Phase Diagrams for Lead-Free Solder Alloys

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 showed that, able for this simple eutectic system. although the agreement between the At 183°C, the liquid with x = 0.619 Sn calculated and measured liquidus data decomposes into the two terminal solid for the binary Ag-Sn and Cu-Sn system solutions, (Pb) and (Sn). (All composi- is satisfactory, it was not possible to use tions are given in weight fraction the descriptions of these binary systems unless otherwise noted.) In addition to obtain an accurate fit of the ternary to the stable phase equilibria data, the tin-rich part of the Ag-Cu-Sn system. It is thermodynamic assessments of Fecht known that the results of the calculation et al.16 and Ohtani et al.17 also used of higher component systems indicate data for metastable equilibria. The whether further refinements of the major differences between the two thermodynamic description of a binary assessments are that Ohtani et al. also system are needed.14 considered the pressure dependence Figure 4. The calculated phase diagram of the phases while Fecht et al. empha- 22 BINARY SYSTEMS of the Sn-Sb system. sized an accurate reproduction of the A summary of the evaluated phase experimental quantities at atmospheric diagrams and available assessments pressure. The phase diagram obtained fact that temperatures between 50°C and of binary and ternary systems in the from the assessment of Fecht et al. is 300°C can be easily measured with high literature has been compiled.12 Most shown in Figure 1. accuracy. However, the interpretation binary phase diagrams that are of interest Sn-Bi of the microstructure of an alloy with for solder alloys are fairly simple near-eutectic composition is not always and most of the phase boundaries are Most parts of the phase diagram of a straightforward task. The phase well established. Since the binary tin- this simple eutectic system are well observation may be misleading due based systems are key systems for the established. At 138°C, the liquid with to a phenomenon caused by a skewed evaluation of candidate solder alloys, x = 0.43 decomposes into the two Sn coupled zone.13 In this case, the fast important features of these systems will terminal solid solutions, (Bi) and (Sn). growth kinetics of the unfaceted phase, be briefly discussed. The elements that However, the solubility limit of tin in (Sn) in most solder alloys, may lead are most often considered as alloying (Bi) is not reliably known although to the formation of dendrites of this elements to tin for lead-free solders are a lower value is preferred.18 Lee et phase even on the other side of the Ag, Bi, Cu, In, and Sb. Although its al.19 accepted this lower value for the eutectic composition where the faceted, susceptibility to corrosion is a concern, (Bi) homogeneity range and also used frequently intermetallic phase, should zinc is occasionally considered as an experimental data of the Sn-Bi-In system form primary dendrites. This phenom- alloying element for solders because for the refinement of the description of enon may result in an overestimation the temperature of the binary Sn-Zn the Sn-Bi system. The phase diagram of the extent of the primary solidifica- eutectic is similar to that of the Sn-Pb obtained from this assessment is shown tion area of the unfaceted phase. The eutectic. The elements gold and nickel in Figure 2. interpretation of the microstructure in are of interest since they are frequently Sn-In higher-component alloys can be even used in substrate materials. Although more difficult. Moon et al.11 found the present goal is to eliminate lead from In addition to liquid and the two that the remaining liquid phase had solder alloys because of its toxicity, the terminal solution phases (In) and (Sn), a tendency for supercooling after knowledge of this system is still relevant the initial formation of the primary for either the study of the mechanisms intermetallic phase in tin-rich alloys of that take place during soldering or the Sn-Ag-Cu system. The sequence of the effects that may be caused in new solidification of this supercooled liquid lead-free solders that are applied to consisted of dendritic (Sn), coupled components that were pre-tinned with formation of non-faceted (Sn), and a classical Sn-Pb solder. The transforma- faceted intermetallic phase (Ag Sn or tion from the high-temperature form, 3 Cu Sn , depending on alloy composi- βSn, to the low-temperature form, αSn 6 5 tion), followed by the ternary eutectic (T = 13°C), can be neglected since β α reaction. This solidification behavior it does not affect the equilibria with resulted in microstructures with a other phases. Also, the transformation, smaller amount of eutectic structure which is kinetically inhibited, rarely than is expected from the equilibrium occurs in solders.15 phase diagram. Sn-Pb The fact that the slope of the liquidus Figure 5. The calculated phase diagram of the Sn-Ag system.24 in tin-based systems with intermetallic The phase diagram of this system phases is usually very steep may lead is well established and a number of

2002 December • JOM 47 established. The two available thermody- this system has six intermediate phases, namic assessments10,24 give very similar Au Sn (β), (ζAu), Au Sn (ζ´), AuSn (δ), 10 5 results. Moon et al.11 pointed out that in AuSn (γ), and AuSn (η). Except for the 2 4 both assessments, the liquidus for ζ´ phase, which forms peritectoidally, primary Ag Sn formation needs further 3 and the δ (AuSn) phase, which forms refinement. The phase diagram obtained congruently, the other intermediate from the assessment of Oh et al.24 is phases form by peritectic reactions. The shown in Figure 5. phase diagram obtained from the thermodynamic assessment by Cheva- Sn-Zn lier28 is consistent with the phase diagram The phase diagram of this system is that was accepted by Okamoto and relatively well established and a number Massalski.29 Since then, the phase of thermodynamic assessments are equilibria involving the Au Sn, (ζAu), 10 available for this simple eutectic system. and Au Sn phases were revised,30 5 Figure 6. The calculated phase diagram At 198.5°C, the liquid decomposes into extending their stability ranges to lower of the Sn-Zn system.25 the two terminal solid solutions, (Zn) temperatures. However, only the AuSn, and (Sn). However, the composition AuSn , and AuSn phases are of interest 2 4 reported for the liquid phase at the for solder applications. The tin-rich this system has two intermediate phases, eutectic temperature varies between x Sn eutectic occurs at a temperature of 217°C β and γ. Both intermediate phases form = 0.906 and x = 0.921. The phase and a liquid composition of x = 0.90 Sn Sn through a peritectic reaction from the diagram obtained from the assessment (x = 0.918 calculated). The tin-rich Sn liquid and one of the terminal solution of Lee25 is shown in Figure 7. AuSn phase decomposes eutectoidally 4 phases. At a temperature of 120°C, the at a temperature above room tempera- Sn-Cu liquid with x = 0.491 decomposes into Sn ture, according to the diagram of the two intermediate phases. The In addition to liquid and the two Okamoto and Massalski. Although the evaluation by Okamoto20 concludes that terminal solution phases, (Cu) and (Sn), exact temperature is not known, the most of the boundaries of the solid phases this system has seven intermediate calculated decomposition temperature need to be better established for phases, β, γ, Cu Sn (δ), Cu Sn (ζ), 41 11 10 3 is 104°C. The phase diagram obtained concentrations with x ≥ 0.75. The two Cu Sn (γ), and Cu Sn /Cu Sn ’ (η/η1, 28 Sn 3 6 5 6 5 from the assessment by Chevalier is thermodynamic assessments19,21 avail- high- and low-temperature forms). All shown in Figure 8. able are based on the evaluation by of the intermediate phases form by Sn-Ni Okamoto. The phase diagram obtained peritectic or peritectoid reactions. All of from the assessment of Lee et al.19 is the copper-rich intermediate phases In addition to liquid and the two shown in Figure 3. decompose in eutectoid reactions at terminal solution phases, (Ni) and (Sn), temperatures above 350°C and, there- this system has five intermediate phases: Sn-Sb fore, only the Cu Sn and Cu Sn /Cu Sn ´ Ni Sn (high- and low-temperature 3 6 5 6 5 3 In addition to liquid and the two phases are of interest for solder applica- forms), Ni Sn (high- and low- 3 2 terminal solution phases, (Sb) and (Sn), tions. The temperature of 227°C for the temperature forms), and Ni Sn . The 3 4 this system has two intermediate phases, eutectic reaction, where liquid decom- high-temperature forms of Ni Sn and 3 SbSn (β) and Sb Sn Both intermediate poses into Cu Sn and (Sn), is well Ni Sn form congruently from the liquid 3 2. 6 5 3 2 phases, as well as (Sn), form by peritectic established. However, various evalua- phase and Ni Sn forms by a peritectic 3 4 reactions. The peritectic reaction forming tions of this system disagree on the exact (Sn) occurs at a temperature of 250°C. composition of the liquid phase, either The two available thermodynamic x = 0.91 or x = 0.93. Moon et al.11 Sn Sn assessments17,22 are based on the experi- showed that the composition of x = Sn mental work of Predel and Schwer- 0.91 is consistent with the eutectic mann.23 The phase diagram obtained temperature and the slope of the liquidus from the assessment of Jönsson and for primary (Sn) formation. The two Ågren22 is shown in Figure 4. available thermodynamic assess- ments26,27 give very similar results. Moon Sn-Ag et al.11 pointed out that the liquidus for In addition to liquid and the two primary Cu Sn formation needs further 6 5 terminal solution phases, (Ag) and (Sn), refinement. The phase diagram obtained this system has two intermediate phases, from the assessment of Shim et al.26 is (ζAg) and Ag Sn. Both intermediate shown in Figure 7. 3 phases form by peritectic reactions. The Sn-Au eutectic reaction in which liquid with Figure 7. The calculated phase diagram of the Sn-Cu system.26 x = 0.965 decomposes into Ag Sn and In addition to liquid and the two Sn 3 (Sn) at a temperature of 221°C is well terminal solution phases, (Au) and (Sn),

48 JOM • December 2002 for a refinement of the description liquid phase. However, it should be of the ternary systems. However, the noted that the intermediate phase, Sb Sn , of the binary Sn-Sb system differences between the results for the 2 3 tin-rich part of the system from the two was not detected in this experimental calculations are not significant. work. Sn-Ag-Cu Sn-Bi-Cu The phase diagram of this system was No experimental phase diagram data experimentally determined by Gebhardt are available for this system. It can and Petzow.37 No ternary phases have be expected that the thermodynamic been reported and the solid phases extrapolation of the description of the have fairly small ternary homogeneity binary systems gives a fairly accurate ranges. The invariant reaction in the prediction of the ternary phase diagram, tin-rich corner was first reported to as was the case for the Sn-Ag-Bi system. Figure 8. The calculated phase diagram be non-eutectic. More recent work38,39 The liquidus projection obtained from of the Sn-Au system.28 showed that this reaction is eutectic the binary descriptions19,26,41 is shown with a temperature of 217 ± 0.2°C, in Figure 10c. The predicted tin-rich and the liquid decomposes into (Sn) eutectic occurs at a temperature of reaction. The tin-rich eutectic occurs at and the binary intermediate compounds 138.8°C and the composition of the Ag Sn and Cu Sn . However, there was eutectic liquid phase is x = 0.428 and a temperature of 231.2°C and a liquid 3 6 5 Bi composition of x = 0.998. The only disagreement on the composition of the x = 0.0004. Sn Cu available thermodynamic assessment of liquid phase at the eutectic temperature. Sn-Cu-Ni this system was carried out by Ghosh;31 The experimental work of Moon et al.11 the calculated phase diagram is shown confirmed the composition of the liquid Extensive phase diagram data are phase as x = 0.035 and x = 0.009 available for the Cu- and Cu,Ni-rich in Figure 9. Ag Cu and the temperature as 217.2°C. The part of this system.34 The experimental TERNARY AND HIGHER calculated tin-rich part of the liquidus data show that the binary Sn-Cu and COMPONENT SYSTEMS projection is shown in Figure 10b. Sn-Ni phases have extended ternary Critical evaluations of experimental homogeneity ranges. A series of ternary Sn-Ag-In data are available for ternary Ag, Al, and phases has been reported but it is not Au systems,32 and Cu systems.33 Also, Few experimental phase diagram data clear whether these phases are true Villars et al.34 compiled a summary of are available for this system. Korhonen ternary phases or extended ternary the available experimental data for most and Kivilahti21 used six ternary alloys homogeneity ranges of the binary ternary systems. Many solder alloys are that were annealed at 250°C to gain phases. No experimental data are included in these references and, information about phase boundary available for tin concentrations larger therefore, only systems that are most locations and differential scanning than an atomic fraction of 0.6. Gupta et relevant for solder applications will be calorimetry (DSC) to investigate the al.42 proposed that a transition (type II) reaction, L + Ni Sn Cu Sn + (Sn), discussed here. melting/solidification behavior of the 3 4 6 5 alloys. The temperature of the ternary occurs in the tin-rich corner. However, Sn-Ag-Bi eutectic where the liquid decomposes without experimental evidence this is into Ag In and the intermediate Sn-In rather speculative. A thermodynamic The phase diagram of this system was 2 experimentally determined by Hassam phases β and γ was reported to be et al.35 No ternary phases have been about 113°C. reported, and the solid phases have fairly Sn-Ag-Sb small ternary homogeneity ranges. The ternary eutectic is reported to occur at A series of 21 alloys was studied with 138.4°C where the liquid with x = differential thermal analysis (DTA) and Ag 0.010 and x = 0.563 decomposes into electron microprobe analysis (EMPA) Bi the two terminal solid solutions, (Bi) and used for the construction of a and (Sn), and the binary Ag-Sn phase, liquidus surface.40 The intermediate Ag Sn. The experimental phase diagram phases of the Ag-Sn ((ζAg) and Ag Sn) 3 3 and Ag-Sb systems ((ζAg) and Ag Sb) is in good agreement with the one 3 predicted by Kattner and Boettinger10 were found to form continuous homo- using thermodynamic extrapolation of geneity ranges in the ternary system. the descriptions of the binary systems. The tin-rich invariant reaction was The tin-rich part of the liquidus projec- reported as a transition (type II) reaction, L + SbSn Ag (Sb,Sn) + Figure 9. The calculated phase diagram tion obtained from this calculation is 3 of the Sn-Ni system.31 36 (Sn), at 234.8°C with x = 0.05 and shown in Figure 10a. Ohtani et al. Ag x = 0.06 as the composition of the used the available experimental data Sb

2002 December • JOM 49 a b c

Figure 10. The composition regimes (shaded) with suitable freezing ranges (<35°C) for candidate solder alloys in the systems (a) Sn-Ag-Bi,10 (b) Sn-Ag-Cu,11 and (c) Sn-Bi-Cu.12 extrapolation of the description of the its liquidus temperature in order to to the sensitivity of the measurement binary systems is not likely to give avoid manufacturing problems, such as technique. This strategy resulted in a a reliable prediction for the ternary defective joints caused by vibrations highly accurate determination of the system since the extrapolation cannot during cooling from the soldering eutectic temperature and composition predict the ternary homogeneity of temperature.43 Alloy compositions that of the liquid phase, as well as two the solid phases. However, once the fulfill the freezing range criterion are temperature-concentration sections. parameters for the description of the shown as shaded areas for the Sn-Ag-Bi, The experimental results were then liquid and solid phases in the ternary Sn-Ag-Cu, and Sn-Bi-Cu systems in used to refine the thermodynamic system have been established utilizing Figure 10. However, it should be noted description. the available experimental data, the that in systems that show noticeable Analysis of contamination effects calculation should reliably predict changes in the maximum solubility of is of great importance in avoiding the liquidus for the tin-rich corner of the alloying element in (Sn) during detrimental effects such as the forma- the system. cooling, the amount of liquid phase tion of low-melting eutectics. Moon during cooling can be larger than et al.45 showed that even though the APPLICATION EXAMPLES predicted by the equilibrium diagram.2 contamination level of a Sn-Bi solder Freezing range evaluation is the most Support of experimental design with lead was not high enough to show basic application of phase diagram through the calculation of phase equilib- the formation of the equilibrium ternary information for the selection of new ria can reduce the number of experiments eutectic, which occurs at 95°C, the candidate solder alloys. The maximum that are necessary for the determina- effects of non-equilibrium solidifica- acceptable component temperature tion of a higher-component system. tion occurred at fairly low levels of during assembly depends on the device, Moon et al.11 presented an example contamination. It was also shown that the package, and PCB material being used, where experimental and computational experimental observation was in accord establishing the maximum temperature methods were used to complement each with predictions obtained assuming for a candidate solder. The processing other in the determination of the tin- the worst case of microsegregation, temperature of a solder should be at rich part of the Sn-Ag-Cu system. The the so-called Scheil path. Moon et al. least 10°C to 20°C above its liquidus phase equilibria information was used performed Scheil calculations for a temperature. The combination of these to understand which signals are likely to range of Sn-Bi solders with a very low criteria limits the maximum liquidus be observed during the thermal analysis level (< 1%) of lead contamination temperature for most solders to so that special attention could be paid (Figure 11). They found that a 0.1% lead 225°C43 and 260°C for high-temperature, fatigue-resistant solders.44 Solders for high-temperature applications are required to perform at operating temperatures up to 160°C. This requirement results in a minimum solidus Figure 11. The effect temperature of 208°C if the operating of lead contamination on the lowest temperature is set to be 90% or less of possible freezing the absolute melting temperature of temperature pre- the solder.1 At the same time, the dicted from Scheil path calculations for solidus temperature should not be Sn-Bi solders.45 much more than 30°C lower than

50 JOM • December 2002 contamination promotes the formation wide range of potential alloys can be 21. T.-M. Korhonen and J.K. Kivilahti, J. Electron. Mater., 27 (1998), pp. 149–158. of ternary eutectic in Sn-Bi solder with relatively rapidly evaluated. The power 22. B. Jönsson and J. Ågren, Mater. Sci. Techn., 2 x ≥ 0.005, although, according to the of the calculation is that information Bi (1986), pp. 913–916. equilibrium diagram, eutectic formation can be provided for higher-component 23. B. Predel and W. Schwermann, J. Inst. Metals, 99 (1971), pp. 169–173. should only occur for concentrations systems using thermodynamic extrapo- 24. C.-S. Oh et al., J. Alloys Compounds, 228 (1996), larger than x = 0.113 and x = 0.007. lation methods where no or little Bi Pb pp. 155–166. Kattner and Handwerker9 applied the experimental information is available. In 25. B.-J. Lee, CALPHAD, 20 (1996), pp. 471–480. 26. J.-H. Shim et al., Z. Metallkd., 87 (1996), pp. Scheil analysis to a series of solder addition, the thermodynamic quantities 205–212. alloys and found that the combination of provide the key to the prediction of other 27. J. Miettinen, Metall. Mater. Trans. A, 33A (2002), bismuth and lead was especially detri- solder properties, such as surface tension pp. 1639–1648. 28. P.-Y. Chevalier, Thermochim. Acta, 130 (1988), mental to the final freezing temperature and viscosity. The coupling of phase pp. 1–3. of solders. diagram calculation and experimental 29. H. Okamoto and T.B. Massalski, Bull. Alloy Reaction with substrate materials is determination of phase equilibria Phase Diagrams, 5 (1984), pp. 492–03; add. ibid, 7 (1986), p. 522. a strong function of processing and provides useful information for interpret- 30. J. Ciulik and M.R. Notis, J. Alloys Compounds, services temperatures and depends ing experimental observations and the 191 (1993), pp. 71–8. on substrate metallization and solder experimental results can be used to 31. G. Ghosh, Metall. Mater. Trans. A, 30A (1999), pp. 1481–494. composition. It is highly desirable to further improve the thermodynamic 32. G. Petzow and G. Effenberg, editors, Ternary control these interfacial reactions to description of the system. Alloys: A Comprehensive Compendium of Evaluated optimize joint properties. For example, it Constitutional Data & Phase Diagrams (Weinheim, was shown that the kind of intermediate Germany: VCH Verlagsgesellschaft, 1988-1995), References pp. 1–15. phase formation on nickel substrates is 33. Y.A. Chang et al., Phase Diagrams and Ther- very sensitive to the copper concentra- 1. F.W. Gayle et al., JOM, 53 (6) (2001), pp. 17–21. modynamic Properties of Ternary Copper-Metal 2. U.R. 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Taskinen, Thermochim. the partial excess Gibbs energies and the 10. U.R. Kattner and W.J. Boettinger, J. Electron. Acta, 173 (1990), pp. 137–150. surface tension to calculate the surface Mater., 23 (1994), pp. 603-610. 42. K.P. Gupta et al., J. Alloy Phase Diagrams, 4 11. K.-W. Moon et al., J. Electron. Mater., 29 (2000), (1988), pp. 160–174. tension of binary solder liquids. In pp. 1122–1136. 43. Lead-Free Solder Project Final Report, NCMS addition to the prediction of the surface 12. U.R. Kattner, NIST, unpublished research (2002); Report 0401RE96 (Ann Arbor, MI: National Center for tension, Ohnuma et al.5 used thermo- see also the web page www.metallurgy.nist.gov/phase/ Manufacturing Science, 1997). solder/solder.html. 44. F. Gayle, Adv. Mater. Processes, 159 (4) (2001), dynamic properties to predict the 13. W. Kurz and D.J. Fisher, Fundamentals of pp. 43–44. activation energy of viscosity. Both Solidification, 3rd ed. (Aedermannsdorf, Switzerland: 45. K.-W. Moon et al., J. Electron. Mater., 30 (2001), of these properties are important for Trans Tech Publications, 1989). pp. 45–52. 14. B. Sundman, Scand. J. Metall., 20 (1991), pp. 46. W.T. Chen, C.E. Ho, and C.R. Koa, J. Mater. Res., understanding the wetting behavior of 79–85. 17 (2002), pp. 263–266. a molten solder. 15. R.J. Klein Wassink, Soldering in Electronics, 47. C.E. Ho et al., J. Electron. Mater., 31 (2002), 2nd ed., (Ayr, Scotland: Electrochemical Publications pp. 584–590. CONCLUSION Ltd., 1989). 16. H.J. Fecht et al., Metall. Trans. A, 20A (1989), Ursula R. Kattner is a physical scientist with the Phase diagram information provides pp. 795–803. Materials Science and Engineering Laboratory basic information for the design and 17. H. Ohtani, K. Okuda, and K. Ishida, J. Phase at the National Institute of Standards and Equilibria, 16 (1995) pp. 416–429. Technology. understanding of solder alloys and 18. M. Hansen and K. Anderko, Constitution of Binary is not limited to the application of Alloys (New York: McGraw-Hill, 1958). For more information, contact Ursula R. Kattner, equilibrium processes. The calculation 20. H. Okamoto, Phase Diagrams of Indium Alloys and National Institute of Standards and Technology, Their Technical Applications, eds. C.E.T. White and Materials Science and Engineering Laboratory, 100 of phase equilibria is a powerful tool H. Okamoto (Materials Park, OH: ASM International, Bureau Drive, Stop 8555, Gaithersburg, Maryland for developing new solders since a 1991), pp. 255–257. 20899; e-mail [email protected].

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