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 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.     

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.     

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 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.     

Effect of Bi content on spalling behavior of Sn–Bi–Zn–Ag/Cu interface

The effect of the Bi content on the formation of intermetallic compounds (IMCs) layers between the Sn-xBi-0.9Zn-0.3Ag lead-free solder (with x = 1, 2, 3 and 4, in weight percent, hereafter) and Cu substrate was investigated. The structure of the IMC layer in the soldered interface varies apparently with increasing the Bi content. When the Bi content is 1 wt%, the interface soldered is consisted of CuZn and Cu₆Sn₅ IMC layers, which are separated by an intermediate solder layer. As the Bi content increases, the spalling phenomenon tends to disappear. Moreover, the layer between the Sn-2Bi-0.9Zn-0.3Ag solder and Cu substrate is thicker than others. The evolution of the soldered interfacial structure could be attributed to the existence of Bi.     

Influence of minor POSS molecules additions on the microstructure and hardness of Sn3Ag0.5Cu–xPOSS composite solders

The influence of minor POSS (polyhedral oligomeric silsesquioxanes) molecules additions on the microstructure and hardness of SnAgCu–xPOSS (x?=?1, 3 and 5) was investigated. A mechanical mixture method was adopted by adding POSS molecules as dispersoids into SnAgCu solder to fabricate SnAgCu–xPOSS composite solders. The microstructural evolution and the hardness of the solders were investigated in details by microstructural observations, Vickers hardness tester and nanoindentation tests. The results showed that the effects of the dispersed POSS molecules in eutectic SnAgCu structure and the refined Ag₃Sn IMC particles increased the hardness of eutectic SnAgCu phase and then increased the hardness of composite solder. In addition, because of the agglomeration of POSS molecule, a coarse lath-shaped structure (composed of POSS molecules, Ag₃Sn phase and minor Cu₆Sn₅ phase) formed in SnAgCu–5POSS solder matrix, which reduced the hardness of SnAgCu–5POSS solder.     

Microstructural and mechanical properties of Sn–Ag–Cu lead-free solders with minor addition of Ni and/or Co

The effects of minor additives, that is, Co and Ni, on the microstructural and mechanical properties of Sn–3.0 mass%Ag–0.5 mass%Cu (SAC305) bulk solder were investigated. The addition of Co and/or Ni resulted in microstructural changes of the SAC305 solder, such as the formation of new intermetallic compounds (IMCs) and the refinement of grain size, as well as the suppression of undercooling. The single addition of Co in SAC305 solder resulted in the formation of CoSn₂ IMCs and undercooling suppression, whereas the single addition of Ni accelerated the appearance of rod-shaped (Cu,Ni)₆Sn₅ IMCs inside the β-Sn dendrites during the solidification process. The dual addition of Co–Ni resulted in refined β-Sn grains and suppression of undercooling, as well as the formation of CoSn₂ IMCs. In tensile tests, Co and/or Ni additives had little effect on the tensile strength of SAC305 solder, but obviously suppressed the elongation ratio and reduction of area. During tensile deformation in samples with existing thin plate-like CoSn₂ IMCs, micro-cracks or cavities were easily initiated through the interface between CoSn₂ and the solder matrix, which was responsible for the decrease of ductility.     

The effects of temperature gradient and growth rate on the microstructure of directionally solidified Sn–3.5Ag eutectic solder

The mechanical properties (microhardness, tensile strength) of alloys are controlled by their microstructure, which depends strongly on temperature gradient (G) and growth rate (V). Thus, it is important to understand the relationships among G, V and microstructure (rod eutectic) of Sn–Ag solders. The Sn–3.5 wt% Ag eutectic alloy was directionally solidified upward with a constant growth rate, V (16.5 μm/s) at different temperature gradients, G (1.43–4.28 K/mm) and with a constant temperature gradient, G (3.93 K/mm) at different growth rates, V (8.3–500 μm/s) in a Bridgman–type directional solidification furnace. The rod spacings (λ) have been measured from both longitudinal section (parallel to the growth direction, λ _L ) and transverse section (perpendicular to the growth direction, λ _T ) of the samples. The undercooling values (ΔT) were calculated by using V, λ and system parameters (K ₁ and K ₂). It was found that the values of λ (λ _T , λ _L ) decrease while V and G are increasing. The relationships between rod spacing and solidification parameters (G and V) were obtained by linear regression analysis. The dependences of eutectic spacings λ on undercooling (ΔT) are also analyzed. λ² V, λΔT, ΔTV ^−0.5 and ΔTG ^−0.5 values were determined by using λ, ΔT, V and G values. The results obtained in this work are compared with the Jackson–Hunt eutectic theory and the similar experimental works. The experimental l_\textT ² \textV \lambda_{\text{T}}^{ 2} {\text{V}} value (159.3 μm³/s) is slightly lower than the result 174.6 μm³/s calculated from Jackson–Hunt eutectic theory.     

Effect of cooling aging on microstructure and mechanical properties of an Al–Zn–Mg–Cu alloy

In the present work, Al–Zn–Mg–Cu alloy was aged by non-isothermal cooling aging treatment (CAT). At high initial aging temperature (IAT), the hardness was decreased with the decreased cooling rate. However, when IAT was lower than 180 °C, the hardness was increased with the decreased cooling rate. Conductivity was increased with the decreased cooling rate regardless of IAT. The tensile strength, yield strength and conductivity of Al alloy after (200–100 °C, 80 °C/h) CAT were increased 2.9%, 8.1% and 8.3% than that after T6 treatment, respectively. With an increase of IAT and decrease of cooling rate, the fine GP zone and η′ phase were transformed to be larger η′ and η precipitates. Moreover, continuous η phase at grain boundary was also grown to be individual large precipitates. Cooling aging time was decreased about 90% than that for T6 treatment, indicating cooling aging could improve the mechanical properties, corrosion resistance and production efficiency with less energy consumption.     

Effect of trace additions of Sn on microstructure and mechanical properties of Al–Cu–Mg alloys

The work is aimed at investigating the influence of trace additions of Tin (Sn) on the microstructure, mechanical properties and age-hardening behavior of Al–6.2%Cu–0.6%Mg alloy system. Al–6.2%Cu–0.6%Mg alloys containing varying weight percentages (from 0 to 0.1 wt.%) of Sn were prepared by casting technique. The mechanical properties and microstructure of these alloys were investigated in the as-cast as well as different heat treated conditions. The composition of the different phases present in the microstructure was determined by energy dispersive X-ray (EDS) analysis. The average grain size of the annealed alloy was found to be maximum with trace content of 0.06 wt.% Sn. The hardness and strength of the alloy increased but the ductility reduced with increase in Sn content up to 0.06 wt.%. Precipitation hardening behavior of the alloys was investigated by analyzing the aging time required to attain the peak hardness value. Addition of trace percentage of Sn was observed to have no significant influence on the peak ageing time of the investigated alloy system.     

Harsh service environment effects on the microstructure and mechanical properties of Sn–Ag–Cu-1 wt% nano-Al solder alloy

This paper investigates the electrical and mechanical performances of eutectic Sn-3Ag-0.5Cu (wt%) solder with the addition of Al nanoparticles. The study revealed that the elastic moduli, electrical resistivity and damping properties of such solder alloy were improved. Further, interfacial reaction phenomena on Au/Ni-plated Cu pad ball grid array substrate during isothermal aging and thermal cycle was evaluated in terms of the formation and growth kinetics of intermetallic compound (IMC) layer. A structural analysis confirmed that at their interfaces a ternary (Cu, Ni)-Sn IMC layer was adhered at the substrate surface. The thickness of this IMC layer was increased with increasing the duration of the isothermal aging and thermal cycle without any defects. In addition, the formation of Ag₃Sn, Cu₆Sn₅, Sn–Al–Ag and AuSn₄ IMC phases were evenly distributed in the solder matrix which acts as the second phase reinforcement. The measured shear strength and microhardness indicated that the exposure of the solder joints to the thermal cycles make the joints degraded faster than the situation in isothermal aging.     

Liquid–solid interfacial reactions of Sn–Ag and Sn–Ag–In solders with Cu under bump metallization

Intermetallic compounds formed during the liquid–solid interfacial reaction of Sn–Ag and Sn–Ag–In solder bumps on Cu under bump metallization at temperatures ranging from 240 to 300 °C were investigated. Two types of intermetallic compounds layer, η Cu₆Sn₅ type and ε Cu₃Sn type, were formed between solder and Cu. It was found that indium addition was effective in suppressing the formation of large Ag₃Sn plate in Sn–Ag solder. During interfacial reaction, Cu consumption rate was mainly influenced by superheat of solder, contact area between solder and Cu and morphology of intermetallic compounds. The growth of η intermetallic compounds was governed by a kinetic relation: ΔX = tⁿ, where the exponent n values for Sn–Ag/Cu and Sn–Ag–In/Cu samples at 240 °C were 0.35 ± 0.01 and 0.34 ± 0.02, respectively. The n values increased with reaction temperature, and it was higher for Sn–Ag/Cu than that for Sn–Ag–In/Cu sample at the same temperature. After Cu was exhausted, ε intermetallic compound was converted to η intermetallic compound. The mechanisms for such growth of interfacial intermetallic compounds during the liquid–solid reaction were investigated.     

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.     

Effect of tin on melting temperature and microstructure of Ag–Cu–Zn–Sn filler metals

Abstract

To develop low melting point filler metals for brazing TiNi shape memory alloy (SMA) and stainless steel (SS), a series of Ag–22Cu–Zn–Sn (wt-%) filler metals have been studied. Using differential thermal analysis (DTA) analysis, the melting temperatures of Ag–22Cu–Zn–Sn filler metals were determined. The results show that the increase of zinc and tin contents drastically decreases the solidus and liquidus temperatures of the Ag–22Cu–Zn–Sn filler metals and the melting temperatures of the Ag–22Cu–18Zn–Sn filler metals with 5–8 wt-%tin are < 650°C. Metallographic observations indicate that the increase of zinc and tin in the Ag–22Cu–Zn–Sn filler metals helps the formation of eutectic structure and inhibits the formation of α-Ag and α-Cu solid solutions, but the increase of tin also causes the formation of Ag₃Sn and Cu₄₁Sn₁₁ brittle compounds. The results of mechanical property tests of the laser brazed joints of TiNi SMA and SS show that the proper increase of zinc and tin in Ag–22Cu–Zn–Sn filler metals is favourable for improving the strength of the laser brazed joints of TiNi SMA and SS.     

Effect of Cu/Zn on microstructure and mechanical properties of extruded Mg–Sn alloys

In this paper, the effect of Cu and Zn addition on mechanical properties of indirectly extruded Mg–2Sn alloy was investigated. Mg–2Sn–0.5Cu alloy exhibits a moderate yield strength (YS) of 225?MPa and an ultimate strength of 260?MPa, which are much higher than those of the binary Mg–2Sn alloy, and the elongation (EL) evolves as ～15.5%. Mechanical properties of the Mg–2Sn–0.5Cu alloy are deteriorated with more 3 wt-% Zn addition, and YS and EL are reduced as 160?MPa and ～10%. The detailed mechanism is discussed according to the work-hardening rate and strengthening effect related to the grain sizes, second phases and macro-textures. Grain refinement and proper texture are believed to play a critical role in both strength and ductility optimisation.     

Effects of Cu content on microstructure and mechanical properties of Al–14.5Si–0.5Mg alloy

This study elucidates how Cu content affects the microstructure and mechanical properties of Al–14.5Si–0.5Mg alloy, by adding 4.65 wt.% and 0.52 wt.% Cu. Different Fe-bearing phases were found in the two alloys. The acicular β-Al₅FeSi was found only in the high-Cu alloy. In the low-Cu alloy, Al₈Mg₃FeSi₆ was the Fe-bearing phase. Tensile testing indicated that the low-Cu alloy containing Al₈Mg₃FeSi₆ had higher UTS and elongation than the high-Cu alloy containing the acicular β-Al₅FeSi. It is believed that the presence of the acicular β-Al₅FeSi in the high-Cu alloy increased the number of crack initiators and brittleness of the alloy. Increasing Cu content in the Al–14.5Si–0.5Mg alloy also promoted solution hardening and precipitation hardening under as-quenched and aging conditions, respectively. The hardness of the high-Cu alloy therefore exceeded that of low-Cu alloy.     

Effect of aging treatment on microstructure and creep behaviour of Sn–Ag and Sn–Ag–Bi solder alloys

Abstract

The present study was undertaken to investigate the influence of aging temperature on the creep behaviour of Sn–Ag and Sn–Ag–Bi solder alloys at testing temperatures ranging from 333 to 363 K under constant stress of 7·80 MPa. The steady state creep rate was found to increase continuously with increasing aging temperature at all testing temperatures. Results show that addition of Bi to the binary Sn–Ag solder alloy led to a significant increase in the strength and improvement in the creep resistance. The activation energy for the creep process of Sn–Ag and Sn–Ag–Bi solder alloys was found to have an average value of 36 and 45 kJ mol^?1 respectively. This might be characterised by diffusion of Ag in Sn. The microstructure of the aged samples for both alloys examined by X-ray diffraction measurements supported the improvement in the creep resistance for Sn–Ag alloy by adding a small trace of Bi.