Impression creep behaviour of tin based lead free solders

Abstract

Growing concern about the toxic effects of lead in conventional solders has prompted the development of lead free solders. Creep owing to heating in service is one of the causes of solder joint failures in electronic packages. The present study deals with the impression creep behaviour of eutectic Sn – 58Bi, Sn – 57Bi – 1˙3Zn and Sn – 38Pb alloys in the temperature range 303 – 393 K and stress range 2˙6 – 180 MPa. Power law creep with stress exponent n varying from 2 to 6˙3 is observed. All the alloys reveal a strong stress dependence of activation enthalpy with values 155, 120 and 112 kJ mol^-1 for Sn – 58Bi, Sn – 57Bi – 1˙3Zn and Sn – 38Pb, respectively, which are well above those for self-diffusion. The steady state impression velocity varies linearly with punch diameter for all three alloys. It is concluded that a mechanism such as forest intersection involving attractive junctions controls the creep flow in these alloys.     

A life prediction model for the chemical failure of FeCrAlRE alloys: preliminary assessment of model extension to lower temperatures

Abstract

FeCrAl alloys are being deployed increasingly for industrial applications at elevated temperatures, primarily because their environment protection derives from the formation of an alumina scale. However, such protection is limited ultimately by chemical failure of the scale resulting in catastrophic, non-protective corrosion rates. The accurate prediction of component failure, therefore, is essential for economic and safety reasons.

A model for the chemical failure of alumina scales has been developed, which predicts the lifetimes of commercial FeCrAlRE alloys in oxidising environments. This model is based on the consumption of aluminium in the alloy through scale growth and also takes into account the effect of mechanical scale failure/spallation accelerating the rate of aluminium depletion – over the temperature range (1100–1400°C) this model has been shown to be applicable, when the scale formed was α-Al₂O₃. Its growth and failure processes are essentially similar over the range, although, of course, rates are temperature dependent.

In other potential technological applications, for example automotive catalytic converters, service temperatures can be much lower, in some instances as low as 750°C, while component sections are considerably thinner than for the high temperature (≥1100°C) structural applications.

At these lower temperatures scale growth mechanisms are more complex, involving the formation initially of transitional aluminas, which in some instances transform with time into the more stable α alumina. Additionally, the role of the reactive element (RE) and also of the mechanical interaction between substrate and scale can vary over the lower temperature range.

Component lifetime prediction at these temperatures is vital also. So, to initiate this action, this paper will present a preliminary assessment of the ability of the existing high temperature model to predict the lifetime of FeCrAlRE alloys in oxidising environments at temperatures in the range 750–1050°C.     

Mechanical alloying – the development of strong alloys

Abstract

Mechanical alloying is a solid-state process for making alloys by high-energy milling, under conditions such that constituent powders are repeatedly fractured and welded together and ever more intimately mixed. After subsequent consolidation at elevated temperature, the alloys can be shaped by rolling, forging, and machining. The process is used to incorporate a fine dispersion of ceramic particles. Mechanically alloyed nickel-base superalloys, combining a dispersion of yttria with conventional precipitation strengthening, have achieved higher strength at 900–1100°C than directionally solidified and single-crystal alloys, and are being used for gas-turbine vanes and blades. Mechanically alloyed ferritic stainless steel, with outstanding strength and corrosion resistance at temperatures as high as 1300°C, has been produced as sheet, tube, plate, rings, and forgings. Mechanically alloyed aluminium alloys also offer higher strength, e.g. in as-forged thick sections of Al–Mg–Li alloy.

MST/567     

Effect of compositional changes and impurities on wetting properties of eutectic Sn–Bi alloy used as solder

Abstract

As the number of layers on multilayer printed-circuit boards increases, the interlayer bonding requirements become progressively more stringent. One way of easing interlayer stresses is to reduce the preheat-soldering temperature interval. To do this by raising the preheat temperature is damaging to both board and components, and one alternative is to choose a solder alloy with a lower melting point than conventional Sn-Pb solders. In this paper, a study is described of the wetting characteristics of a range of near-eutectic Sn-Bi alloys, whose melting points fall near 139°C, ~46 K lower than Sn-Pb solders, making possible a reduction of 55 K in soldering temperature. The effect of divergence of composition from the 42Sn-58Bi eutectic composition on wetting, wetting speed, and spread area is described for two flux systems on plain copper, immersiontinned copper, and palladium-coated substrates. Additionally, the effects of various impurity levels of Sb, As, Cu, Fe, Pb, Ni, and Pd on solderability are investigated, the type of defects which each produces being listed. Some difficulties involved in contamination investigations on Al and S are also discussed. Finally, the effects of these impurities on Sn-Pb and Sn-Bi eutectic solder alloys are compared.

MST/88     

Superplastic behaviour of Inconel 718 sheet

Abstract

Inconel 718 is a nickel based alloy used extensively in the aerospace industry, having good service capabilities, in terms of strength and fatigue resistance, at high temperatures. Inconel 718, in the form of sheet, has the capability of being shaped using gas pressure forming techniques similar to those used for a number of aluminium and titanium based alloys. An extensive research programme has been carried out to investigate the high temperature formability of this alloy. This has involved both uniaxial tensile testing to determine such parameters as flow stress and strain rate sensitivity, and microstructural examination to investigate grain stability under both static heating and following deformation. The forming characteristics of the material have been correlated with the δ phase solvus temperature determined using SEM techniques. Optimum forming temperatures and strain rates are discussed.     

Erosion–corrosion of preoxidised Incoloy 800H in fluidised bed environments: effects of temperature,velocity, and exposure time

Abstract

The effects of preoxidation as a potential protective measure for alloys exposed to erosion–corrosion have been evaluated for Incoloy 800H in laboratory simulated fluidised bed conditions. The performance of the specimens after exposure to such environments was estimated from weight change data, electron microscopy, and X-ray diffraction patterns. The results showed that, in ‘corrosion dominated’ conditions, i.e. at relatively high temperatures and low velocities, preoxidation was successful in reducing the erosion–corrosion of the underlying alloy. However, when the velocity was increased and the temperature decreased, preoxidation afforded only some short term protection to the alloy. In these more ‘erosion–corrosion dominated’ regimes, preoxidation delayed the incubation period for erosion–corrosion of the alloy, but, once the scale was removed, the erosion–corrosion rates were similar to those of the non-preoxidised alloys. The effects of temperature, velocity, and exposure time are discussed for the preoxidised and non-preoxidised alloys. Comparisons are made between the results of the present and other erosion–corrosion studies, to explain the distinctive pattern of alloy wastage in these environments.

MST/1432     

The mechanism of early oxidation stages of Fe20Cr5Al-type alloys at 1123 K

Abstract

Fe20Cr5Al-type alumina forming alloys are known as α alumina-forming materials at relatively high temperatures (≥1273 K). This paper reports on the oxidation mechanism of commercial and synthetic Fe20Cr5Al alloys, at a considerably lower temperature, 1123 K. Some of the alloys were implanted with oxygen or yttrium ions prior to the oxidation exposure.

Two-stage-oxidation, secondary ion mass spectrometry, secondary electron microscopy and lowangle X-ray diffraction were used as experimental tools. Results showed that even prolonged exposures are not sufficient to ensure that the scale consists essentially of α-Al₂O₃. Mechanisms involved from the observed scale morphology, microstructure and phase composition are discussed     

Mechanical properties of In-based eutectic alloy solders used in Josephson packaging

InBiSn and InSn eutectic alloy solders are used in the packaging of the cryogenic Josephson processor. The ductile behaviour of these alloys is important to the application of joints which experience large thermal stresses. In order to characterize the mechanical behaviour at cryogenic temperatures, tensile and shear strengths of bulk solders were measured at and below room temperature. It is found that the ultimate tensile and shear strengths of bulk solders increase as temperature decreases. At low temperatures, the ultimate tensile strength of InBiSn alloy is about three times less than that of InSn alloy. The ductility of both alloys reduces at low temperatures. The fracture surfaces of both bulk solders and solder joints having an interface material (Pd and Au thin films) to the electrical contact pads (Nb thin film) were examined using SEM. Ductile fracture mode was observed for all the specimens down to liquid nitrogen temperature.     

Designing metal matrix composites to meet their target: particulate reinforced aluminium alloys for missile applications

Abstract

Aluminium alloys have traditionally been used for the manufacture of missile structural parts. As the performance of missiles improves, kinetic heating will increase and the continued use of conventional monolithic aluminium alloys for the manufacture of structural parts for short to medium range missiles will be limited by their elevated temperature performance. Steel or titanium alloys could be used but these may add weight or be more expensive. The alternative is to exploit the potential benefits of the high specific properties of aluminium based metal matrix composites (MMCs) which may be substantially retained during short term exposure to elevated temperatures. The improvements in strength and stiffness could enable weight savings to be made by reductions in wall thickness of the missile structure or alternatively, the improved stiffness of components such as wings and fins may prove beneficial for improved missile accuracy. The aim of the present paper is to highlight the results of demonstrator programmes which have been designed to assess the potential benefits of both particulate and fibre reinforced aluminium based MMCs for missile applications.     

Development of intermetallic materials for aerospace systems

Abstract

Certain intermetallic materials have undergone an evolutionary process whereby application of some of them could provide major benefits in aerospace systems. The realisation of the potential of intermetallic alloys based on Ti₃Al has provided significant hope for making still greater advances in turbine performance through further developments in other intermetallic materials. However, examination of the past four years of progress toward this goal has highlighted the problem that much of the fundamental understanding of process–structure–property relationships in these materials, which is the technology base upon which their application relies, has simply not been developed and suggests that widespread implementation of these materials lies in the distant future. This paper briefly discusses the problems of employing intermetallics in aerospace systems, reviews recent research progress on selected intermetallic alloys currently under investigation as high temperature structural materials, and assesses the status of intermetallics as turbine engine materials. Specifically, advances and findings from studies carried out during the past few years on alloys of the titanium and nickel aluminides and on intermetallics that are intended for service at temperatures above 1000°C are discussed. Technical challenges and factors affecting the pace of development are highlighted throughout.

MST/1562     

Microstructure evolution in bulk and surface states of chromium rich nickel based cast alloys reinforced by hafnium carbides after exposure to high temperature air

Abstract

Three alloys based on nickel, with a high level of chromium (25 wt-%) and containing varied quantities of carbon, 0·25 and 0·50 wt-%, and hafnium, 3·7 and 5·6 wt-%, fabricated by casting, were subjected to a 46 h long exposure at 1200°C in dry industrial air. The aim of the work was to investigate the thermal stability of their carbide interdendritic network and to discover their general behaviour in high temperature oxidation. The volume fraction of the hafnium carbides decreased slightly during high temperature exposure but their fragmentation was rather limited. In contrast, chromium carbides appeared in the two alloys, which initially contained exclusively HfC, and this may result in a decrease in their high temperature properties. The evolution of the carbides appeared to be responsible for a moderate lowering of room temperature hardness. The behaviour of the three alloys during high temperature oxidation was very good, despite the unusually high content of hafnium. All were chromia-forming, although oxidation of Hf led to HfO₂ islands in the external scale and in the subsurface region. In summary, the behaviour of these three alloys showed that the HfC containing Ni–25Cr family is potentially interesting for applications at very high temperatures.     

Dynamic recrystallisation in hot deformed oxide dispersion strengthened MA956 and MA957 steels

Abstract

The hot deformation behaviour of the oxide dispersion strengthened stainless steels MA956 and MA957 has been studied. The alloys are made by a mechanical alloying process which leaves them in a very fine grained, cold deformed state immediately after consolidation. It is found that deformation is accompanied by dynamic recrystallisation, even when the deformation temperatures are far less than the ordinary recrystallisation temperatures of the two alloys. These and other results on the strength and anisotropy of the alloys are interpreted in terms of their microstructures.

MST/3043     

Extrusion processing of Al–Li–Mg–Zr alloy

Abstract

The hot deformation behaviour of an Al–Li–Mg–Zr alloy was characterised in hot torsion and extrusion. The alloy was found to have similar hot ductility to existing high strength aluminium alloys, but this could be maintained at higher temperatures. Billets were extruded over a range of process conditions and a limit diagram was constructed for surface cracking. All the extrusions were found to be partially recrystallised after deformation, but the volume fraction of recrystallisation was a strong function of billet temperature and extrusion ratio. In addition, the unrecrystallised areas contained a recovered substructure where the subgrain size was inversely proportional to the temperature compensated strain rate. The as extruded structure was retained during solution treatment and as a result final mechanical properties were strongly dependent on the extrusion conditions. The use of high billet temperatures and low extrusion ratios gave the best combination of strength and toughness.

MST/839     

Creep crack growth in AI–Li alloy 8090 sheet

Abstract

Recrystallised and unrecrystallised Al–Li–Cu–Mg–Zr alloy 8090 sheet and Al–Cu alloy 2024 sheet have been tested to determine their relative resistance to creep cracking. Creep cracking in recrystallised 8090 sheet occurred at much lower stress intensity factors K, at higher rates for a given value of K, and at lower temperatures, compared with unrecrystallised 8090 or 2024 sheet. For recrystallised sheet, significant rates of creep cracking were observed at temperatures as low as 60°C, at K values of ~10%K_c (the critical value of K for overload fracture). Creep crack growth was predominantly intergranular for recrystallised 8090 and 2024 sheet, and a mixture of intersubgranular and transgranular for unrecrystallised 8090 sheet. The fractographic features, as well as other observations, suggest that 8090 alloys contain low melting point sodium-rich phases which are not found in conventional alloys. It was concluded that the presence of these phases and the continuous, intergranular crack paths approximately normal to the applied stress in recrystallised 8090 sheet were responsible for its poor resistance to creep cracking.

MST/1482     

Low temperature processing of NiZn ferrite by citrate precursor method and study of properties

Abstract

Nickel zinc ferrite of composition Ni_0·5 Zn_0·5 Fe₂O₄ was prepared by the citrate precursor method at temperatures as low as 100°C. This is much lower than those used in the conventional ceramic method for the preparation of ferrites (~1000°C). The dc resistivity of the sintered specimens was observed to be ~10⁸ Ω cm which is greater, by at least two orders of magnitude, than that for specimens prepared by the conventional method. Although the initial permeability values are similar to those for conventionally prepared specimens, the losses are lower and the operational frequency range is larger. The significance of the citrate method lies, therefore, in producing ferrites with better properties at reduced processing temperatures.     

Synergism between effects of velocity,temperature, and alloy corrosion resistance in laboratory simulated fluidised bed environments

Abstract

In studies of the erosion of alloys at elevated temperature, the combined effects of velocity, temperature, and alloy corrosion resistance are not well understood. Wide variations in the effects of velocity have been observed for alloys of different corrosion resistance in various erosion–corrosion environments. There is also some evidence that temperature can affect this relationship. The object of the present work was to undertake a systematic study of the effects of erodent velocity for two alloys, mild steel and 310 stainless steel, at elevated temperatures (300 and 600°C). The velocity was controlled at values between 1·5 and 4·5 m S^?l. Weight change data and analytical scanning electron microscopy were used to characterise the degradation in the various conditions. The results showed that the ranking order of the erosion–corrosion rates of the two different alloys varied as a function of velocity. The velocity at which the ranking of the two alloys reversed increased with increasing temperature. The reasons for such behaviour are discussed in terms of the dependence of the erosion–corrosion rate on the velocity in the various erosion–corrosion regimes.

MST/3188     

Reliability of Pb-free solders for harsh environment electronic assemblies

Abstract

Since 2006 and the implementation of environmental regulations, the electronic industry has moved to Pb-free solders. Harsh environment industries that were exempted from the regulations will soon have to follow suit. However, a suitable replacement solder for use in harsh environments still has to be validated and reliability models are yet to be established. In this review, research that led to the selection of currently used Pb-free alloys and the continuing search for high reliability alloys are described. Sn pest and Sn whiskers, potential major threats for electronics operating in harsh environments, are highlighted. This review also focuses on the microstructure, mechanical properties and deformation mechanisms of Pb-free alloys. Emphasis is placed on Sn–Ag–Cu alloys, now considered to be the alloys of choice for replacement of Sn–Pb solders. The reliability of Pb-free electronic assemblies is studied, focusing on thermal fatigue, believed to be the main source of failure through creep–fatigue mechanisms. The validity of models for Pb-free solder joints life time prediction is assessed and the lack of cohesiveness among the available reliability data is examined.     

Short communication Effects of tantalum addition on transformation behaviour of (Ni51 Ti49 )1-x Tax and Ni50 Ti50-y Tay shape memory alloys

Abstract

The effect of Ta content on the transformation characteristics of Ni–Ti–Ta ternary alloys has been studied. In (Ni₅₁ Ti₄₉ )_1-x Ta_x type alloys, the phase transformation temperatures increase with Ta content, especially when the Ta content is less than 4 at.-%. In Ni₅₀ Ti_50-x Ta_x type alloys, the phase transformation temperatures decrease as Ta content increases. The martensite start temperature is less sensitive to changes in Ni content in ternary Ni–Ti–Ta alloys than that in Ni–Ti binary alloys. The phase transformation temperatures of Ni–Ti–Ta ternary alloys are mainly controlled by the Ni/Ti ratio in the Ni–Ti matrix.     

NiTiHf-based shape memory alloys

Abstract

NiTiHf-based shape memory alloys have been receiving considerable attention for high temperature, high strength and two-way shape memory applications since they could have transformation temperatures above 100°C, shape memory effect under high stress (above 500 MPa) and superelasticity above 100°C. Moreover, their shape memory properties can be tailored by microstructural engineering. However, NiTiHf-based alloys have some drawbacks such as low ductility and high slope in stress induced martensite transformation region. In order to overcome these limitations, studies have been focused on microstructural engineering by aging, alloying and processing. It has been revealed that microstructural control is crucial to govern the shape memory properties (e.g. transformation temperatures, matrix strength, shape recovery strain, twinning type, etc.) of NiTiHf-based alloys. A summary of the most recent improvements on selected NiTiHf-based systems is presented to point out their significant shape memory properties, effects of alloying, aging and microstructure of transforming phases and precipitates.     

Influence of nanoparticle addition on the formation and growth of intermetallic compounds (IMCs) in Cu/Sn–Ag–Cu/Cu solder joint during different thermal conditions

Abstract

Nanocomposite lead-free solders are gaining prominence as replacements for conventional lead-free solders such as Sn–Ag–Cu solder in the electronic packaging industry. They are fabricated by adding nanoparticles such as metallic and ceramic particles into conventional lead-free solder. It is reported that the addition of such nanoparticles could strengthen the solder matrix, refine the intermetallic compounds (IMCs) formed and suppress the growth of IMCs when the joint is subjected to different thermal conditions such as thermal aging and thermal cycling. In this paper, we first review the fundamental studies on the formation and growth of IMCs in lead-free solder joints. Subsequently, we discuss the effect of the addition of nanoparticles on IMC formation and their growth under several thermal conditions. Finally, an outlook on the future growth of research in the fabrication of nanocomposite solder is provided.