Zn Solder Research Articles

Effects of coupled stressing and solid-state aging on the mechanical properties of Sn–58Bi–0.7Zn solder joint

The mechanical properties of coupled and aged Sn–58Bi–0.7Zn solder joint were respectively investigated in this study. During tensile tests, the ultimate tensile strength of electromigration and thermal aging coupled solder joint was ascribed to the weakening of the anode interface between intermetallic compounds and Bi layer. It was due to the Bi segregation which led to the formation of vacancies and stress concentration at the Cu–Sn–Zn/Cu–Zn interface in the aged solder joint. Nanoindentation results revealed that the creep resistance of coupled solder joint was higher than that of the aged solder joint, which was because the Bi layer could play a role of load transfer at the solder joint interface in coupled sample.

Improved drop reliability of Sn–Ag–Cu solder joints by Zn addition to a Cu wetting layer

Cu–20 wt% Zn solder wetting layers were fabricated by electroplating in cyanide and non-cyanide solutions, and board-level drop impact reliability was evaluated for Sn–Ag–Cu (SAC) solder joints formed on Cu–Zn layers. Adding Zn to the Cu wetting layer suppressed the formation of a large Ag3Sn plate in the solder and delayed interfacial intermetallic compound (IMC) growth in the solder/Cu–Zn interfaces. Formation of Cu3Sn IMC and microvoids was not observed in the Cu6Sn5/Cu–Zn interface during thermal aging. The drop impact reliability of the Cu–Zn/SAC/Cu–Zn solder joints was superior to that of Cu/SAC/Cu solder joints, both before and after aging. The drop impact performance of the Cu–Zn specimens was similar whether their Cu–Zn layers were electroplated in cyanide or non-cyanide solution. Drop impact reliability of both Cu and Cu–Zn specimens decreased after aging due to IMC growth and/or microvoid formation in the solder interfaces. Interfacial microstructure influenced the failure mode of the solder joints. For Cu specimens, the dominant failure sites were a Cu6Sn5/Cu interface before aging and a Cu3Sn/Cu interface after aging. The formation of a large Ag3Sn plate facilitated crack growth. For Cu–Zn specimens, the crack always propagated inside the Cu6Sn5 layer. Microstructural changes in the solder interfaces arising from the addition of Zn contributed to the improvement of drop impact reliability of SAC solder joints.

The effect of undercooling on the microstructure and tensile properties of hypoeutectic Sn–6.5Zn–xCu Pb-free solders

Abstract Hypoeutectic Sn–6.5Zn alloy may be regarded as a better choice than eutectic Sn–9Zn lead-free solder in microelectronics industry. In this study, the properties of hypoeutectic Sn–6.5Zn lead-free solder were modified with minor additions of Cu. SEM investigations reveal that the plain Sn–6.5Zn solder exhibits large number of undesirable acicular structure of angular needle-like Zn particles at the solder matrix. The acicular-shape morphology of Zn was remarkably suppressed after Cu modification. Moreover, a new type of small flower-like γ-Cu5Zn8 intermetallic compound (IMC) was detected with 0.5 wt.% Cu added specimens. The flower-like morphology of γ-Cu5Zn8 IMC appears to cause a sharp increase in Young’s modulus, yield strength (YS) and ultimate tensile strength (UTS) of Cu modified solder. However, this effectiveness is reduced when 1.5 wt.% Cu addition starts to enhance the growth of coarse dendrite morphology of γ-Cu5Zn8 phase with enlarged β-Sn matrix. In addition, a 1.5 wt.% Cu addition was found to induce undesirable effects on the degree of undercooling, melting temperature and pasty range. Constitutive Garofalo model was assembled based on the experimental data of Sn–6.5Zn lead-free solders.

Ti addition to enhance corrosion resistance of Sn–Zn solder alloy by tailoring microstructure

The effect of trace addition of Ti on the corrosion behavior of Sn–9Zn (wt.%) solder alloy in NaCl solution was investigated using polarization and electrochemical impedance spectroscopy techniques. It is found that the corrosion resistance of Sn–9Zn alloy can be significantly enhanced by adding 0.05wt.% of Ti, evidenced by much lower corrosion current density, lower passive current density and higher impedance. Such enhancement results from the refinement of Zn-rich precipitates within the microstructure, which is conducive to forming a relatively more protective passive film on the surface of the modified alloy. This would be an important finding in the design of novel Sn–Zn solder alloys in electronic assemblies operating under aggressive conditions.

Effects of Zn addition on mechanical properties of eutectic Sn–58Bi solder during liquid-state aging

Interfacial reaction, tensile strength and creep resistance of Sn–58Bi–xZn (x=0, 0.7, mass fraction, %) solder samples during liquid-state aging were investigated. The coarsening of Bi and the growth of Cu–Sn intermetallic compounds (IMCs) in Sn–58Bi–0.7Zn solder sample were both effectively suppressed. With the addition of 0.7% Zn, ultimate tensile strengths (UTSs) of the Sn–58Bi solder slabs were respectively increased by 6.05% and 5.50% after reflow soldering and liquid-state aging, and those of the Cu/Sn–58Bi/Cu solder joints were also increased by 21.51% and 29.27%, respectively. The increase in strengthening effect of Cu/Sn–58Bi–xZn/Cu solder joints could be attributed to the fracture surface which was changed from the Cu/IMC interface to the IMC/solder interface due to the finer Bi grain. Nanoindentation results revealed that the creep behavior of Sn–58Bi–0.7Zn solder was significantly improved compared with that of the eutectic Sn–58Bi solder after reflow soldering and liquid-state aging.

Solid-state reactions between Sn-20.0 wt.%In-x wt.%Zn solders and Ag and Ni substrates

The Sn-20 wt.%In (Sn–20In) alloy is a promising base material for low-temperature Pb-free solders. Zn is usually added in solders to reduce the extent of undercooling during reflow, while Ag and Ni are commonly seen under bump metallurgy in contact with solder in electronic products. In this study, solid-state reactions at 150 °C between Zn-doped Sn–20In solders and Ag and Ni substrates are investigated. In Sn–20In–xZn/Ag couples, when the Zn-doping level is low (x ≤ 1.0), the reaction path is γ-InSn4/ζ-AgZn/ζ-(Ag,In)/ζ-AgZn/Ag, that the ζ-AgZn layer near the solder has non-uniform composition, the ζ-(Ag,In) layer has a porous microstructure, and the ζ-AgZn phase near the substrate is composed of small grains. When the Zn doping level is high (x ≥ 2.0), the reaction path becomes γ-InSn4/ε-AgZn3/γ-Ag5Zn8/ζ-AgZn/Ag, that all intermetallic compounds (IMCs) are planar and neither Sn nor In participate in the reactions. Although the reactions are very sensitive to Zn contents, the overall thicknesses of IMCs do not vary much with different Zn-doping levels. In Sn–20In–xZn/Ni couples, the planar Ni5Zn21 phase is the only reaction product with a very slow growth rate. The interfacial liquation in Sn–20In/Ni contacts can be fully mitigated with a minor Zn addition of 0.5 wt.%.

The role of Zn precipitates and Cl− anions in pitting corrosion of Sn–Zn solder alloys

Electrochemical corrosion behavior of Sn–Zn solder alloys in aerated and quiescent 0.5M NaCl solution was investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques, aiming to assess the role of Zn precipitates and Cl− anions in pitting corrosion. The results reveal that increasing in Zn contents leads to increasing in pitting susceptibility, ascribed to the coarsening Zn precipitates with larger intergranular boundaries between Zn-rich precipitates and Sn matrix. The EIS results provide a quantitative description of the electrochemical interface behavior. The mechanism of pitting and the role of Cl− anions are discussed.

Evaluation of cooling rate on electrochemical behavior of Sn–0.3Ag–0.9Zn solder alloy in 3.5 wt% NaCl solution

The corrosion resistance of Sn–0.3Ag–0.9Zn alloy solidified under different cooling rates in 3.5 wt% NaCl solution was evaluated based on potentiodynamic polarization, electrochemical impedance spectroscopy and electrochemical noise. It is found that the corrosion resistance of this alloy improves with the increase of the applied cooling rate, which is attributed to the distinct microstructure. The furnace-cooled and air-cooled alloy with active Zn-rich phase and a microstructure formed by a coarser dendritic array associated with larger size of AgZn3 intermetallic compounds exhibit a worse corrosion resistance due to a serious galvanic corrosion. Besides, the Mott–Schottky measurement and the X-ray photoelectron spectroscopy were performed to analyze the semiconductor properties and composition of the passive film formed on the surface of the alloy. It confirms that the stability and protective ability of the passive films formed on furnace-cooled and air-cooled alloy are worse than that of water-cooled alloy due to their higher oxygen vacancy defects and concentration of unstable SnO. Furthermore, the major corrosion product on the surface of the Sn–0.3Ag–0.9Zn alloys is tin oxide chloride hydroxide.

IMC Growth at the Interface of Sn–2.0Ag–2.5Zn Solder Joints with Cu, Ni, and Ni–W Substrates

Growth of intermetallic compounds (IMC) at the interface of Sn–2.0Ag–2.5Zn solder joints with Cu, Ni, and Ni–W substrates have been investigated. For the Cu substrate, a Cu5Zn8 IMC layer with Ag3Sn particles on top was observed at the interface; this acted as a barrier layer preventing further growth of Cu–Sn IMC. For the Ni substrate, a thin Ni3Sn4 film was observed between the solder and the Ni layer; the thickness of the film increased slowly and steadily with aging. For the Ni–W substrate, a thin Ni3Sn4 film was observed between the solder and Ni–W layer. During the aging process a thin layer of the Ni–W substrate was transformed into a bright layer, and the thickness of bright layer increased with aging.

Improving the shear strength of Sn–Ag–Cu–Ni/Cu–Zn solder joints via modifying the microstructure and phase stability of Cu–Sn intermetallic compounds

This study focuses on the correlation between high-speed impact tests and the interfacial reaction in Sn-3.0Ag-0.5Cu-0.1Ni/Cu (wt%) and Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints. Adding Ni into the Sn–Ag–Cu solder alters the interfacial morphology from scallop type to layer type and exhibits high shear strength after reflow in both solder joints. However, the shear strength of Sn-3.0Ag-0.5Cu-0.1Ni/Cu solder joints degrades significantly after thermal aging at 150 °C for 500 h. It is notable that Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints still present higher shear strength after aging at 150 °C. The weakened shear strength in Sn-3.0Ag-0.5Cu-0.1Ni/Cu solder joints is due to stress accumulation in the interfacial (Cu,Ni)6Sn5 compound induced by the phase transformation from a high-temperature hexagonal structure (η-Cu6Sn5) to a low-temperature monoclinic structure (η'-Cu6Sn5). However, doping small amounts of Zn into (Cu,Ni)6(Sn,Zn)5 can inhibit the phase transformation during thermal aging and maintain strong shear strength. These experiments demonstrate that Sn-3.0Ag-0.5Cu-0.1Ni/Cu-15Zn solder joints can act as a stable connection in the micro-electronic packaging of most electronic products at their average working temperatures.

Effect of polarizations on Sn–Zn solders alloys in alkaline electrolyte

The corrosion properties of Sn–9Zn lead-free solder was studied through potentiodynamic polarization in 6M potassium hydroxide at different scanning rates. The corrosion rates of Sn–9Zn solder remained nearly constant at 2.1×10−4Acm−2 from scan rates of 2.0mVs−1 to 3.5mVs−1 before significantly increasing as the rates increased to higher than 3.5mVs−1. The morphological analysis showed that two distinctive structures of a lamellar-like groove and irregularly shaped corrosion product compounds co-existed on the solder surface after being polarized at different rates. In addition, phase and elemental analyses revealed the formation of mixed corrosion products of SnO, SnO2, and ZnO on the surface after polarization. Moreover, the ultimate tensile stress endured by the solder joint decreased by 26% after polarization.

Effect of doping Al on the liquid oxidation of Sn–Bi–Zn solder

The liquid oxidation behaviors of Sn–40Bi–2Zn and Sn–40Bi–2Zn–0.005Al solders were investigated from thermal dynamics and kinetics analysis. The characteristics of surface oxidation film at 170 °C were studied by thermo gravimetric analysis and X-ray photoelectron spectroscopy (XPS). Sn–40Bi–2Zn solder performed inferiorly in oxidation prevention performance, due to the formation of ZnO, which exhibits lower Gibbs free energy of formation and higher growth rate. Trace amount of Al addition, however, alleviated the oxidation behavior of Zn. XPS depth profile results indicated that the surface layer of Sn–40Bi–2Zn–0.005Al consisted of oxides of Al and Zn formed on the outer surface of the solder film and in the subsequent layer, mainly formed by the oxides of Sn, Bi. Al, basically formed as Al2O3, segregated towards the outer surface, seemed to deter the Zn oxidation on the solder surface.

Effects of zinc on the interfacial reactions of tin–indium solder joints with copper

Sn-20.0 wt%In (Sn-20.0In) alloy is a promising base material in Pb-free solders for low-temperature applications. Zn is often used as an additive to Pb-free solders to reduce the extent of undercooling during reflow. Cu is the most commonly used substrate in electronics industry. Interfacial stability at Sn–In–Zn/Cu joints is crucial to reliability of electronic products. In this study, interfacial reactions between Sn-20.0 wt%In-x wt%Zn (Sn-20.0In-xZn) solders and Cu where x = 0.5, 0.7, 1.0, 2.0, 3.0, and 5.0 at 150, 230, and 260 °C were experimentally examined. It is found that the reaction phase formation and interfacial morphologies are strongly influenced by Zn concentrations. The reaction phases evolve from the Cu6Sn5 phase, CuZn and Cu5Zn8 phase, to Cu5Zn8 phase with higher Zn doping in the solders. The Cu5Zn8 phase acted as a diffusion barrier and suppressed the growth of the Cu6Sn5 phase. The results indicate that 2.0 wt%Zn addition resulted in the gentlest reactions during both soldering and solid-state ageing in Sn-20.0In-xZn/Cu couples.

Design of lead-free candidate alloys for low-temperature soldering applications based on the hypoeutectic Sn–6.5Zn alloy

The coupling effect of both minor alloying addition and reducing the amount of Zn phase have been proposed as an important strategy to improve the integrity and reliability of eutectic Sn–9Zn solder joints. In this work, the changes in microstructures, thermal behaviors and mechanical properties associated with the alloying of Ni and Sb to eutectic Sn–Zn after reducing the amount of Zn phase were explored. Thermal analysis confirmed that Ni and Sb additions being effective in reducing the amount of undercooling, while the melting temperature and pasty range remained at the hypoeutectic Sn–6.5Zn level. The resulting ultimate (UTS), yield tensile strength (YS) and elongation (El) of Sn–6.5Zn–0.5Ni and Sn–6.5Zn–0.5Sb alloys were experimentally determined and compared with the corresponding results of plain Sn–6.5Zn solder alloy. It was found that the Sn–6.5Zn–0.5Ni and Sn–6.5Zn–0.5Sb alloys examined comply with the compromise between high mechanical strength and ductility. Microstructural analysis revealed that the origin of change in mechanical properties was attributed to the enhanced solid solution effect of Sb and the flower shaped (Ni, Zn)3Sn4 intermetallics (IMC) phase produced by Ni addition. The Sn–6.5Zn–0.5Sb alloy has the highest UTS and appropriate ductility of all alloys examined. This finding indicates the capability of newly developed ternary solder alloys to serve a much wider array of value-added applications.

Creep deformation behavior of Sn–Zn solder alloys

Creep properties of three Sn–Zn solder alloys (Sn–9Zn, Sn–20Zn, and Sn–25Zn, wt%) were studied using the impression creep technique. Microstructural characteristics were examined using a scanning electron microscope. The alloys exhibited stress exponents of about 5.0. The activation energy for creep was calculated to be ~50–75 kJ/mol with a mean value of 66.3 kJ/mol. The likely creep mechanism was identified to be the low temperature viscous glide of dislocations.

Improving the impact toughness of Sn–Ag–Cu/Cu–Zn Pb-free solder joints under high speed shear testing

Abstract This study aims to investigate the interfacial reaction and the impact reliability of Sn–3.0Ag–0.5Cu/Cu–15Zn (wt.%; SAC/Cu–Zn) solder joints. The Cu–Zn under bump metallization (UBM) was developed to replace the traditional Cu UBM in the Pb-free solder joints. In the Sn–Ag–Cu/Cu solder joint, thick Cu 6 Sn 5 and Cu 3 Sn intermetallic compounds (IMCs) formed at the interface after thermal aging at 150 °C for 1000 h. In contrast, the SAC/Cu–Zn solder joint shows a thinner Cu 6 (Sn,Zn) 5 layer, and the growth of Cu 3 Sn IMC was absolutely suppressed at the SAC/Cu–Zn interface. To evaluate the bonding energy, the high speed shear testing was carried out to correlate the microstructure of solder joints. It is noteworthy that the SAC/Cu–Zn solder joint can enhance the impact toughness. The higher impact toughness of SAC/Cu–Zn is attributed to the thinner and tougher Cu 6 (Sn,Zn) 5 IMC at the joint interface. Accordingly, Cu–Zn alloy is a potential UBM material for Pb-free solder joints to improve the impact reliability.

Retarding intermetallic compounds growth of Zn high-temperature solder and Cu substrate by trace element addition

Since the thickness of layers of intermetallic compounds (IMCs) at the joining interface has an apparent effect on the joint reliability, understanding the growth mechanisms of IMCs is important for die-attachment applications. In the present study, the interfacial reaction between elements (Ca, Mn, Cr, and Ti) added to Zn alloys and Cu substrate were investigated in detail, focusing on IMCs grown by isothermal aging at 150 °C. The reaction layers included two types of Cu–Zn IMCs i.e., γ-Cu5Zn8 and e-CuZn5 phases. The joining interface with trace elements containing Zn solder reduced the growth rate of the IMCs. In particular, the addition of 0.1 wt% Cr to pure Zn solder decreased the growth rate of the IMCs by approximately 50 %. The mechanism we propose assumes that a phase containing the small Cr atoms exists between the solder and e-CuZn5, which suppresses the diffusion of metal atoms. To summarize, the Cr additive showed beneficial effects in terms of suppressing the growth of IMCs during the solid-state isothermal aging.

Electronics Assembly and High Temperature Reliability Using Sn-3.8Ag-0.7Cu Solder Paste With Zn Additives

In this paper, we report a comparison of interfacial reactions of Sn-3.8Ag-0.7Cu (SAC 387) and SAC (0-1.5 Zn) solder pastes on Cu (organic solderability preservative finish) and Au/Ni-P/Cu [electroless Ni immersion gold (ENIG)] substrate metallizations with Ni/Sn and Cu/Sn plated component leads. Zn added to the paste in the form of surface-coated micrometer-sized particles dissolves into the solder during reflow. High-temperature aging (150 °C and 185 °C), thermal cycling experiments ( -20°C to 175 °C for FR4 substrate, -40°C to 185 °C for ENIG polyimide substrate), and shear testing of the solder joints were carried out. At a Cu interface, adding Zn to the solder joint improves the shear strength and suppresses Cu3Sn and overall interfacial intermetallic compound (IMC) and Kirkendall void formation . However, above this temperature, the presence of Zn accelerates IMC growth. At a Ni interface, IMC suppression with Zn was noted at all temperatures. The amount of IMC suppression depends on the Zn concentration in the IMCs, which in turn depends on the geometry of joint as well as the original concentration of Zn in the solder.

Solidification of Sn-0.7Cu-0.15Zn Solder: In Situ Observation

High resolution time-resolved X-ray imaging with synchrotron radiation was used for in situ observation of four distinct events during solidification of a Sn-0.7Cu-0.15Zn solder despite small composition and density differences. These included βSn dendrite growth, Sn-Cu6Sn5 univariant eutectic growth, microporosity formation, and a polyphase reaction in the last stages of freezing. The development of microstructure was described quantitatively by tracking the loci of dendrite tips during grain growth. The results have implications for microstructure control and the understanding of structure–property relationships in Sn-Cu-Zn lead-free solders.

Enhancing mechanical response of hypoeutectic Sn–6.5Zn solder alloy using Ni and Sb additions

In this paper, we report a convenient and informative procedure for improving the integrity and reliability of eutectic Sn–9Zn solder using minor alloying additions of Ni and Sb after reducing the amount of Zn phase. Results indicate that alloying of Ni or Sb into hypoeutectic Sn–6.5Zn resulted in slightly reduced the undercooling and pasty range with maintaining its melting temperature at the Sn–9Zn level. After 0.5% Sb addition, the enhanced solid solution effect and enlarged eutectic area with small needle-like α-Zn phase raised the creep resistance of hypoeutectic Sn–6.5Zn alloy, while the flower shaped (Ni,Zn)3Sn4 intermetallics (IMCs) and small spacing needle-like α-Zn phase produced by 0.5% Ni alloying results in small increase in its creep resistance. These results show that the creep strain rate of the entire alloys follows Garafalo hyperbolic sine equation, and the calculated creep stress exponent for three solders is reasonably close to other published data. Moreover, with the addition of Sb and Ni into Sn–6.5Zn solder, significant improvement in creep resistance of ∼270% and ∼182% is realized, respectively when compared with the hypoeutectic Sn–6.5Zn solder. This finding indicates the capability of newly developed ternary solder alloys to serve a much wider array of electronic applications.