Progress in the design of new lead-free solder alloys

Various alloy design approaches have been employed to develop new lead-free solder alloys that can not only substitute for the lead-tin solders, but also offer significantly improved mechanical properties. Three new alloys are described in this article. In Sn-3.5Ag-1Zn (melting point ~217°C), the solidification structure and the eutectic precipitate morphology are6 refined by the addition of zinc. As a result, a high-strength, high-ductility solder with significantly improved creep resistance is obtained. In Bi-43Sn+2.5Fe, a eutectic alloy (melting point ~137°C), dispersion hardening by magnetically distributed iron particles retards both high-temperature deformation and microstructural coarsening, thus widening the useful service range of Bi-Sn eutectic alloys to much higher homologous temperatures than are typical for the Sn-Pb eutectic alloy. Lastly, Sn-Zn-In based alloys (melting point ~185°C) have been developed for consideration as a drop-in replacement for the neareutectic Sn-Pb alloy(melting point ~183°C).     

Creep properties of Sn-Ag solder joints containing intermetallic particles

The creep behavior of the eutectic tin-silver joints and tin-silver composite solder joints containing 20 vol.% of Cu₆Sn₅, Ni₃Sn₄, and FeSn₂ intermetallic reinforcements introduced by in-situ methods was investigated. These creep tests were carried out using single shear lap solder joints at room temperature, 85°C, and 125°C. The creep resistance was similar in magnitude for all alloys, and with increasing temperature, the stressexponents decreased in a manner consistent with power-law breakdown behavior. The FeSn₂ intermetallic reinforced composite solder was found to be the most creep-resistant alloy at room temperature. Creep failure was observed to occur within the solder matrix in all these solder joints. Although a detailed analysis of the processes involved was difficult because of smearing of the features in the fracture surface, there were indications of grain-boundary separation, ductile fracture, and interfacial separation. For more information, contact K.N. Subramanian, Michigan State University, Department of Materials Science and Mechanics, 3536 Engineering Building, East Lansing, Michigan 48824-1226; (517) 353-5397; fax (517) 353-9842; e-mail subraman@egr.msu.edu.     

Wetting of Cu and Al by Sn-Zn and Zn-Al Eutectic Alloys

Wetting properties of Sn-Zn and Zn-Al alloys on Cu and Al substrates were studied. Spreading tests were carried out for 3 min, in air and under protective atmosphere of nitrogen, with the use of fluxes. In the case of Zn-Al eutectic, spreading tests were carried out at 460, 480, 500, and 520 °C, and in the case of Sn-Zn eutectic at 250, 300, 350, 400, 450, and 500 °C, respectively. Solidified solder/substrate couples were cross-sectioned and subjected to microstructure examination. The spreading tests indicated that the wetting properties of eutectic Sn-Zn alloys, on copper pads do not depend on temperature (up to 400 °C), but in the lack of protective atmosphere, the solder does not wet the pads. Wettability studies of Zn-Al eutectic on aluminum and copper substrates have shown a negative effect of the protective nitrogen atmosphere on the wetting properties, especially for the copper pads. Furthermore, it was noted that with increasing temperature the solder wettability is improved. In addition, densities of liquid solders were studied by means of dilatometric technique.     

Nanomechanical characterization of Sn–Ag–Cu/Cu joints—Part 1: Young’s modulus,hardness and deformation mechanisms as a function of temperature

Nanoindentation has been used at room and elevated temperature to measure the spatial distribution of mechanical properties within Pb-free solder ball joints. The hardness, Young’s modulus and creep behaviour of the phases formed in a Sn–Ag–Cu/Cu solder joint have been characterized at temperatures from 25 to 175 °C. The hardness and Young’s modulus of Cu₆Sn₅ and Cu₃Sn had a weak dependence on temperature, while the hardness and modulus of primary β-Sn, eutectic regions and electroplated Cu were sensitive to temperature. Nanoindentation was able to detect both mechanical anisotropy in Cu₆Sn₅ and the sluggish η′ → η phase transformation at 150–175 °C. In a second part of this study, Pb-free solder creep behaviour has been investigated by nanoindentation and compared with Pb-free creep behaviour models from the literature. A two-dimensional finite-element analysis of solder nanoindentation creep has been used to compare creep measurement methods, and the insights from this analysis can be then implemented in commercial finite-element codes for creep behaviour prediction at the microscale in microelectronic solder joints.     

The creep properties of lead-free solder joints

This paper describes the creep behavior of three tin-rich solders that have become candidates for use in lead-free solder joints: Sn-3.5Ag, Sn-3Ag-0.5Cu, and Sn-0.7Cu. The three solders show the same general behavior when tested in thin joints between copper and Ni/Au metallized pads at temperatures between 60‡C and 130°C. Their steady-state creep rates are separated into two regimes with different stress exponents. The low-stress exponents range from ∼3–6, while the high-stress exponents are anomalously high (7–12). Strikingly, the high-stress exponent has a strong temperature dependence near room temperature, increasing significantly as the temperature drops from 95°C to 60°C. The anomalous creep behavior of the solders appears to be due to the dominant tin constituent. Research on creep in bulk samples of pure tin suggests that the anomalous temperature dependence of the stress exponent may show a change in the dominant mechanism of creep. Whatever its source, it has the consequence that conventional constitutive relations for steady-state creep must be used with caution in treating tin-rich solder joints, and qualification tests that are intended to verify performance should be carefully designed.     

Understanding the Influence of Copper Nanoparticles on Thermal Characteristics and Microstructural Development of a Tin-Silver Solder

This paper presents and discusses issues relevant to solidification of a chosen lead-free solder, the eutectic Sn-3.5%Ag, and its composite counterparts. Direct temperature recordings for the no-clean solder paste during the simulated reflow process revealed a significant amount of undercooling to occur prior to the initiation of solidification of the eutectic Sn-3.5%Ag solder, which is 6.5 °C, and for the composite counterparts, it is dependent on the percentage of copper nanopowder. Temperature recordings revealed the same temperature level of 221 °C for both melting (from solid to liquid) and final solidification (after recalescence) of the Sn-3.5%Ag solder. Addition of copper nanoparticles was observed to have no appreciable influence on melting temperature of the composite solder. However, it does influence solidification of the composite solder. The addition of 0.5 wt.% copper nanoparticles lowered the solidification temperature to 219.5 °C, while addition of 1.0 wt.% copper nanoparticles lowered the solidification temperature to 217.5 °C, which is close to the melting point of the ternary eutectic Sn-Ag-Cu solder alloy, Sn-3.7Ag-0.9Cu. This indicates the copper nanoparticles are completely dissolved in the eutectic Sn-3.5%Ag solder and precipitate as the Cu₆Sn₅, which reinforces the eutectic solder. Optical microscopy observations revealed the addition of 1.0 wt.% of copper nanoparticles to the Sn-3.5%Ag solder results in the formation and presence of the intermetallic compound Cu₆Sn_5. These particles are polygonal in morphology and dispersed randomly through the solder matrix. Addition of microsized copper particles cannot completely dissolve in the eutectic solder and projects a sunflower morphology with the solid copper particle surrounded by the Cu₆Sn₅ intermetallic compound coupled with residual porosity present in the solder sample. Microhardness measurements revealed the addition of copper nanopowder to the eutectic Sn-3.5%Ag solder resulted in higher hardness.     

Creep behavior of nonburning Ti-35V-15Cr-<Emphasis Type="Italic">x</Emphasis>C alloys

A comparison is made between the creep behavior of the Ti-35V-15Cr and Ti-35V-15Cr-0.2C alloys at 500 to 580 °C within the stress range of 200 to 300 MPa. The creep resistance of Ti-35V-15Cr-0.2C is considerably improved by the incorporation of Ti₂C particulates into the Ti-35V-15Cr-0.2C matrix. This paper was presented at the Beta Titanium Alloys of the 00’s Symposium sponsored by the Titanium Committee of TMS, held during the 2005 TMS Annual Meeting & Exhibition, February 13–16, 2005 in San Francisco, CA.     

Effect of Ca addition on the oxidation resistance of AZ91 magnesium alloys at elevated temperatures

AZ91 magnesium alloys containing 0.27–5.22 wt.% Ca, were melted and cast to study the effects of Ca addition on oxidation resistance at elevated temperatures. An ignition temperature test showed that the ignition of AZ91 alloy occurred at about 350–450 °C below the melting point, whereas that of the Ca-containing AZ91 alloys did so at above 650 °C. Weight gain measurements indicated that the oxidation resistance of the AZ91 alloys improved with Ca addition. The oxidation rate was dependent on the oxidation temperature. In the temperature range of 300–400 °C, the oxidation rate increased linearly. By contrast, the weight of 5 wt.% Ca-containing AZ91 alloy increased slowly due to the formation of a protective oxide layer. The oxidized surfaces were analyzed with low-angle XRD, FE-SEM equipped with EDS and AES. Complex structures were found in the oxide layers of the Ca-containing alloys: the outer layer mainly consisted of CaO, which was of uniform thickness, and the inner layer was a mixture of CaO, MgO, and Al₂O₃. In contrast to the loose and porous MgO formed on the surface of AZ91, the compact and dense oxide layers acted as an effective barrier to the further oxidation of the Ca-containing AZ91 alloys.     

Pb-Free solders: Part 1. Wettability testing of Sn−Ag−Cu alloys with Bi additions

Maximum bubble pressure, dilatometric, and meniscographic methods were used in the investigations of the surface tension, density, wetting time, wetting force, contact angle, and interfacial tension of liquid alloys of Sn−Ag−Cu eutectic composition with various additions of Bi. Density and surface tension measurements were conducted in the temperature range 250–900 °C. Surface tensions at 250 °C measured under a protective atmosphere of Ar−H₂ were combined with data from meniscographic studies done under air or with a protective flux. The meniscographic data with a nonwetted teflon substrate provided data on interfacial tension (solder-flux), surface tension in air, and meniscographic data with a Cu substrate allowed determinations of wetting time, wetting force, and calculation of contact angle. The calculated wetting angles from meniscographic studies for binary Sn−Ag eutectic and two ternary Sn−Ag−Cu alloys were verified by separate measurements by the sessile drop method under a protective atmosphere with a Cu substrate. Additions of Bi to both ternary alloys improve the wettability and move the parameters somewhat closer to those of traditional Sn−Pb solders.     

Relationship Between Mechanical Properties of Lead-Free Solders and Their Heat Treatment Parameters

The research was undertaken to establish mechanical properties of as-cast and heat-treated Sn-Zn-based alloys of binary and ternary systems as candidates for lead (Pb)-free solder materials for high-temperature applications. The heat treatment of as-cast alloys was made under different combinations of processing parameters (168 h/50 °C, 42 h/80 °C, and 24 h/110 °C). The systematic study of structure-property relationships in Sn-Zn, Sn-Zn-Ag, and Sn-Zn-Cu alloys containing the same amount of Zn (4.5, 9, 13.5 wt.%) and 1 wt.% of either Ag or Cu was conducted to identify the effects of chemical composition and heat treatment processing parameters on the alloy microstructure and mechanical behavior. Structural characterization was made using optical microscopy and scanning electron microscopy techniques coupled with EDS analysis. Mechanical properties (initial Young’s modulus E, ultimate tensile strength UTS, elastic limit R _0.05, yield point R _0.2, elongation A ₅, and necking Z) were determined by means of static tensile tests. All the examined Sn-Zn-based alloys have attractive combination of mechanical characteristics, especially tensile strength, having values higher than that of common leaded solders and their substitutes of Pb-free SAC family. The results obtained demonstrate that the Sn-Zn-based alloys present competitive Pb-free solder candidates for high-temperature applications.     

Effect of thermal exposure on microstructure and mechanical properties of Al−Si−Cu−Ni−Mg alloy produced by different casting technologies

The effect of thermal exposure at 350 °C for 200 h on microstructure and mechanical properties was investigated for Al−Si−Cu−Ni−Mg alloy, which was produced by permanent mold casting (PMC) and high pressure die casting (HPDC). The SEM and IPP software were used to characterize the morphology of Si phase in the studied alloys. The results show that the thermal exposure provokes spheroidization and coarsening of eutectic Si particles. The ultimate tensile strength of the HPDC alloy after thermal exposure is higher than that of the PMC alloy at room temperature. However, the TEPMC and TEHPDC alloys have similar tensile strength around 67 MPa at 350 °C. Due to the coarsening of eutectic Si, the TEPMC alloy exhibits better creep resistance than the TEHPDC alloy under studied creep conditions. Therefore, the alloys with small size of eutectic Si are not suitably used at 350 °C.     

Reinvestigation of Ni-solid solution/liquid equilibria in Ni-Al binary and Ni-Al-Zr ternary systems

The present work reports the results of a reinvestigation of the γ liquidus and solidus temperatures of the Ni-Al system and the γ/Ni₅Zr eutectic reaction temperatures in the Ni-Al-Zr system. In the Ni-Al binary system, it is found that the stability of the Ni-rich γ solid solution phase is enhanced by small additions of Al with the melting temperature of the γ phase and that the melting temperature reaches a congruent maximum at a few at.% Al. The temperature of the eutectic reaction L→γ+Ni₅Zr in the Ni-Zr binary edge is confirmed to be 1196 °C by differential thermal analysis (DTA), rather higher than the value reported previously. The reaction temperature increases with Al addition to reach or exceed 1206 °C, forming a “saddle point,” then decreasing to reach 1187 °C or below by flowing into a ternary invariant reaction. These findings can be explained by γ/liquid equilibrium in the Ni-Al binary system.     

Reinvestigation of Ni-solid solution/liquid equilibria in Ni-Al binary and Ni-Al-Zr ternary systems

The present work reports the results of a reinvestigation of the γ liquidus and solidus temperatures of the Ni-Al system and the γ/Ni₅Zr eutectic reaction temperatures in the Ni-Al-Zr system. In the Ni-Al binary system, it is found that the stability of the Ni-rich γ solid solution phase is enhanced by small additions of Al with the melting temperature of the γ phase and that the melting temperature reaches a congruent maximum at a few at.% Al. The temperature of the eutectic reaction L→γ+Ni₅Zr in the Ni-Zr binary edge is confirmed to be 1196 °C by differential thermal analysis (DTA), rather higher than the value reported previously. The reaction temperature increases with Al addition to reach or exceed 1206 °C, forming a “saddle point,” then decreasing to reach 1187 °C or below by flowing into a ternary invariant reaction. These findings can be explained by γ/liquid equilibrium in the Ni-Al binary system.     

Observations of nucleation catalysis effects during solidification of SnAgCuX solder joints

While modification of a strong (high Cu) Sn-Ag-Cu (SAC) solder alloy with a substitutional alloy addition (X=Co, Fe, Zn, and Ni) for Cu has been demonstrated to enhance solder joint strength and ductility after aging at 150°C for 1,000 h, control of the as-solidified SAC+X solder joint microstructure is also needed to inhibit under cooling and nucleation of brittle pro-eutectic phases (e.g., Ag₃Sn). Bulk undercooling measurements of SAC+X alloys and microstructural analysis of SAC+X solder joints were used to rank the effectiveness and consistency of low-level (X < 0.15 wt.%) substitutional additions to a base SAC composition, Sn-3.5Ag-0.95Cu (wt.%). This SAC composition was selected to favor thermodynamically the nucleation of pro-eutectic Cu₆Sn₅ over that of Ag₃Sn and the formation of an enhanced ternary eutectic fraction in the joint microstructure, while retaining a pasty range that is only 3°C. Using differential scanning calorimetry with sample pans that serve as either inert (aluminum) or actively wetting (copper) substrates, reflow cycles were studied that simulated surface mount (1.5°C/s) and ball-grid array (0.17°C/s) cooling rates. Of the SAC+X solders tested with copper pans, X = Zn appeared to be most effective and consistent, providing catalytic enhancement of the nucleation temperature for even the minimum concentration (0.05 wt.%) and lowest cooling rate.     

The development of alumina-forming austenitic stainless steels for high-temperature structural use

A new family of alumina-forming austenitic stainless steels is under development at Oak Ridge National Laboratory for structural use in aggressive oxidizing environments at 600–900°C. Data obtained to date indicate the potential to achieve superior oxidation resistance compared to conventional Cr₂O₃-forming iron-and nickel-based heat-resistant alloys, with creep strength comparable to state-of-the-art advanced austenitic stainless steels. A preliminary assessment also indicated that the newly developed alloys are amenable to welding. Details of the alloy design approach and composition-microstructure-property relationships are presented. Author’s Note: Part of this research summary is based on a recent review paper (see Reference 1) and findings first reported in References 2–7.     

Thermodynamic study of the phase equilibria in the Sn−Ti−Zn ternary system

The Sn−Ti−Zn ternary phase diagram has been constructed using the CALPHAD technique. The Ti−Zn binary system phase boundaries were determined using differential scanning calorimetry and the solid-liquid diffusion couples method. In addition, the formation energy of some stoichiometric compounds was obtained using first-principle band energy calculations. For the ternary system, some alloys were prepared by equilibration at 600 or 700 °C, and the compositions of the precipitates were analyzed using electron probe microanalysis. Thermodynamic assessment of the Ti−Zn and Sn−Ti−Zn systems was performed based on the experimental information and by adopting reported values of the thermodynamic properties of the Sn−Zn and Sn−Ti binary systems. Microstructural observation showed that Sn₃Ti₅Zn₁₂ exists in the ternary system. Seven types of invariant reaction on the Sn-rich liquidus surface of the ternary system are predicted by the phase diagram calculations. The ternary eutectic point falls at 0,0009 mass% Ti and 8.69 mass% Zn, at T=192.40°C, which is slightly lower than the calculated eutectic point of Sn−Zn binary alloy (T=192.41°C). Based on these results, a nonequilibrium solidification process using the Scheil model was simulated. This paper was presented at the International Symposium on User Aspects of Phase Diagrams, Materials Solutions Conference and Exposition, Columbus, Ohio, 18–20 October, 2004.     

Carburization resistance of nickel-base, heat-resisting alloys

Cast heat-resisting alloys containing 45 or 60 wt% nickel were isothermally carburized in flowing gas mixtures of H₂–5CH₄ (volume percent) at temperatures of 900–1,100 °C, and their performance compared with that of standard commercial grades containing 30–35% nickel. Chromium-rich M₇C₃ and M₂₃C₆ precipitated internally in all materials, to depths which increased according to parabolic kinetics. Comparison of the rate constants with those predicted from Wagner’s diffusion theory showed that carbon diffusion through a chromium-depleted alloy matrix was the rate-controlling process. The 45% nickel alloys carburized more slowly than the 30–35% nickel grades as a result of decreased carbon solubility and diffusivity at the higher nickel level. The 60% nickel alloys also contained aluminum. At higher temperatures, diffusion of aluminum to the surface led to A1₂O₃ scale formation and enhanced carburization resistance. The degree of protection obtained depended on alloy aluminum content. P. Becker—On secondment from Schmidt & Clemens GmbH, Post Fach 1140, 51779 Lindlar, Germany.     

Isothermal Oxidation Behavior of Ti-50Al Alloy with Y Additions at 800 and 900°C

The isothermal-oxidation behavior of Al-rich TiAl alloys containing Y up to 1.0 at.% was studied in synthetic air with a flow of 200–250 mL/min at 800 and 900°C. Oxidation kinetics and scale adherence were studied in terms of the morphological features and microstructural evolution of the oxide scale. In the specimens oxidized at 800°C, all alloys containing 0.3–1.0 at.%Y showed reduced mass gain compared to the Y-free alloy, especially for the 0.3 at.%Y alloy. Under isothermal exposure at 900°C, the addition of small amounts of Y (0.1 and 0.3 at.%) was effective in enhancing the oxidation resistance. The alloys with higher Y contents (0.6 and 1.0 at.%), on the contrary, had a reverse effect on the oxidation resistance by providing rapid diffusion paths in the form of coarse Y₂O₃ particles close to the substrate. The improvement of oxidation resistance of the alloy with Y additions was due partly to the improved adhesion of the scale and due partly to the formation of a continuous α-Al₂O₃ layer in the outer scale. Y segregation and/or Y₂O₃ precipitation at the oxide grain boundaries was effective in decreasing the oxidation rate and refining the oxide grains. The thinner scale was responsible for relaxing the thermal stress and, thus the cohesion between the scale and substrate was greatly improved in Y-containing alloys.     

Evaluation on the characteristics of tin-silver-bismuth solder

Tin-silver-bismuth solder is characterized by its lower melting point, good wetting behavior, and good mechanical property for which it is expected to be a new lead-free solder to replace tin-lead solder. In this article, Sn-3.33Ag-4.83Bi solder was investigated concerning its physical, spreading, and mechanical properties under specific conditions. Cooling curves and DSC results showed that it was close to eutectic composition (m.p. 210°–212 °C). Coefficiency of thermal expansion (CTE) of this solder, between that of PCBs and copper substrates, was beneficial to alleviate the thermal mismatch of the substrates. It was also a good electrical and thermal conductor. Using a rosin-based, mildly activated (RMA) flux, a spreading test indicated that SnAgBi solder paste had good solderability. Meanwhile, the solder had high tensile strength and fracture energy. Its fracture mechanism was a mixture of ductile and brittle fracture morphology. The metallographic and EDAX analyses indicated that it was composed of a tin-based solid solution and some intermetallic compound (IMC) that could strengthen the substrate. However, these large needle-like IMCs would cut the substrate and this resulted in the decreasing of the toughness of the solder.     

Wetting and interfacial reaction characteristics of Sn-1.2Ag-0.5Cu-xIn quaternary solder alloys

Through the use of a wetting balance technique, the wetting characteristics of Sn-1.2Ag-0.5Cu-xIn quaternary solder alloys with respect to the In content and soldering temperature were investigated to validate the applicability of compositions with a low Ag content as solder material. It was found that a small addition (0.4–0.6 wt.%) of In significantly improved the wetting properties of the Sn-1.2Ag-0.5Cu-xIn composition at soldering temperatures ranging from 230 °C to 240 °C due to the excellent wetting property of In. In an observation of the interfacial reaction, it was found that the added In element did not participate in the interfacial reaction with a Cu or Ni pad, unlike in the Sn-Ag-Cu-In case, which has a high In content. The package or boardside IMC layers in Sn-1.2Ag-0.5Cu-0.4In joints were thinner than those of Sn-3.0Ag-0.5Cu, especially after aging.