Reactions and mechanical properties between AuSn20 solders and metalized Al–Si alloys for electronic packaging application

AuSn20 (mass fraction) lead-free solder reacting with the Au/Ni-metalized AlSi50 substrate during reflowing and aging processes were investigated in this study. The single lap shear strength, fracture behavior and microstructure evolution characteristics of the joints are detected. It is found that only a thin (Ni,Au)₃Sn₂ layer forms at the interface between the AuSn20 solder and Ni metalized AlSi50 alloy. But a composite Intermetallic compound (IMC) layer of (Ni,Au)₃Sn₂ and (Au,Ni)Sn is formed in the aged joints, due to the continuous interfacial reactions during aging process. The growth of the composite IMC layer is governed by the volume diffusion of the constituent elements at 120, 160 and 200 °C. The shear strength decreases with the increasing aging time and temperature, which is caused primarily by the growth of the IMC layer. The presence of faceted structures on the fracture surfaces of these specimens is indicative of a brittle failure mode for the joints.     

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.     

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.     

Whisker microcrystals on the surface of tantalum–cadmium alloy films

Technical Physics Letters - Whiskers crystals were discovered in the tantalum–cadmium (56.6 at % Cd) solid solutions formed by in-turn deposition of the superfine tantalum and cadmium...     

Time and temperature dependent mechanical characterization of polymer‐based materials in electronic packaging applications

Abstract

The thermo‐mechanical testing of high performance polyimide films Type HPPST supplied by Dupont^® was conducted at different strain rates and in different temperature environments. The stress‐strain behavior of materials was investigated, and the dependence of Young's modulus on temperature and strain rate is reported. In view of the uncertainty of the Young's modulus determination, the specimens were tested with unloading‐reloading to verify the test results. Constant strain rate uniaxial tensile tests and long‐time creep tests at various temperatures were performed to characterize the time‐temperature‐dependent mechanical property precisely. Cyclic loading tests were also implemented on specimens to investigate cyclic stress‐strain behaviors. This research is expected to enhance finite‐element‐modeling accuracy and characterize material properties precisely.     

Microstructure and thermal expansion behavior of spray-formed Al–27Si alloy used for electronic packaging

The hypereutectic Al–27Si (mass fraction) alloys are prepared by spray deposition and extrusion. The effect of thermal aging process on the coefficient of thermal expansion (CTE) and microstructure of the Al–27Si alloys are investigated. The results show that the distribution of Si particles in α-Al matrix is uniform, and the primary Si phase grows gradually during the process of thermal aging. The CTE between room temperature to 100 °C increases gradually with the ascending of aging temperature, attributed to the relaxation of residual thermal stress and the coarsening of primary Si phases in the alloys. On the other hand, the CTE increases linearly as the cycling temperature increases up to 500 °C, and the measured values are in good agreement with the Kerner model.     

Grain growth behaviour on reheating Al–Nb-containing steel in the homogenised condition

ABSTRACT

Grain size development during reheating is important to the mechanical properties of steels, and non-uniform grain growth including abnormal and bimodal grain growth are undesirable for steel manufacturing. In this paper, reheating treatments for Al–Nb-containing steel (0.057?wt-% Al and 0.019?wt-% Nb) were selected based on Thermo-Calc predictions for the dissolution temperatures of precipitates in the steel for a fully homogenised condition. Abnormal grain growth occurred in the homogenised Al–Nb sample during reheating, associated with random dissolution of the grain boundary-pinning AlN precipitates, while bimodal grain growth was not observed. So material in a homogenised condition would not be expected to provide a fully uniform grain structure during reheating treatment.     

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.     

Effect of TiO2 addition concentration on the wettability and intermetallic compounds growth of Sn3.0Ag0.5Cu–xTiO2 nano-composite solders

In the present study, addition of TiO₂ nanoparticles with a concentration in the range from 0 to 0.75 wt% into eutectic Sn3.0Ag0.5Cu solders were prepared. The effect of TiO₂ addition concentration on intermetallic compounds (IMC) growth in solder matrix, wettability of the composite solder, and interfacial IMC growth at composite solder/Cu interface were studied respectively. The microstructure images show that both IMC growth in solder matrix and interfacial IMC growth at solder/Cu interface were suppressed when TiO₂ nanoparticles are added into the Sn3.0Ag0.5Cu solder system, meanwhile, wettability test results show wetting time reduction and wetting force enlargement with TiO₂ nanoparticles addition concentration increasing. These results reveal that the added TiO₂ nanoparticles in solders work as reinforce agent and enhance solder performance by reducing IMC dimension and improving wettability. However, TiO₂ addition concentration is critical to the improvement extent. The matrix IMC size and interfacial IMC thickness were reduced significantly with the TiO₂ addition concentration increasing in small addition range (0, 0.1 and 0.25 wt%). The most significant suppression appears when TiO₂ concentration is about 0.25 wt%. Beyond this concentration, the matrix IMC size and interfacial IMC thickness increase, but still smaller than non-added Sn3.0Ag0.5Cu solder. Sn–Ag–Cu (SAC)–0.25TiO₂ exhibits most obviously refined solder microstructure. The variation of wetting time and wetting force with the change of TiO₂ concentration are similar. Addition of 0.25 wt% TiO₂ shortens wetting time and strengthens wetting force most effectively. SAC–0.25TiO₂ exhibit best wettability performance as well. The IMC variation consistent with wettability variation revels there is an optimal TiO₂ addition concentration which is about 0.25 wt%. Both insufficient adding and excessive adding will weaken the TiO₂ nanoparticle reinforcement extent. Based on the theory of adsorption and agglomeration, a mechanism of the TiO₂ nanoparticle concentration effect and the optimal addition point was proposed.     

The effects of solder deformation on the wetting characteristics and interfacial reaction of Sn–3.5Ag solders on Cu substrates in fluxless soldering

The effects of solder deformation on the wetting characteristics during fluxless soldering were studied when deformed Sn–3.5Ag solder balls were reacted with Cu or oxidized Cu substrates. The Cu surfaces were oxidized at 100 °C for 2 or 4 h in air. After the 760 μm diameter solder balls were deformed on the substrates under 0–30 N, they were then reflowed at 300 °C for 30 s without flux. An optical microscope and a scanning electron microscope equipped with energy dispersive spectroscopy were used to measure the wetting angles and to characterize interfacial microstructures. As solder deformation increased, the wetting angle of solder bumps on the Cu or oxidized Cu substrates decreased and the spreading area increased. The oxide layer on the Cu surface decreased the wettability of the solders. Intermetallic compound (IMC) growth was suppressed in the solder interface when the solder reacted with oxidized Cu, while the IMC thickness increased with solder deformation. Solder deformation exposed a fresh Sn surface and improved contact between the solder and Cu substrate, thereby increasing the wettability of the solders.     

Interfacial reaction and growth behavior of IMCs layer between Sn–58Bi solders and a Cu substrate

This paper reports on the interfacial reaction and growth behavior of intermetallic compounds (IMCs) layer (η-Cu₆Sn₅ + ε-Cu₃Sn) between molten Sn–58Bi solder and Cu substrate for various liquid–solid soldering temperatures and times. In addition, the Bi segregation at the Cu₃Sn/Cu interface was also discussed, too. It was found that the Cu₆Sn₅ IMC could be observed as long as the molten solder contacted with the Cu substrate, while the Cu₃Sn IMC was formed at the interface between Cu₆Sn₅ and Cu substrate as the higher soldering temperature and/or longer soldering time were applied. Both thickness of total IMCs layer and Cu₆Sn₅ grains size increased with increased soldering temperature or time. The growth of the Cu-Sn IMCs layer during soldering exhibited a linear function of the soldering temperature and 0.27 power of soldering time. With soldering temperature increasing (above 280 °C in this present study), Bi was accumulated at the Cu₃Sn/Cu interface and resulted in some isolated Bi particles were formed.     

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.     

Effects of solidification structure on fatigue crack growth in rheocast and thixocast Al–Si–Mg alloys

Fatigue crack growth test was performed for rheocast and thixocast Al–Si–Mg aluminum alloys. At small stress intensity factor range (ΔK), fatigue crack growth (FCG) rate of sample with coarse acicular Si particles decreased slightly compared with specimen with small acicular Si particles. However, at large ΔK, fatigue crack growth rate of specimen with coarse acicular Si particles drastically increased. This is because large acicular Si particles induce high strain hardening at small ΔK, but such particles are easily cracked with the increase in ΔK. Morphology of the Si particles strongly affects striation formation.     

Phase-field modelling on effect of pressure on growth kinetics of Mg–Al–Sn alloy

The effects of pressure on dendritic growth kinetics of Mg–Al–Sn alloy were investigated using a ternary phase-field model coupled with thermodynamics with pressure effects and experiments. The results showed that the improved growth velocity and nucleation rate caused by pressure had the opposite effects on grain size. For one-grain growth, the dendrite was larger and more developed under pressure of 85?MPa than that under ambient pressure owing to larger thermodynamic driving force, and thus higher growth velocity. However, when nucleation was considered in multigrain growth case, the average grain size under pressure was smaller owing to less growth space. Growth velocity decreased with the increase in Sn content, on which pressure had no great influence.     

Post-impact fatigue damage growth in fiber–metal laminates

Fiber–metal laminates (FMLs) are a family of hybrid materials currently being considered for use in airframe structural applications. Post-impact fatigue strength tests were carried out on several varieties of GLAss REinforced (GLARE) aluminum laminates. The panels were impacted in a drop weight impact tower located at the Institute for Aerospace Research of the National Research Council of Canada. Observations made by other researchers that the internal impact damage in FMLs is confined to the immediate impact site were confirmed. The impacted specimens were cycled in tension–tension fatigue until failure. Cracks developed along side the dent and also at the edges of the gauge section of the specimen. Aluminum baseline specimens had significantly lower fatigue lives than the FML specimens. The stress-state surrounding the dent is complicated and contributed to unusual fatigue crack initiation behavior in some GLARE variants.     

Pearlite growth rate in Fe–C and Fe–Mn–C steels

The kinetic theory for the growth of pearlite in binary and ternary steels is implemented to ensure local equilibrium at the transformation front with austenite, while accounting for both boundary and volume diffusion of solutes. Good agreement is on the whole observed with published experimental data, although the reported growth rate at the lowest of temperatures is much smaller than predicted. To investigate this, experiments were conducted to replicate the published data. It is found that the cooperation between cementite and ferrite breaks down at these temperatures, and surface relief experiments are reported to verify that the resulting transformation product is not bainite.     

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.     

Shear strength of the Zn–Sn high-temperature lead-free solders

This study examines the shear strength behavior of the high-temperature Zn–20 wt% Sn, Zn–30 wt% Sn, and Zn–40 wt% Sn solders in the temperature range of 298–425 K. The results showed that increasing the Sn content of the alloys decreases both shear yield stress (SYS) and ultimate shear strength (USS) at all test temperatures. This can be attributed to the higher volume fraction of the softer β-Sn matrix and the eutectic α-Zn + β-Sn structure, which replaces the colonies of the harder α-Zn phase in the microstructure. The high shear strength of these high temperature solder alloys makes them suitable for application in harsh environments.