Effects of Ag addition on microstructures and mechanical properties of Mg–6Zn–2Sn–0.4Mn-based alloy system

We investigated the effects of Ag addition on the microstructure and mechanical properties of hot-extruded Mg–6Zn–2Sn–0.4Mn-based alloys. Ag addition resulted in the formation of fine submicron-sized Mg–Ag particles, grain refinement, and weaker basal texture in the alloys. The Ag-containing extruded alloys had better mechanical properties than the alloys without Ag. The ultimate tensile strength (UTS) and elongation of alloys containing 1 wt.% Ag were 352 MPa and 19%, respectively.     

Effects of Zn content on microstructures and mechanical properties of as cast Mg–Zn–Y–Zr alloys

Abstract

In the present work, the effects of Zn content on the microstructures and mechanical properties of as cast Mg–xZn–5Y–0·6Zr alloys (x?=?2, 5, 8 and 13 wt-%) have been investigated. The results show that the ternary Mg–Zn–Y phase compositions change with Zn/Y ratios induced by the change in Zn content. It is found that the fracture is mainly decided by the characteristics and distribution of second phase rather than the grain size. The influences of these phases, especially the W phase, on the mechanical properties of the alloys have been discussed. Both ultimate tensile strength (UTS) and elongation decrease with the increase in Zn content, while the instance of yield strength (YS) is just the reverse. The W phase is easily cracked because of its brittleness and easy to result in decohesion from the matrix because of the weak atomic bonding, which greatly degrade the UTS and elongation. It can be concluded that the YS closely depends on the grain size, while UTS and elongation closely depend on the volume fraction of eutectic compound (α-Mg+W phase).     

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.     

Wetting properties and interfacial microstructures of Sn–Zn–xGa solders on Cu substrate

The wetting properties and interfacial microstructures of Sn–9Zn–xGa lead-free solders with Cu substrate were investigated. The wetting property is improved remarkably with the increase of Ga content in the Sn–9Zn lead-free solder. The lower surface tension, which results from the decrease of the oxidation of the Zn atoms owing to the formation of the Ga-rich protective film covered on the liquid solder, is the key reason for the better wettability. During soldering, the Cu₅Zn₈ compounds layer form at the interface of Sn–9Zn/Cu and the IMCs formed at the solder/Cu surface become much thicker when the Ga content is from 0.1 wt.% to 3 wt.%. However, neither Cu–Sn compounds nor Ga-rich phases are observed at the solder/Cu surface.     

Review on microstructure evolution in Sn–Ag–Cu solders and its effect on mechanical integrity of solder joints

The use of Pb-bearing solders in electronic assemblies is avoided in many countries due to the inherent toxicity and environmental risks associated with lead. Although a number of “Pb-free” alloys have been invented, none of them meet all the standards generally satisfied by a conventional Pb–Sn alloy. A large number of reliability problems still exist with lead free solder joints. Solder joint reliability depends on mechanical strength, fatigue resistance, hardness, coefficient of thermal expansion which are influenced by the microstructure, type and morphology of inter metallic compounds (IMC). In recent years, Sn rich solders have been considered as suitable replacement for Pb bearing solders. The objective of this review is to study the evolution of microstructural phases in commonly used lead free xSn–yAg–zCu solders and the various factors such as substrate, minor alloying, mechanical and thermo-mechanical strains which affect the microstructure. A complete understanding of the mechanisms that determine the formation and growth of interfacial IMCs is essential for developing solder joints with high reliability. The data available in the open literature have been reviewed and discussed.     

Effects of Ga,Al, Ag,and Ce multi-additions on the properties of Sn–9Zn lead-free solder

Wetting balance method is used to evaluate the effects of Ga, Al, Ag, and Ce multi-additions on the solderability of Sn–9Zn lead-free solders, results show that the optimal addition amounts of Ga, Al, Ag, and Ce is 0.2, 0.002, 0.25, and 0.15 wt% respectively. The surface property of Sn–9Zn–0.2Ga–0.002Al–0.25Ag–0.15Ce solder is studied by X-ray photoelectron spectroscopy and auger electron spectroscopy analysis; results indicate that Al aggregates on the surface as a compact aluminum oxide film which prevents the further oxidation. The aggregation of Ce on the subsurface can reduce the surface tension of solder, and improve the solderability accordingly. Meanwhile, SEM and XRD analysis indicate that Cu₅Zn₈ and AgZn₃ intermetallic compounds form at the interface between Sn–9Zn–0.2Ga–0.002Al–0.25Ag–0.15Ce solder and Cu substrate, while AuZn₃ and AuAgZn₂ form at the interface between solder and Cu/Ni/Au substrate. Moreover, results also indicate that the mechanical property of soldered joints is improved duo to the dispersion strengthening effects of AgZn₃ in Sn–9Zn–0.2Ga–0.002Al–0.25Ag–0.15Ce solder.     

Effects of Ga–Ag, Ga–Al and Al–Ag additions on the wetting characteristics of Sn–9Zn–X–Y lead-free solders

The effects of Ga–Al, Ga–Ag and Al–Ag binary additions on the wetting characteristics of Sn–9Zn–X–Y lead-free solders are studied by the wetting balance method. Experimental results show that Sn–9Zn–1.0Ga–0.3Ag, Sn–9Zn–0.005Al–0.3Ag, and Sn–9Zn–0.3Ga–0.002Al possess better wettability than the other alloys tested. The mechanism by which Ga, Al, and Ag additions improve the wettability is also proposed. It appears that dense aluminum oxide film formation and the enrichment of Ga on the surface may protect the bulk liquid solder from further oxidation. Moreover, results also indicate that, AgZn₃ IMCs layer formed at the interface, which may release reaction energy during the wetting, results in improving the wettability of the solder.     

Interfacial microstructures and mechanical properties of Sn–9Zn–0.5Ga–xNd on Cu substrate with aging treatment

The interfacial microstructures and mechanical properties of Sn–9Zn–0.5Ga–xNd on Cu substrate with aging treatment were investigated. Unlike the previous results, no Sn–RE phase was formed near the interface of Sn–9Zn–0.5Ga–xNd soldered joint after aging treatment at 150 °C for 1200 h. The combined action of Ga and Nd inhibited the formation of Sn–RE phase and resulted in an enhanced reliability of the soldered joint. The shear force of Sn–Zn–0.5Ga–0.08Nd soldered joint after aging treatment for 1200 h was twice the amount of Sn–9Zn joint and approaching the origin as-soldered joint of Sn–9Zn. Further, the fracture type of soldered joint still performed ductile after aging treatment for 1200 h. Synthesized the results of interfacial microstructures, mechanical test and fracture morphologies, Sn–9Zn–0.5Ga–0.08Nd solder has shown great potential to satisfy the increasing reliability requirements in electronics industry.     

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.     

Effects of rare earth Ce on properties of Sn–9Zn lead-free solder

The influences of different Ce content on the properties of Sn–9Zn lead-free solder were investigated. The results indicate that Ce plays an important role not only in the structure and the solderability, but also in the interfacial structure of Sn–9Zn–xCe/Cu and mechanical property of soldered joint. Sn–9Zn–0.08Ce shows finer and more uniform microstructure than Sn–9Zn, and when the quantity of Ce is 0.5–1 wt%, some dark Sn–Ce compounds appear in the solder. With the addition of 0.08 wt% Ce, the solderability of solder is significantly improved because the surface tension of molten solder is decreased. Adding Ce makes the Cu5Zn8 IMCs formed at the interface of solder/Cu become much thicker than that of Sn–9Zn/Cu because much more content of Zn diffuse to the interface of solder/Cu to react with Cu. Results also indicate that adding 0.08 wt% Ce to the solder enhances mechanical property of soldered joint. When the Ce content is 0.1–0.5 wt%, some hard and brittle Cu–Zn IMCs appear in the bottom of dimples and the pull force of soldered joint decreases.     

Effects of Zn on microstructure,mechanical properties and corrosion behavior of Mg–Zn alloys

In this study, binary Mg–Zn alloys were fabricated with high-purity raw materials and by a clean melting process. The effects of Zn on the microstructure, mechanical property and corrosion behavior of the as-cast Mg–Zn alloys were studied using direct observations, tensile testing, immersion tests and electrochemical evaluations. Results indicate that the microstructure of Mg–Zn alloys typically consists of primary α-Mg matrix and MgZn intermetallic phase mainly distributed along grain boundary. The improvement in mechanical performances for Mg–Zn alloys with Zn content until 5% of weight is corresponding to fine grain strengthening, solid solution strengthening and second phase strengthening. Polarization test has shown the beneficial effect of Zn element on the formation of a protective film on the surface of alloys. Mg–5Zn alloy exhibits the best anti-corrosion property. However, further increase of Zn content until 7% of weight deteriorates the corrosion rate which is driven by galvanic couple effect.     

Effects of Sn on microstructure and mechanical properties of as-rolled Mg–5Zn–1Mn alloy

Effects of Sn on microstructure and mechanical properties of Mg–5Zn–1Mn alloy subjected to high strain rate rolling (9.1?s^-1), 300°C and 80% pass reduction are investigated. With higher Sn content, the dynamic recrystallisation (DRX) grain size gradually decreases due to the stronger pinning of nano-scale precipitates at grain boundaries and the DRX fraction first increases due to the enhanced effect on DRX by decreasing stacking fault energy and then decreases due to more precipitates at grain boundaries. Ultimate tensile strength (UTS) and elongation to rupture (Er) of as-rolled alloys increase and then decrease. Alloy with 0.9 mass% Sn exhibits the highest DRX fraction (95?vol.-%), the finer DRX grain size (1.22?µm), UTS of 358?MPa and Er of 20.4%.     

Effects of shear test temperatures and conditions on mechanical properties of Sn–Ag flip-chip solder bumps

Journal of Materials Science: Materials in Electronics - Lately, as electronic devices are continually becoming smaller, thinner, and multifunctional, there has been a growing demand for highly...     

Combined effects of Ag content and cooling rate on microstructure and mechanical behavior of Sn–Ag–Cu solders

Sn–0.7 wt%Cu–1.0 wt%Ag and Sn–0.7 wt%Cu–2.0 wt%Ag alloys were directionally solidified under transient conditions undergoing cooling rates varying from 0.1 to 25 K/s. The microstructure was characterized along the castings lengths and the present experimental results include the secondary dendrite arm spacing (λ₂) and its correlation with: the tip cooling rate ($\dot{T}$) during solidification and microhardness (HV), yield tensile strength (σ_y), ultimate tensile strength (σ_u) and elongation to fracture (δ). The aim is to examine the effects of Ag content and tip cooling rate on both the microstructure and mechanical properties. The initiation of tertiary branches within the dendritic arrangement, as well as the distinct morphologies of the intermetallic compounds (IMC) related to the solidification cooling rate was also assessed for both examined alloys. While the Cu₆Sn₅ phase appeared as large faceted crystals along the entire casting length, very fine Ag₃Sn spheroids prevailed at higher cooling rates (>7.5 K/s and > 4.0 K/s for 1.0 wt%Ag and 2.0 wt%Ag alloying, respectively) with a mixture of Ag₃Sn coarser spheroids and fibers predominating at lower cooling rates. The Sn–0.7 wt%Cu–2.0 wt%Ag alloy exhibited smaller dendritic spacings and HV of about two times higher than the corresponding values of the Sn–0.7 wt%Cu–1.0 wt%Ag alloy. A single Hall–Petch equation is proposed relating δ to λ₂ for both alloys, which means that the increase in Ag content from 1.0 to 2.0 wt% does not affect the elongation. It is shown that δ decreases with the increase in λ₂.     

Effect of Ce on microstructures and mechanical properties of rapidly solidified Mg–Zn alloy

Abstract

The rapidly solidified (RS) Mg–Zn based alloys with Ce addition were produced via atomising the alloy melt and subsequent splat quenching on the water cooled copper twin rollers in the form of flakes. The effects of Ce additions on the microstructures, phase compositions, thermal stability and isochronal age hardening behaviour of the RS Mg–Zn alloy were systematically investigated. The RS Mg–6Zn alloy is characterised by fine grains in the size of 6–10 μm and is composed of α-Mg, Mg₅₁Zn₂₀ and a small quantity of MgZn₂ and Mg₂Zn₃ phases. With the increment of Ce, the microstructures of the alloys are refined, and the volume fractions of dispersions are increased remarkably. The stable intermetallic compounds, i.e. the Mg_xZn_yCe_z ternary phases, are formed in the RS Mg–Zn–Ce alloys at the expense of the Mg₅₁Zn₂₀ phases, which leads to the enhanced thermal stability of the alloys, especially for the Mg–6Zn–5Ce alloy. In the alloy, the atomic percentage ratio of Zn/Ce in the Mg_xZn_yCe_z phase is close to two, and the maximum hardness is 91·5±7 HV after annealing at 200°C for 1 h. However, the age hardening behaviour of the alloys decreases with the increment of Ce, and the main reason is discussed.     

Evolution of interfacial IMCs and mechanical properties of Sn–Ag–Cu solder joints with Cu-modified carbon nanotube

Cu-modified carbon nanotube (Cu-CNT) was mechanical mixed with Sn-3.0Ag-0.5Cu (SAC305) solder powder to fabricate composite solder. Experiments were carried out on the composite solder to investigate its thermal behavior and mechanical properties. Evolution of interfacial intermetallic compounds (IMCs) and shear fracture behaviors of composite solder joints were explored as well. Differential scanning calorimetry results showed that the addition of Cu-CNT increased the melting point of SAC305. Nanoindentation indicated that the hardness and modulus of composite solder increased with the increase of Cu-CNT content, indicating that the addition of Cu-CNT could improve the mechanical properties of SAC305 solder. SAC-xCu-CNT/Cu (x?=?0, 0.1 and 0.2 wt%) composite solder joints were obtained by reflow soldering, aging experiment was carried out subsequently. The results showed that Cu-CNT as the reinforcement phase of SAC305 solder could inhibit the growth of IMC. The suppression effect was enhanced when the content of Cu-CNT increased from 0.1 to 0.2 wt% in SAC305. The growth rate of the IMC layer of the composite solder joint with 0.2 wt% Cu-CNT was the lowest. Shear tests showed that adding Cu-CNT could improve the shear strength of solder joints and the fracture mode of the solder joint changed from mixed fracture to ductile fracture.

     

Study on properties of Sn–9Zn–Ga solder bearing Nd

The effects of Nd on wettability, microstructure and mechanical properties of Sn–9Zn–Ga–xNd lead-free solder were investigated. The results indicate that adding moderate amount of rare earth Nd, the wettability as well as mechanical properties of Sn–9Zn–0.5Ga solder were evidently improved, and when the content of Nd is at 0.08 wt%, the best wettability and comprehensive properties of soldered joint were obtained. It was also found that the addition of rare earth Nd could refine the microstructure of the solder, but some dark NdSn₃ phase appeared when the addition of Nd exceeded 0.15 wt%. Moreover, the IMCs thickness at the solder/Cu interface was reduced with the addition of Nd which gave a favorable influence on the mechanical property of the soldered joints.     

Effects of indium addition on properties and wettability of Sn–0.7Cu–0.2Ni lead-free solders

This paper reports the investigation on indium addition into Sn–0.7Cu–0.2Ni lead-free solder to improve its various performances. The effects of indium addition on melting temperature, coefficient of thermal expansion (CTE), wettability, corrosion resistance and hardness of the solder alloys were studied. The results showed that when the addition of indium was ⩽0.3 wt.%, the change in melting temperature of Sn–0.7Cu–0.2Ni–xIn solders was negligible, but the melting range of the solder alloy increased. The CTE and spreading area of Sn–0.7Cu–0.2Ni–xIn solders on copper both increased with the addition of indium. An optimal CTE was 17.5 × 10⁻⁶/°C by adding 0.3 wt.% indium. At this concentration, the spreading area of solder on copper was about 15.6% larger than that of Sn–0.7Cu–0.2Ni indium-free solder. The corrosion resistance also increased with the addition of indium increasing, and the corrosion rate of Sn–0.7Cu–0.2Ni–0.3In solder was reduced by 32.8% compared with Sn–0.7Cu–0.2Ni alloy after 14 days in 5% hydrochloric acid solution at room temperature. However, a decrease of 11.7% in hardness of the solder was found when 0.3 wt.% indium was added.