Growth mechanisms of intermetallic compounds and Bi-rich layer in ball grid array structure Cu/Sn-58Bi/Cu solder joints during solid–solid and liquid–solid electromigration

Journal of Materials Science: Materials in Electronics - Sn-58Bi solder has attracted much attention due to its low melting temperature and low cost in recent years. However, Sn-58Bi solder might...     

Effects of Zn addition on electromigration behavior of Sn–1Ag–0.5Cu solder interconnect

Effects of electromigration on microstructure and tensile property were studied in the Sn–1Ag–0.5Cu and Sn–1Ag–0.5Cu–1Zn solder interconnects. While the polarity effect and strength reduction from electromigration occurred in the Sn–1Ag–0.5Cu solder interconnects, they were suppressed by the Zn addition in the Sn–1Ag–0.5Cu–1Zn solder interconnects. Such a strong effect of Zn was explained by the strong binding of Zn with Cu, which prevented the dissolution of the IMC at the cathode, and by the reverse migration of the Zn elements, which counteracted the increase in the vacancy concentration so that the strength reduction was successfully inhibited.     

Interfacial evolution in Sn–58Bi solder joints during liquid electromigration

For Sn–58Bi low temperature solder alloy, local molten induced from electromigration Joule heating might change the atomic diffusion and interfacial behavior. In this paper, the diffusion behavior and interfacial evolution of Cu/Sn–58Bi/Cu joints were studied under liquid–solid (L–S) electromigration in molten solder and were compared with the interfacial behaviors in solid–solid (S–S) electromigration in solid solder. L–S or S–S electromigration was realized by applying a current density of 1.0?×?10⁴ A/cm² to molten solder at 150 °C or solid solder at 25 °C, respectively. During S–S electromigration, Bi atoms were driven towards anode side under electromigration induced flux and then accumulated to form Bi-rich layer near anode interface with current stressing time increasing. During L–S electromigration, Bi atoms were reversely migrated from anode to cathode to produce Bi segregation at cathode interface, while Cu atoms were rapidly dissolved into molten solder from cathode and migrated to form large amounts of Cu₆Sn₅ rod-like phases near anode interface. The reversal in the direction of Bi atoms may be attributed to the reversal in the direction of electromigration induced flux and correspondingly the change on effective charge number of Bi atoms from negative to positive.     

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.     

Interfacial reaction and properties of Sn/Cu solder reinforced with graphene nanosheets during solid–liquid diffusion and reflowing

In this paper, different mass fractions (0, 0.01, 0.03, 0.05, 0.07, and 0.09 wt.%) of graphene nanosheets (GNSs) were selected as a strengthening phase to promote the performances of Sn/Cu solder joint. The wettability and shear performance of Sn-xGNSs/Cu solder and the growth behavior of intermetallic compound (IMC) during solid–liquid diffusion at 250 °C and under multiple reflows (1, 2, 4, and 8 times) were systematically discussed. Results exhibited that the wettability of GNSs doped solder improved as GNSs added and 0.05% GNSs addition would promote the spreading area of the composite solder effectively. The scalloped Cu₆Sn₅ IMC layer was formed at the interfacial Sn-xGNSs/Cu. Based on the adsorption theory, GNSs doping was conducive to prohibiting the interfacial IMC growth of Sn solder reinforced with GNSs during soldering process and reflowing process. Meanwhile, the IMC overtly became thinner in Sn-0.05GNSs solder with comparison to plain solder. What’s more, GNSs addition brought about an enhancement in the mechanical performance of GNS-containing solder. The fracture surface of solder joint after doping GNSs transformed from a brittle pattern to a ductile pattern.

     

Interfacial interaction of solid cobalt with liquid Pb-free Sn–Bi–In–Zn–Sb soldering alloys

Dissolution kinetics of cobalt in liquid 87.5%Sn–7.5%Bi–3%In–1%Zn–1%Sb and 80%Sn–15%Bi–3%In–1%Zn–1%Sb soldering alloys and phase formation at the cobalt–solder interface have been investigated in the temperature range of 250–450 °C. The temperature dependence of the cobalt solubility in soldering alloys was found to obey a relation of the Arrhenius type c _s = 4.06 × 10² exp (−46300/RT) mass% for the former alloy and c _s = 5.46 × 10² exp (−49200/RT) mass% for the latter, where R is in J mol⁻¹ K⁻¹ and T in K. For tin, the appropriate equation is c _s = 4.08 × 10² exp (−45200/RT) mass%. The dissolution rate constants are rather close for these soldering alloys and vary in the range (1–9) × 10⁻⁵ m s⁻¹ at disc rotational speeds of 6.45–82.4 rad s⁻¹. For both alloys, the CoSn₃ intermetallic layer is formed at the interface of cobalt and the saturated or undersaturated solder melt at 250 °C and dipping times up to 1800 s, whereas the CoSn₂ intermetallic layer occurs at higher temperatures of 300–450 °C. Formation of an additional intermetallic layer (around 1.5 μm thick) of the CoSn compound was only observed at 450 °C and a dipping time of 1800 s. A simple mathematical equation is proposed to evaluate the intermetallic-layer thickness in the case of undersaturated melts. The tensile strength of the cobalt-to-solder joints is 95–107 MPa, with the relative elongation being 2.0–2.6%.     

Growth mechanisms of interfacial intermetallic compounds in Sn/Cu–Zn solder joints during aging

The interfacial reactions of Sn/Cu–xZn (x = 15 and 30 at.%) solder joints were investigated. Before aging, [Cu₆(Sn,Zn)₅] and [Cu₆(Sn,Zn)₅/Cu–Zn–Sn] intermetallic compounds (IMCs) formed at the [Sn/Cu–15Zn] and [Sn/Cu–30Zn] interfaces, respectively. After thermal aging at 150 °C for 80 days, [Cu₆(Sn,Zn)₅/Cu₃(Sn,Zn)/Cu(Zn,Sn)/CuZn] and [Cu₆(Sn,Zn)₅/Cu(Zn,Sn)/CuZn] IMCs, respectively, formed at the [Sn/Cu–15Zn] and [Sn/Cu–30Zn] interfaces. Increasing the amount of Zn in the Cu–Zn substrates evidently suppresses the growth of Cu₃Sn and Kirkendall voids at the solder joint interfaces. Transmission electron microscopy images show the different microstructure of CuZn and Cu–Zn–Sn phases in Sn/Cu–Zn joints. These Cu–Zn phases act to inhibit the growth of Cu₆Sn₅ and Cu₃Sn IMCs. As the content of Zn increased in Cu–Zn substrates, both CuZn and Cu(Zn,Sn) grew significantly. In addition, the growth of the Cu₆(Sn,Zn)₅/Cu₃Sn IMCs approached a reaction-controlled process. The formation mechanisms of the CuZn and Cu(Zn,Sn) phases were probed and proposed with regard to the interfacial microstructure, elemental distribution, and the compositional variation at Sn/Cu–xZn interfaces.     

Wettability and interfacial whiskers of Sn–9Zn–0.5Ga–0.08Nd solder with Sn,SnBi and Au/Ni coatings

The wettability and interfacial whiskers of Sn–9Zn–0.5Ga–0.08Nd solder were investigated. The results indicated that Sn–9Zn–0.5Nd–0.08Nd solder has shown good wettability using water soluble flux and self made flux based on wetting balance testing, and solder with SnBi coating has shown superiority wettability among all three coatings. Moreover, no whisker was observed in Sn–9Zn–0.5Ga–0.08Nd solder with different coatings. But in Sn–9Zn–0.5Ga–1Nd solder, fewer whiskers were observed in solder with SnBi coating compared with Sn and Au/Ni coatings, which can be attributed to the refinement effect of Bi.     

Electrical conductivity and viscosity of liquid Sn–Sb–Cu alloys

Tin-antimony-copper alloys are under intense consideration as favourable lead-free solders for consumer electronics and telecommunications. The electrical conductivity and viscosity studies were carried out in a wide temperature range above the liquidus. A melting-solidification region was investigated by Differential Scanning Calorimetry (DSC). The scaling relations have been proposed. A comparison with data available in literature is given.     

Interfacial reaction of Sn–2.0Ag–2.5Zn solder on Cu and Ni–W substrates

The interfacial reactions of Sn–2.0Ag–2.5Zn solder on Cu and Ni–W substrates after soldering and subsequent aging have been investigated in this study. Ni–W alloy layers with tungsten content of 3.0 and 10.0 at.% were electrodeposited on copper substrate. The interfacial micrographs of solder joints prepared at 250 °C for 15 s and aged at 150 °C for 24, 96 and 216 h are shown. Double-layer IMC composed of Cu₅Zn₈ and Ag₃Sn was observed at the interface of Sn–2Ag–2.5Zn and Cu couple, which was compact and acted as a barrier layer to confine the further growth of Cu–Sn IMC. On Ni–W barrier layer, a thin Ni₃Sn₄ film appeared between the solder and Ni–W layer, whose thickness decreases with the increase of W content. During the aging process, a thin layer of the Ni–W substrate transforms into an amorphous bright layer, and the thickness of amorphous layer increased as aging time extended. Referring to the elemental line-distribution and the thickness of different layers at the interface, the formation of the bright layer is caused by the fast diffusion of Sn into Ni–W layer.     

Formation of microstructure resulting from quasi-peritectic reactions during freezing of Bi–Pb–Sn and Cu–Ni–Sn ternary alloys

Abstract

The quasi-peritectic reaction, frequently observed in ternary alloy systems, takes the form L + α?β + γ, where L indicates the liquid phase, and α, β and γ indicate solid phases. The formation of microstructure resulting from the kinetics of this reaction is considered from a theoretical point of view and compared with experimental observations of microstructures formed by quasi-peritectic reactions in two ternary alloy systems, Bi–Pb–Sn and Cu–Ni–Sn. Based on these considerations, an explanation is proposed for experimental observations previously reported in literature concerning phase transformations from the liquid in multicomponent ferrous alloys.     

The influence of Pd on growth behavior of a quaternary (Cu,Ni,Pd)6Sn5 compound in Sn–3.0Ag–0.5Cu/Au/Pd/Ni–P solder joint during a liquid state reaction

This study investigated the liquid state reaction of a Sn–3.0Ag–0.5Cu solder jointed with electroless Ni–P/immersion Au (ENIG) and electroless Ni–P/electroless Pd/immersion Au (ENEPIG) surface finishes. Treatments with various soldering temperatures (240, 250, and 260 °C) and times (60, 180, 300, and 600 s) were performed to study the microstructure evolution. Detailed interfacial images revealed that the morphology of (Cu,Ni)₆Sn₅ affects the formation of Ni₃P and the curvature of the interface between them. In addition, the growth kinetics of (Cu,Ni)₆Sn₅ and (Cu,Ni,Pd)₆Sn₅ were studied and compared. The effect of grain coarsening during extended reflow modified the diffusion transport mechanism. However, because of the refinement of Pd on the grain structure, reduced IMC growth and a lower degree of transition from grain boundary diffusion to volume diffusion could be observed in the growth kinetics of (Cu,Ni,Pd)₆Sn₅. Moreover, the activation energy of IMC growth was evaluated using the Arrhenius equation. Pd may act as heterogeneous nucleation sites in the initial stage of soldering and lower the activation energy of (Cu,Ni,Pd)₆Sn₅, compared to (Cu,Ni)₆Sn₅. The lower activation energy of (Cu,Ni,Pd)₆Sn₅ growth ensured that no phase transformation occurred in the SAC305/ENEPIG joints, which may benefit the solder joint reliability. Finally, the detailed influence of Pd on the growth kinetics of IMC formation was investigated and discussed.     

Investigation of intermetallic compound and voids growth in fine-pitch Sn–3.5Ag/Ni/Cu microbumps

Micron level Cu-pillar microbumps, which appear as the size of three-dimensional packaging shrinks down, have to introduce many unexpected reliability problems. The fast growth of intermetallic compounds (IMC) and voids tend to be a serious one. In this paper, the growth behaviors of IMC and voids were investigated in Sn–3.5Ag/Ni/Cu bumps, which were in the diameter ranging from 6 to 11 μm and fabricated under same reflow process. The consequence manifested that the growth rate of interfacial IMC increased from 0.448 to 0.578 μm/min as the bump diameter decreased from 11 to 6 μm and the acquired IMC could be divided into two layers: the (Cu, Ni)₆Sn₅ layer and Ni₃Sn₄ layer. As a result of the migration of Ni atoms, many voids were left in the interface between (Cu, Ni)₆Sn₅ and Ni₃Sn₄. In the edge of Cu pillar, side wall wetting reaction was detected, which was confirmed by the formation of voids and (Cu, Ni)₆Sn₅ phase. Further, as the bump diameter decreased, the side wall wetting reaction aggravated, which partly contributed to the acceleration of IMC growth.     

Phase transformation and fracture behavior of Cu/In/Cu joints formed by solid–liquid interdiffusion bonding

In this paper, microstructure evolution and phase transformation of Cu–In intermetallic compounds in Cu/In/Cu joints formed by solid–liquid interdiffusion bonding at 260 and 360 °C were investigated respectively. The shearing properties and fracture behaviors of the Cu/In/Cu joints formed under different bonding conditions were also studied. For Cu/In/Cu joints bonded at 260 °C, Cu₁₁In₉ phase firstly generated and then Cu₂In phase formed between Cu₁₁In₉ layer and Cu substrate. For Cu/In/Cu joints bonded at 360 °C, Cu₂In phase firstly formed and then parts of Cu₂In grains transformed to Cu₇In₃ phase, and this transition from incomplete to complete coverage of Cu₂In/Cu₂In grain boundaries by Cu₇In₃ phases was observed with the bonding time increasing. The shear test results show that Cu₂In was high-quality phase which could improve the mechanical properties of Cu/In/Cu joints. After shear test, the fractures in Cu/In/Cu joints bonded at 260 °C were found at Cu₁₁In₉ layers and the fracture mode was cleavage fracture. In the case of the joints bonded at 360 °C, the intergranular fractures were found at the interface between Cu₂In layer and Cu₇In₃ layer while the cleavage fractures were found at Cu₇In₃ layer.     

Growth behavior of interfacial Cu–Sn intermetallic compounds of Sn/Cu reaction couples during dip soldering and aging

The formation and growth of intermetallic compound (IMC) layer at the interface between pure Sn and a Cu substrate during dip soldering and aging were studied. The soldering was conducted at 250 °C using dipping method, followed by aging treatment at 150 °C for up to 10 days. The results showed that the IMC layer flattened with aging duration because the grooves in scallop-like IMC provide a more convenient access for Cu atoms to dissolve and react with solders and previous IMCs. And when isothermal aging was subjected, the growth rate of Cu₆Sn₅ was lower than that of Cu₃Sn due to Cu₃Sn growing rapidly with aging time by consuming Cu₆Sn₅ at the interface of Cu₃Sn/Cu₆Sn₅. Kirkendall voids were observed at Cu₃Sn/Cu interface as well as inside the Cu₃Sn layer as the Sn/Cu couple was aged at 150 °C for prolonged time, with which the Cu₃Sn IMC dominates the interfacial IMCs growth. During solid-state aging, the mean diameter (d) of interfacial Cu₆Sn₅ grains increased dramatically with the increasing time (t). The relationships between d and t were given to be d = 1.22 t ^0.291 for samples formed at 250 °C for 1 min and d = 1.53 t ^0.259 for samples formed at 250 °C for 5 min, respectively.     

Microstructure and magnetic studies of Mg–Ni–Zn–Cu ferrites

Soft Mg–Ni–Zn–Cu spinel ferrites having general chemical formula Ni_xMg_0.5−xCu_0.1Zn_0.4Fe₂O₄ (where x = 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by standard double sintering ceramic method. The samples were characterized by X-ray diffraction at room temperature. The X-ray diffraction (XRD) study revealed that lattice parameter decreases with increase in Ni content, resulting in a reduction in lattice strain. The electrical and magnetic properties of the synthesized ferrites have been investigated as a function of temperature. The variation of initial permeability and AC susceptibility with temperature exhibits normal ferrimagnetic behavior. The variation of initial permeability with frequency is studied. The Curie temperature (T_C) in the present work was determined from initial permeability and AC susceptibility. The Curie temperature increases with Ni content.     

Effect of Ni Content on Mechanical Properties and Corrosion Behavior of Al/Sn－9Zn－xNi/Cu Joints

The effects of Ni content on the microstructure and the wetting behavior of Sn-9Zn-xNi solders on Al and Cu substrates, as well as the mechanical properties and electrochemical corrosion behavior of Al/Sn-9Zn-xNi/Cu solder joints, were investigated. The microstructure of Sn-9Zn-xNi revealed that tiny Zn and coarsened Ni 5 Zn 21 phases dispersed in the β-Sn matrix. The wettability of Sn-9Zn-xNi solders on Al substrate was much better than that on Cu substrate. With increasing Ni content, the wettability on Cu substrate was slightly improved but became worse on Al substrate. In the Al/Sn-9Zn-xNi/Cu joints, an Al4.2Cu3.2Zn0.7 intermetallic compound (IMC) layer formed at the Sn-9Zn-xNi/Cu interfaces, while an Al-Zn-Sn solid solution layer formed at the Sn-9Zn-xNi/Al interface. The mixed compounds of Ni3Sn4 and Al3Ni dispersed in the solder matrix and coarsened with increasing Ni content, thus leading to a reduction in shear strength of the Al/Sn-9Zn-xNi/Cu joints. Al particles were segregated at both interfaces in the solder joints. The corrosion potentials of Sn-9Zn-xNi solders continuously increased with increasing Ni content. The Al/Sn-9Zn-0.25Ni/Cu joint was found to have the best electrochemical corrosion resistance in 5% NaCl solution.     

The measurement of interfacial energies for solid Sn solution in equilibrium with the Sn–Bi–Ag liquid

The equilibrated grain boundary groove shapes of solid Sn solution in equilibrium with Sn–Bi–Ag liquid were observed from a quenched sample by using a radial heat flow apparatus. The Gibbs–Thomson coefficient, solid–liquid interfacial energy, and grain boundary energy of the solid Sn solution were determined from the observed grain boundary groove shapes. The thermal conductivity of the solid phase for Sn-10 at.%Bi-2 at.%Ag alloy and the thermal conductivity ratio of the liquid phase to the solid phase for Sn-10 at.%Bi-2 at.%Ag alloy at the melting temperature were also measured with a radial heat flow apparatus and a Bridgman-type growth apparatus, respectively. A comparison of present results for solid Sn solution in the Sn–10 at.%Bi–2 at.%Ag alloy with the results obtained in previous works for similar solid Sn in equilibrium with different binary or ternary liquid was made. From the comparison, it can be concluded that for solid Sn solution in equilibrium with different liquid, the Gibbs–Thomson coefficient seems to be constant and does not depend on the composition of liquid but solid–liquid interfacial energy changes little bit with composition of liquid at a constant temperature.