Liquid Solder Research Articles

Effects of thermal aging on growth behavior of interfacial intermetallic compound of dip soldered Sn/Cu joints

Intermetallic compounds (IMCs) formations of Sn/Cu reaction couple prepared by dip soldering were studied with isothermal aging at four levels of temperatures (120, 150, 170 and 200 °C, respectively). It is found that an obvious scallop-type layer of Cu6Sn5 IMC formed between the liquid Sn solder and Cu substrates and a planar Cu3Sn IMC layer between Cu6Sn5 IMC layer and Cu substrates appears after solid-state aging treatment. A significant increment of IMC layer thickness is observed with increasing aging time as well as aging temperature. And it is further found that the thickness of IMC layer is linearly related to the square root of aging time during the aging process. The values of growth rate constants for the different solid state aging temperatures (120, 150, 170 and 200 °C) were calculated as 6.578 × 10−18, 1.595 × 10−17, 9.89 × 10−17 and 4.521 × 10−16 m2 s−1, respectively. The apparent activation energy calculated for the growth of the total IMC is 84.43 kJ mol−1. Moreover, The mean diameters of the Cu6Sn5 grains increased linearly with cubic root of aging time.

Phenomenological Investigations on Diffusion Kinetics in Multicomponent Metallic Melts

Multicomponent diffusion in metallic melts is a very important phenomenon during the solidification/casting process of the metallic alloys. However, there exist extremely limited reports on the diffusivity information in multicomponent metallic liquids. In this chapter, a universal and effective phenomenological approach to predict the composition– and temperature–dependent diffusivities in liquid multicomponent systems is systematically proposed. The presently proposed phenomenological method is then adopted to construct the diffusivity/mobility databases of liquid solders, cemented carbides, Co–Cr–Fe–Mn–Ni high entropy alloys and Al–Ce–Ni alloys. Then, the accurate diffusivity/mobility data are further utilized to perform the simulations of the dissolutions of the substrate into the solders, the gradient layer formation of the cemented carbides, the diffusion behavior of liquid Co–Cr–Fe–Mn–Ni high entropy alloys and the rapid solidification of Al–Ce–Ni system. The simulated results indicate that the presently proposed phenomenological method is applicable to investigate the diffusion kinetics in multicomponent metallic melts.

Estimation of the Viscosities of Liquid Sn-Based Binary Lead-Free Solder Alloys

The viscosity of a binary Sn-based lead-free solder alloy was calculated by combining the predicted model with the Miedema model. The viscosity factor was proposed and the relationship between the viscosity and surface tension was analyzed as well. The investigation result shows that the viscosity of Sn-based lead-free solders predicted from the predicted model shows excellent agreement with the reported values. The viscosity factor is determined by three physical parameters: atomic volume, electronic density, and electro-negativity. In addition, the apparent correlation between the surface tension and viscosity of the binary Sn-based Pb-free solder was obtained based on the predicted model.

Effect of bonding time on the microstructure and mechanical properties of Co/Sn/Cu joint

The bonding process of Co/Sn/Cu couple has been conducted at 270 °C. The microstructure evolution was observed. The spreading test of Cu/Sn–xCo and Co/Sn–xCu was conducted to explain the microstructure evolution of Co/Sn/Cu couple. The results showed that the main interfacial intermetallic compounds (IMCs) layer of Cu/Sn side of Co/Sn/Cu couple was (Cu, Co)6Sn5, as the dissolution of Co from Co substrate to solder layer. The (Cu, Co)6Sn5 IMCs layer only thickened slightly during bonding process as large amount of the (Cu, Co)6Sn5 IMCs layer dissociated into solder alloy layer. As the adsorbing effect of Co substrate on the dissociating (Cu, Co)6Sn5 IMCs, there was a thick (Cu, Co)6Sn5 IMCs layer covered on the surface of Co substrate at the early stage of bonding process. For longer bonding time (> 30 min), the (Co, Cu)Sn and (Co, Cu)Sn3 IMCs generated on the Co/(Cu, Co)6Sn5 interface as the diffusion of Sn atom from liquid solder alloy to Co/(Cu, Co)6Sn5 interface. The Co/Sn/Cu joint bonded for 60 min could work at the temperature of 300 °C. The shear strength of joints bonded for 20 and 30 min was approximate 45 MPa. The shear strength of joints bonded for a time more than 30 min declined gradually as the generation of (Co, Cu)Sn3 IMCs.

Binding of indocyanine green in polycaprolactone fibers using blend electrospinning for in vivo laser-assisted vascular anastomosis.

The clinical application of laser-assisted vascular anastomosis is afflicted by unreliable and low bonding strengths as well as tedious handling during microvascular surgery. The challenge to be met arises from the flow-off of the chromophore during soldering that changes the absorption and stains the surrounding tissue, leading to an uncontrollable thermal damage zone. In this study, we investigated the feasibility to produce an indocyanine green (ICG)-loaded patch by electrospinning and tested its applicability to both in vitro and in vivo microvascular laser soldering. A blend of polycaprolactone and ICG was electrospun to produce a pliable patch. Prior to soldering, the patch was soaked in 40% wt. bovine serum albumin solution. The solder patch was wrapped in vitro around blood vessel stumps of rabbit aortas. An intraluminal balloon catheter enabled an easy alignment and held the setup in place. The soldering energy was delivered via a diffusor fiber from the vessel lumen using a diode laser at 810 nm. During the procedure, the surface temperature was observed with an infrared camera. Afterward, samples were embedded in methylmethacrylate and epon to study thermal damage. The quality of the fusion was assessed by measuring the tensile strength. After in vitro tests with rabbit aortas, eight large white pigs were subjected to an acute in vivo experiment, and the artery of the latissimus dorsi flap was anastomosed to the distal femoral artery. The ICG-loaded patch, produced by electrospinning, has a thickness of 279 ± 62 μm, a fiber diameter of 1.20 ± 0.19 μm, and an attenuation coefficient of 1,119 ± 183 cm-1 at a wavelength of 790 nm. The patch was pliable and easy to handle during surgery. No leakage of the chromophore was observed. Thermal damage was restricted to the Tunica adventitia and Tunica media and the area of the vessel wall that was covered with the patch. Six pigs were successfully treated, without any bleeding and with a continuous blood flow. The in vivo flap model yielded a similar tensile strength compared to in vitro laser-assisted vascular anastomoses (138 ± 52 vs. 117 ± 30 mN/mm2 ). Our study demonstrated the applicability of the ICG-loaded patch for laser-assisted vascular anastomosis. By using electrospinning, ICG could be bound to polymer fibers, avoiding its flow-off and the staining of the surrounding tissue. This patch demonstrated several advantages over liquid solder as it was easier to apply, ensured a high and reliable bonding strength while maintaining a constant concentration of ICG concentration during the surgery. Lasers Surg. Med. 49:928-939, 2017. © 2017 Wiley Periodicals, Inc.

Force and collapsed shape of a liquid solder bump under load

It is shown that a collapsed solder bump carries load through a bump force that depends on the collapsed shape in a way that closely resembles the Young-Laplace (YLP) equation. The relation between force and shape allows deriving the force constant of the bump, which is important in solder bump reliability. For the commonly used ellipsoid shape model, the new force method gives similar results as the results derived conventionally from the increase in free surface energy but in a mathematically simpler way, omitting the difficult surface integral. The results confirm that the ellipsoid shape model is not a true solution for the collapsed shape. The new force based approach enables developing an improved shape model and it is shown that this satisfies both the YLP equation and minimum increase in surface energy better.

Coupling effect of thermomigration and cross-interaction on evolution of intermetallic compounds in Cu/Sn/Ni ultrafine interconnects undergoing TLP bonding

Abstract

Effect of Graphene Layers on Phenomena Occurringat Interface of Sn-Zn-Cu Solder and Cu Substrate

Cu and graphene-coated Cu substrates spread with Sn-Zn eutectic-based alloy containing 0.5 wt.%, 1 wt.%, or 1.5 wt.% Cu have been studied at 250°C. Soldering experiments were performed for wetting time of 3 min, 8 min, 15 min, 30 min, and 60 min in (1) presence of flux without argon protective atmosphere and (2) fluxless in argon atmosphere. Solidified solder–pad couples were cross-sectioned and examined using scanning electron microscopy with energy-dispersive spectroscopy to study their interfacial microstructure. To assess the effect of graphene coating on solder, Cu pads were covered by graphene using chemical vapor deposition. The results revealed that the liquid solder did not wet the graphene-coated copper in the absence of flux. Wetting took place only with use of flux, because it destroyed the graphene layer and enabled contact of the liquid solder with the copper. Experiments were designed to demonstrate the effect of Cu addition and graphene coating on the kinetics of the formation and growth of Cu5Zn8 and CuZn4 phases identified by x-ray diffraction analysis, Raman spectroscopy, and energy-dispersive spectroscopy. A decrease of about 60% in the thickness of the intermetallic layer was observed when applying the graphene interlayer in presence of flux. Addition of copper to the Sn-Zn alloy improved the wettability as the copper content was increased.

Evaluation of Detachable Ga-Based Solder Contacts for Thermoelectric Materials

Low electrical and thermal contact resistances are a prerequisite for highly efficient thermoelectric generators. Likewise, certain measurement setups for characterization of thermoelectric materials rely on good-quality contacts between sample and setup. Detachable contacts are an interesting alternative to permanent contacting solutions due to ease of handling and nondestructive disassembly of valuable samples. Therefore, the applicability of gallium-based liquid metal solder as detachable contact material was studied, particularly with regard to compatibility of the solder with state-of-the-art thermoelectric materials CoSb3, Mg2Si, and FeSi2. Tungsten, nickel, chromium, and titanium were tested as protective coatings between the thermoelectric material and liquid metal solder. Electrical measurements showed that some materials form excellent and stable contacts with the solder for a limited temperature range. At higher temperatures, application of a protective layer was found to be necessary for all investigated materials. Tungsten and nickel showed promising results as protective layer.

Large-scale anodic bonding mediated by a liquid tin solder

Large area seals were prepared between a metal foil frame (Fe-Ni alloy with 48wt% Ni) and a 901×814mm2 glass pane through the simultaneous application of activated liquid tin solder anodic bonding and soft soldering. An experimental facility including a liquid solder injector was built. Prototype seals were produced with tin solder alloys with variable Al contents and the hermiticity and interface integrity was determined. Solder alloys with an Al content of 112.5ppm resulted in the strongest adhesion forces, broadly consistent with a previous study at the laboratory scale. The resulting prototypes consist of a bonded area of ∼450cm2 along the perimeter of a 0.73m2 glass pane and display consistently strong bonds for sections over a meter in length, demonstrating that activated liquid solder anodic bonding is not restricted to small-area applications.

In situ study on liquid structure of Sn during Sn/Cu liquid–solid interfacial reaction by fluorescence XAFS

Fluorescence X-ray Absorption Fine Structure (XAFS) was used to in situ study the local structure around Cu atoms in liquid Sn solder during Sn/Cu liquid–solid interfacial reaction. The extended edge data were analyzed by cumulant expansion method. The results showed that both the atomic distance and the coordination number in the first shell decreased with the increase of temperature. It is an intrinsic trend that high-coordinated polyhedrons could transform into low-coordinated ones with relatively high stability in the liquid Sn solder with increasing temperature, and the high-coordinated polyhedrons always had larger atomic distance. The atomic distance in the first shell decreased with decreasing coordination number. Based on the fitting results, the proportion of Cu–Sn atomic coordination increased with rising soldering temperature from 250 to 310 °C. That is, Cu atoms preferred to form Cu–Sn interaction with Sn atoms, which promoted the nucleation and growth of Cu6Sn5 intermetallic compound at the Sn/Cu interface during soldering process.

Effect of Indium Additions on the Formation of Interfacial Intermetallic Phases and the Wettability at Sn-Zn-In/Cu Interfaces

The wettability of copper substrates by Sn-Zn eutectic solder alloy doped with 0, 0.5, 1, and 1.5 at.% of indium was studied using the sessile drop method, with flux, in air, at 250°C and reflow time of 3, 8, 15, 30, and 60 min. Wetting tests were performed at 230, 250, 280, 320, and 370°C for an alloy containing 1.5 at.% of indium, in order to determine activation energy of diffusion. Solidified solder/substrate couples were studied using scanning electron microscopy (SEM), the intermetallic phases from Cu-Zn system which formed at the solder/substrate interface were identified, and their growth kinetics was investigated. The ε-CuZn4 was formed first, as a product of the reaction between liquid solder and the Cu substrate, whereas γ-Cu5Zn8 was formed as a product of the reaction between ε-CuZn4 and the Cu substrate. With increasing wetting time, the thickness of ε-CuZn4 increases, while the thickness of ε-CuZn4 does not change over time for indium-doped solders and gradually disappears over time for Sn-Zn eutectic solder.

The role of physico-chemical properties of liquid solder in reactive wetting: the Cu/SnZnIn system

The measurements of surface tension, density and viscosity of liquid Sn-Zn eutectic alloys containing 0, 5, 10, 15 and 25 mole fraction of In were carried out using the sessile drop method, dilatometric technique and capillary method. The measurements were performed at temperature range between 493 and 843 K. The technique of sessile drop was applied in the measurements of wetting angles and spreading test in the SnZnIn/ Cu system. Surface tension, density and viscosity measurements were carried out in a protective argon-hydrogen atmosphere. Wettability tests were performed in air in the presence of flux Alu33, at 250?C for 2 minutes. Subsequently, the microstructure of solder and the resulting joints was studied. The addition of In to eutectic Sn-Zn alloy improved the wetting properties and causes a reduction of thickness of the intermetallic compounds layer created at the interface between the liquid solder and the Cu substrate.

Formation mechanism and kinetic analysis of the morphology of Cu6Sn5 in the spherical solder joints at the Sn/Cu liquid–solid interface during soldering cooling stage

The morphology evolution mechanism and dynamics of Cu6Sn5 intermetallic compound (IMC) in cooling stage were studied by using pure Sn solder ball with a diameter of 1 mm to react with polycrystalline Cu substrate and form Cu6Sn5. A nearly uniform height of the scallop-like IMC grains is attained by using high pressurized air to remove excess liquid solder (that is, for acquisition of the IMC morphology identical to heat preservation stage). But the morphology evolution of IMC in the solder joint was greatly affected by the cooling phase of interfacial reaction, and IMC morphology partition phenomenon was found in the spherical joints. The IMC at the edge portion characterized scallop morphology whereas the central portion possessed prismatic shape morphology. Finite element simulation for the temperature field distribution in the solder ball-substrate domain showed temperature gradient in solder’s internal core was significantly greater than that at the edge. Using this simulation results with kinetic equations, it could be understood that the central core region is conducive to the formation of small plane structure morphology. These small planes would eventually form the prismatic morphology IMC in the central region. As the soldering temperature increases, the area ratio of IMC with scallop-like morphology to the prismatic shaped IMC reduced from 0.2178 down to 0.1680. Moreover, for an elevated temperature of 300 °C, it is observed that the central region IMC (with prismatic structure) grows upto an average thickness of 7.7245 μm, whereas the thickness value for scalloped IMC at the edge portion is around 4.556 μm.

Amorphous Copper-Based Alloys: Structure, Technology and Properties

The scientific interest in amorphous and nanocrystalline materials is associated with the expectation of different structural and size effects on the properties of solids, as applied attention due to the possibility of fundamental changes in the properties of polycrystalline materials. Numerous experimental studies indicate the following regularities: the higher the degree of equilibrium of the melt prior to crystallization is, the better quality has a solid metal. In industrial conditions the equilibrium state of the melt is achieved by the technology for heat treatment of the melt. The scientific rationale for this technology is the results of studying the temperature and time dependence of physical properties. The present work describes the analysis of the physical properties of liquid amorphous solder, conducted heat treatment of the melt and the study of the samples mechanical properties in the solid state. Amorphous ribbons obtained by the technology for melt heat treatment, ceteris paribus withstand higher loads in tension, exhibit better ductility and are characterized by high stability of the results. This fact confirms a more uniform material structure.

Hexagonal-Rod Growth Mechanism and Kinetics of the Primary Cu6Sn5 Phase in Liquid Sn-Based Solder

A hexagonal-rod growth mechanism is proposed to describe the growth behavior of the primary Cu6Sn5 phase in liquid Sn-based solder. After Sn-6.5 at.%Cu solder had been maintained at 250°C for 10 h, a large number of hexagonal-rod-type Cu6Sn5 grains were found to have separated within it. Our observations show that these hexagonal rods had side facets in the \(\{ 10\overline{1} 0\}_{\mathrm{\eta} }\) family and round ends close to the {0002}η family. Moreover, the nucleation of the hexagonal rods was studied, and the corresponding growth kinetics found to be governed by a Cu-supply-controlled mechanism rather than an interfacial-reaction-controlled or Cu-diffusion-limited mechanism. More importantly, the anisotropic growth of the Cu6Sn5 phase was confirmed to be the dominant reason for production of these primary hexagonal rods with high aspect ratio. This may represent an avenue for synthesis of nanosized Cu6Sn5 single crystals for use as anode materials in lithium-ion batteries. Additionally, our Cu6Sn5 hexagonal-rod growth mechanism may provide insight into morphological and kinetic studies on interfacial Cu6Sn5 grains and similar intermetallics.

Optimized solder alloy for glass-to-metal joints by simultaneous soldering and anodic bonding

Strong glass/solder/metal seals were prepared between float glass, tin solder and Fe-Ni alloys by the Activated Liquid Tin Solder Anodic Bonding (ALTSAB) process that combines classical anodic bonding with a liquid solder to ensure a near perfect contact between the bonding surfaces. The optimization of the Al content of the solder enables the production of glass/solder/metal joints without pre-treatment of the metal or glass surfaces. Strong glass/solder bonds are formed by anodic bonding across the oxide/metal interface and strong solder/metal bonds are formed simultaneously by soft soldering. The combined process consists of two very different processes and the optimal solder composition represents the middle ground between the requirements for both interfaces. The glass/solder bond strength increases with increasing Al content at the expense of the solder/metal bond strength: solders with a very low Al content of 75ppm constitute the best compromise and result in the strongest glass/solder/metal joints. The optimal solder composition was used to anodically bond Dilaton Fe-48wt%Ni frames to the edge of 500×500mm2 float glass panes for vacuum glazing applications.

Interaction of Zn and Zn–4Al, Zn–15Al (wt-%) solder alloys with aluminium

The interaction of pure zinc and Zn–4Al, Zn–15Al (wt-%) solder alloys with aluminium has been investigated. Two different experimental techniques were used: (i) the aluminium samples soldering and (ii) the holding of liquid solder alloys inside the hollow in an aluminium base. The aluminium content was determined in samples after contact with aluminium at various temperatures and various holding times. It is shown that in soldered seams and solder alloys that had been held inside the aluminium base, the content of the aluminium was higher than at the points on the liquidus line on the Zn–Al equilibrium phase diagram at the corresponding temperatures and increases along with an increase in the holding time. This was apparently due to isothermal solidification.

Thermal and Microstructure Characterization of Zn-Al-Si Alloys and Chemical Reaction with Cu Substrate During Spreading

The problems associated with the corrosion of aluminum connections, the low mechanical properties of Al/Cu connections, and the introduction of EU directives have forced the potential of new materials to be investigated. Alloys based on eutectic Zn-Al are proposed, because they have a higher melting temperature (381 °C), good corrosion resistance, and high mechanical strength. The Zn-Al-Si cast alloys were characterized using differential scanning calorimetry (DSC) measurements, which were performed to determine the melting temperatures of the alloys. Thermal linear expansion and electrical resistivity measurements were performed at temperature ranges of −50 to 250 °C and 25 to 300 °C, respectively. The addition of Si to eutectic Zn-Al alloys not only limits the growth of phases at the interface of liquid solder and Cu substrate but also raises the mechanical properties of the solder. Spreading test on Cu substrate using eutectic Zn-Al alloys with 0.5, 1.0, 3.0, and 5.0 wt.% of Si was studied using the sessile drop method in the presence of QJ201 flux. Spreading tests were performed with contact times of 1, 8, 15, 30, and 60 min, and at temperatures of 475, 500, 525, and 550 °C. After cleaning the flux residue from solidified samples, the spreadability of Zn-Al-Si on Cu was determined. Selected, solidified solder/substrate couples were cross-sectioned, and the interfacial microstructures were studied using scanning electron microscopy and energy dispersive x-ray spectroscopy. The growth of the intermetallic phase layer was studied at the solder/substrate interface, and the activation energy of growth of Cu5Zn8, CuZn4, and CuZn phases were determined.

Effect of Reflow Time on Wetting Behavior, Microstructure Evolution, and Joint Strength of Sn-2.5Ag-0.5Cu Solder on Bare and Nickel-Coated Copper Substrates

The effect of reflow time on wetting behavior of Sn-2.5Ag-0.5Cu lead-free solder on bare and nickel-coated copper substrates has been investigated. The solder alloy was reflowed at 270°C for various reflow times of 10 s, 100 s, 300 s, and 500 s. On bare copper substrate, the intermetallic compound (IMC) thickness increased with increase in reflow time, whereas on Ni-coated Cu substrate, the IMC thickness increased up to 300 s followed by a drop for solder alloy reflowed for 500 s. The spreading behavior of the solder alloy was categorized into capillary, gravity (diffusion), and viscous zones. Gravity zone was obtained from 3.8 ± 0.43 s to 38.97 ± 3.38 s and from 5.99 ± 0.5 s to 77.82 ± 8.84 s for the Sn-2.5Ag-0.5Cu/Cu and Sn-2.5Ag-0.5Cu/Ni/Cu system, respectively. Sn-2.5Ag-0.5Cu solder alloy was also reflowed for the period corresponding to the end of the gravity zone (40 s and 80 s on bare and Ni-coated Cu, respectively). The joint strength was maximum at reflow time of 40 s and 80 s for the Sn-2.5Ag-0.5Cu/Cu and Sn-2.5Ag-0.5Cu/Ni/Cu system, respectively. The dynamic contact angle at the end of the gravity (diffusion) zone (θgz) was found to be a better parameter compared with the stabilized contact angle (θf) to assess the effect of the wettability of the liquid solder on the microstructure and joint strength. The present investigation reveals the significance of the gravity zone in assessment of optimum reflow time for lead-free solder alloys.