Investigations of interfacial reactions of Sn–Zn based and Sn–Ag–Cu lead-free solder alloys as replacement for Sn–Pb solder

The interfacial reactions of Sn–Zn based solders and a Sn–Ag–Cu solder have been compared with a eutectic Sn–Pb solder. During reflow soldering different types of intermetallic compounds (IMCs) are found at the interface. The morphologies of these IMCs are quite different for different solder compositions. As-reflowed, the growth rates of IMCs in the Sn–Zn based solder are higher than in the Sn–Ag–Cu and Sn–Pb solders. Different types of IMCs such as γ-Cu₅Zn₈, β-CuZn and a thin unknown Cu–Zn layer are formed in the Sn–Zn based solder but in the cases of Cu/Sn–Pb and Cu/Sn–Ag–Cu solder systems Cu₆Sn₅ IMC layers are formed at the interface. Cu₆Sn₅ and Cu₃Sn interfacial IMCs are formed in the early stages of 10 min reflow due to the limited supply of Sn from the Sn–Pb solder. The spalling of Cu–Sn IMCs is observed only in the Sn–Ag–Cu solder. The size of Zn platelets is increased with an increase of reflow time for the Cu/Sn–Zn solder system. In the case of the Sn–Zn–Bi solder, there is no significant increase in the Zn-rich phases with extended reflow time. Also, Bi offers significant effects on the wetting, the growth rate of IMCs as well as on the size and distribution of Zn-rich phases in the β-Sn matrix. No Cu–Sn IMCs are found in the Sn–Zn based solder during 20 min reflow. The consumption of Cu by the solders are ranked as Sn–Zn–Bi > Sn–Ag–Cu > Sn–Zn > Sn–Pb. Despite the higher Cu-consumption rate, Bi-containing solder may be a promising candidate for a lead-free solder in modern electronic packaging taking into account its lower soldering temperature and material costs.     

Single-wall carbon nanotube (SWCNT) functionalized Sn–Ag–Cu lead-free composite solders

Sn–3.8Ag–0.7Cu-based composite solders functionalized with single-wall carbon nanotubes (SWCNTs) with various weight proportions ranging from 0.01 to 1 wt% were successfully produced. The microstuctural, melting and mechanical properties of Sn–3.8Ag–0.7Cu-based composite solders were evaluated as a function of different wt% of SWCNT addition. The microstructures of the composite specimens were studied by means of field-emission scanning electron microscope (FE-SEM). It was observed that SWCNTs were homogeneously distributed at the edges of Ag₃Sn compounds that are distributed evenly in the β-Sn solder matrix. Energy dispersion X-ray (EDX) analysis method was employed to reveal the presence of the phases existed in the solder composites. The mechanical properties of the composite solders were evaluated by Vickers-microhardness measurements and tensile tests performed at room temperature. The different wt% and addition of SWCNTs to Sn–3.8Ag–0.7Cu produced a dramatic increase in tensile strength, hardness, and better melting characteristics. A slight decrease in elongation to failure was observed. FE-SEM observations of the fracture surface, revealed the overall failure mechanism as the ductile manner of failure.     

Phase transformation and morphology of the intermetallic compounds formed at the Sn–9Zn–3.5Ag/Cu interface in aging

The morphology and phase transformation of the intermetallic compounds (IMCs) formed at the Sn–9Zn–3.5Ag/Cu interface in a solid-state reaction have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), electron diffraction (ED), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The monoclinic η′-Cu₆Sn₅ transforms to the hexagonal η-Cu₆Sn₅ and the orthorhombic Cu₅Zn₈ transforms to the body-centered cubic (bcc) γ-Cu₅Zn₈ as aged at 180 °C. The scallop-shaped Cu₆Sn₅ layer is retained after aging at 180 °C for 1000 h. In the solid-state reaction, Ag is repelled from η′-Cu₆Sn₅ and reacts with Sn to form Ag₃Sn, and the Cu₅Zn₈ layer decomposes. Kirkendall voids are not observed at the Sn–9Zn–3.5Ag/Cu interface even after aging at 180 °C for 1000 h.     

Thermoelectric characteristics and tensile properties of Sn–9Zn–xAg lead-free solders

This study investigates the structural characteristics and tensile properties of the Sn–9Zn–xAg alloy under an electrical current test and oil bath treatment. Experimental results indicate that alternating current (ac) and direct current (dc) led to different values for the fusing electrical current value. Electromigration of direct current (dc) reduced the fusing electrical resistance. In addition, the electrical conductivity of the Sn–9Zn–xAg alloys deteriorated significantly by adding more 2 wt.% Ag. The Sn–9Zn–1Ag specimen underwent no transformation during oil–silicon heat-treatment, however its structure experienced electromigration under electrical current testing. After electrical current testing, the needle-like Zn-rich phases had decomposed, the Ag–Zn compounds had grown, the amount of Sn–Zn eutectic phases had increased, and the content of Sn-rich phase had decreased. Also, prolonging the duration of thermoelectric testing led to a deterioration in the tensile mechanical properties of the specimens. Raising the heat-treatment temperature enhanced the solid solution effect and raised the tensile strength of the as-cast specimens.     

钎料熔滴与焊盘界面反应及再重熔时的界面组织演变

采用熔滴直接凸点制作方法,对共晶SnPb及SnAgCu钎料熔滴与Au/Ni/Cu焊盘所形成的凸点/焊盘界面组织进行了研究,并与激光重熔条件下获得的凸点/焊盘界面组织进行了比较,考察了凸点/焊盘界面组织在随后的再重熔过程中的演变.结果表明:钎料熔滴与焊盘在接触过程中形成了Au-Sn化合物,Au层并未完全反应.在随后的再重熔过程中,Au层被完全消耗,全部溶入钎料基体中,Ni层与钎料发生反应.无铅钎料(SnAgCu)和SnPb钎料所形成的界面组织明显不同;再重熔后SnPb钎料/焊盘的界面组织为Ni3Sn1,SnAgCu钎料/焊盘界面组织为(CuxNi1-x)6Sn5.     

Effects of small addition of In on the structure of the rapidly cooled Sn–Ag–Zn solder

The effect of small additions of In, up to 1 wt.%, on the microstructure of the eutectic Sn–3.7 wt.%Ag–0.9 wt.%Zn solder was investigated. As observed by microstructural analysis, the increase of In content made β-Sn easy to form but suppressed the formation of the AgZn phase in the Sn–3.7Ag–0.9Zn solder. After annealing at 473 K for 20 and 50 h, the microstructure varied a lot in the morphology of the investigated Sn–Ag–Zn–In solder. The β-Sn dendrites grew coarser but dimmer accompanied with the segregation of the intermetallic compounds (IMCs) along their boundaries. Furthermore, the suppressed Ag–Zn IMCs formed in the Sn–3.7Ag–0.9Zn–1In solder. And the coarsening of the β-Sn dendrites and the growth of IMCs particles in the microstructure of the samples brought a significant softening during annealing of the investigated Sn–Ag–Zn–In alloys.     

Intermetallic compound growth suppression at high temperature in SAC solders with Zn addition on Cu and Ni-P substrates

The effect of adding 0.5-1.5 wt.% Zn to Sn-3.8Ag-0.7Cu (SAC) solder alloy during reflow and solid state ageing has been investigated. In particular, the role of the Zn addition in suppressing interfacial Intermetallic Compound (IMC) growth on Cu and Ni-P substrates has been determined. Solder-substrate couples were aged at 150 °C and 185 °C for 1000 h. In the case of 0.5-1.0 wt.% Zn on Cu substrate, Cu₃Sn IMC was significantly suppressed and the morphology of Cu₆Sn₅ grains was changed, leading to suppressed Cu₆Sn₅ growth. In the SAC-1.5Zn/Cu substrate system a Cu₅Zn₈ IMC layer nucleated at the interface followed by massive spalling of the layer into the solder, forming a barrier layer limiting Cu₆Sn₅ growth. On Ni-P substrates the (Cu,Ni)₆Sn₅ IMC growth rate was suppressed, the lowest growth rate being found in the SAC-1.5Zn/Ni-P system. In all cases the added Zn segregated to the interfacial IMCs so that Cu₆Sn₅ became (Cu,Zn)₆Sn₅ and (Cu,Ni)₆Sn₅ became (Ni,Cu,Zn)₆Sn₅. The effect of Zn concentration on undercooling, wetting angles and IMC composition changes during ageing are also tabulated, and a method of incorporating Zn into the solder during reflow without compromising solder paste reflow described.     

The 260 °C phase equilibria of the Sn–Sb–Ag ternary system and interfacial reactions at the Sn–Sb/Ag joints

The isothermal section of the Sn–Sb–Ag ternary system at 260 °C has been determined in this study by experimental examination. Experimental results show no existence of ternary compounds in the Sn–Sb–Ag system. Two extensive regions of mutual solubility have been determined. The one located between the two binary isomorphous phases, Ag₃Sn and Ag₃Sb, is labeled as and the other one located between the two binary isomorphous phases, Ag₄Sn and Ag₄Sb, is labeled as ξ. The phase is a very stable phase and is in equilibrium with ξ, Sb, SbSn, Sb₂Sn₃, and liquid Sn phases. Each of the Sb and SbSn phases has a limited solubility of Ag. Only one stoichiometric compound, Sb₂Sn₃, exists. Besides phase equilibria determination, the interfacial reactions between the Sn–Sb alloys and the Ag substrate were investigated at 260 °C. It was found that the phase formations in the Sn–Sb/Ag couples are very similar to those in the Sn/Ag couples.     

Kinetics of intermetallic phase formation at the interface of Sn-Ag-Cu-X (X = Bi, In) solders with Cu substrate

The effects of Bi and In additions on intermetallic phase formation in lead-free solder joints of Sn-3.7Ag-0.7Cu; Sn-1.0Ag-0.5Cu-1.0Bi and Sn-1.5Ag-0.7Cu-9.5In (composition given in weight %) with copper substrate are studied. Soldering of copper plate was conducted at 250 °C for 5 s. The joints were subsequently aged at temperatures of 130-170 °C for 2-16 days in a convection oven. The aged interfaces were analyzed by optical microscopy and energy dispersive X-ray spectroscopy (EDX) microanalysis. Two intermetallic layers are observed at the interface - Cu₃Sn and Cu₆Sn₅. Cu₆Sn₅ is formed during soldering. Cu₃Sn is formed during solid state ageing. Bi and In decrease the growth rate of Cu₃Sn since they appear to inhibit tin diffusion through the grain boundaries. Furthermore, indium was found to produce a new phase - Cu₆(Sn,In)₅ instead of Cu₆Sn₅, with a higher rate constant. The mechanism of the Cu₆(Sn,In)₅ layer growth is discussed and the conclusions for the optimal solder chemical composition are presented.     

Effect of surface finish on the fracture behavior of Sn–Ag–Cu solder joints during high-strain rate loading

This study assesses the reliability of eutectic Sn–Pb, Sn–1.0Ag–0.5Cu, Sn–3.0Ag–0.5Cu and Sn–4.0Ag–0.5Cu solder bumps on three different pad surface finishes (ENIG, electrolytic Ni/Au and Cu-OSP) with and without an aging treatment at 150 °C for 100 h. This study focused primarily on how the pad surface finish and solder alloy composition affects the reliability of solder joints using a high-speed ball pull test method. The fracture forces and failure mechanisms were also examined. The results showed that the electrolytic Ni/Au surface finish had the highest fracture forces for all four different solder alloys with and without the aging process.     

Concurrent nucleation, formation and growth of two intermetallic compounds (Cu6Sn5 and Cu3Sn) during the early stages of lead-free soldering

This study investigates the concurrent nucleation, formation and growth of two intermetallic compounds (IMCs), Cu₆Sn₅ (η) and Cu₃Sn (ε), during the early stages of soldering in the Cu-Sn system. The nucleation, formation and growth of the IMC layers is simulated through a multiphase-field model [1] and [2] in which the concurrent nucleation of both IMC phases is considered to be a stochastic Poisson process with nucleation rates calculated from classical nucleation theory [3]. CALPHAD thermodynamic models are used to calculate the local contributions to the free energy of the system and the driving forces for precipitation of the IMC phases. The nucleation parameters of the η phase are estimated from experimental results [4] and those of the ε phase are assumed to be similar. A parametric investigation of the effects of model parameters (e.g. grain boundary (GB) diffusion rates, interfacial and GB energies) on morphological evolution and IMC layer growth rate is presented and compared with previous works in which nucleation was ignored [5]. In addition, the resulting growth rates are compared with the available literature and it is found that, for a certain range in the model parameters, the agreement is quite satisfactory. This work provides valuable insight into the dominant mechanisms for mass transport as well as morphological evolution and growth of IMC layers during early stages of Pb-free soldering.     

Electrochemical corrosion behaviour of Sn–Ag–Cu (SAC) eutectic alloy in a chloride containing environment

The corrosion behaviour of the Sn_94.5Ag_3.8Cu_1.5 (SAC) eutectic alloy was investigated in 0.1 M NaCl solution by potentiodynamic polarization and impedance spectroscopy measurements and compared with that of the conventional Sn_73.9Pb_23.1 eutectic solder employed for a long time in the packaging of microelectronic components and devices. Scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to characterize the SAC eutectic alloy prior to and after the electrochemical tests. The electrochemical results indicated that the Sn–Ag–Cu eutectic alloy exhibits better corrosion behaviour than the Sn–Pb eutectic solder in NaCl solution. The presence of a corrosion products layer constituted by tin oxy‐chloride was detected at the surface of both alloys investigated after the electrochemical tests. The better corrosion behaviour of SAC eutectic alloy compared to Sn–Pb eutectic solder is ascribed to the formation of a more compact surface film of corrosion products with improved protective properties owing to the presence of copper and silver, as revealed by EPMA.     

Effect of reflow numbers on the interfacial reaction and shear strength of flip chip solder joints

The effects of multiple reflows on the microstructural variation and joint strength of the flip chip solder joints were investigated. Conventional Sn–37Pb and a representative Pb-free Sn–3.0Ag–0.5Cu were used, and a popular low cost under bump metallurgy (UBM) of immersion Au/electroless Ni–P plating was employed. In case of Sn–37Pb, only Ni₃Sn₄ intermetallic compound layer was formed during multiple reflows. In case of Sn–3.0Ag–0.5Cu, (Cu,Ni)₆Sn₅ intermetallic compound layer was observed after a single reflow, while (Ni,Cu)₃Sn₄ layer was additionally settled at the interface between the (Cu,Ni)₆Sn₅ layer and Ni–P UBM. The thicknesses of the intermetallic compound layers increased and coarsened. The shear force of the both joints slightly decreased during the multiple reflows. The decrease of the joint strength was caused by different softening phenomena between the two cases, and that is discussed in the main text.     

Effect of a trace of Cr on intermetallic compound layer for tin–zinc lead-free solder joint during aging

Influence of Cr on growth of interfacial intermetallic compound (IMC) at the interface of Sn–9Zn/Cu substrate during aging at 85 °C/20%RH and 85 °C/85%RH for 500 h has been investigated. After aging treatment, IMC layer at the Sn–9Zn/Cu joint is much thicker than that at the Sn–9Zn–Cr/Cu joint. Estimation according to experimental data presents that IMC growth rate of Sn–9Zn–Cr/Cu interface is about 70–75% lower than that of Sn–9Zn/Cu interface.     

Investigation of interfacial reactions between Sn–Zn solder with electrolytic Ni and electroless Ni(P) metallization

In this study, interfacial reactions of electrolytic Ni and electroless Ni(P) metallization of the ball-grid-array (BGA) substrate with the molten Sn–9Zn (wt.%) eutectic solder alloy were investigated, focusing on the shear strengths and the identification of the intermetallic compound (IMC) phases at various reflow periods. Zn-containing Pb-free solder alloys were kept in molten condition (240 °C) on the bond pads for different durations ranging from 1 to 60 min to render the ultimate interfacial reaction and to observe the consecutive shear strength. After the shear test, fracture surfaces were investigated by scanning electron microscopy equipped with an energy dispersive X-ray spectrometer. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. The solder ball shear-load for the Ni(P) system during extended reflow increased with the increase of reflow time. The consumption of the electroless Ni(P) layer in Sn–9Zn was also lower than that of the electrolytic Ni. It was evident that the Sn–Zn solder/electrolytic Ni system was more vulnerable than the Sn–Zn solder/electroless Ni(P) system in high temperature long time liquid state annealing. Sn–Zn solder with electroless Ni(P) metallization appeared as a good combination in soldering technology.     

Interfacial reactions between Sn–8Zn–3Bi–xAg lead-free solders and Cu substrate

The formation and the growth of the intermetallic compounds (IMCs) at the interface between the Sn–8Zn–3Bi–xAg (x = 0, 0.5, and 1 wt.%) lead-free solder alloys and Cu substrate soldered at 250 °C for different durations from 5 to 60 min were investigated. It was found that Cu₅Zn₈ and CuZn₅ formed at Sn–8Zn–3Bi/Cu interface, and Cu₅Zn₈ and AgZn₃ formed at the solder/Cu interface when the solder was added with Ag. The thickness of IMC layers in different solder/Cu systems increased with increasing the soldering time. And the growth of the IMCs was found to be mainly controlled by a diffusion mechanism. Additionally, the growth of the IMC layers decreased with increasing content of Ag in the soldering process.     

Influence of interfacial reaction between molten SnAgCu solder droplet and Au/Ni/Cu pad on IMC evolution

Intermetallic compounds（IMC） formed at Sn-Ag-Cu solder droplet/pad interface during wetting reaction were investigated. Comparative studies of the IMC evolution during reflow and aging were also conducted. The results show that the wetting reaction between molten solder droplet and pad leads to the formation of Au-Sn compound at interface, but Au element is not fully consumed during wetting reaction. After reflow, all Au layer disappears from the interface, Au element is dissolved into solder and Au-Sn intermetallic compounds are precipitated in the bulk. Reaction between Ni layer and Sn-Ag-Cu solder leads to the formation of （CuxNi1-x）6Sn5 layer at interface during reflow. According to the thermodynamic-kinetic of interfacial reaction, the wetting reaction at solder droplet/pad interface influences the phase selectivity of IMC evolution during reflow and aging process.