Sn Interface Research Articles

Kinetics of intermetallic phase formation at the interface of Sn–Ag–Cu–X (X = Bi, In) solders with Cu substrate

The effects of Bi and In additions on intermetallic phase formation in lead-free solder joints of Sn–3.7Ag–0.7Cu; Sn–1.0Ag–0.5Cu–1.0Bi and Sn–1.5Ag–0.7Cu–9.5In (composition given in weight %) with copper substrate are studied. Soldering of copper plate was conducted at 250 °C for 5 s. The joints were subsequently aged at temperatures of 130–170 °C for 2–16 days in a convection oven. The aged interfaces were analyzed by optical microscopy and energy dispersive X-ray spectroscopy (EDX) microanalysis. Two intermetallic layers are observed at the interface – Cu 3Sn and Cu 6Sn 5. Cu 6Sn 5 is formed during soldering. Cu 3Sn is formed during solid state ageing. Bi and In decrease the growth rate of Cu 3Sn since they appear to inhibit tin diffusion through the grain boundaries. Furthermore, indium was found to produce a new phase – Cu 6(Sn,In) 5 instead of Cu 6Sn 5, with a higher rate constant. The mechanism of the Cu 6(Sn,In) 5 layer growth is discussed and the conclusions for the optimal solder chemical composition are presented.

Evolution of Ag 3Sn at Sn–3.0Ag–0.3Cu–0.05Cr/Cu joint interfaces during thermal aging

The intermetallic compounds (IMC) in the solder and at the interface of Sn–3.0Ag–0.5Cu (SAC)/Cu and Sn–3.0Ag–0.3Cu–0.05Cr (SACC)/Cu joints were investigated after isothermal aging at 150 °C for 0, 168 and 500 h. Different shaped Ag 3Sn phases were found near the IMC layer of the latter joint. Interestingly, fine rod-shaped and branch-like Ag 3Sn were detected near the interface after soldering and long Ag 3Sn changed into shorter rods and small particles during aging. It is investigated that the Cr addition and thermal aging have effect on the evolution of Ag 3Sn morphologies and it is controlled by interfacial diffusion. Energy minimization theory and the redistribution of elements are used to explain the morphological evolution of Ag 3Sn. Small Ag 3Sn particles were also found on the IMC layer after aging, unlike the large Ag 3Sn at that of SAC/Cu joints. In conclusion, a favorable morphology of the joint interface leads to better bonding properties for SACC/Cu joints against thermal aging than that for SAC/Cu.

Effect of Interfacial Reaction on Joint Strength of Semiconductor Metallization and Sn-3Ag-0.5Cu Solder Ball

Semiconductor packages that use metallization and lead-free solders are increasingly being used in electronic products. In this study, interface reactions and joint-strength reliability were investigated for Sn-3W%Ag-0.5W%Cu solder ball joints joined to Cr/Cu and Cr/Ni-40W% metallization layers that were heat treated at 260°C. The strength of the joint with the Cr/Cu metallization layer decreased as the duration of the heat treatment increased. Sn and Cr interface reactive layers were generated after the loss of Cu in the Cr/Cu metallization layer, but the connection was maintained. By contrast, the connection of the joint to the Cr/Ni-40W metallization layer was relatively stable under the heat treatment conditions investigated.

Properties and microstructure of Sn–0.7Cu–0.05Ni solder bearing rare earth element Pr

Effects of trace amount of rare earth element Pr on properties and microstructure of Sn–0.7Cu–0.05Ni solder were investigated in this paper. The solderability of Sn–Cu–Ni–xPr alloy and shear strengh of Sn–Cu–Ni–xPr soldered micro-joints were determined by means of the wetting balance method and shear test, respectively. Moreover, microstructure of solder alloys bearing Pr, as well as intermetallic compound (IMC) layer formed at solder/Cu interface after soldering were observed. It was concluded that the major benefits of rare earth element Pr on Sn–Cu–Ni lead-free solder are: improving solderability, refining microstructure, and depressing IMC (IMC) growth, which exhibited improved mechanical properties. It also revealed that (Cu,Ni)6Sn5 is the majority IMC phase at the interface of Sn–Cu–Ni–xPr/Cu solder joints. Ni added into the solder effectively suppressed the growth of Cu3Sn and consequently also the total IMC layer thickness. Above all, the thickness and morphology of the interfacial (Cu,Ni)6Sn5 IMC were optimized due to alloying Pr. It can be inferred that Pr and Ni would play an important role in improving the reliability of Sn–Cu–Ni lead-free solder joints.

Carbothermal synthesis of Sn-based composites as negative electrode for lithium-ion batteries

The composite [Sn–BPO 4/ xC] to be used as negative electrode material for the storage of electrochemical energy was obtained by dispersing electroactive tin species onto a BPO 4 buffer matrix by carbothermal reduction of a mixture of SnO 2 and nanosized BPO 4. This composite material was thoroughly characterized by X-ray diffraction, Scanning Electron Microscopy, 119Sn Mössbauer spectroscopy and Raman spectroscopy. The electrochemical tests of this new material highlight its very interesting electrochemical properties, i.e., a discharge capacity of 850 mAh g −1 for the first cycle and reversible capacity around 585 mAh g −1 at C/5 rate. These electrochemical performances are attributed to the very high dispersion and stabilisation of tin metal particles onto the BPO 4 matrix. The irreversible capacity observed for the first charge/discharge cycle is due the reduction of interfacial Sn II species and to the passivation of the anode surface by liquid electrolyte decomposition (formation of the SEI layer).

Josephson effect in superconductor/ferromagnet structures with a complex weak-link region

The critical currents ${I}_{C}$ of SNF-FN-FNS, SN-FN-NS, and SNF-N-FNS Josephson junctions (S---superconductor, F---ferromagnetic, N---normal metal) with complex SNF or SN electrodes (N or NF bilayer are situated under a superconductor) are calculated in the framework of linearized Usadel equations for arbitrary overlap length $d$ of SN interface. We demonstrate that in these geometries, in the case of large resistances of SN interfaces, the critical current can exceed that in ramp-type junctions. Based on these results, the choice of the most practically applicable geometry is discussed. We predict that in a certain parameter range there is single $0\text{\ensuremath{-}}\ensuremath{\pi}$ transition with the increase in the overlap length $d$. This single transition can be realized also in SFN-N-FNS Josephson junctions, where the coherence length in the weak-link region is a real quantity. Further, we predict that in SNF-N-FNS Josephson junctions $0\text{\ensuremath{-}}\ensuremath{\pi}$ transition may take place with increase in distance between superconducting electrodes.

Weakening of the Cu/Cu3Sn(100) Interface by Bi Impurities

We have performed density functional theory calculations to examine the role of Bi impurities in modifying the structure and properties of the Cu/Cu(3)Sn(100) interface. Analysis of the heat of supercell formation reveals that Bi atoms preferentially replace Cu atom in the Cu slab. Substitution of Cu by Bi reduces the adhesion energy of the interface from 1.5 J/m(2) to 1.1 J/m(2), primarily due to the atomic size effect. The size effect leads to expansion of the interface, because surrounding Cu and Sn atoms are pushed away from the misfit Bi atoms. Analysis of electronic density indicates that the local charge density is dispersed around Bi atoms. The presence of a Bi atom makes surrounding atoms less active, which is attributed to the reduction of hybridizations.

Effects of Current Stressing on Formation and Evolution of Kirkendall Voids at Sn–3.5Ag/Cu Interface

Kirkendall voids (KVs) are known to be formed at the Cu/Cu 3 Sn interface, which can remarkably weaken solder joints. In this paper, the formation and evolution processes of KVs at Sn-3.5Ag/Cu joints were systematically investigated under isothermal aging and current stressing. It was found that the processes develop faster when joints are subjected to current stressing as opposed to thermal aging. This can be illuminated by the high KV densities caused by current stressing at both cathode and anode Cu/Cu 3 Sn interfaces. Moreover, KVs formed under current stressing showed some polarity characteristics, namely that higher KV density was observed on the anode side compared with the cathode side. The interfacial reaction generated at the Cu 3 Sn/Cu 6 Sn 5 interface, which was partly affected by current stressing, contributed to this polarity effect. As the holding time was prolonged, microvoids coalesced into larger porosities and microcracks. These defects will greatly threaten the reliability of the interface.

Morphology and chemical composition of Ag/Sn/Ag interconnections

The main goal of the present contribution was to describe morphology and chemical composition of the intermetallic phases, which were formed during diffusion soldering process of the silver using tin. The Ag(3)Sn intermetallics is the main constituent of the joint after diffusion soldering at 235 degrees C and 265 degrees C. A closer inspection of the Ag/Ag(3)Sn interface revealed also the small crystallites of the second intermetallic phase, Ag(5)Sn, which was not previously observed using scanning electron microscope. Both phases are characterized by high melting temperatures: 480 degrees C and 724 degrees C, respectively. Therefore, their presence guarantees high thermal stability of the interconnection, which can be even three times higher than the temperature used for soldering.

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 Cu 5Zn 8 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.

Reliability studies of Sn–9Zn/Cu and Sn–9Zn–0.3Ag/Cu soldered joints with aging treatment

In this study, the interfacial reactions and joint reliabilities of Sn–9Zn/Cu and Sn–9Zn–0.3Ag/Cu were investigated during isothermal aging at 150 °C for aging times of up to 1,000 h. Cu5Zn8 IMCs layer is formed at the as-soldered Sn–9Zn/Cu interface. Adding 0.3wt.% Ag results in the adsorption of AgZn3 on the Cu5Zn8 IMCs layer. The as-soldered Sn–9Zn/Cu and Sn–9Zn–0.3Ag/Cu joints have sufficient pull strength. The thickness of the IMCs layer formed at the interface of Sn–9Zn/Cu and Sn–9Zn–0.3Ag/Cu both increase with increasing aging time. Correspondingly, both the pull forces of the Sn–9Zn and Sn–9Zn–0.3Ag soldered joints gradually decrease as the aging time prolonged. However, the thickness of the IMCs layer of Sn–9Zn–0.3Ag/Cu increases much slower than that of Sn–9Zn/Cu and the pull force of Sn–9Zn–0.3Ag soldered joint decreases much slower than that of Sn–9Zn soldered joint. After aging for 1,000 h, some Cu–Sn IMCs form between the Cu5Zn8 IMC and the Cu substrate, many voids form at the interface between the Cu5Zn8 layer and solder alloy, and some cracks form in the Cu5Zn8 IMCs layer of Sn–9Zn/Cu. The pull force Sn–9Zn soldered joint decreases by 53.1% compared to the pull force measured after as-soldered. Fracture of Sn–9Zn/Cu occurred on the IMCs layer on the whole and the fracture micrograph implies a brittle fracture. While the pull force of Sn–9Zn–0.3Ag soldered joint decreases by 51.7% after aging at 150 °C for 1,000 h. The fracture mode of Sn–9Zn–0.3Ag soldered joint is partially brittle at the IMCs layer, and partially ductile at the outer ring of the solder.

Effect of isothermal aging on intermetallic compounds and Kirkendall void growth kinetics of Au stud bumps

Intermetallic compounds (IMCs) and Kirkendall void growth kinetics at various interfaces in Au stud bumps were studied in terms of isothermal aging at 120 °C, 150 °C, and 180 °C for 300 h. Phases of AuSn, AuSn2, and AuSn4 form at the interface of Au studs and Sn after a reflow. The thickness of an Au-Sn IMC was quantified with an image analyzer as a function of aging temperature and time. The growth of the Au-Sn IMC increases linearly with the square root of the aging time at 120 °C, 150 °C, and 180 °C. The growth of the Au-Sn IMC at 180 °C differs from the growth of the Au-Sn IMC at 120 °C and 150 °C because of excessive Au consumption by the Au studs. Kirkendall voids form at the interface of the Au stud and the Au-Sn IMC and increase linearly with the square root of the aging time. The apparent activation energies for the growth of the Au-Sn IMC and the Kirkendall voids were determined to be 0.57 eV and 0.2 eV, respectively, from measurement of the thickness of the Au-Sn IMCs and the width of the Kirkendall voids in relation to the aging temperature and time.

Fundamental Study of the Intermixing of 95Pb-5Sn High-Lead Solder Bumps and 37Pb-63Sn Pre-Solder on Chip-Carrier Substrates

This study investigated the intermixing of 95Pb-5Sn solder bumps and 37Pb-63Sn pre-solder in flip-chip solder joints. The reaction conditions included multiple reflows (up to ten) at 240°C, whereby previously solder-coated parts are joined by heating without using additional solder. We found that the molten pre-solder had an irregular shape similar to a calyx (i.e., a cup-like structure) wrapped around a high-lead solder bump. The height to which the molten pre-solder ascended along the solid high-lead solder bump increased with the number of reflows. The molten pre-solder was able to reach the under bump metallurgy (UBM)/95Pb-5Sn interface after three to five reflows. The molten pre-solder at the UBM/95Pb-5Sn interface generated two important phenomena: (1) the molten solder dewetted (i.e., flowed away from the soldered surface) along the UBM/95Pb-5Sn interface, particularly when the number of reflows was high, and (2) the molten pre-solder transported Cu␣atoms to the UBM/95Pb-5Sn interface, which in turn caused the Ni-Sn compounds at the chip-side interface to change into (Cu0.6Ni0.4)6Sn5.

Intermetallic compounds at the interface between Sn–Cu(–Ni) solders and Cu substrate

Abstract The influence of Cu content and Ni addition on the morphology and the growth kinetics of the intermetallic compound (IMC) layer formed at the interface of liquid Sn – Cu-based solders and the Cu substrate were investigated. The effects of temperature and reaction times on IMC morphology and thickness were studied. The reaction of a Sn-1.4Cu alloy with the substrate leads to the formation of a layer of Cu6Sn5 (-phase) of typical scallop morphology. At long reaction times, a Cu3Sn (-phase) is observed at the interface between the Cu substrate and the Cu6Sn5 layer. A small nickel addition to a Sn-0.6Cu alloy significantly changes the IMC morphology, accelerates its growth kinetics, prevents the formation of the Cu3Sn layer and reduces the rate of substrate dissolution. The effective activation energy of growth of the IMC layer in the Sn – Cu – Ni alloy was determined.

Influence of Ag on Kinetics of Solid-State Reactive Diffusion between Pd and Sn

The influence of Ag on the kinetics of the solid-state reactive diffusion between Pd and Sn was experimentally examined using Sn/ (Pd-Ag)/Sn diffusion couples with a Ag concentration of 75 at% in the present study. The diffusion couples were isothermally annealed at temperatures of 433, 453 and 473 K for various periods up to 1365 h. During annealing, a compound layer dominantly consisting of polycrystalline PdSn 4 and Ag 3 Sn lamellae is formed at the (Pd-Ag)/Sn interface in the diffusion couple. The square of the thickness of the compound layer increases in proportion to the annealing time. This relationship is called the parabolic relationship. On the other hand, the interlamellar spacing in the compound layer is proportional to a power function of the annealing time, and thus grain growth occurs in the compound layer. The exponent of the power function is close to 1/3. The parabolic relationship of the layer growth and the occurrence of the grain growth guarantee that the growth of the compound layer is controlled by volume diffusion. The addition of Ag with 75 at% into Pd decreases the parabolic coefficient by 93, 88 and 78% at 433, 453 and 473 K, respectively. Hence, Ag works as an effective suppressant against the growth of the compound during the solid-state reactive diffusion between Pd and Sn.

Kinetics of reactive diffusion between Pd–Ag alloys and Sn at solid-state temperatures

Abstract In order to examine the influence of Ag on the growth behavior of compounds at the interconnection between the Sn-base solder and the multilayer Pd/Ni/Cu conductor during energization heating, the kinetics of the reactive diffusion in the (Pd–Ag)/Sn system was experimentally studied at solid-state temperatures. Diffusion couples consisting of pure Sn and binary Pd–Ag alloys with concentrations of 25 and 50 at.% Ag were prepared by a diffusion bonding technique, and then isothermally annealed in the temperature range between T = 433 and 473 K for various times up to 312 h. During annealing, a PdSn 4 compound layer dispersed with fine particles of Ag 3 Sn is formed at the (Pd–Ag)/Sn interface in the diffusion couple. The square of the mean thickness l of the compound layer is proportional to the annealing time t as follows: l 2 = Kt , where K is the parabolic coefficient. Since the grain size of PdSn 4 monotonically increases with increasing annealing time, the growth of the compound layer is controlled by volume diffusion in each phase. The value of K becomes less than half due to addition of Ag with 50 at.% into Pd. Hence, the addition of Ag into the Pd layer inhibits the deterioration of the electrical and mechanical properties at the interconnection between the Sn-base solder and the multilayer Pd/Ni/Cu conductor during energization heating.

Dislocation-assisted intermetallic layer formation at the interface of Sn–Pb solder and Cu

Dislocation-assisted intermetallic layer formation at the interface of Sn–Pb solder and Cu

Effect of a trace of Cr on intermetallic compound layer for tin–zinc lead-free solder joint during aging

Abstract Influence of Cr on growth of interfacial intermetallic compound (IMC) at the interface of Sn–9Zn/Cu substrate during aging at 85 °C/20%RH and 85 °C/85%RH for 500 h has been investigated. After aging treatment, IMC layer at the Sn–9Zn/Cu joint is much thicker than that at the Sn–9Zn–Cr/Cu joint. Estimation according to experimental data presents that IMC growth rate of Sn–9Zn–Cr/Cu interface is about 70–75% lower than that of Sn–9Zn/Cu interface.

Investigation of relation between intermetallic and tin whisker growths under ambient condition

The relation between the whisker growth and intermetallic on various lead-free finish materials that have been stored at ambient condition for 2 yrs (6.3 × 10 7 s) is investigated. The matte Sn plated leadframe (LF) had the needle-shaped whisker and the nodule-shaped whisker was observed on the semi-bright Sn plated LF. Both the Sn plated LFs had a same columnar grain structure and both whiskers were grown in connection with the scalloped intermetallic compound (IMC) layer. The morphology of the IMC layer is similar, regardless of the area which has whisker or not. On the Sn–Bi finish and bright Sn plated LF, hillock-shaped and sparsely grown branch-shaped whiskers were observed, respectively. The IMC grew irregularly under both the areas with or without whisker. The IMC growth along the Sn grain boundaries generated inner compressive stress at the plating layer. Atomic force microscopy (AFM) profiling analysis is useful for characterization the IMC growth on the Sn and Cu interface. The measured root mean square (RMS) values IMC roughness on semi-bright Sn, matte Sn, and bright Sn plated LF were 1.82 μm, 1.46 μm, and 0.63 μm, respectively. However, there is no direct relation between whisker growth and the RMS value. Two layers of η′-Cu 6Sn 5 were observed using field emission transmission electron microscopy (FE-TEM): fine grains and coarse grains existed over the fine grains.

Kinetics of reactive diffusion in the (Au–Ni)/Sn system at solid-state temperatures

The kinetics of the reactive diffusion in the ternary (Au–Ni)/Sn system was experimentally studied at solid-state temperatures. Binary Au–Ni alloys containing 10 and 20 mass% of Ni were used to prepare sandwich Sn/(Au–Ni)/Sn diffusion couples by a diffusion bonding technique. The diffusion couples were isothermally annealed at temperatures of T = 433, 453 and 473 K for various times up to 1246 h. Due to annealing, Au 1.5Ni 0.5Sn 8 and AuNi 2Sn 4 compound layers are formed along the (Au–Ni)/Sn interface in the Au–20Ni diffusion couple, but Au 1.7Ni 0.3Sn 8, AuSn 2 and Au 6Ni 4Sn 15 compound layers dispersed with fine particles of AuNi 2Sn 4 are produced along the interface in the Au–10Ni diffusion couple. The total thickness l of the compound layers monotonically increases with increasing annealing time t according to the relationship l = k( t/ t 0) n , where t 0 is unit time, 1 s. For both diffusion couples, the exponent n increases with decreasing annealing temperature. However, for the Au–20Ni diffusion couple, n is close to 0.5 at T = 453–473 K, and takes a value of 0.7 at T = 433 K. This means that volume diffusion is the rate-controlling process for the growth of the compound layers at higher annealing temperatures, but interface reaction contributes to the rate-controlling process at lower annealing temperatures. On the other hand, for the Au–10Ni diffusion couple, n is smaller than 0.5 at T = 453–473 K, but close to 0.5 at T = 433 K. The values n < 0.5 indicate that grain boundary diffusion as well as volume diffusion contributes to the rate-controlling process and grain growth occurs at certain rates in the compound layers. Consequently, for the reactive diffusion in the ternary (Au–Ni)/Sn system, the rate-controlling process considerably varies depending on the composition of the Au–Ni alloy and the annealing temperature.