Solder Interconnects Research Articles

Research on microstructure and mechanical behaviour of smaller microbumps for high density solder interconnects

PurposeThe purpose of this paper is to study the microstructure and mechanical behaviour of smaller microbumps for high density solder interconnects.Design/methodology/approachThe microstructure was analyzed by scanning electron microscopy and electron backscatter diffraction tests to determine the Sn grain number of the resultant microbumps. The nanomechanical properties of Sn microbumps were investigated by the nanoindentation and shearing tests to understand the failure mechanism and assess the reliability of ultra-high density solder interconnects with numbered grains.FindingsOnly one Sn grain is observed in the interconnect matrix when the microbumps are miniaturized to 40 μm or less. Because of the body-centred tetragonal lattice of ß-Sn unit cell, the mechanical properties of the one-grain Sn microbumps are remarkably anisotropic, which are proved by the difference of the elastic modulus and the stiffness in the different orientations. The shearing tests show that the one-grain Sn microbump has a typical brittle sliding fracture of monocrystal at different shearing speeds.Practical implicationsThe paper provides a comparable study for the performance of the bigger solder joints and also makes preliminary research on the microstructure and mechanical behaviour of Sn microbumps with the diameter of 40 μm.Originality/valueThe findings in this paper provide methods of microstructure study by combination of EBSD test and metallographic analysis, mechanical study by combination of nanoindentation test and shearing test, which can provide good guidelines for other smaller microbumps. The strain rate sensitivity exponent of the one-grain Sn microbumps is consistent with the Pb-free bulk solder. This implies that the one-grain Sn microbump has a comparable flow stress to Sn37Pb solder, which is beneficial for Pb-free replacement in higher density microelectronic packaging.

Effects of β-Sn grain c-axis on electromigration behavior in BGA Sn3.0Ag0.5Cu solder interconnects

The anisotropy of β-Sn grain can significantly affect the electromigration (EM) behavior in Sn3.0Ag0.5Cu (SAC305) solder interconnects. A real ball grid array (BGA) specimen with a cross sectioned edge row suffered electromigration for 600 h to investigate the effects of β-Sn c-axis on the behavior of electromigration in SAC305 solder interconnects. Scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) were used to obtain the microstructure and orientation of β-Sn grains in as-reflowed and low current density conditions. Besides, the orientation of c-axis had a great effect on the growth direction of IMCs in solder matrix. The solder interconnect with the Sn grain c-axis pointing the positive direction of ND would emerge serious electromigration phenomena. The density of Cu6Sn5 IMCs distributing at the surface of solder matrix increased obviously. However, when Sn grain c-axis was in the same direction with the opposite direction of ND, the original Cu6Sn5 IMCs in as-reflowed solder interconnect disappeared. Therefore, the results show that the solder interconnects will performance a different electromigration behavior due to the direction of c-axis in Sn grain: the growth direction of Cu6Sn5 IMCs in solder matrix will along the c-axis accompanied growing into solder matrix or gathering at the surface of the cross section.

Microstructural Evolution and Migration Mechanism Study in a Eutectic Sn-37Pb Lap Joint Under High Current Density

The microstructural evolution in eutectic Sn-37Pb solder under high current density seriously threatens the reliability of solder interconnections, but atomic electromigration has often been confused with thermomigration. In this paper, after decoupling the effect of the non-uniform temperature distribution in a Cu/Sn-37Pb/Cu lap joint from the current stress, the microstructural evolution was investigated under an average current density of 1.84 × 104 A cm−2 for 0–24 h. The decomposition and recombination of the Pb-rich phase occurred at the cathode and the anode, respectively. The corresponding migration mechanism was proposed from the viewpoint of energy and was explained by the interactions among the potential energies of ripening, electron wind force, and back stress. Our study may be helpful for understanding the migration mechanism and reliability of eutectic two-phase solder joints and provides supporting data for interpreting the acceleration tests of Sn-37Pb solder joints under electromigration.

Numerical simulations of migration and coalescence behavior of microvoids driven by diffusion and electric field in solder interconnects

A diffuse interface model is developed to simulate the effect of electric field on the morphological evolution and migration behavior of the microvoid in the solder interconnect consisting of the Sn based solder and Cu substrate (i.e., Sn/Cu system). The model takes into account the coupled effect of surface diffusion and electric field, and the validity of the model is confirmed by good agreement between the simulation and theoretical predictions in terms of evolution behavior of the noncircular microvoid driven by surface energy. The results show that the coalescence of microvoids driven only by surface energy occurs when the microvoids contact each other. The evolution and migration of the microvoid under electric field are governed by the magnitude of electric field and the initial size of the microvoid. The microvoid migrates at a constant velocity under a weak electric field, while the strong electric field results in the shape change of the microvoid from circular to narrow crack-like. In addition, the migration velocity of the microvoid increases linearly with the voltage and is inversely proportional to the size of the microvoid; a small microvoid can catch up with a large one, and finally they merge to form a larger microvoid, which may promote the open circuit failure near the solder/Cu interface in solder interconnects.

A theoretical analysis of the electromigration-induced void morphological evolution under high current density

In this work, analysis of electromigration-induced void morphological evolution in solder interconnects is performed based on mass diffusion theory. The analysis is conducted for three typical experimentally observed void shapes: circular, ellipse, and cardioid. Void morphological evolution is governed by the competition between the electric field and surface capillary force. In the developed model, both the electric field and capillary force on the void’s surface are solved analytically. Based on the mass conversation principle, the normal velocity on the void surface during diffusion is obtained. The void morphological evolution behavior is investigated, and a physical model is developed to predict void collapse to a crack or to split into sub-voids under electric current. It is noted that when the electric current is being applied from the horizontal direction, a circular void may either move stably along the electric current direction or collapse to a finger shape, depending on the relative magnitude of the electric current and surface capillary force. However, the elliptical-shaped void will elongate along the electric current direction and finally collapse to the finger shape. On the other hand, the cardioid-shaped void could bifurcate into two sub-voids when the electric current reaches a critical value. The theoretical predictions agree well with the experimental observations.

Formation of highly preferred orientation of β-Sn grains in solidified Cu/SnAgCu/Cu micro interconnects under temperature gradient effect

β-Sn grain orientation and configuration are becoming crucial factors to dominate the lifetime of solder interconnects in three-dimensional integrated circuit packaging. In this paper, we found that a temperature gradient during solidification significantly dominated the orientation and configuration of the final β-Sn grains in Cu/SnAgCu/Cu micro interconnects. Being different from the random orientations and growth fronts meeting or cyclic twin boundary forming near the center after homogeneous temperature bonding, the β-Sn grains solidified under a certain temperature gradient were observed to follow a highly preferred orientation with their c-axis departing from the direction of temperature gradient by about 45°–88°. Meanwhile, these preferred oriented β-Sn grains consisted of low angle grain boundary structures with misorientation in the range of 0°–15°. The mechanism was explained in terms of the anisotropy and directional growth of β-Sn grains. The results pave the way for grain orientation control in 3D packaging technology.

Electromigration-induced growth mode transition of anodic Cu6Sn5 grains in Cu|SnAg3.0Cu0.5|Cu lap-type interconnects

The rapid accumulation of the Cu6Sn5 phase at the anode is one of the major electromigration-induced phenomena characterizing solder interconnections; however, the outcome of the growth mode has always been conflated with that of thermomigration. In this work, after the effects of the non-uniform thermal distribution of the Cu|SnAg3.0Cu0.5|Cu lap-type joints are decoupled from the influence of the current stress, the microstructural evolution of the anodic Cu6Sn5 grains is studied under an average current density of 7.12 × 107 A m−2 for 0–300 h. The results show that, due to the anisotropy of the Cu6Sn5 in the absorption factor and the action of the electron wind, the [0001] directions of the Cu6Sn5 grains at the anodic Sn|Cu6Sn5 interface gradually reorient toward the current density vectors with the stress time, and certain original Cu6Sn5 grains (termed normal grains) with unfavorable surface orientations are replaced by newly generated grains (termed abnormal grains) with favorable surface orientations. Consequently, the growth mode of the anodic Cu6Sn5 grains is not invariable, and the corresponding transition process is conjectured to transition from the reaction-controlled mode to the diffusion-controlled mode in three stages. Finally, the anodic Cu6Sn5 grains, both normal and abnormal, grow to assume elongated rod-type shapes and may further form a reliable interconnection layer to improve the joint reliability.

Effects of the degradation of methane sulfonic acid electrolyte on the collapse failure of Sn–Ag alloy solders for flip-chip interconnections

The degradation mechanism of the methanesulfonic acid based electroplating bath used for the electrodeposition of Sn–Ag alloy solder bumps and its effects on the collapse failure of flip-chip solder bumps.

Effects of Bismuth in Sn-Cu Based Solder Alloys and Interconnects

Effects of Bismuth in Sn-Cu Based Solder Alloys and Interconnects

Evaluation of thermo-mechanical damage and fatigue life of solar cell solder interconnections

The soldering process of interconnecting crystalline silicon solar cells to form photovoltaic (PV) module is a key manufacturing process. However, during the soldering process, stress is induced in the solar cell solder joints and remains in the joint as residual stress after soldering. Furthermore, during the module service life time, thermo-mechanical degradation of the solder joints occurs due to thermal cycling of the joints which induce stress, creep strain and strain energy. The resultant effect of damage on the solder joint is premature failure, hence shortened fatigue life. This study seeks to determine accumulated thermo-mechanical damage and fatigue life of solder interconnection in solar cell assembly under thermo-mechanical cycling conditions. In this investigation, finite element modelling (FEM) and simulations are carried out in order to determine nonlinear degradation of SnAgCu solder joints. The degradation of the solder material is simulated using Garofalo-Arrhenius creep model. A three dimensional (3D) geometric model is subjected to six accelerated thermal cycles (ATCs) utilising IEC 61215 standard for photovoltaic panels. The results demonstrate that induced stress, strain and strain energy impacts the solder joints during operations. Furthermore, the larger the accumulated creep strain and creep strain energy in the joints, the shorter the fatigue life. This indicates that creep strain and creep strain energy in the solder joints significantly impacts the thermo-mechanical reliability of the assembly joints. Regions of solder joint with critical stress, strain and strain energy values including their distribution are determined. Analysis of results demonstrates that creep energy density is a better parameter than creep strain in predicting interconnection fatigue life. The use of six ATCs yields significant data which enable better understanding of the response of the solder joints to the induced loads. Moreover, information obtained from this study can be used for improved design and better-quality fabrication of solder interconnections in solar cell assembly for enhanced thermo-mechanical reliability. Thermo-mechanical damage of solder joints in Si solar cell assembly evaluated.FEM and simulations carried out to determine nonlinear degradation of solder joints.Results show that induced stress and strain impacts solder joints reliability.Creep energy density better than creep strain in solder joint life prediction.Results useful for improved design of solder joints and better-quality fabrication.

Evaluation of the impact of the physical dimensions and material of the semiconductor chip on the reliability of Sn3.5Ag solder interconnect in power electronic module: A finite element analysis perspective

This paper primarily focuses on an evaluation study for the temperature cycling capability of tin silver solder interconnect in power electronic applications by the impact of die dimensions and die material properties. The study was investigated on finite element analysis perspective on chip/solder/substrate structure. A commercially available chip was chosen in the finite element analysis (FEA) as the nominal base die. Two thermal cycle profiles were utilised. The effect of die area, die thickness and material properties (Si and SiC) on the thermal cycling capability of the solder layer was investigated from FEA perspective. From the FEA, it was concluded that decrease in die thickness resulting in increment of thermal cycling capability of solder layer for both material (Si and SiC). Increase in die area increases the thermal cycling capability of solder. For higher ΔT thermal cycle, solder under SiC die perform better than solder under Si die in terms of thermal cycling capability. When the die thickness become smaller than a threshold value of the thermal cycle regime, solder under Si die have better thermal cycling capability than solder under SiC die. Additionally a parametric study was undertaken for a SiC chip/substrate structure under high ∆T temperature cycling profile for solder layer geometric parameter (wetting angle, titling angle and thickness). From the parametric study which utilised design of experiments (DoE), a wavelet radial basis surrogate model was generated. A sensitivity analysis was performed on surrogate model in order to identify the most influencing parameter. From the sensitivity analysis, it was concluded that wetting angle and solder layer thickness of solder layer have significant impact on the thermal cycling capability of the solder layer.

A statistical mechanics model to predict electromigration induced damage and void growth in solder interconnects

Abstract Electromigration is an irreversible mass diffusion process with damage accumulation in microelectronic materials and components under high current density. Based on experimental observations, cotton type voids dominate the electromigration damage accumulation prior to cracking in the solder interconnect. To clarify the damage evolution process corresponding to cotton type void growth, a statistical model is proposed to predict the stochastic characteristic of void growth under high current density. An analytical solution of the cotton type void volume growth over time is obtained. The synchronous electromigration induced damage accumulation is predicted by combining the statistical void growth and the entropy increment. The electromigration induced damage evolution in solder joints is developed and applied to verify the tensile strength deterioration of solder joints due to electromigration. The predictions agree well with the experimental results.

Measurement of underfill interfacial and bulk fracture toughness in flip-chip packages

Flip-chip package reliability is greatly improved by encapsulating the solder interconnections between a polymeric encapsulant or underfill. However, thermo-mechanical stresses within such packages often lead to failures initiating in the vicinity of chip and underfill interface. In this study, we present experimental results geared towards measuring and understanding such failure mechanisms. We provide the bulk fracture toughness of the underfill material and interfacial fracture toughness between the underfill material and the silicon die. The bulk and interfacial fracture toughness measurements are performed as a function of temperature. We use the single edge notch bending test to calculate the bulk fracture toughness of the underfill and to measure the interfacial fracture toughness, we use a novel technique referred to as the wedge delamination method. The wedge delamination method provides substantial advantage in measuring the interfacial fracture toughness for brittle materials over traditional methods. Using the wedge delamination method we compare the fracture strength between the underfill and silicon at the front-face and side-wall interfaces. Additionally, the influence of dicing technique on fracture toughness is also investigated.

Microstructure and reliability of hybrid interconnects by Au stud bump with Sn-0.7Cu solder for flip chip power device packaging

With miniaturization of the interconnect solder bumps, high current density causes serious reliability issues (stress, electromigration etc.) in electronic packages. Through Au stud bumping on the chips and following reflow of solder to produce hybrid interconnects, the eletromigration resistance may be improved by the intermetallics formed inside them due to their barrier effects on the atoms migration. Here, microstructures and reliabilities of Au stud with serial amounts of Sn-0.7Cu solder paste were studied through controlling size of stencil printing aperture. After reflow, AuSn, AuSn2 and AuSn4 formed from the surface of Au stud bump to the solder. A layer of (Cu,Au)6Sn5 with thickness of 3μm existed at the interface near the Cu substrate with a scallop shape similar to Cu6Sn5. The fraction of intermetallics to the mixed joints varied with the solder amount. Shear strength decreased slightly when comparing with the sole solder joint due to large amounts of brittle intermetallics. Thermal aging resulted in many Kirkendall voids generated at the interfaces of Au stud and the solder, which further decreased the shear strength. The effect of solder amount on microstructural evolution and fracture modes was discussed. The hybrid interconnects showed a good electromigration resistance.

Transient liquid phase sintering materials (TLPS) vs. conventional solders for high temperature MLCC interconnects.

Abstract There is a growing need within the electronics industry for interconnect materials that can meet higher processing and operating temperatures. The solidus to liquidus range of current tin (Sn) based solders and other lead (Pb) solders are narrower than traditional SnPb solders as the industry is moving to higher temperatures. Transient Liquid Phase Sintering (TLPS) is a technology that offers low processing temperatures while having high temperature capabilities but has had limited exposure outside of the Semiconductor industry. Results of mechanical testing as well as product environmental data is presented herein as TLPS is used as a mechanical and electrical interconnect for Multi-Layer Ceramic Capacitors (MLCC).

Investigation of soldering for crystalline silicon solar cells

Purpose Tabbing and stringing are the critical process for crystalline silicon solar module production. Because of the mismatch of the thermal expansion coefficients between silicon and metal, phenomenon of cell bowing, microcracks formation or cell breakage emerge during the soldering process. The purpose of this paper is to investigate the effect of soldering on crystalline silicon solar cells and module, and reveal soldering law so as to decrease the breakage rates and improve reliability for crystalline silicon solar module. Design/methodology/approach A microscopic model of the soldering process is developed by the study of the crystalline silicon solar cell soldering process in this work. And the defects caused by soldering were analyzed systematically. Findings The defects caused by soldering are analyzed systematically. The optimal soldering conditions are derived for the crystalline silicon solar module. Originality/value The quality criterion of soldering for crystalline silicon solar module is built for the first time. The optimal soldering conditions are derived for the crystalline silicon solar module. This study provides insights into solder interconnection reliability in the photovoltaic (PV) industry.

Microstructural Indicators for Assessment of Effect of Prolonged and Intermittent Storage on Reliability of Lead-free Electronics

Electronic systems may be subjected to prolonged and intermittent periods of storage prior to deployment or usage. Prior studies have shown that the lead-free solder interconnects show measurable degradation in the mechanical properties even after the brief exposures to high temperature. In this paper, a method has been developed for determining the equivalent storage time to produce identical damage at a different temperature. Electronics subjected to accelerated tests often have a well-defined thermal profile for a specified period of time. Quantification of the thermal profile in field-deployed electronics may be often difficult because of the variance in the environment conditions and usage profile. There is a need for tools and techniques to quantify the damage in deployed systems in the absence of macroindicators of damage without the knowledge of prior stress history. The approach for mapping damage in the lead-free second-level interconnects between different thermal conditions is new. High-reliability applications, such as avionics and missile systems, may be often exposed to long periods of storage prior to deployment. The effect of storage at different temperature conditions can be mapped using the presented approach. A framework has been developed to investigate the system state and estimate the remaining useful life of the solder ball subjected to a variety of isothermal aging conditions, including 60 °C, 75 °C, and 125 °C for periods of time between 1 and 4 weeks. Data on damage precursors, including the rate of change in the normalized phase growth and the normalized IMC thickness, has been collected and analyzed to derive physics-based damage mapping relationships for aging. Mathematical relationships have been derived for the damage mapping to various thermal storage environments to facilitate determining an appropriate time–temperature combination to reach a particular level of damage state. Activation energy for the leading indicators of failure is also computed. Specific damage proxies examined include the phase-growth indicator and the intermetallic thickness. The viability of the approach has been demonstrated for the lead-free test assemblies subjected to multiple thermal aging at 60 °C, 75 °C, and 125 °C. Damage mapping relationships are derived from the data based on the two separate leading indicators.

Investigation of interfacial behavior in miniaturized solder interconnects

In the present study the influence of intermetallic compound growth on mechanical and thermal response of miniaturized lead free solder joints was investigated. The focus of the study was the relationship between the solder gap size and thickness of the intermetallic compound (IMC) on tensile behavior of Cu/Sn3.5Ag0.75Cu/Cu solder joints with different ratios of IMC to the gap size and temperature boundary conditions. Independent of the IMC to solder gap thickness ratio a steady increase of tensile strength with decreasing gap size was observed. The reason for this behavior is the so called constraint effect which is well explained by plasticity models. The variation of the IMC size was realized by different reflow times or heat treatments. An increasing ratio of IMC thickness to the gap size results in a transition of the fracture mode from ductile to brittle and affects the strength of the solder joint. Thereby, embrittlement of the material seems to be a consequence of short diffusion paths introduced by defects and failures as confirmed by finite element analysis. Furthermore, thermal experiments revealed that the overall coefficient of thermal expansion of the joints decreased with increasing the IMC proportion of the joints. This study shows that the mechanical response and failure modes of thin solder joints are strongly dependent on the interaction between the proportional thickness, inherent defects and morphology of the IMC layer in addition to the constraint effect.

Advances in Pb-free Solder Microstructure Control and Interconnect Design

New electronics applications demanding enhanced performance and higher operating temperatures have led to continued research in the field of Pb-free solder designs and interconnect solutions. In this paper, recent advances in the microstructural design of Pb-free solders and interconnect systems were discussed by highlighting two topics: increasing β-Sn nucleation in Sn-based solders, and isothermally solidified interconnects using transient liquid phases. Issues in β-Sn nucleation in Sn-based solders were summarized in the context of Swenson’s 2007 review of the topic. Recent advancements in the areas of alloy composition manipulation, nucleating heterogeneities, and rapid solidification were discussed, and a proposal based on a multi-faceted solidification approach involving the promotion of constitutional undercooling and nucleating heterogeneities was outlined for future research. The second half of the paper analyzed two different approaches to liquid phase diffusion bonding as a replacement for high-Pb solders, one based on the application of the pseudo-binary Cu-Ni-Sn ternary system, and the other on a proposed thermodynamic framework for identifying potential ternary alloys for liquid phase diffusion bonding. All of the concepts reviewed relied upon the fundamentals of thermodynamics, kinetics, and solidification, to which Jack Smith substantially contributed during his scientific career.

Dominant effect of high anisotropy in β-Sn grain on electromigration-induced failure mechanism in Sn-3.0Ag-0.5Cu interconnect

The effect of high diffusivity anisotropy in β-Sn grain on electromigration behavior of micro-bumps was clearly demonstrated using Sn-3.0Ag-0.5Cu solder interconnects with only two β-Sn grains. The orientation of β-Sn grain (θ is defined as the angle between the c-axis of β-Sn grain and the electron flow direction) is becoming the most crucial factor to dominate the different electromigration-induced failure modes: 1) the excessive dissolution of the cathode Cu, blocking at the grain boundary and massive precipitation of columnar Cu6Sn5 intermetallic compounds (IMCs) in the small angle θ β-Sn grain occur when electrons flow from a small angle θ β-Sn grain to a large one; 2) void formation and propagation occur at the cathode IMC/solder interface and no Cu6Sn5 IMCs precipitate within the large angle θ β-Sn grain when electrons flow in the opposite direction. The EM-induced failure mechanism of the two β-Sn grain solder interconnects is well explained in viewpoint of atomic diffusion flux in β-Sn.