Tensile properties of Cu/Sn–58Bi/Cu soldered joints subjected to isothermal aging

The effects of isothermal aging on the tensile properties of Cu/Sn–58Bi/Cu soldered joints were investigated. Experimental results show that the scallop-shaped Cu₆Sn₅ and planar Cu₃Sn formed at the interface between solder and Cu substrate during reflowing and aging. The thickness of the intermetallic compounds (IMCs) increased almost linearly with the square root of aging time, and aging at 120 °C yielded a much faster growth of the IMCs layer than that of samples aged at 100 °C. The IMCs growth rate constants were 6.02 × 10^?18 and 1.85 × 10^?18 m² s^?1 for solder joints aged at 120 and 100 °C, respectively. The tensile strength of the Sn–58Bi/Cu soldered joints decreased slightly with the increasing aging time and temperature. The failure was dominated by the mixed fracturing in both the solder and the Cu₆Sn₅ grains irrespective of their thermal aging conditions. However, the fracture pattern tended to transform from ductile to brittle with increasing aging time and temperature. The Bi segregation and voids were observed around the Cu/Cu₃Sn interface as the long term aging at high aging temperature was carried out, which resulted in reduction of tensile strength of solder joints.     

Reliability studies of Sn–9Zn/Cu and Sn–9Zn–0.3Ag/Cu soldered joints with aging treatment

In this study, the interfacial reactions and joint reliabilities of Sn–9Zn/Cu and Sn–9Zn–0.3Ag/Cu were investigated during isothermal aging at 150 °C for aging times of up to 1,000 h. Cu₅Zn₈ IMCs layer is formed at the as-soldered Sn–9Zn/Cu interface. Adding 0.3wt.% Ag results in the adsorption of AgZn₃ on the Cu₅Zn₈ IMCs layer. The as-soldered Sn–9Zn/Cu and Sn–9Zn–0.3Ag/Cu joints have sufficient pull strength. The thickness of the IMCs layer formed at the interface of Sn–9Zn/Cu and Sn–9Zn–0.3Ag/Cu both increase with increasing aging time. Correspondingly, both the pull forces of the Sn–9Zn and Sn–9Zn–0.3Ag soldered joints gradually decrease as the aging time prolonged. However, the thickness of the IMCs layer of Sn–9Zn–0.3Ag/Cu increases much slower than that of Sn–9Zn/Cu and the pull force of Sn–9Zn–0.3Ag soldered joint decreases much slower than that of Sn–9Zn soldered joint. After aging for 1,000 h, some Cu–Sn IMCs form between the Cu₅Zn₈ IMC and the Cu substrate, many voids form at the interface between the Cu₅Zn₈ layer and solder alloy, and some cracks form in the Cu₅Zn₈ IMCs layer of Sn–9Zn/Cu. The pull force Sn–9Zn soldered joint decreases by 53.1% compared to the pull force measured after as-soldered. Fracture of Sn–9Zn/Cu occurred on the IMCs layer on the whole and the fracture micrograph implies a brittle fracture. While the pull force of Sn–9Zn–0.3Ag soldered joint decreases by 51.7% after aging at 150 °C for 1,000 h. The fracture mode of Sn–9Zn–0.3Ag soldered joint is partially brittle at the IMCs layer, and partially ductile at the outer ring of the solder.     

Influence of phase inhomogeneity on the mechanical behavior of microscale Cu/Sn–58Bi/Cu solder joints

Journal of Materials Science: Materials in Electronics - Electronics are becoming smaller and more versatile, and the size of solder joints has decreased to tens of microns, inducing obvious...     

Electromigration-enhanced intermetallic growth and phase evolution in Cu/Sn–58Bi/Cu solder joints

This paper presents some experimental observations relative to the influence of elevated current densities on the intermetallic growth and phase evolution in Cu/Sn–58Bi/Cu solder joints. Three samples were stressed with different current densities of 10⁴, 1.2 × 10⁴, and 1.4 × 10⁴ A/cm², respectively, for 80 h. The abnormal polarity effect of electromigration (EM) on chemical reactions at the cathode and the anode was investigated as well as the effect of EM on phase segregation in the two-phase eutectic microstructure. Results indicate that electric current enhances the growth of IMC layer at the cathode and retards it at the anode due to the Bi accumulation acting as a barrier layer with current density of 10⁴ A/cm². However, when current density increases, the electrical force dissolves the IMC at the cathode into the solder. More and more intermetallic precipitates formed due to the dissolution of Cu into the solder at the cathode side with increased current densities, leading to a very different morphology at the anode and the cathode interfaces, one being planar and the other being very irregular. It can be concluded that the chemical force and the electrical force are the main driving forces contributing to the IMC growth at both interfaces.     

Effects of Zn and Y on hot-tearing susceptibility of Mg–xZn–2xY alloys

The effect of Zn and Y on hot-tearing susceptibility (HTS) of Mg–xZn–2xY (x?=?1, 1.67, 2.67) alloys is investigated. It is found that the microstructure of the alloys is mainly composed of α-Mg, long-period stacking-ordered (LPSO) phase and W-phase. Both theoretical and experimental results illustrated that HTS of the investigated alloys is in the following order: Mg–1Zn–2Y > Mg–1.67Zn–3.34Y > Mg–2.67Zn–5.34Y. For Mg–2.67Zn–5.34Y alloy, LPSO phase content reaches at the maximum and its grain size reaches at minimum of 16.4 µm, and the pinning effect of the LPSO phase on grain boundaries is considered to be an important reason for reducing HTS of the alloy.     

Electromigration-induced Bi-rich whisker growth in Cu/Sn–58Bi/Cu solder joints

Effect of current stressing on whisker growth in Cu/Sn–58Bi/Cu solder joints was investigated with current densities of 5 × 10³ and 10⁴ A/cm² in oven at different temperatures. Two types of whiskers, columnar-type and filament-type, were observed on the solder film propagating along the surface of the Cu substrate and at the cathode interface, respectively, accompanied with many hillocks formation. Typically, these whiskers were 5–15 μm in length and 0.06–2 μm in diameter. EDX revealed that these whiskers and hillocks were mixtures of Sn and Bi rather than single crystal. It should be noted that the sprouted whiskers would not grow any more even if the current-stressing time increased again when the solder joint was stressed under lower current density. Nevertheless, when the current density was up to 10⁴ A/cm², the whiskers would melt along with the increasing current-stressing time. Results indicated that the compressive stress generated by precipitation of Cu₆Sn₅ intermetallics provides a driving force for whisker growth on the solder film, and the Joule heating accumulation should be responsible for whisker growth at the cathode interface.     

Influence of Sn and Bi Additions on the Structures and Wear Properties of Cu–Sn–P–Ce cBN Tools

In this work Sn and Bi were added to Cu–Sn–P–Ce for improving its microstructures and properties. Structures of the three matrices were investigated by XRD, SEM and EDS. The Cu₄₁Sn₁₁ became the main microstructure with some pores, the grinding ratio increased, and the grinding efficiency improved slightly with the addition of Sn to Cu–Sn–P–Ce. Bi was distributed in the form of simple substance, the grinding ratio increased, and the grinding efficiency greatly reduced with the addition of Bi to Cu–Sn–P–Ce.     

Effects of Al,Pr additions on the wettability and interfacial reaction of Zn–25Sn solder on Cu substrate

Zn–25Sn alloy suffers from easy oxidation during soldering. This study investigated the feasibility of Al and rare earth Pr addition for enhancing the wettability of the Zn–25Sn solder. The wettability and interfacial reaction of Zn–25Sn, Zn–25Sn–0.05Al, Zn–25Sn–0.05Al–XPr (X = 0.01, 0.05, 0.08, 0.15 wt%) on Cu substrate were investigated. The additions of 0.05 wt% Al in Zn–25Sn and 0.01, 0.05 wt% Pr in Zn–25Sn–0.05Al enhanced the wettability of solders and depressed the growth of intermetallic compounds. However, the additions of 0.08, 0.15 wt% Pr in Zn–25Sn–0.05Al degraded the wettability and enhanced the growth of the intermetallic compound. Pr and Al were shown to accumulate at the surface of solders and the interfaces between solder and substrate by the Secondary Ion Mass Spectroscopy, Scanning Electron Microscope and Transmission Electron Microscopy. The phases PrZn₃, Pr₃Sn₅ and (Cu, Al)₄Zn were formed at the solder/substrate interface.     

Effects of pre-treatments on aging precipitates and corrosion resistance of a creep-aged Al–Zn–Mg–Cu alloy

The effects of pre-treatments (solution and retrogression) on aging precipitates and corrosion resistance of a creep-aged Al–Zn–Mg–Cu alloy are investigated by means of transmission electron microscope (TEM), scanning electron microscope (SEM) and cyclic potentiodynamic polarization experiments. It is found that the aging precipitates and corrosion resistance are greatly affected by the pre-treatments. For the creep-aged alloy after solution pre-treatment, fine aging precipitates with high density are formed within grains. Meanwhile, large and continuously-distributed aging precipitates appear along grain boundaries. Also, this creep-aged alloy is strongly sensitive to the electrochemical corrosion, and the corrosion pits are easily induced in the 3.5 wt.% NaCl solution. For the creep-aged alloy after retrogression pre-treatment, when the retrogression pre-treatment time is increased, the density of intragranular aging precipitates first increases and then decreases, while the size of grain boundary precipitate and the width of precipitate free zone continuously increase. Compared with the creep-aged alloy after solution pre-treatment, the corrosion resistance of the creep-aged alloy after retrogression pre-treatment is greatly improved.     

Growth mechanisms of interfacial intermetallic compounds in Sn/Cu–Zn solder joints during aging

The interfacial reactions of Sn/Cu–xZn (x = 15 and 30 at.%) solder joints were investigated. Before aging, [Cu₆(Sn,Zn)₅] and [Cu₆(Sn,Zn)₅/Cu–Zn–Sn] intermetallic compounds (IMCs) formed at the [Sn/Cu–15Zn] and [Sn/Cu–30Zn] interfaces, respectively. After thermal aging at 150 °C for 80 days, [Cu₆(Sn,Zn)₅/Cu₃(Sn,Zn)/Cu(Zn,Sn)/CuZn] and [Cu₆(Sn,Zn)₅/Cu(Zn,Sn)/CuZn] IMCs, respectively, formed at the [Sn/Cu–15Zn] and [Sn/Cu–30Zn] interfaces. Increasing the amount of Zn in the Cu–Zn substrates evidently suppresses the growth of Cu₃Sn and Kirkendall voids at the solder joint interfaces. Transmission electron microscopy images show the different microstructure of CuZn and Cu–Zn–Sn phases in Sn/Cu–Zn joints. These Cu–Zn phases act to inhibit the growth of Cu₆Sn₅ and Cu₃Sn IMCs. As the content of Zn increased in Cu–Zn substrates, both CuZn and Cu(Zn,Sn) grew significantly. In addition, the growth of the Cu₆(Sn,Zn)₅/Cu₃Sn IMCs approached a reaction-controlled process. The formation mechanisms of the CuZn and Cu(Zn,Sn) phases were probed and proposed with regard to the interfacial microstructure, elemental distribution, and the compositional variation at Sn/Cu–xZn interfaces.     

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.     

Effect of Ni,Bi concentration on the microstructure and shear behavior of low-Ag SAC–Bi–Ni/Cu solder joints

This paper investigated the effect of Bi, Ni concentration on the microstructure and interfacial intermetallic compounds of low-Ag Sn–0.7Ag–0.5Cu–xBi–yNi/Cu solder joints by comparing with Sn–0.7Ag–0.5Cu (SAC0705)/Cu and Sn–3Ag–0.5Cu (SAC305)/Cu. Meanwhile, the shear behavior of the solder joints at both the bulk solder and soldering interface with various Bi, Ni content were also studied. Experimental results indicated that SAC0705–3.5Bi showed coarse microstructure due to the excessive growth of β-Sn dendritic crystal, which can be obviously suppressed by small amount of Ni element addition. Needle-like (Cu, Ni)₆Sn₅ appeared in the bulk solder of SAC–Bi–Ni/Cu, instead of the pipe-like Cu₆Sn₅ in SAC/Cu. Compare with SAC0705/Cu and SAC305/Cu, SAC–Bi–Ni/Cu showed higher shear strength at both the bulk solder and soldering interface. The increase of Bi content significantly increased the shear strength of Sn–0.7Ag–0.5Cu–xBi–yNi/Cu solder joints at the soldering interface. Brittle fracture appeared in the bulk solder of Sn–0.7Ag–0.5Cu–3.5Bi–0.05Ni/Cu solder joint. But this brittle failure can be suppressed by increasing the concentration of Ni in the solder alloys.     

Effects of tin particles addition on structural and mechanical properties of eutectic Sn–58Bi solder joint

Due to the inherent environmental and health toxicities associated with lead, the use of environmental friendly lead-free solder materials has become an unavoidable trend in the electronic packaging industry. Sn-58Bi alloy is gaining attention for its good material properties such as low melting point, reliability and high tensile strength. The presence of the bismuth-rich phase increases the brittleness of Sn-58Bi alloy. The purpose of this study is to suppress the brittleness of Sn-58Bi alloy by the addition of different wt% (0, 10, 20, 30) of Sn powder. The powder metallurgy method was used to prepare the samples. Scanning electron microscopy and energy-dispersive X-ray analysis were done to study the structural properties and a tensile test was done by a universal tensile machine to study the mechanical properties. The results reveal that the Sn particles partially dissolved in the Sn-58Bi solder matrix. The dissolution of Sn particles significantly improved the mechanical strength by 30%, suppressed the brittleness and improved the strain value by 1.3 times.

     

Influences of ZrO2 nano-particles on the microstructures and microhardness of Sn8Zn1Bi–xZrO2/Cu solder joints

ZrO₂ nano-particles reinforced Sn8Zn1Bi–xZrO₂ (x = 0.25, 0.5 and 1) composite solder balls were prepared by a mechanical mixing method and then they were reflowed on Cu substrates. The influences of ZrO₂ nano-particles on the microstructures and microhardness of the composite solder joints was investigated. Zn-rich phase was refined in the SnZnBi–xZrO₂ composite solder matrices due to the adsorption of the ZrO₂ nano-particles on them. The refined Zn-rich phase increased the microhardness of the composite solder matrices. Furthermore, the adsorption effect of ZrO₂ nano-particles to the SnZnBi–xZrO₂/Cu interface suppressed the growth of intermetallic compound (IMC) layers by decreasing the growth driving force of them. However, Zn atoms were impeded from gathering on the SnZnBi–xZrO₂/Cu interfaces to form Cu₅Zn₈ phase due to the refinement of Zn-rich phase. Therefore, this gave excessive growth of the Cu₆Sn₅ IMC layer.     

Effects of reflow atmosphere and flux on Sn whisker growth of Sn–Ag–Cu solders

We evaluated the Sn whisker growth behavior of Sn–Ag–Cu solder fillets on lead frames of quad flat packages (QFPs) upon OSP printed circuit boards that were exposed to 85 °C/85% relative humidity (RH) exposure. Three different concentrations of halogen flux for activated Sn-3.0wt%Ag–0.5wt%Cu were used to solder in air and in an inert N₂ reflow atmosphere. The lead frames of the QFPs consisted of Sn plated Cu and Fe-42wt%Ni (alloy 42). Sn whiskers were observed on the surface of the QFP solder fillet joints that were reflowed with halogen containing flux in an air atmosphere. A substantial amount of Sn oxides were formed in those solder fillets while whisker growth and the amount of Sn oxides increased with the halogen content. Sn oxide formation apparently enhanced whisker formation. The combination of air reflow atmosphere and high halogen flux was the worst combination for solder fillet oxidation resulting in Sn whisker formation regardless of the electrode’s lead frame composition of Cu or alloy 42. In contrast, an inert N₂ reflow atmosphere obviously prevented Sn whisker formation on Sn–Ag–Cu solder fillets under all conditions used in this work.