Sn-rich Solder Research Articles

The role of Sn grain orientation in Cu depletion of Sn-based solders

The miniaturization of solder bumps in flip-chip packages results in large variability in their microstructure making the prediction of joint lifetime challenging. This is of particular importance in Sn-rich solders due to the anisotropic diffusion behavior of Sn and the presence of fast diffusion pathways, such as grain boundaries and twins that vary from joint to joint. We developed a multi-physics phase-field model to predict the electromigration-enhanced evolution of intermetallic compounds in Cu/Sn system with different grain structures. We present simulations of solder joints with polycrystalline Sn with grains with different orientations to observe the effect of Sn anisotropy and grain boundaries on electromigration failure. The simulations predict Cu depletion in the cathode and the formation and accumulation of intermetallic compounds along Sn grain boundaries dependent on the Sn grain orientation. Additionally, Cu6Sn5 clusters are formed in the Sn matrix when high diffusion rates at the interfaces are considered.

Void-free interface between In and high-impurity Cu joint

The effect of impurities in the Cu film on the void formation at the interface of the Sn-rich solder joint has been extensively studied. However, this has rarely been studied for In solder joints. In this study, Sn-3.0Ag-0.5Cu (SAC305) and In-solder/ Cu films with high impurities were aged at 150 °C to analyze the films at various aging times. A metastable CuIn2 phase appeared and transformed into a stable Cu11In9 phase. Noticeable voids and cracks were observed at the SAC305/Cu interface during the solid-state aging. In contrast, there was a void-free interface between In and Cu-CP. This was primarily due to the difference in diffusion behaviour between In/Cu and SAC305/Cu. The mechanism of the above phenomenon was revealed.

Stacking-disordered CoSn3 and tetragonally stacked CoSn4 formed during solid-state interdiffusion of Co and Sn

The reactive interaction of Sn-rich solders with transition metals at temperatures close or above the melting point of the Sn leads to formation of stable or metastable intermetallic phases. Adequate microstructure analysis of solder joints relies on correct identification of the phases and their microstructures. The present work, thereby, focusses on the intermetallic forming from Co and Sn under solid-state conditions (150°C–190 °C) in model diffusion couples. The crystal structures and microstructures having developed were characterized using by X-ray diffraction (XRD) and scanning electron microscopy) supplemented by electron-dispersive X-ray spectroscopy and electron-backscatter diffraction (EBSD). While at 190 °C virtually exclusively a CoSn3 layer develops, at 170 °C and 150 °C an additional CoSn4 layer is formed. As evidenced by XRD, both the CoSn3 and CoSn4 having formed here show crystal structures different from previous works. The CoSn3 shows a random stacking of Co2Sn6 double layers, intermediate between the known equilibrium polytypes α-CoSn3 and β-CoSn3. The CoSn4 has the tetragonal β-IrSn4 structure (also I41/acd) with a largely regular stacking of CoSn4 layers (referred to as β-CoSn4), whereas until now a different polytype (orthorhombic α-CoSn4) has been reported. Including also results from first-principles calculations on the stable polytypes, criteria for accurate phase identification by XRD and EBSD methods have been elaborated, involving strategies to reliably distinguish the polytypes of CoSn3 or CoSn4, but also to distinguish (whatever form of) CoSn3 from (whatever form of) CoSn4.

Numerical investigation on Sn-rich solder joints failure mode: Effects of the thermomechanical anisotropic performance

In this paper, a thermomechanical anisotropic constitutive model was employed to investigate the failure mode of multiple Sn-rich solder joints in system in package under thermal cycle loading. As a comparison, traditional isotropic constitutive model was also employed to investigate the failure mode. Results showed that the anisotropic constitutive model predicted a new failure mode that failure would occur earlier at inner solder joint rather than the outmost solder joint, which was different from traditional perspective. This new failure mode indicated that traditional fatigue life prediction model just based on the outermost solder joint was not enough in view of the thermomechanical anisotropic performance of Sn-rich solder.

Effect of Sn Grain Orientation on Reliability Issues of Sn-Rich Solder Joints

Sn-rich solder joints in three-dimensional integrated circuits and their reliability issues, such as the electromigration (EM), thermomigration (TM), and thermomechanical fatigue (TMF), have drawn attention related to their use in electronic packaging. The Sn grain orientation is recognized as playing an important role in reliability issues due to its anisotropic diffusivity, mechanical properties, and coefficient of thermal expansion. This study reviews the effects of the Sn grain orientation on the EM, TM, and TMF in Sn-rich solder joints. The findings indicate that in spite of the failure modes dominated by the Sn grain orientation, the size and shape of the solder joint, as well as the Sn microstructures, such as the cycling twining boundary (CTB), single crystals, and misorientations of the Sn grain boundary, should be considered in more detail. In addition, we show that two methods, involving a strong magnetic field and seed crystal layers, can control the Sn grain orientations during the solidification of Sn-rich solder joints.

A Novel Design of High-Temperature Lead-Free Solders for Die-Attachment in Power Discrete Applications

Abstract Development of high-temperature lead-free (HTLF) solders to replace high-lead solders for die-attachment in power device applications is driven by (1) the harmful effects of lead to human health and the environment, and (2) the demand of the improved bonding materials serving under high-power density and high-junction temperatures, especially for wide-band-gap power devices. A novel design, based on a mixed solder powder paste technology—Durafuse™—has been developed to deliver a Sn-rich HTLF paste, presenting the merits of both constituent powders. The combination of the rigid, high-melting SnSbCuAgX and the ductile, low-temperature Sn-rich solder in one paste enables reflow at a relatively low temperature (barely above the liquidus temperature of the final joint composition) and maintains the joint strength above 15MPa in the temperature range between 270°C and 295°C. The sufficient high-temperature strength has demonstrated the capability of maintaining the joint integrity during subsequent multiple SMT reflows below the 270°C peak temperature, regardless of the existence of a partial melting phase. Both X-ray inspection and cross-section microstructure have not shown any damage in the Si die or any noticeable cracks in the bonding joint, even after 3000 cycles of TCT (−40 to 150°C). In summary, Durafuse™ HT, the novel design of the high-temperature lead-free pastes, has shown the feasibility as a drop-in solution to replace high-lead solders for die-attachment in power discrete applications.

Anisotropic constitutive model coupled with damage for Sn-rich solder: Application to SnAgCuSb solder under tensile conditions

With the rapid development of microelectronics and nanoelectronics, Moore law has significantly slowed down and More than Moore based system in packaging (SiP) is expected to be more and more important, at least for next one to two decades. Mechanical behaviors of interconnect materials such as solders are critical for yield in processes and reliability in testing and operation. Based on the framework of crystal plastic theory and continuum damage mechanics, an anisotropic constitutive model coupled with damage was developed to describe the deformation behaviors of Sn-rich solder. In the proposed model, the inelastic shear rate function was presented by hyperbolic sinusoidal form and power law form. For the damage evolution law, the total shear strain was chosen as the damage function variable. The proposed model was implemented into the general finite element software ABAQUS by forward Euler integration procedure. Some simulation examples were performed to verify the proposed model by comparing the simulation results with the experiments at uniaxial tensile conditions with SnAgCuSb solder chosen as the Sn-rich solder. The tensile stress-strain curves of the simulation results agreed well with the experiments at small strain under different temperatures and strain rates. The simulated stress-rupture stages showed reasonable accuracy with the experiments under four representative tensile conditions. Different tensile stress-strain curves of single grains with orientation of (0-0-0)°, (0-45-0)°, and (0-90-0)° were obtained under the same loading conditions, with an inverse relationship between the tensile strength and elongation. This relationship was in accordance with a referable literature. All these results indicate that the proposed model can describe the deformation behaviors of SnAgCuSb solder well under the tensile conditions in consideration of the mechanical anisotropy and the damage evolution.

Electrodeposition of Twinned Cu with Strong Texture Effect on Voiding Propensity in Electroplated Cu Solder Joints

Electrodeposition of Cu receives considerable attention due to its wide application in microelectronic products. Sn-rich alloys are commonly used to join the electroplated Cu to form solder joints, while their interactions give rise to undesired Kirkendall voids. Suppression of voids is imperative to ensure reliable solder joints with mechanical robustness. In this study, twinned Cu with various crystal orientations are constructed using electroplating and their efficacy on void suppression are investigated. Four Cu electroplated films (Cu A, B, C, and D) with numerous twin boundaries and different (111) and (110) ratios are joined with Sn-rich solder (SAC305) and thermally aged at 200 °C. The voiding propensity is in an order of Cu D > Cu C > Cu B > Cu A, inversely corresponding to their (111) ratios (Cu A > Cu B > Cu C > Cu D). Particularly, a void-free solder joint constructed by the electroplated Cu A film with plenty of twinned bamboo structures is observed. The findings demonstrate that Σ3 twin boundary in the bamboo structure with 〈111〉-preferred orientation has much higher efficiency at suppressing the Kirkendall effect than that in 〈110〉-preferred Cu films.

Effect of Trace Addition of In on Sn-Cu Solder Joint Microstructure Under Electromigration

Recently In has been considered as an additional alloying element in Sn-rich solders primarily due to its abilities to decrease the solder melting temperature and to modify mechanical properties and microstructure. While In is an attractive candidate for addition to solder, its effect on solder microstructure is not well understood. In order to study the effect of minor In additions on Sn-rich solder alloys, solder joints were prepared using Sn-0.7 wt.% Cu and Sn-0.7 wt.% Cu- < 1 wt.% In alloys. Thermal aging and electromigration testing were done, followed by post-mortem microstructure characterization including composition, morphology, and grain structure. The addition of In did not appear to affect the microstructure under thermal aging conditions, but slowed interfacial intermetallic growth under electromigration, particularly of the compound Cu6Sn5. Transmission electron microscope analysis revealed the formation of Cu7In3 IMC nanoparticles, which were semicoherent with the surrounding Sn matrix.

Whisker Growth in Sn Coatings: A Review of Current Status and Future Prospects

Whiskering is a spontaneous filamentary growth of material, and it is a major long-term reliability issue affecting microelectronic packages comprising Sn plating and Sn-rich solders. In particular, whisker growth out of Sn-plated surfaces has been studied extensively in recent years due to the advent of next-generation, environment-friendly, Pb-free microelectronic packaging. Here, we review this scientifically challenging and technologically important problem, especially in the light of relatively new insights gained in the recent past, intending to provide a quick overview of the important results and stimulating future studies. In particular, we discuss the mechanisms of whisker growth by critically examining the roles of stress and its regeneration, oxide layer, diffusion conduits, and crystal anisotropy in creating conditions conducive for whiskering. We also discuss the recent proposals for effectively mitigating whisker growth in Sn coatings. Finally, an outlook is provided, with details of important unresolved issues related to whisker growth.

Mechanisms of thermal cycling damage in polycrystalline Sn-rich solder joints

Solder joints will experience elevated temperatures during operation due to resistive heating and ambient conditions. Device components have different thermal expansion characteristics, leading to straining of the solder joint and development of surface damage features. The anisotropic thermal expansion of βSn grains and discrepancy between the thermal expansion of intermetallic compounds causes degradation to be localized near grain boundaries and phase interfaces. Three experiments were conducted to investigate thermal effects on solder joint microstructure. Interrupted low temperature aging revealed the formation and movement of grain boundary ledges and uninterrupted thermal aging confirmed that the ledges formed during the cooling cycle of the experiment. Finally, high temperature thermal cycling demonstrated the acceleration of surface damage that accompanies an increase in peak temperature and cycling frequency. The orientation of the c-axis of the Sn matrix grains proved to be the critical feature controlling the direction of grain boundary ledge migration and type of surface deformation surrounding Cu6Sn5 intermetallic needles.

Elastic Behavior of Coarse-Grained SnAgCu (SAC) Solder Joints Based on an Anisotropic Multi-scale Predictive Modeling Approach

Lead-free electronic interconnections made of Sn-rich solder alloys usually contain only a few highly anisotropic grains, rendering their overall thermo-mechanical behavior highly dependent on each crystal’s morphology and orientation. Miniaturization and heterogeneous integration will further reduce the length-scale of interconnections and exacerbate this piece-to-piece variation among different solder joints. The objective of this paper is to predict the elastic response and intergranular stresses in coarse-grained SAC (SnAgCu) solder joints, starting with orientation sensitive anisotropic predictive models of the elastic behavior of single crystals. The properties of the single crystal are, in turn, predicted by a multi-scale method, where each grain is modeled as a collection of Sn dendrites embedded in a eutectic Sn-Ag matrix. The eutectic phase, in turn, is modeled as an effective homogenized composite material consisting of nano-scale Ag3Sn particles embedded in a highly anisotropic tin matrix. All homogenization for prediction of single-crystal elastic properties is based on composite micromechanics, using Eshelby’s method and Mori–Tanaka homogenization. Elastic response of a few-grained solder joints is then numerically predicted from the anisotropic behavior of the single crystal, by explicitly modeling the grain morphology in finite element analysis (FEA). Stress distributions along grain boundaries are simulated and analyzed in terms of the misorientation angle between neighboring grains. The mismatch in von Mises stress along the boundaries varies up to 55% for grain structures considered here. This multi-scale predictive modeling approach is explicitly sensitive to microstructural features such as the morphology of: (1) the intermetallic compound (IMC) reinforcements in the eutectic phase; (2) dendrites; and (3) grains. Thus, this modeling approach is ideally suited for: (1) parametric studies of the effect of microstructural tailoring on solder joint thermo-mechanical behavior and (2) examining the effects of microstructural evolution caused by static and cyclic thermo-mechanical aging. This work forms the starting point for future development of microstructurally based anisotropic models of crystal-scale inelastic behavior.

Electroplating of 3D Sn-rich solder for MEMS packaging applications

Electroplating is a popular method to produce solders in microelectromechanical systems (MEMS) encapsulation and interconnection applications. In this work, a fabrication process is introduced for the generation of three-dimensional (3D) Sn solder microstructures. An aluminum membrane was employed as the conducting layer while electroplating. Based on the proposed method, Sn solders with different heights can be easily obtained after reflow. Taking advantage of these 3D structures, MEMS packaging, electrical interconnections, hermetic seals and metal-armoring can be formed simultaneously in one step. Metal-armoring provided by the solder–solder contact can be used at the extremes of the suspension travel, greatly increasing the capability of shock resistance. The helium leak rates of the packaged samples are lower than the reject limit (5 × 10−8 atm cc s−1 based on MIL-STD-883E standard). The bonding strength of samples is measured by a die shear strength tests with an average value of 7.4 MPa. In addition, the ohmic behavior of the Sn solder bonding is verified as well. These excellent results show an outstanding packaging quality in MEMS encapsulation applications.

Microstructure and opto-electronic properties of Sn-rich Au-Sn diffusive solders

Microstructural and opto-electronic properties of Au⧹Sn and Sn⧹Au bilayers, obtained by sequential evaporating of metals on the Si substrate, were investigated by means of atomic force microscopy, X-ray diffraction and spectroscopic ellipsometry. Thicknesses of individual films were established to obtain the atomic ratio of Au:Sn atoms 1:1, 1:2 and 1:4, which were favor the formation of AuSn, AuSn2 and AuSn4, respectively. However, the produced intermatallic compounds were detected as AuSn and AuSn2. Additionally, the unbounded Sn was found. The sequence of deposition of Au and Sn films as well as their thickness strongly affect on the composition, microstructure, optical and electrical properties of the produced layers. The Au⧹Sn (Sn on the top) layers were more smooth than Sn⧹Au (Au on the top) films. Generally, the Au⧹Sn layers exhibit a better electrical and optical properties than Sn⧹Au films. The optical parameters: plasma energy, free-carrier damping, mean relaxation time of conduction electrons and optical resistivity were determined from the effective complex dielectric function of the formed Au, Sn and Au-Sn films. The optical resistivity values are in the range from 17.8 μΩ cm to 85.1 μΩ cm and from 29.6 μΩ cm to 113.3 μΩ cm for Au⧹Sn and Sn⧹Au layers, respectively.

Mössbauer studies of β → α phase transition in Sn-rich solder alloys

Hyperfine interactions in tin-base alloys Sn-x%Ag-y%Cu (x = 0, 0.3, 3; y ∈ 0 ÷ 3) were studied using 119Sn Mössbauer spectroscopy in order to detect the conditions that promote α-Sn phase formation in metallic tin (β-Sn). In many engineering applications, so-called grey tin or tin pest (α-Sn) is a parasitic phase that weakens the mechanical properties of solder joints. In particular, the chemical composition of commercially available Sn-rich solders and their working temperature conditions could significantly affect the durability of tin joints. The Mössbauer results confirmed that the above mentioned factors have a significant impact on the growth rate of tin pest. The most visible α-Sn phase increase (28.5%) was observed for alloy containing 1% Cu. In turn, the addition of even a small amount of Ag could effectively suppress tin pest formation, which indicates that the composition of tin solder alloys is still demanding for commercial applications due to their time stability. On the other hand, the temperature dependence of Sn-rich solder degradation is a less dominant factor, although the thermal treatment effect can be measured quite well using Mössbauer spectroscopy.

Micro-EDXRF, SEM-EDS and OM characterisation of tin soldering found in handle attachments of Roman situlae from Conimbriga (Portugal)

Tin (Sn) or a Sn-rich solder applied to copper-based artefacts has been frequently used at least, since the Ancient Greece, although scarce studies have been published concerning the technology of this metallurgical joining technique.Several filler remnants were reported to be found in a Roman collection of handle attachments of situlae or cauldrons (2nd century BCE–5th century CE) from the archaeological site of Conimbriga, a Roman city from the Lusitania Province (Portugal). All these artefacts were cast in high leaded coppers and bronzes.The present study aims to contribute to the knowledge of Sn-rich soldering, an ancient metallurgical joining technique, by the characterisation of the fusible metallic alloy present in 10 Roman artefacts by means of micro-energy dispersive X-ray fluorescence spectrometry (micro-EDXRF), scanning electron microscopy with energy dispersive X-ray (SEM-EDS) microanalysis and optical microscope (OM) observations.Results of studied solders show the presence of Cu-Sn alloys, with Sn contents ranging from δ to η phase composition (30–60wt% Sn). As the attachments were made in leaded copper alloys, it was also observed, in some cases, the melting of the interdendritic Pb-rich chains with long-range diffusion of the solder alloy into the substrate.The fillers compositions suggest that the handle attachments have been joined to a situla body by the soldering metallurgical process with Sn or a Sn-rich alloy. The studied leaded Cu-Sn attachments, probably formulated by local craftsman, were joined into the body of a situla or cauldron with a soft solder (soldering), a common metallurgical joint from Antiquity, although no relation was found between composition or typology and the Sn or Sn-rich solder.

The Relationship between XRD Peak Intensity and Mechanical Properties of Irradiated Lead-Free Solder

The variation on mechanical properties and crystalline structure of gamma-irradiated Sn-rich lead-free solder (SAC) were intensively investigated using nanoindentation and X-ray diffraction (XRD) techniques. Samples of solder on a printed circuit board (PCB) with copper substrate were irradiated at low dose (5 Gray) of gamma from Co-60 source. The nanoindentation hardness for β-Sn phase of the solder was found to increase from 0.1935 GPa to 0.2210 GPa after the irradiation. Furthermore XRD peak intensity was also observed to increase as well indicating the occurrence of defect in β-Sn crystal structure due to gamma radiation. The defect contributes to the increment of the hardness by indicating the change in crystallite size of the grains. Microstructure analysis by FESEM-EDAX has also confirmed the indentation was performed with no cracks in subsurface on β-Sn area.

Effects of Complexing Reagent on Electroless Nickel Iron Alloy Plating for the Diffusion Barrier of UBM

Electro and electroless nickel platings have been extensively used as a diffusion barrier in under bump metallization (UBM) or as a surface finish on printed circuit boards (PCB). Even though a thin Ni layer on top of Cu can reduce the interfacial reactions with Sn-rich solders at a low reflow temperature, it may not be so effective when a reflow process is performed at a higher temperature and for a longer period. To provide a more robust diffusion barrier layer than electrodes Ni, electroless NiFe alloy system has been proposed. Since the electroless Ni-Fe plating bath has been rarely studied, the proper baths were not available commercially. In this research, the electroless plating of Ni-Fe alloys was investigated on the basis of complexing reagent. The stability of complexing reagent was studied. The compositions of electroplating layers were analyzed with EDS. The desired Ni-Fe alloy were obtained by controlling the chemical composition of the bath and its operating parameters.

A zero-level vacuum encapsulation technique with less parasitic effect for VHF MEMS resonators

This paper presents a zero-level vacuum encapsulation technique with the development of effective parasitic optimization methods for VHF MEMS resonator. Sn-rich Au-Sn solder bonding is used to achieve vacuum hermeticity packaging with high shear strength. The measured average leak rate and shear strength are (2.2±0.9)×10−8atm-ccs−1 and 40.65±8.25MPa among 12 packaged chips, respectively. An equivalent circuit model of the packaged resonator is established to analyze the parasitic effect caused by encapsulation. A significant reduction of the parasitic effect is achieved via grounding sealing ring, and decreasing parasitic capacitances between the sealing ring and signal feedthroughs. The measured frequency spectrum of a packaged resonator is not distorted by parasitic effect, which is well consistent with the simulation result. Besides, the Q value of the resonator increases from 6400 to 10,300, as well as the transmission gain at harmonic peak from −73.92dB to −70.36dB due to an excellent electrical performance of the encapsulation structure and a high vacuum in this packaging cavity.

Growth Kinetics of Intermetallic Compounds Between Sn and Fe Liquid–Solid Reaction Couples

Growth behavior of the intermetallic compound (IMC), FeSn2, was investigated in the liquid Sn/solid Fe reaction couple over the annealing temperatures from 250 °C to 400 °C. Low-carbon steel AISI 1018 was chosen to make Fe samples. The morphology and thickness of the IMC formed between Sn and Fe were examined using scanning electron microscopy (SEM). In addition, energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were used to confirm that the IMC is FeSn2. The growth kinetics of FeSn2 was modeled by parabolic law and empirical power law. Based on the models, the growth constants, the activation energy, and the time exponents were established at different annealing temperatures. It was found that the time exponent values obtained by fitting with empirical power law deviate from 0.5, meaning that volume (bulk) diffusion is not the only rate-controlling process in the liquid Sn/solid Fe reaction couple. Also, a variation in the time exponent values is indicative that the growth behavior is correlated with grain size growth and irregular grain morphology at different annealing stages. The results of this research show that AISI 1018 steel can readily react with Sn to form IMC on the interface. This is an essential requirement of soldering action using Sn-rich solders.