Effect of microstructure and Ag3Sn intermetallic compounds on corrosion behavior of Sn–3.0Ag–0.5Cu lead-free solder

In this paper, the effects of microstructure on the corrosion behavior of Sn–3.0Ag–0.5Cu (SAC305) lead-free solder were investigated by potentiodynamic polarization and atmospheric corrosion test. Scanning electron microscopy and X-ray diffraction were used to characterize the samples after the electrochemical and atmospheric corrosion tests. Results showed that commercial SAC305 solder exhibits better corrosion resistance than air-cooled and furnace-cooled SAC305 solders both in 3.5 wt% NaCl solution and at 60 °C/100 % relative humidity condition.     

Effect of axial ratcheting deformation on torsional low cycle fatigue life of lead-free solder Sn–3.5Ag

Multiaxial fatigue tests were conducted on Sn–3.5Ag solder specimens under axial/torsional loading at room temperature. It was found that the ratcheting strain increased while the fatigue life decreased with the increase of axial stress and shear strain amplitude. A power relationship of ratcheting strain rate versus fatigue life was observed. Equivalent strain approach and critical plane approaches were evaluated with fatigue life data obtained in the tests. Since those approaches excluded the consideration of the ratcheting strain and mean stress, the methods for fatigue life prediction were improper for multiaxial fatigue with ratcheting strain. Coffin model, considered the effect of ratcheting on fatigue life depending on the ratio of ratcheting strain to material ductility, brought the fatigue life predictions on non-conservative side if the ratcheting deformation was large. For this reason, a model with the maximum shear strain range and axial ratcheting strain rate was proposed as a new damage parameter. The new model could not only describe the fatigue life in torsion test, but also predicted torsional fatigue life of the lead-free solder with axial ratcheting.     

Effects of microstructure and temperature on corrosion behavior of Sn–3.0Ag–0.5Cu lead-free solder

The heterogeneous microstructure of solder could be obtained when cooling rate of the solder joint was not even, which would affect the corrosion behavior of solder during service. The ambient temperature would also affect the corrosion behavior of solder joint. In this paper, the effects of microstructure and temperature on the corrosion behavior of Sn–3.0Ag–0.5Cu (SAC305) lead-free solder were investigated. The various microstructures of SAC305 lead-free solder were obtained by cooling specimens in air and furnace. Compared to the fine-fibrous Ag₃Sn phase inside the commercial SAC305 solder, platelet-like Ag₃Sn formed as cooling speed decreasing. The polarization behavior of SAC305 solders in 3.5 wt.% NaCl solution was not significantly affected by various microstructures, but sensitive to temperature.     

Interface behavior and performance of Sn–Ag–Cu lead-free solder bearing Tb

Journal of Materials Science: Materials in Electronics - To further improve the performance of the Sn–3.5Ag–0.5Cu solder, trace amount of rare earth Tb was added. The wettability, shear...     

Tensile creep behavior of Sn–Ag–Cu–Ni multicomponent lead-free solder alloy

In the process of electronic packaging, the dissolution of under bump metallizations, such as Cu and Ni, into liquid solder occurs during soldering, which can change the original solder to a multicomponent one. Under the trend of miniaturization, it is quite necessary to evaluate the properties of multicomponent solder with excessive Cu and Ni compositions. In this study, the tensile creep behavior of Sn–3.5Ag–2.0Cu–0.5Ni multicomponent lead-free solder alloy is investigated at three temperatures, i.e., 303, 348 and 393 K. The steady-rate creep rates are obtained in the range of 10^?4–10^?8 s^?1, when the normalized stress, σ/E, is in the range of 10^?4–10^?3. Based on the Dorn equation, the apparent stress exponent (n _a), threshold stress (σ _th), and activation energy of creep (Q _C) are calculated at the three temperatures. It is found that the Sn–3.5Ag–2.0Cu–0.5Ni solder alloy shows a better creep performance than pure tin and eutectic Sn–3.5Ag solder due to the strengthening effect of Ag₃Sn and (Cu,Ni)₆Sn₅ IMC precipitations. The true stress exponent for creep is identified to be 7, indicating that the creep behave is controlled by the dislocation-pipe diffusion in the tin matrix.     

Structural characterization and creep resistance of nano-silicon carbide reinforced Sn–1.0Ag–0.5Cu lead-free solder alloy

Nano-sized, non-reacting, non-coarsening SiC particles were successfully fabricated by high energy ball milling. Mechanically mixing was adopted to prepare SiC-particulate reinforced Sn–1.0Ag–0.5Cu (SAC105) composite solders. The effects of SiC addition on the melting behavior, microstructure and the corresponding creep properties were explored. It is found that the addition of 0.35–0.75 wt.% SiC nano-sized particles can effectively decrease the undercooling, while the melting temperature is sustained at the SAC(105) level, indicating that the novel composite solder is fit for existing soldering process. After the addition of 0.35% SiC nano-particles, a fine microstructure of Ag₃Sn and Cu₆Sn₅ IMCs with small spacing appeared in the β-Sn matrix. Moreover, the creep rate of the composite solder exhibited a consistently lower value than that of plain SAC(105) solder due to a second phase dispersion strengthening mechanism as well as a refinement of IMCs. Hence, the composite SAC(105)/0.35% SiC solder displayed a higher creep resistance (3.1 times) and fracture lifetime (3 times) than that of plain solder. However, this effectiveness is reduced when 0.75% SiC addition starts constricting the growth Ag₃Sn and Cu₆Sn₅ IMC and forming a weak interface with the enlarged β-Sn matrix.     

Thermal creep and fracture behaviors of the lead-free Sn–Ag–Cu–Bi solder interconnections under different stress levels

In our previous study, the creep behavior of the lead-free Sn–Ag–Cu–Bi solder joints has been proven to follow the Arrhenius power-law relationship, and the thermal fatigue behavior of the solder joints exhibits the typical creep deformation characteristics with a superposition of the pulsating features. In this study, the thermal creep and fracture behaviors of the lead-free Sn–Ag–Cu–Bi solder interconnections were characterized under different stress levels, with a systematical comparison to that of a traditional Sn60Pb40 near-eutectic solder. The results show that the creep strain rate of both solder connections follows Weertman-Dorn equation, and the calculated creep stress exponent for two solders is reasonably close to other published data. The SEM inspection and analysis of fractographies of creep fractured solder joints manifest that the creep failure of the lead-free Sn–Ag–Cu–Bi solder joint shows obviously intergranular fracture mechanism, while the Sn60Pb40 joint ruptures dominantly by a transgranular sliding mechanism.     

Annealing effect to constitutive behavior of Sn–3.0Ag–0.5Cu solder

Sn–Ag–Cu based solder alloys are replacing Sn–Pb solders in electronic packaging structures of commercial electric devices. In order to evaluate the structural reliability, the mechanical property of solder material is critical to the numerical simulations. Annealing process has been found to stabilize material properties of Sn–37Pb solder material. In the current study, the annealing effect on tensile behaviour of Sn–3.0Ag–0.5Cu (SAC305) solder material is investigated and compared with Sn–37Pb solder. It is found that the tensile strength for both materials are more stabilized and consistent after the annealing process, nevertheless, the annealing process will improve the plasticity of SAC305 solder dominated by dislocation motion, and impede the occurrence of hardening deformation in Sn–37Pb solder dominated by grain-boundary sliding mechanism. Furthermore, the annealing effect is quantified in the proposed constitutive model based on unified creep–plasticity theory. The parameters are calibrated against the measured stress–strain relationships at the tensile strain rates ranging from 1?×?10^?4 to 1?×?10^?3 s^?1. The numerical regressions for dominant parameters in the proposed model reveal the intrinsic differences between SAC305 and Sn–37Pb solders under annealing treatment.     

Development of a lead-free composite solder from Sn–Ag–Cu and Ag-coated carbon nanotubes

The microelectronic applications of lead-free solders pose ever-increasing demands. We seek to improve the solder by forming composites with Ag-coated single-walled carbon nanotubes (Ag-coated SWCNTs). These were incorporated into 96.5Sn–3.0Ag–0.5Cu solder alloy with an ultrasonic mixing technique. Composite solder pastes with 0.01–0.10 wt% nanotube reinforcement were prepared. The wettability, melting temperature, microstructure and mechanical properties of the composite solders were determined, and their dependency on nanotube loading assessed. Loading with 0.01 wt% Ag-coated SWCNTs improved the composite solder’s wetting properties, and the contact angle was reduced by 45.5 %, while over loading of the coated nanotubes up to 0.10 wt% degraded the wettability. DSC results showed only slight effects on the melting behavior of the composite solders. Cross-section microstructure analysis of the spreading specimens revealed uniform distribution of the intermetallic compounds throughout the solder matrix, and EDS analysis identified the phases as β-Sn, Ag₃Sn and Cu₆Sn₅. The mechanical properties of composite specimens, compared with those of unloaded 96.5Sn–3.0Ag–0.5Cu solder, had a maximal improvement in the shear strength of 11 % when the nanotube loading was 0.01 wt% of Ag-coated SWCNTs.     

Dependency of eutectic spacings and microhardness on the temperature gradient for directionally solidified Sn–Ag–Cu lead-free solder

Sn–3.5 wt.%Ag–0.9 wt.%Cu alloy was directionally solidified upward at a constant growth rate (V = 7.20 μm s⁻¹) with different temperature gradients (G = 2.48–6.34 K mm⁻¹) by using a Bridgman type directional solidification furnace. The eutectic microstructures of directionally solidified Sn–3.5 wt.%Ag–0.9 wt.%Cu alloy were observed to be plate and rod structures from quenched samples. The values of eutectic spacings (λ) and microhardness (H_V) were measured from both transverse and longitudinal sections of the samples. The dependence of eutectic spacings (λ) and microhardness (H_V) on the temperature gradient (G) were determined by using linear regression analysis. According to these results, it has been found that, the value of λ decreases with the increasing the value of G and whereas, the value of H_V increases for a constant growth rate. The results obtained in the present work were also compared with the previous similar experimental results obtained for binary and ternary alloys.     

Development of a Sn–Ag–Cu solder reinforced with Ni-coated carbon nanotubes

In this study, Ni-coated carbon nanotubes (Ni-CNTs) were incorporated into the 95.8Sn-3.5Ag-0.7Cu solder alloy using the powder metallurgy route. Up to 0.3 wt% of Ni-CNTs were successfully incorporated. The effects of Ni-CNTs on the physical, thermal and mechanical properties of Sn–Ag–Cu solder alloy were investigated. With the addition of increasing weight percentages of Ni-CNTs, the composite solders showed a corresponding decrease in density values and improved wetting properties. The thermomechanical property results showed an improvement in thermal stability for the composite solders. Mechanical characterization revealed an improvement in ultimate tensile strength (up to 12%) and 0.2% yield strength (up to 8%) with the addition of 0.05 wt% Ni-CNTs in the solder.     

The morphology and kinetic evolution of intermetallic compounds at Sn–Ag–Cu solder/Cu and Sn–Ag–Cu-0.5Al2O3 composite solder/Cu interface during soldering reaction

In this work, Sn3.0Ag0.7Cu (SAC) composite solders were produced by mechanically intermixing 0.5 wt% Al₂O₃ nanoparticles into Sn3.0Ag0.7Cu solder. The formation and growth kinetics of the intermetallic compounds (IMC) formed during the liquid–solid reactions between SAC-0.5Al₂O₃ composite solder and Cu substrates at various temperatures ranging from 250 to 325 °C were investigated, and the results were compared to the SAC/Cu system. Scanning electron microscopy (SEM) was used to quantify the interfacial microstructure for each processing condition. The thickness of interfacial intermetallic layers was quantitatively evaluated from SEM micrographs using imaging software. Experimental results showed that IMC could be dramatically affected by a small amount of intermixing 0.5 wt% Al₂O₃ nanoparticles into Sn3.0Ag0.7Cu solder. A continuous elongated scallop-shaped overall IMC layer was found at SAC/Cu interfaces. However, after the addition of Al₂O₃ nanoparticles, a discontinuous rounded scallop-shaped overall IMC layer appeared at SAC-0.5Al₂O₃/Cu interfaces. Kinetics analyses showed that growth of the overall IMC layer in SAC/Cu and SAC-0.5Al₂O₃/Cu soldering was diffusion controlled. The activation energies calculated for the overall IMC layer were 44.2 kJ/mol of SAC/Cu and 59.3 kJ/mol for SAC-0.5Al₂O₃/Cu soldering, respectively. This indicates that the presence of a small amount of Al₂O₃ nanoparticles is effective in suppressing the growth of the overall IMC layer.     

Corrosion behavior of Sn–3.0Ag–0.5Cu solder under high-temperature and high-humidity condition

The aim of this study is to evaluate the corrosion behavior of Sn–3.0Ag–0.5Cu (SAC305) solder alloy under high-temperature and high-humidity condition. The corrosion of SAC305 alloy was attributed to the oxidation of Sn, which formed SnO₂ and SnO, and SnO₂ existed on the outer layer of the oxide film. After a period exposure, a stable and dense protective oxide film formed on the specimen surfaces, and the specimen which exposed at 75 °C had the thickset oxide film.     

Enhanced ductility and mechanical strength of Ni-doped Sn–3.0Ag–0.5Cu lead-free solders

In this paper, the tensile tests were conducted to investigate the effect of adding a small amount of Ni on the microstructure, thermal and mechanical properties of 3.0Ag–0.5Cu(SAC 305) solder. The results indicated that addition of Ni can effectively decrease both the undercooling and the onset melting temperature of SAC(305) solder alloy. The strength and ductility of the SAC(305) solder depend significantly on Ni content. In general, the SAC(305)–0.5%Ni solder reveals superior mechanical properties in terms of maximum strength and ductility when compared to the high Ni-content or plain solders. Microstructure analysis revealed that a new η-(Cu,Ni)₆Sn₅ intermetallic compound (IMC) phase containing large amount of Ni was generated, while the initial Cu₆Sn₅ phase was converted into (Cu,Ni)₆Sn₅ phase after 0.5%Ni addition. Besides, the fine fiber-like Ag₃Sn and finer dot-shaped precipitates rather than needle-like morphology have occurred at the surface of β-Sn matrix easily, which could provide more obstacles for dislocation pile up in the adjacent grains and enhanced the mechanical property. With increasing Ni addition, the Ni-doped SAC(305) solder showed a corresponding deterioration in their mechanical property due to the coarsening of (Cu,Ni)₆Sn₅ IMCs and increasing the inter-particle spacing of Ag₃Sn IMCs in the eutectic colony.     

Effects of package geometry on thermal fatigue life and microstructure of Sn–Ag–Cu solder joint

Abstract

Soldering experiments of chip scale package devices were carried out by means of diode laser soldering system with Sn–Ag–Cu solders. In addition, pull tests and a scanning electron microscope were used to analyse the effect of processing parameters on mechanical strength of solder joints. Viscoplastic finite element simulation was utilised to predict solder joint reliability for different package geometry under accelerated temperature cycling conditions. The results indicate that under the conditions of laser continuous scanning mode as well as the fixed soldering time, an optimal power and package geometry exists, while the optimal mechanical properties of microjoints are gained.     

Effects of Zn addition on electromigration behavior of Sn–1Ag–0.5Cu solder interconnect

Effects of electromigration on microstructure and tensile property were studied in the Sn–1Ag–0.5Cu and Sn–1Ag–0.5Cu–1Zn solder interconnects. While the polarity effect and strength reduction from electromigration occurred in the Sn–1Ag–0.5Cu solder interconnects, they were suppressed by the Zn addition in the Sn–1Ag–0.5Cu–1Zn solder interconnects. Such a strong effect of Zn was explained by the strong binding of Zn with Cu, which prevented the dissolution of the IMC at the cathode, and by the reverse migration of the Zn elements, which counteracted the increase in the vacancy concentration so that the strength reduction was successfully inhibited.     

Microstructural evolution of 96.5Sn–3Ag–0.5Cu lead free solder reinforced with nickel-coated graphene reinforcements under large temperature gradient

In this study, 96.5Sn–3Ag–0.5Cu (SAC305) lead-free composite solder containing graphene nanosheets (GNS) decorated with Ni nanoparticles (Ni-GNS) was prepared using a powder metallurgy method. A lab-made set-up and a corresponding Cu/solder/Cu sample design for assessing thermo-migration (TM) was established. The feasibility of this setup for TM stressing using an infrared thermal imaging method was verified; a temperature gradient in a solder joint was observed at 1240 K/cm. Microstructural evolution and diffusion of Cu in both plain and composite solder joints were then studied under TM stressing conditions. Compared to unreinforced SAC305 solder, the process of diffusion of Cu atoms in the composite solder joint was significantly reduced. The interfacial intermetallic compounds (IMCs) present in the composite solder joint also provide a more stable morphology after the TM test for 600 h. Furthermore, during the TM test, the Ni-GNS reinforcement affects the formation, migration and distribution of Ni–Cu–Sn and Cu–Sn IMCs by influencing the dissolution rate of Cu atoms.     

Effects of Ga addition on microstructure and properties of Sn–0.5Ag–0.7Cu solder

The effects of rare element Ga on solderability, microstructure, and mechanical properties of Sn–0.5Ag–0.7Cu lead-free solder were investigated. The experimental results show that Ga plays a positive role in improving the wettability and the microstructure of the solder. When the content of Ga is at 0.5 wt%, the grain size of the solder is smaller and the shear force is enhanced greatly. It is also found that the thickness of the IMCs at the solder/Cu interface is reduced with proper addition of Ga. The increase of mechanical properties may be related to the refining of IMCs of the solder due to Ga addition.     

Solder joint reliability of Sn–0·7Cu and Sn–0·3Ag–0·7Cu lead-free solder alloys solidified on copper substrates with different surface roughnesses

Abstract

The bond shear test was used to assess the integrity of Sn–0·7Cu and Sn–0·3Ag–0·7Cu lead-free solder alloy drops solidified on copper substrates with smooth and rough surface finishes. Solder alloys solidified on smooth substrates required higher shear force compared to that on rough substrates. Sn–0·3Ag–0·7Cu alloy required higher shear energy than Sn–0·7Cu alloy. Solder alloys solidified on smooth substrate surfaces exhibited complete ductile failure. On rough copper surfaces, solder alloys showed a transition ridge characterized by sheared intermetallic compounds (IMCs) and the presence of dimples. The peak shear strength decreased with increase in contact area of the solder bond on the substrate. Smooth surface and the presence of minor amount of Ag in the solder alloy enhance the integrity of the solder joint.