phase can also be described tolerances of these solders. The liquidus 20 to 35, and the number of quaternary as a stoichiometric phase. temperature changes little with composi- systems from 15 to 35. Experimental The usual strategy for the assessment tion in many tin-based systems when information for the entire phase diagram of a multi-component system is to (Sn) forms as a primary phase, while the is available for most of the binary systems first derive thermodynamic model temperature dependence on composition that are of interest for solders, but descriptions that are consistent with the can be relatively large for other phases, experimental information becomes experimental data of the binary systems. especially for intermetallic compounds. increasingly sparse as the number of These descriptions are then used If the liquidus is steep, composition constituent elements increases (i.e., for together with a standard thermodynamic fluctuations in the solder alloy can ternary, quaternary, and higher- extrapolation method to calculate ternary cause the solder to have a significantly component systems). It has been shown and higher-order systems. If indicated higher liquidus temperature than for the that thermodynamic calculation of phase by experimental data, it is possible to nominal composition. equilibria with the CALPHAD method2,3 add ternary interaction terms to the The invariant temperatures of solder is extremely useful for obtaining thermodynamic models to obtain a alloys are usually known with fairly high quantitative information about these more accurate calculation of the ternary accuracy while there is usually a larger higher-component systems. system. This strategy is usually followed variation in the reported composition The thermodynamic descriptions that until the constituent systems of a of the liquid phase at the invariant are used with the CALPHAD method higher-order system have been assessed. temperature. This is, in part, due to the can also be used to obtain data of other Experience has shown that no or very properties that are important for under- minor corrections are necessary for a standing the wetting behavior of a molten reasonable prediction of quaternary and solder, such as surface tension and higher-component systems. viscosity.4,5 In addition, Lee et al.6 used The quality of the results that are the calculation of metastable equilibria obtained from calculations with the and the driving forces for phase forma- CALPHAD method depends not only tion to predict the phase that forms first on the quality of the thermodynamic at a solder/substrate interface. Further- models but also on the quality of the more, thermodynamic calculation also available experimental information that provides information that is needed for was used to derive the model parameters the simulation of kinetic processes. For of the individual phases. The quality of example, tie-line information (i.e., the the extrapolation of a ternary system compositions of two phases in equilib- from the three constituent binary systems rium) and thermodynamic factors for depends on the magnitude of possible Figure 3. The calculated phase diagram the calculation of diffusion coefficients ternary interactions and the occurrence of the Sn-In system.19 are necessary for the simulation of of ternary intermetallic compounds. For diffusion processes.7 most binary systems that are relevant 46 JOM • December 2002 to insufficiently accurate calculation of thermodynamic assessments are avail- a system. Moon et al.11