Search

To gain better understanding of rheological transitions from suspension flow to granular deformation and shear cracking, this research conducted shear-deformation on globular semi-solid Al-Cu alloys to study the rheological behavior of semi-solid as a function of solid fraction (38% - 85%) and shear rate (10-4 – 10-1 s-1) under real-time synchrotron radiography observation. By analyzing 17 X-ray imaging datasets, we define three rheological transitions: (i) the critical solid fraction from a suspension to a loosely percolating assembly; (ii) from the net contraction of a loose assembly to the net dilation of a densely packed assembly, and (iii) to shear cracking at high solid fraction and shear rate. Inspired by in-situ observations of semi-solid deformation showing a disordered assembly of percolating crystals in partially-cohesive contact with liquid flow, we reproduced a two-phase sample using the coupled lattice Boltzmann method-discrete element method (LBM-DEM) simulation approach for granular micromechanical modeling. In DEM, each globular Al grain is represented by a discrete element, and the flow of interstitial liquid is solved by LBM. The LBM-DEM simulations show quantitative agreement of semi-solid strain localization with the experiments and are used to explore the components involved in the shear rate dependence of the transitions, and the role of liquid pressure on the initiation of shear cracking.

Multi-axial compression of the mushy zone occurs in various pressurized casting processes. Here, we present a drained triaxial compression apparatus for semi-solid alloys that allow liquid to be drawn into or expelled from the sample in response to isotropic or triaxial compression. The rig is used to measure the pressure-dependent flow stress and volumetric response during isothermal triaxial compression of globular semi-solid Al-15 wt pct Cu at 70 to 85 vol pct solid. Analysis of the stress paths and the stress–volume data show that the combination of the solid fraction and mean effective pressure determines whether the material undergoes shear-induced dilation or contraction. The results are compared with the critical state soil mechanics (CSSM) framework and the similarities and differences in behavior between equiaxed semi-solid alloys and soils are discussed.

Antimony is attracting interest as an addition to Pb-free solders to improve thermal cycling performance in harsher conditions. Here, we investigate microstructure evolution and failure in harsh accelerated thermal cycling (ATC) of a Sn-3.8Ag-0.9Cu solder with 5.5 wt.% antimony as the major addition in two ball grid array (BGA) packages. SbSn particles are shown to precipitate on both Cu6Sn5 and as cuboids in β-Sn, with reproducible orientation relationships and a good lattice match. Similar to Sn-Ag-Cu solders, the microstructure and damage evolution were generally localised in the β-Sn near the component side where localised β-Sn misorientations and subgrains, accelerated SbSn and Ag3Sn particle coarsening, and β-Sn recrystallisation occurred. Cracks grew along the network of recrystallised grain boundaries to failure. The improved ATC performance is mostly attributed to SbSn solid-state precipitation within β-Sn dendrites, which supplements the Ag3Sn that formed in a eutectic reaction between β-Sn dendrites, providing populations of strengthening particles in both the dendritic and eutectic β-Sn.

The coarsening of Ag3Sn particles occurs during the operation of joints and plays an important role in failure. Here, Ag3Sn coarsening is studied at 125°C in the eutectic regions of Sn-3Ag-0.5Cu/Cu solder joints by SEM-based time-lapse imaging. Using multi-step thresholding segmentation and image analysis, it is shown that coalescence of Ag3Sn particles is an important ripening process in addition to LSW-like Ostwald ripening. About 10% of the initial Ag3Sn particles coalesced during ageing, coalescence occurred uniformly across eutectic regions, and the scaled size distribution histograms contained large particles that can be best fit by the Takajo model of coalescence ripening. Similar macroscopic coarsening kinetics were measured between the surface and bulk Ag3Sn particles. Tracking of individual surface particles showed an interplay between the growth/shrinkage and coalescence of Ag3Sn.

Rheological transitions from suspension flow to granular deformation and shear cracking are investigated in equiaxed-globular semi-solid alloys by combining synchrotron radiography experiments with coupled lattice Boltzmann method, discrete element method (LBM-DEM) simulations. The experiments enabled a deformation mechanism map to be plotted as a function of solid fraction and shear rate, including a rate dependence for the transition from net-contraction to net-dilation, and for the initiation of shear cracking. The LBM-DEM simulations are in quantitative agreement with the experiments, both in terms of the strain fields in individual experiments and the deformation mechanism map from all experiments. The simulations are used to explore the factors affecting the shear rate dependence of the volumetric strain and transitions. The simulations further show that shear cracking is caused by a local liquid pressure drop due to unfed dilatancy, and the cracking location and its solid fraction and shear rate dependence were reproduced in the simulations using a criterion that cracking occurs when the local liquid pressure drops below a critical value.

Orientation relationships (ORs) can form during solidification by a variety of mechanisms that are often difficult to distinguish after solidification. Here we study three ORs formed by the nucleation of Al3Ti on TiB2, and by the pushing and engulfment of TiB2 by growing Al3Ti facets in hyperperitectic Al-rich melts. The nucleation OR is identified by growing a relatively large TiB2 crystal, solidifying multiple small Al3Ti crystals on one (0001) facet of TiB2, and measuring the resulting OR by electron backscatter diffraction (EBSD). Pushing and engulfment ORs are investigated by statistical analysis of EBSD measurements, density functional theory (DFT) calculations of interface energies, and imaging of cross-sections of TiB2 particles being pushed and engulfed by Al3Ti facets. It is shown that the lowest energy OR is formed by nucleation as well as by pushing/engulfment. The higher energy ORs, formed by pushing and engulfment, correspond to local interfacial energy minima and can be explained by rotation of TiB2 particles on Al3Ti facets during pushing.

Multiple primary and eutectic intermetallic compounds (IMCs) often form during the solidification of multicomponent magnesium alloys. Here, we comprehensively explore eutectic intermetallic formation in Mg-4Sn-3Al-1.5Zn-0.6Mn (wt%) as-cast alloy, in comparison with Mg-3Al-1Zn-0.3Mn, to better understand Mg2Sn formation during complex multiphase solidification and how it affects the formation of other IMCs. Various IMCs formed at different stages of solidification: τ-AlMn and Al8Mn5 early during solidification and Mg2Sn, β-Mg17Al12, and τ-MgAlZn as late forming eutectic phases. Most Mg2Sn phase shared a faceted interface with β-Mg17Al12 and τ-MgAlZn grains, forming a fully divorced eutectic particle containing multiple phases. EBSD-based trace analysis in combination with XCT and TEM results revealed that β-Mg17Al12 and τ-MgAlZn phase grew adjacent to the {100} growth facets of Mg2Sn. Within multi-phase particles, seven types of reproducible orientation relationships (ORs) were measured between each intermetallic. The ORs between Mg2Sn and τ-MgAlZn were found to have the best lattice match among the observed phase pairs. For the defined ORs, Mg2Sn can be correlated with the icosahedral coordination in τ-MgAlZn.

In-depth understanding of microstructure development is required to fabricate high quality products by additive manufacturing (for example, 3D printing). Here we report the governing role of side-branching in the microstructure development of alloys by laser powder bed fusion. We show that perturbations on the sides of cells (or dendrites) facilitate crystals to change growth direction by side-branching along orthogonal directions in response to changes in local heat flux. While the continuous epitaxial growth is responsible for slender columnar grains confined to the centreline of melt pools, side-branching frequently happening on the sides of melt pools enables crystals to follow drastic changes in thermal gradient across adjacent melt pools, resulting in substantial broadening of grains. The variation of scan pattern can interrupt the vertical columnar microstructure, but promotes both in-layer and out-of-layer side-branching, in particular resulting in the helical growth of microstructure in a chessboard strategy with 67° rotation between layers., Understanding metal component microstructure during 3D printing remains a challenge. Here, the authors use local thermal parameters and the solidification microstructure to better understand how the printed microstructure varies with the laser scan strategy.

Al8Mn5 particles form as primary solidification phases in Mg-Al-based alloys and are important for ensuring adequate corrosion resistance. However, excessive Al8Mn5 formation above the α-Mg liquidus temperature can lead to sludge formation, and the clustering of Al8Mn5 particles on oxide films can generate deleterious casting defects. Here, we use analytical SEM and real-time synchrotron x-ray radiography to study Al8Mn5 particle settling, the formation of a sludge layer, and the mechanism of Al8Mn5 clustering on oxides in AZ91 containing two Fe levels. It is shown that settling Al8Mn5 can become trapped in entrained oxide and that new Al8Mn5 particles also seem to nucleate on oxide films. On cooling, these Al8Mn5 particles continue to grow, creating large Al8Mn5 clusters.

The mechanical properties of $\beta$-Sn single crystals have been systematically investigated using a combined methodology of micropillar tests and rate-dependent crystal plasticity modelling. The slip strength and rate sensitivity of several key slip systems within $\beta$-Sn single crystals have been determined. Consistency between the numerically predicted and experimentally observed slip traces has been shown for pillars oriented to activate single and double slip. Subsequently, the temperature-dependent, intermetallic-size-governing behaviour of a polycrystal $\beta$-Sn-rich alloy SAC305 (96.5Sn-3Ag-0.5Cu wt%) is predicted through a multi-scale homogenization approach, and the predicted temperature- and rate-sensitivity reproduce independent experimental results. The integrated experimental and numerical approaches provide mechanistic understanding and fundamental material properties of microstructure-sensitive behaviour of electronic solders subject to thermomechanical loading, including thermal fatigue.

The intermetallic Cu6Sn5 is ubiquitous in electronic interconnects where research has focused on controlling the size and distribution of this phase for improved performance. Cu6Sn5 also finds application as an anode material for advanced lithium-ion batteries. Cu6Sn5 anodes can be fabricated via an in-situ growth method involving the reaction between molten Sn and the Cu current collector. This manufacturing route offers some advantages over traditional anode fabrication however the process is slow, limiting its practical application. In this work we show the addition of 6 wt% Ni to the Cu current collector greatly accelerates the growth of (Cu,Ni)6Sn5 in Cu-xNi/Sn solid-melt couples, leading to a growth rate of up to 50x faster, reducing the processing time above 200 °C to less than 10 min. This research studies the dynamics of the formation of (Cu,Ni)6Sn5 between Cu-xNi alloys and liquid Sn through real-time observation using synchrotron X-ray imaging. The (Cu,Ni)6Sn5 growth dynamics are characterised, and the growth kinetics are analysed. Subsequently, the mechanism of the accelerated growth is investigated with electron backscatter diffraction and transmission electron microscopy. The results show the accelerated growth is due to the formation of η-(Cu,Ni)6Sn5 grains with two distinct Ni concentration ranges, leading to finer grains and spalling, which in turn facilitates the diffusion of Sn, enhancing the η-(Cu,Ni)6Sn5 formation kinetics.

Creep of directionally solidified Sn-3Ag-0.5Cu wt.% (SAC305) samples with near- orientation along the loading direction and different microstructural lengthscale is investigated under constant load tensile testing and at a range of temperatures. The creep performance improves by refining the microstructure, i.e. the decrease in secondary dendrite arm spacing (λ2), eutectic intermetallic spacing (λe) and intermetallic compound (IMC) size, indicating a longer creep lifetime, lower creep strain rate, change in activation energy (Q) and increase in ductility and homogeneity in macro- and micro-structural deformation of the samples. The dominating creep mechanism is obstacle-controlled dislocation creep at room temperature and transits to lattice-associated vacancy diffusion creep at elevated temperature ($$ \frac{T}{{T_{M} }} $$ T T M > 0.7 to 0.75). The deformation mechanisms are investigated using electron backscatter diffraction and strain heterogeneity is identified between β-Sn in dendrites and β-Sn in eutectic regions containing Ag3Sn and Cu6Sn5 particles. The size of the recrystallised grains is modulated by the dendritic and eutectic spacings; however, the recrystalised grains in the eutectic regions for coarse-scaled samples (largest λ2 and λe) is only localised next to IMCs without growth in size.

The microstructure of Sn-Ag-Cu (SAC) solder joints plays an important role in the reliability of electronics, and interlaced twinning has been linked with improved performance. Here, we study the three-dimensional (3-D) shape of interlaced regions in Sn-3.0Ag-0.5Cu (SAC305) solder balls by combining serial sectioning with electron backscatter diffraction. In solder balls without large Ag3Sn plates, we show that the interlaced volume can be reasonably approximated as a hollow double cone with the common 〈100〉 twinning axis as the cone axis, and the 〈110〉 from all three twinned orientations making up the cone sides. This 3-D morphology can explain a range of partially interlaced morphologies in past work on 2-D cross-sections.

Magnesium alloys are commonly melted and held in steel or cast iron crucibles and small but important amounts of Fe and C can dissolve into Mg-rich melts. Here, carbide formation is studied during interface reactions between solid Fe-xC alloys (x = 0–3.6 wt%) and liquid Mg-9Al-0.7Zn-0.2Mn (wt.%, AZ91) at temperatures from 700 to 800 °C. Two ternary carbides, AlFe3C and Al2MgC2, formed in the reaction layers between the Fe-C and AZ91, and T2-Al2MgC2 additionally formed within the AZ91 alloy due to carbon pickup. T2-Al2MgC2 grew in liquid AZ91 as hexagonal plates that were commonly twinned. A reproducible orientation relationship was measured between T2-Al2MgC2 and α-Mg, and the grain refinement of magnesium by heterogeneous nucleation on T2-Al2MgC2 is explored.

The creep behaviour and microstructural evolution of a Sn-3Ag-0.5Cu wt% sample with a columnar microstructure have been investigated through in-situ creep testing under constant stress of 30 MPa at ~298 K. This is important, as 298 K is high temperature within the solder system and in-situ observations of microstructure evolution confirm the mechanisms involved in deformation and ultimately failure of the material. The sample has been observed in-situ using repeat and automatic forescatter diode and auto electron backscatter diffraction imaging. During deformation, polygonisation and recrystallisation are observed heterogeneously with increasing strain, and these correlate with local lattice rotations near matrix-intermetallic compound interfaces. Recrystallised grains have either twin or special boundary relationships to their parent grains. The combination of these two imaging methods reveal grain 1 (loading direction, LD, 10.4° from [100]) deforms less than the neighbour grain 2 (LD 18.8° from [110]), with slip traces in the strain localised regions. In grain 2, (1 1 ¯ 0)[001] slip system is observed and in grain 1 (1 1 ¯ 0)[ 1 ¯ 1 ¯ 1]/2 and (110)[ 1 ¯ 11]/2 slip systems are observed. Lattice orientation gradients build up with increasing plastic strain and near fracture recrystallisation is observed concurrent with fracture.

The faceted growth of primary Al13Fe4 intermetallic compounds was studied using in-situ X-ray radiography in a solidifying Al-3Fe alloy. Microscopic twins were frequently observed in the growing intermetallics and were confirmed by post-solidification electron backscatter diffraction. A twin plane re-entrant growth mechanism was suggested, where repeated formation of re-entrant corners facilitated crystal growth along a preferential direction, forming elongated plates. In contrast, for intermetallics where this preferential growth was constrained by surrounding crystals, formation of layered twins perpendicular to the preferential direction was promoted and led to lower aspect ratios, known to be less deleterious to tensile properties.

Manganese-bearing intermetallic compounds (IMCs) are important for limiting micro-galvanic corrosion of magnesium-aluminum alloys and can initiate cracks under tensile load. Here, we use electron backscatter diffraction (EBSD), deep etching, and focussed ion beam (FIB) tomography to investigate the types of Al-Mn phases present, their faceted growth crystallography, and their three-dimensional distribution at different locations in high-pressure die cast (HPDC) AZ91D. The Al-Mn particle size distributions were well-described by lognormal distributions but with an additional population of externally solidified crystals (ESCs) formed in the shot chamber analogous to α-Mg ESCs. The large Al8Mn5 particles were cyclic twinned. Differences in the particle size distributions and number density in the center compared with the HPDC skin are identified, and the spatial relationship between Mg17Al12 and Al-Mn particles is explored.

The influence of Ni on the size and twinning of primary Cu6Sn5 crystals in Sn-0.7Cu-0.05Ni and Sn-xCu (x = 0.7, 0.9, 1.1) (mass%) solder joints is studied using synchrotron radiography and SEM-based EBSD. It is shown that the Ni addition does not cause significant refinement of primary Cu6Sn5 if the alloy is fully melted. However, for peak temperatures ≤250 °C relevant to industrial soldering, primary Cu6Sn5 are not completely melted in Sn-0.7Cu-0.05Ni and there are 10–100 times more numerous and smaller crystals than in Sn-0.7Cu. X-shaped Cu6Sn5 crystals with an angle of ∼70° commonly formed in Sn-0.7Cu-0.05Ni/Cu joints and are shown to be penetration twins. This type of growth twinning was only found in partially melted samples, both in Sn-0.7Cu-0.05Ni/Cu joints and binary Sn-1.1Cu alloy. The frequency of twinned crystals was significantly higher in Ni-containing solders. The results are discussed in terms of the influence of Ni on the Cu6Sn5 liquidus slope and on the lattice parameters of (Cu,Ni)6Sn5.

Sn-0.7Cu-0.05Ni is a widely used Pb-free solder that solidifies into a near-eutectic microstructure and a small fraction of primary Cu6Sn5. This paper overviews in-situ time-resolved imaging experiments on the solidification of Sn-0.7Cu-0.05Ni solder under three conditions: (i) directional solidification, (ii) continuous cooling in a near-uniform thermal field, and (iii) solder joint solidification on a Cu substrate. Primary Cu6Sn5 grow as rods along [0001] in each case but can also grow as X-shaped crystals in (iii). There are significant differences in eutectic growth due to nucleation difficulties for tin in conditions (ii) and (iii).

Twinned crystal growth is studied in primary Al45Cr7 and Al13Fe4 intermetallic compounds (IMCs) to explore how different twin types and twin variants affect the growth morphology during solidification. In both IMCs, the number of twin variants increased as the cooling rate increased and, by ~ 5 K/s, both IMCs formed cyclic twins with combined icosahedral (Al45Cr7) or decagonal (Al13Fe4) pseudosymmetry. The growth morphology depended on which twin variants were present. When all twin domains shared a common direction that was a rod growth direction in single crystals, twinning did not prevent crystals from growing as rods. This was the case for both IMCs at slow cooling rate and Al13Fe4 at all cooling rates. Cyclic twinning of Al13Fe4 generated many re-entrant corners but resulted in only a modest reduction in rod aspect ratio. In contrast, when twin domains in Al45Cr7 had common directions along multiple pseudo-i(2) axes, crystal growth transitioned from rod-like to a near-equiaxed morphology.

The crystallography and kinetics of Al11Mn4 formation on Al8Mn5 have been studied in magnesium alloys AZ80 / AZ91 and AZ31. During solidification, Al11Mn4 formation was promoted by low cooling rates where triclinic Al11Mn4 nucleated on rhombohedral Al8Mn5 particles with one of multiple related orientation relationships (ORs) and their variants. Al11Mn4 grew as (010) plates that were commonly twinned and the interrelationships amongst Al11Mn4 twins, Al8Mn5 twins and Al8Mn5-Al11Mn4 ORs are discussed. During solid state heat treatment at 410 °C, Al8Mn5 particles transformed into Al11Mn4 by a core-shell reaction with cracking in the Al11Mn4 shell. The solid-state reaction kinetics were consistent with interface reaction controlled growth. The results show that heat treatment can be used to tailor the Al-Mn compound in contact with the matrix (Mg) phase which may enable some control of corrosion performance.

A method is presented to control the size of primary Cu6Sn5 in ball grid array (BGA) joints while keeping all other microstructural features near-constant, enabling a direct study of the size of primary Cu6Sn5 on impact properties. For Sn-2Cu/Cu BGA joints, it is shown that larger primary Cu6Sn5 particles have a clear negative effect on the shear impact properties. Macroscopic fracture occurred by a combination of the brittle fracture of embedded primary Cu6Sn5 rods and ductile fracture of the matrix βSn. Cleavage of the Cu6Sn5 rods occurred mostly along (0001) or perpendicular to (0001) with some crack deflection between the two. The deterioration of shear impact properties with increasing Cu6Sn5 size is attributed to (1) the larger microcracks introduced by the brittle fracture of larger embedded Cu6Sn5 crystals, and (2) the less numerous and more widely spaced rods when the Cu6Sn5 crystals are larger, which makes them poor strengtheners.

Sn-Ag-Cu electronic solder joints typically solidify from a single nucleation event producing either a single βSn grain or twinned grains. With so few and variable βSn orientations, each joint is mechanically unique due to the anisotropy of tetragonal βSn. Here we explore the potential of increasing the number of βSn orientations in solder joints by promoting heterogeneous nucleation during solidification. It is shown that large (60 g) samples can be grain refined effectively by combining growth restricting solute with potent heterogeneous nucleant particles introduced by alloying additions of Zn, Ti, Co, Ir, Pd, or Pt. These mechanisms are discussed with reference to the grain refinement of structural casting alloys. In 500 μm solder balls, these grain refinement approaches were less effective and a relatively high cooling rate of 17 K/s combined with solute and nucleant particles were required to trigger multiple nucleation events. With this approach, up to 12 independent grains formed in 500 μm balls of Sn-3Ag-0.5Cu alloyed with Pt, Co, or Ti, compared with one independent grain in balls without nucleant particle additions.

The potential of cobalt substrates to control the microstructure of 550 μm Sn-3Ag-0.5Cu ball grid array (BGA) joints is explored. It is shown that cobalt substrates give a small and reproducible nucleation undercooling for tin and prevent the formation of large primary Ag 3 Sn blades and Cu 6 Sn 5 rods. βSn dendrites grew from the CoSn 3 reaction layer and the βSn grains are shown to inherit their orientation from the CoSn 3 layer due to heterogeneous nucleation with the following orientation relationship: (100) Sn ||(100) CoSn3 with [001] Sn ||[001] CoSn3 . Changes in the dendrite and eutectic microstructure are shown to be caused by the reduced nucleation undercooling for βSn and the altered solidification path due to cobalt substrate dissolution.

The improvement of solder joint mechanical performance due to bismuth additions is of ongoing interest. This paper investigates the influence of 0–14 wt% Bi on microstructure formation in Sn-0.7 wt%Cu-0.05 wt%Ni-xBi solder balls on Cu substrates. It is shown that Bi additions reduce the Ni content of (Cu,Ni)6Sn5, increase the size of primary (Cu,Ni)6Sn5 rods and reduce the thickness of the Cu6Sn5 reaction layer, although these effects are subtle until the Bi content reaches ∼5 wt%Bi and Bi additions of ≤5 wt% did not significantly affect the positive effects of Ni. Higher Bi additions (≥8 wt% Bi) caused a transition of the tin growth texture from columnar grains with a fibre texture towards a single grain with subgrains. Bi additions increased the non-equilibrium freezing range and caused a nonequilibrium ternary eutectic reaction in joints containing ≥2 wt% Bi. Bi contents less than 2 wt% are required to prevent (Bi) phase formation during solidification.

The growth mechanisms of primary Cu6Sn5 are studied in Sn-Cu alloys and solder joints by combining EBSD, FIB-tomography and synchrotron radiography. With increasing cooling rate and Cu content, Cu6Sn5 crystals developed from faceted hexagonal rods to grooved rods, in-plane branched faceted crystals and, finally, to nonfaceted dendrites. This range of growth morphologies has been rationalised into a kinetic microstructure map. Cu6Sn5 hexagonal rods grew along [0001] bounded by { 10 1 ¯ 0 } facets and Cu6Sn5 dendrites branched along in the { 10 1 ¯ 0 } planes. The faceted to nonfaceted transition indicates a kinetic interface roughening transition and a gradual change in mechanism from lateral growth governed by anisotropic attachment kinetics to continuous growth governed by diffusion and curvature. Finally, it is shown that the full range of Cu6Sn5 morphologies that grew for different composition and cooling rate combinations in bulk alloys can be engineered to grow in solder joints made with a single composition (Sn-0.7 wt%Cu/Cu) by altering the peak temperature and the cooling rate.

The heterogeneous evolution of microstructure in a single grain Cu/SAC305/Cu solder joint is investigated using in-situ thermal cycling combined with electron backscatter diffraction (EBSD). Local deformation due to thermal expansion mismatch results in heterogeneous lattice rotation within the joint, localised towards the corners. This deformation is induced by the constraint and the coefficient of thermal expansion (CTE) mismatch between the β-Sn, Cu6Sn5 and Cu at interfaces. The formation of subgrains with continuous increase in misorientation is revealed during deformation, implying the accumulation of plastic slip at the strain-localised regions and the activation of slip systems (110)[11]/2 and (0)[111]/2.

The Sn-0.7Cu-0.05Ni solder composition was first introduced in 1999 as a Pb-free interconnection material for the electronics industry and quickly earned widespread popularity as a cost-effective alternative to the Ag-containing solders then being widely promoted as the best way of meeting the requirement of the impending EU RoHS regulations. Recently there has been ongoing industrial interest in the development of next-generation Pb-free and Ag-free electronic solder alloys with enhanced reliability in a wider range of applications. In the search for improved reliability, first generation Pb-free solders are being modified with alloying additions, such as Bi, Sb and In to improve their performance in harsh environments. The present investigation explores microstructures formed in Sn-0.7Cu-0.05Ni-X/Cu solder joints (where X = 0-5wt%Bi, Sb, In or combinations thereof) and explores the correlation between the resulting solder joint microstructures and their mechanical response during shear impact testing.

When two solid metals are in contact at high temperature, interdi usion occurs leading in some cases to the growth of intermetallic compounds. The study of nucleation, growth and properties of these intermetallic compounds are of interest since it can be critical for many applications in industries. Yet, the e ect of these reactions on the initial surfaces of both metals is not well understood and particularly when surfaces are not perfectly flat and for short contact time. The purpose of the present study is to demonstrate that the growth of an intermetallic compound layer between to solid metals can lead to the surface reconstruction of one of them. The silver–zirconium system will be presented in order to illustrate this new phenomenon. The e ect of contact point on the di usion- reaction process has been modelled by patterning the Zr surface. The nucleation and growth of the intermetallic compounds occur along the contact points which leads to silver surface reconstruction with the growth of the preferential crystal planes f 111 g ad f 100 g . A model explaining this new phenomenon is developed based on the minimisation of Gibbs energy and the di usion rates af both Ag & Zr atoms in the binary system Ag / Zr.

Sn-3Ag-3Bi-3In solder has been investigated to improve the understanding of microstructure formation in this solder during solidification and soldering to Cu and Ni substrates. The as-solidified microstructures of Sn-3Ag-3Bi-3In samples were found to consist of a significant fraction of βSn dendrites with a complex eutectic between the dendrites. In total five phases were observed to form during solidification: βSn, Ag3Sn, Bi, ζAg and a “Sn-In-Bi” ternary compound. Soldering of Sn-3Ag-3Bi-3In to substrates changed the phase equilibria in the system and caused the formation of additional phases: Cu6Sn5 during soldering to Cu and Ni3Sn4 and metastable NiSn4 during soldering to Ni. It is shown that metastable NiSn4 forms as a primary phase in a complex 5-component Sn-3Ag-3Bi-3In-Ni system. In and Bi were detected in solid solution in the βSn matrix in amounts of ~1.5-2at% and ~1.2at% respectively. Bi also existed as fine particles of two distinct types. (i): sub-micron (

The solidification of Sn-3Ag-0.5Cu and Sn-0.7Cu-0.05Ni are overviewed and compared. In joints on Cu substrates, both solders begin solidification with primary Cu6Sn5 growing in the bulk liquid prior to tin nucleation. In freestanding balls and joints, SAC305 generally solidifies with a single tin nucleation event and exhibits a mutually-twinned tin grain structure. In contrast, SN100C BGA balls and joints exhibit multiple independent tin grains that grow as a columnar array in joints.

The creep behaviour of directionally solidified SAC305 (96.5Sn-3Ag-0.5Cu wt%) alloy has been investigated with integrated particle matrix composite (PMC) crystal plasticity modelling and quantitative experimental characterisation and test. In this manuscript, the mechanistic basis of creep rate dependence is shown to be influenced by plastic strain gradients, and the associated hardening due to geometrically necessary dislocation (GND) density. These gradients are created due to heterogenous deformation at the β-Sn phase and intermetallic compound (IMCs) boundaries. The size and distribution of IMCs is important, as finer and well dispersed IMCs leading to higher creep resistance and lower creep rates, and this agrees with experimental observations. This understanding has enabled the creation of a new microstructurally homogenised model which captures this mechanistic link between the GND hardening, the intermetallic size, and the corresponding creep rate. The homogenised model relates creep rates to the microstructure found within the solder alloy as they evolve in service, when ageing and coarsening kinetics are known.

The mechanisms by which Ti additions catalyse the nucleation of β-Sn are studied in 550 μm Sn-3Ag-0.5Cu (wt%) solder balls and joints on Cu and Ni substrates. It is shown that at least two new intermetallic compounds (IMCs), Ti2Sn3 and (Ti,Fe,Cu)Sn2, form as a result of a 0.2 wt% Ti addition. The nucleation potential of each IMC was studied by electron backscatter diffraction (EBSD) of tin droplets solidified on the cross sectioned facets of each IMC. It is found that reproducible orientation relationships (ORs) form only between β-Sn and Ti2Sn3 and that the two ORs generate good atomic matching between the Sn atoms in Ti2Sn3 and the closest packed plane in β-Sn, {100}. β-Sn cyclic twinning occurred in droplets on Ti2Sn3 where the twinning axis 〈100〉Sn was always parallel with the lowest disregistry direction in the ORs. In solder balls and joints, the Ti2Sn3 addition triggered up to 12 independent β-Sn grains, whereas Ti-free SAC305 always solidified with one independent grain.

In recent years there has been increased interest in Bi-containing solders to improve the reliability of electronics and drive down processing temperatures. When the Bi content is more than ~ 2 wt% in Sn–Ag–Cu–Bi and Sn–Cu–Ni–Bi solders, (Bi) phase forms by a non-equilibrium eutectic reaction and also precipitates in the solid state. Here, we study the development of bismuth plates during room temperature storage in a range of Bi-containing solders. It is shown that Bi plates precipitate with one of two (Bi)–βSn orientation relationships (ORs). During coarsening the OR with the better lattice match grows at the expense of the other, leading to a single OR. The plate coarsening kinetics scale with the cube root of holding time and are relatively rapid, with plates exceeding 1 µm in length within 1 day at room temperature. Prolonged room temperature storage of more than 1 year resulted in Bi accumulation in 5–20 µm (Bi) particles at βSn grain boundaries.

The reliability of solder joints is affected significantly by thermomechanical properties such as creep and thermal fatigue. In this work, the creep of directionally solidified (DS) Sn-3Ag-0.5Cu wt.% (SAC305) dog-bone samples (gauge dimension: 10 × 2 × 1.5 mm) with a controlled or fibre texture is investigated under constant load testing (stress level: ∼ 30 MPa) at a range of temperatures from 20°C to 200°C. Tensile testing is performed and the secondary creep strain rate and the localised strain gradient are studied by two-dimensional optical digital image correlation (2-D DIC). The dominating creep mechanisms and their temperature dependence are explored at the microstructural scale using electron backscatter diffraction (EBSD), which enables the understanding of the microstructural heterogeneity of creep mechanisms at different strain levels, temperatures and strain rates. Formation of subgrains and the development of recrystallization are observed with increasing strain levels. Differences in the deformation of β-Sn in dendrites and in the eutectic regions containing Ag3Sn and Cu6Sn5 are studied and related to changes in local deformation mechanisms.

This paper overviews methods to catalyse the nucleation of tin in BGA solder joints and explores the potential of engineering tin nucleation to control joint microstructures. A range of heterogeneous nucleants for Sn is overviewed. It is shown that CoSn 3 , PtSn 4 , PdSn 4 and IrSn 4 can all catalyse Sn nucleation and substantially reduce the nucleation undercooling. The nucleation mechanisms are discussed in terms of a crystallographic lattice matching analysis, where each nucleant is shown to have good planar matching across the closest packed planes in each crystal structure. We then demonstrate an approach to incorporate the nucleants into solder joints as seed crystals to control the orientation of the tin nucleation event. With this approach, it is possible to generate single-crystal joints all with the same c-axis orientation.

Manganese is widely added to Mg-Al-based alloys to form Al-(Mn,Fe) intermetallics that combat impurity Fe. Here EBSD, deep etching and FIB-tomography are combined to study the nucleation and growth crystallography of Al-Mn-Fe particles in Mg-9Al-0.7Zn-0.15Mn-0.002Fe (wt%) solidified at 1 K⋅s−1. It is shown that Al8Mn5 nucleates on B2-Al(Mn,Fe) particles and an incomplete peritectic transformation results in a Fe-richer B2-Al(Mn,Fe) core enveloped by a low-Fe Al8Mn5 shell. A reproducible orientation relationship (OR) is measured, and the close lattice match and OR are discussed in terms of the group-subgroup relationship between these phases. It is found that most rhombohedral Al8Mn5 particles are twinned, consisting of four orientations related by ∼90° rotations around three common 1 1 ¯ 02 > , which is discussed in terms of the pseudo-cubic axes of the Al8Mn5 rhombohedral gamma brass. Al8Mn5 grew as equiaxed particles that are explained by polyhedron models based on {100}, {110} and {112} facets of the pseudo-cubic Al8Mn5 cell. The results indicate that, at low Fe:Mn ratio, most impurity Fe is dissolved in B2 particles that are encapsulated by low-Fe Al8Mn5 which may be important for corrosion resistance.

The CoCrFeMnNi high-entropy alloy is a promising candidate for metal additive manufacturing. In this study, single-layer and multi-layer builds were produced by laser powder bed fusion to study microstructure formation in rapid cooling and its evolution during repeated metal deposition. CoCrFeMnNi showed good printability with high consolidation and uniform high hardness. It is shown that microstructure in the printed alloy is governed by epitaxial growth and competitive grain growth. As a consequence, a bi-directional scanning pattern without rotation in subsequent layers generates a dominant alternating sequence of two crystal orientations.

Sn-Cu-Ni solders were introduced to electronic assembly in 1999 and the Sn-0.7Cu-0.05Ni (wt%) composition was widely accepted as a Pb-free and Ag-free alternative to Sn-Ag-Cu alloys for wave and reflow soldering. The minor Ni addition of 0.05wt% was shown to have several beneficial effects on the Cu6Sn5 intermetallic that form during soldering. In recent years, Bi additions have been identified as a beneficial alloying component that significantly improves the mechanical properties of solder alloys. The present investigation explores BGA joint microstructures that form in Sn-0.7Cu-xNi-yBi solders and seeks to identify the Ni and Bi levels that deliver optimum performance in soldering processes and reliability in service.

This paper investigates the effect of 0.05 wt.% Ni on the formation and growth of primary Cu6Sn5 in Sn-0.7 wt.%Cu solder paste soldered on a Cu substrate, using a real-time synchrotron imaging technique. It was found that small additions of Ni significantly alter the formation and growth of the primary Cu6Sn5 intermetallics, making them small. In contrast, without Ni, primary Cu6Sn5 intermetallics tend to continue growth throughout solidification and end up much larger and coarser. The primary effect of the Ni addition appears to be in promoting the nucleation of a larger amount of small Cu6Sn5. The results provide direct evidence of the sequence of events in the reaction of Ni-containing Sn-0.7 wt.%Cu solder paste with a Cu substrate, and in particular the formation and growth of the primary Cu6Sn5 intermetallic.

We show that dilute Al additions can control the size of primary Cu6Sn5 rods in Sn-0.7Cu/Cu ball grid array joints. In Sn-0.7Cu-0.05Al/Cu joints, the number of primary Cu6Sn5 per mm2 is ∼7 times higher and the mean three-dimensional length of rods is ∼4 times smaller than in Al-free Sn-0.7Cu/Cu joints, while the area fraction of primary Cu6Sn5 is similar. It is shown that epitaxial nucleation of primary Cu6Sn5 occurs on δ-Cu33Al17 or γ 1-Cu9Al4 particles, which are stable␣in the Sn-0.7Cu-0.05Al melt during holding at 250°C. The observed facet relationships agree well with previously determined orientation relationships between δ-Cu33Al17 and Cu6Sn5 in hypereutectic Sn-Cu-Al alloys and result in a good lattice match with

The solidification of Pb-free solder joints is overviewed with a focus on the formation of the βSn grain structure and grain orientations. Three solders commonly used in electronics manufacturing, Sn-3Ag-0.5Cu, Sn-3.5Ag, and Sn-0.7Cu-0.05Ni, are used as case studies to demonstrate that (I) growth competition between primary dendrites and eutectic fronts during growth in undercooled melts is important in Pb-free solders and (II) a metastable eutectic containing NiSn4 forms in Sn-3.5Ag/Ni joints. Additionally, it is shown that the substrate (metallization) has a strong influence on the nucleation and growth of tin. We identify Co, Pd, and Pt substrates as having the potential to control solidification and microstructure formation. In the case of Pd and Pt substrates, βSn is shown to nucleate on the PtSn4 or PdSn4 intermetallic compound (IMC) reaction layer at relatively low undercooling of ~4 K, even for small solder ball diameters down to

Metastable Sn-NiSn4 eutectic can form in electronic solder joints and is a reliability concern. Here the competition between stable Sn-Ni3Sn4 and metastable Sn-NiSn4 eutectic growth is studied during unidirectional solidification. The stable and metastable eutectic points are measured, the dynamics of eutectic growth and the transition between the two eutectics is investigated by synchrotron radiography, and the crystallography of eutectic growth is measured by EBSD. It is shown that the Sn-Ni3Sn4 eutectic has a highly irregular morphology with diverging and converging rods because monoclinic Ni3Sn4 grows only along [010] without a reproducible orientation relationship (OR) with Sn. The metastable Sn-NiSn4 eutectic has a more regular broken-lamellar morphology and a reproducible OR with Sn. The critical velocity between the two eutectics is less than 1 μm/s which is discussed in terms of the small temperature difference between the two eutectic points and the kinetic growth advantages of the metastable eutectic.

The morphology and distribution of Al8Mn5 is studied in AM50 and AZ91 produced by hot and cold chamber high pressure die casting (HPDC). It is found that, in HPDC, primary Al8Mn5 particles take a wide range of morphologies within the same casting spanning from faceted polyhedra to weakly-faceted dendrites. These different morphologies exist across the whole cross-section without any clear trend in morphology versus radial position. A comparison with Al8Mn5 in samples solidified at low cooling rate suggests that the larger polyhedral particles are externally solidified crystals (ESCs) that nucleate and grow in the shot chamber analogous to αMg ESCs, and that the dendritic Al8Mn5 nucleated and grew at high cooling rate in the die cavity.

Cu6Sn5 is a common intermetallic in Pb-free soldering. We explore the influence of dilute aluminium additions on the heterogeneous nucleation and grain refinement of primary Cu6Sn5 in Sn–4Cu–xAl solders. For cooling rates relevant to soldering, it is found that 0.02 and 0.2 wt%Al cause significant grain refinement of Cu6Sn5 with 0.2 wt%Al increasing the number of Cu6Sn5 grains per unit area by a factor of ∼8. Grain refinement is shown to be due to heterogeneous nucleation of Cu6Sn5 on either delta-Cu33Al17 or gamma1-Cu9Al4, coupled with significant constitutional supercooling ahead of growing Cu6Sn5 crystals. Reproducible orientation relationships are measured between the Cu–Al intermetallics and Cu6Sn5, with a planar lattice mismatch of less than 2.5%. The role of potent nuclei and solutal growth restriction are discussed with reference to the grain refinement of structural casting alloys.