Solder Interconnects Research Articles

Sintered Nanocopper Paste for High-Performance 3D Heterogeneous Package Integration

Fifth-generation (5G) communications have been driving major package innovations to enable low-loss interconnects between the integrated circuits (IC) and other system components such as antenna arrays. Antenna-in-package with three-dimensional (3D) or double-side component is widely pursued as the front-up architecture to realize this vision. The interconnect height and losses are critical parameters in these 3D package structures. Copper sintering paste is emerging as an ideal candidate to replace solders for both off-chip and on-package interconnects because of the resulting higher electrical conductivity and relatively simple manufacturability with additive processes. This article focuses on computational modeling, optimization of sintering conditions, and electrical conductivity measurements of highly conductive copper paste to verify the proposed model. The model is based on two-particle sintering theory, which describes the neck growth evolution of copper paste at the initial stage of sintering. The neck-growth model shows good consistency with the measured neck size of copper, and hence can be used to provide guidelines for the sintering conditions of copper paste. The celectrical conductivity measurements suggest that the copper paste sintered at 260°C for 30 min shows a electrical conductivity of 1.4 × 107 S/m, which is 82% higher than that of solder. In addition, this article investigates the potential of copper paste interconnect technology in high-frequency applications such as in 5G millimeter-wave communications. The transmission lines patterned with the copper paste show good correlation with simulated results in the millimeter-wave frequency band. The high-frequency characterization also indicates that the copper paste enables simple circuit patterning, offering equivalent signal losses with plated copper. The detailed analyses discussed in this article suggest the eligibility for multi-applications of copper sintering paste in IC packaging.

Experimental and numerical simulation of mechanical behavior of micro-scale SAC305 solder joint based on joint height

Lead-free “copper wire/Sn-3.0Ag-0.5Cu (SAC305) solder/copper wire” sandwich-structured micro-scale solder joints with a constant diameter (d = 400 μm) and various heights (125, 225, and 325 μm) were adopted to carried out the mechanical behavior research. Experimentally, quasi-static shear and tensile experiments were conducted by using a precision dynamic mechanics analyzer (DMA Q800). Then, the micro-scale solder joint model was built to simulate shear and tensile behavior of solder joints using the finite element analysis software ABAQUS. The experimental results show that, when the solder joint diameter is fixed, the smaller the joint height is, the stronger the shear strength and tensile strength. Under the same test conditions (225 μm), the shear strength of micro-scale solder joints of the same size is lower than the tensile strength, which indicates that the service environment of electronic packaging interconnection solder joints under shear stress is more severe. In addition, the micro-scale solder joints fracture at the interface of the solder joint and copper wire under shear force, but in the middle of the solder joint under tensile force. Different fracture failure mechanisms are found by observing the cross sections under different stress states.

Continuous growth and coarsening mechanism of the orientation-preferred Cu6Sn5 at Sn-3.0Ag/(001)Cu interface

Orientation-preferred Cu6Sn5 usually offers solder interconnections superior mechanical and physical performances. In this work, morphology and orientation evolutions of Cu6Sn5 at Sn-3.0Ag/(001)Cu interface were investigated in the 300 °C–300 °C, 230 °C–230 °C and 300 °C–230 °C reflow processes. Results demonstrated that the continuous growth of orientation-preferred Cu6Sn5 was realized in the temperature-decreased reflow process of 300 °C–230 °C and the fraction of 0° boundaries in preferred Cu6Sn5 decreased from 0.81 to 0.60 with time while the fraction of 90° boundaries increased from 0.19 to 0.40; both curvature difference and misorientation angle of grain boundaries should be valued in the growth of preferred Cu6Sn5; the coarsening of preferred Cu6Sn5 includes two methods: coalescence and annexation, induced by the disappearance and migration of grain boundaries respectively. These results are of great significance in the orientation and size control of Cu6Sn5 crystals in 3D packaging.

Failure mechanism of lead-free component boards in thermomechanical test based on recrystallization enhanced cracking

The purpose of this paper is to make a comprehensive understanding of the trends between the lifetime and the thermomechanical test setup parameters, based on the physical failure mechanism of SnAgCu solder interconnections in the component boards. The thermomechanical tests, including thermal cycling and thermal shock, were employed to investigate the failure of the component boards under different loading conditions. The in situ observation during the thermomechanical tests was employed to study the microstructure evolution and the crack propagation of solder interconnections. Electrical failures of the component boards under all setup conditions are mainly caused by the cracks, which propagate along the continuous network of the high-angle grain boundaries produced by the distinguished recrystallization of solder interconnections. The propagation of cracks is highly dependent on the expansion of the recrystallized volume in the solder interconnections. In addition, the expansion of the recrystallization would be influenced by the setup parameters of the thermomechanical test. The paper discusses the relationship among the four factors of the lifetime, the crack growth, the restoration phenomenon and the test setup parameters, in order to explain the complicated trend between the component board lifetime and the setup parameters.

Modelling and analysis of vibration on power electronic module structure and application of model order reduction

Modelling and analysis of vibration of an IGBT power electronic module (PEM) structure were undertaken. PEM structure considered in this study was without molding compound and wirebonds. The most critical resonant frequency was identified by modal analysis. At the critical frequency of 1345 Hz, for the vertical displacement of the base excitation, subsequent stress distribution on the PEM structure was analysed. Concurrent vibration and thermo-mechanical fatigue loads on the reliability of PEM structure solder interconnects were also estimated by widely used linear damage superposition approach. It was concluded that the at critical resonant frequency the vibration induced damage is more severe than the thermo-mechanical fatigue loading. In addition, a quarter car model (QCM) was used to mimic the dynamic interaction between the rough road surface and an electric vehicle (EV) in order to analyse the road surface roughness induced excitation on the PEM structure in the engine compartment. Stress and strain distribution on the PEM structure due to road surface roughness were analysed. Furthermore, three Krylov subspace based model order reduction (MOR) techniques were applied to the resulting dynamic system in vibration analysis. Due to the limits on computing resources, a submodel was utilized for MOR analysis. Within the three MOR techniques, Passive Reduced order Interconnect Macromodeling Algorithm (PRIMA) MOR technique performs better than the other techniques. Computational time ratio between reduced system iteration and the full system iteration is 1: 53.

A phase field computational procedure for electromigration with specified contact angle and diffusional anisotropy

Electromigration in electrical interconnects with high current density causes voids to form and grow near the cathode. These voids can possibly grow large enough to cause open circuit failure. The simulation of electromigration void nucleation and growth is challenging because of the complex interaction of electrical, thermal and mechanical fields that lead to the observed voids. In this paper, a phase-field approach is developed to study void growth due to electromigration taking into account diffusional anisotropy and specified contact angle. Consideration of anisotropy and contact angle is critical in applications including electronic assemblies that most commonly use Sn-based solder interconnects. In such systems, Sn exhibits significant anisotropy in its self-diffusivity, which assumes importance in small solder joints that are known to contain only a few grains. Furthermore, the contact angle at the void-bonding pad-solder triple junction is known to influence the void size and shape. To the best of the authors’ knowledge, only anisotropy in surface diffusivity has been considered in phase-field electromigration literature, whereas the contact angle at a triple junction has not been considered. The developed phase-field model incorporates effects of both contact angle and anisotropy in self-diffusivity. The model is used to simulate systems with different contact angles and dominant directions of diffusion. It is observed that diffusional anisotropy plays a more dominant role in failure, while the contact angle dictates evolution of the shape of voids.

Phase-Field Study of Thermomigration in 3-D IC Micro Interconnects

We studied the evolution of the Cu6Sn5 and Cu3Sn intermetallic compounds under large thermal gradients for assisted directional solidification in Cu/Sn/Cu micro solder units for fast bonding in 3-D IC packaging. Thermomigration-enhanced growth and dissolution of the phasesare observed and tracked using a CALPHAD-reinforced multiphase-field model that accounts for the thermodynamics and kinetics of the Cu(Sn) reacting system. The results show that the process of bonding follows a reaction-controlled growth regime ( $n{\sim }1$ ) during thermal gradient bonding, proceeding faster than isothermal conditions. In addition, the asymmetric phase evolution, i.e., simultaneous growth in hot side and dissolution in the cold side of the interconnect, is modeled. The developed model can be used for the proactive control of the complex heat treatment processes for the design of multicomponent low-volume solder interconnects in electronic packaging.

Development of a cycle counting algorithm with temporal parameters

Load cycle counting is an important tool for estimating the fatigue life of structures. Rainflow counting methods have been developed to count load reversals and extract load cycles from load time histories. Rainflow counting methods were originally developed to assess fatigue damage associated with mechanical cycling where time-dependent factors such as dwell time and loading rate were not considered significant contributors to fatigue failure. However, loading rate and hold times can be significant factors in some fatigue failures, such as solder interconnects under temperature cycling. In these cases, fatigue life models may require the load transition time and load hold times to account for time-dependent material changes, such as creep. This paper develops a new multi-parameter cycle counting algorithm that provides a range-based hold time estimation for use with time-dependent fatigue damage models. To show the applicability of the new method, the life of solder joints under a complex thermal cycling regime was estimated using the data extracted by the new multi-parameter cycle counting algorithm. This life estimate was then compared with test results and found to be within the estimated uncertainty bounds.

Effect of voids on thermomechanical cracking in lead-free Sn3Ag0.5Cu interconnections of power modules

In this article, the effect of voids on thermomechanically-induced failure in the lead-free solder interconnections of power amplifier (PA) modules is investigated. The interconnection of interest is between the module's flange and substrate (baseplate) initially having a large contact area, and whose cracking has strong detrimental effects on the RF performance and reliability of the PA. PA modules were attached onto baseplates with lead-free SAC305 solder and put to a thermal cycling test (TCT) in the 15 °C to 95 °C range. X-ray imaging was used to characterize the number of voids in interconnections after reflow soldering. The cross-sections of the pristine reflow-soldered and tested interconnections were inspected with cross-polarized light microscopy to reveal the shapes of the voids and crack paths. It was noted that the voids forming in interconnections during the final stages of the reflow process take an elliptical shape, leaning towards the module's outer edge, due to differences in the thermal expansions of the module and baseplate. During the TCT, high thermomechanical stresses caused localized recrystallization of the as-soldered SAC matrix in the vicinity of the elliptical voids. Finally, creep related intergranular cracks formed in these recrystallized areas. Anand's viscoplastic model was used to model the void formation and the effect of the formed voids on crack evolution in solder. Simulations of the high creep energy of the elliptical, tilted voids and their correlation with the recrystallization behavior of the SAC305 interconnections were consistent with the observed creep related failures. Confirmed by simulations, these voids are detrimental to the reliability of solder interconnection because the creep strain is strongly localized to the sharp curvatures of the elliptic void.

Mechanism and control of preferred Cu6Sn5 growth on single crystal (001)Cu

Preferred orientation Cu6Sn5 prisms are usually formed on (001)Cu single crystal, making use of which can highly improve the packaging reliability of electronic devices. A practical method for morphology controlling of preferred orientation Cu6Sn5 was put forward preliminarily in this article based on temperature variation. The roof-type Cu6Sn5 prisms actually behaved as scallops of highly preferred orientation relationship with (001)Cu substrate in isothermal heating of reflow process and began to transform into prismatic roofs without grain number and orientation changing following it cooled down to solidification. This new phenomenon undiscovered before can pave the way for understanding the precise mechanism of roof-type Cu6Sn5 formation and provide a scientific viewpoint that orientation, size, and outline control of oriented Cu6Sn5 should be realized in different stages of reflow process. This is quite significant for manufacture of morphology-controllable solder interconnections in electronic industry.

Microstructural Characterization and Unified Reliability Assessment of Aged Solder Joints in a PV Module

In this article, the finite-element method (FEM) along with experimental analysis was used to establish a mission profile-based reliability assessment of solder joints of the photovoltaic module under thermal cycling conditions. While considering the uncertainties of the solder joint lifetime model that leads to more realistic results, this method is able to estimate the time if the probability of solder joint failure reaches 10%. Several conditions, including standard and nonstandard cases, were simulated and performed to obtain the fatigue lifetime models. The experimental results, that is, scanning electron microscopy (SEM) along with X-ray tomography, showed that more intermetallic compounds and microvoids are formed in the nonstandard conditions (STCs). As a result, the non-STC leads to the accelerated failure in the solder interconnection, which is consistent with the FEM outcomes. In the end, a real case study was considered to investigate the efficiency of thermomechanical fatigue reliability model.

Improvement of Sintering Performance of Nanosilver Paste by Tin Doping

Nanosilver paste, an interconnect solder, is a common choice in the electronics packaging industry. However, higher sintering temperature and lower sintering strength limit its application. At present, doped nanosilver paste has been studied for use in chip interconnection. In order to improve the sintering properties and shear strength of nanosilver paste, we have developed a new tin-doped nanosilver paste (referred to as silver tin paste), and according to the decomposition temperature of the organic dispersant in the slurry, a corresponding sintering process with a maximum temperature of 300°C was developed. The product after sintering of the silver tin paste is a mixture of a solid solution of Ag and an Ag3Sn phase. Among them, the hard and brittle phase Ag3Sn diffuse distribution in the silver matrix for strengthening, and the solid solution of Ag acts as a replacement solid solution strengthening. As the content of doped Sn increases, the sintering strength increases remarkably. When the Sn content is 5%, the joint shear strength reaches the highest value of 50 MPa. When it exceeds 5%, the sintering strength gradually decreases, which may be caused by the excessive formation of the intermetallic compound IMC as the dopant content increases. This new tin-doped nanosilver technology has the characteristics of low-temperature sintering and high-temperature service, so it is expected to be widely used in semiconductor power devices.

Characterization of Electrical Current Stress and Indentation Creep of Carbon Nanotubes- reinforced Low Melting Temperature Sn-58Bi Composites

Sn-58Bi eutectic alloy emerges as one of the potential lead-free solder material candidates due to its low melting temperature that enables low temperature soldering reflow process during the manufacturing of fine-pitch and low z-height advanced microelectronic devices. Its low melting temperature provides feasible solution for a two-step reflow process in the assembly of 3D-stacked advanced packaging technology, and at the same time reduces dynamic warpage-induced quality and reliability issues. However, miniaturization of these microelectronic devices has triggered significant reliability risk to its solder interconnects due to the requirement of high current density that resulted in severe physical failures, such as electromigration (EM), and thermomigration (TM). To mitigate the risk, carbon nanotubes (CNT) are incorporated into elemental Sn and Bi powders using planetary ball milling technique, and subject to liquid phase melting to form low melting temperature CNT- reinforced eutectic Sn-58Bi composites. Electrical resistance and current density of the eutectic alloy and its composites were investigated, and results show that Sn-58Bi-0.03CNT composite has the lowest electrical resistance and was able to survive 26 times longer duration of current stressing, even with the supply of higher stressed current. Creep performance of eutectic Sn-58Bi system was also examined in this study due to its high homologous temperature of 0.72 at room temperature. Marginal improvement in indentation creep performance was seen in the composites.

Experimental Approach to Determine Damage Curves for SnAgCu Solder Under Sequential Cyclic Loads

Miner’s rule assumes that damage in solder interconnects accumulates linearly under cyclic loading and is independent of the load sequence. Under these constraints, damage is equivalent to cycle ratio, defined as the ratio of the applied cycles to the cycles to failure for the specific cyclic loading condition. Due to these inherent assumptions, Miner’s rule can inaccurately estimate solder interconnect life under sequential loading conditions. A nonlinear damage model with load-dependent damage exponents would take into account the effect of loading sequence under sequential loading conditions. In the nonlinear damage model, damage is related to cycle ratio using a power law relationship with a load-dependent damage exponent. This paper presents an experimental approach to determine the load-dependent exponents under three load levels. Load drop in the specimens, as a result of cyclic loading induced changes, is used as the criterion for the damage state. The tests consisted of a series of constant amplitude (standalone) and blocks of sequential variable amplitude cyclic shear tests in a thermo-mechanical micro-scale analyzer. The load-dependent damage exponents were developed for SAC305 (96.5%Sn + 3.0%Ag + 0.5Cu) solder. The result of the study can be used for SAC305 damage accumulation model formulation. Further, the experimental approach can be used to generate additional fatigue data under variable amplitude loads.

Highly solderability of FeP film in contact with SnAgCu solder

New under bump metallization (UBM) is required for future microelectronics to minimize reliability concerns with solder interconnects. In this study, electroless FeP film was prepared as UBM. Solderability of electroless NiP, electrodeposite Fe and electroless FeP films with eutectic SnAgCu solder was investigated by static wetting tests and wetting balance measurements. Results revealed that the solderability of NiP film has almost no change with thermal aged for the film in air atmosphere increasing. The contact angles for FeP and Fe films with SnAgCu solder increased and the maximum wetting force decreased with heating time increasing from 0 h, 48 h, 89 h–160 h at 180 °C. But the solderability of FeP film was found to be better than those of NiP and Fe whether the films in base or thermal aged at 180 °C for 89 h. Even after thermal aged in air atmosphere at 180 °C for 160 h, the FeP also performed the smallest contace angle compared with the NiP and the Fe. Oxide layer on the surface of FeP was much easier to be removed by rosin mildly activated flux during reflow process because loose structure. It was proved to be the key for the highly solderability of electroless FeP.

Thermomechanical Analysis of Conformally Coated QFNs for High-Reliability Applications

The use of quad flat no-leads (QFNs) in high-reliability applications is receiving increased interest. To address concerns of harsh working environments, conformal coatings are used to protect the components and associated printed circuit board (PCB). However, using conformal coatings can pose a reliability risk for the components on the PCB. This paper details an investigation into the effect of conformal coatings on the reliability of QFN second-level solder interconnections. Five QFN package types and two types of conformal coatings were investigated. In the experiments, a large population of QFN assemblies coated with conformal coatings were subjected to a thermal cycling test condition. These components, designed with daisy-chain connections, were monitored during the thermal cycling tests, and the exact time for failure to occur for each component was recorded and analyzed. Detailed finite-element models for the tested QFN assemblies were developed; the inelastic strain energy density was used as the damage indicator to evaluate the impact of conformal coating on the solder joint reliability. Modeling results reveal that the conformal coating plays an intricate role in QFN solder joint reliability. The presence of conformal coating results in a reduction in the shear stress in the solder joint and plays a positive role in solder reliability. However, it also induces more stress in the solder joint in the out-of-plane direction and plays a negative role. These two mechanisms are competing, and their relative magnitude will determine the overall effect of conformal coating on solder joint reliability.

Fatigue Behavior of SAC-Bi and SAC305 Solder Joints With Aging

Reliability of microelectronic assemblies is typically limited by the fatigue failure of one of the interconnected solder joints. The fatigue behavior of the lead-free solder joints doped with bismuth under different stress loading and aging conditions is not yet understood. This article investigates the effect of adding bismuth on the mechanical reliability of SAC alloys considering different loading and aging conditions. The fatigue behavior and shear strength of individual SAC305 (Sn–3.0Ag–0.5Cu) and SAC-Q (Sn–3.4Ag–0.5Cu–3.3Bi) solder joints are examined and compared. A unique experiment is designed to test the individual solder joints using a micromechanical testing system. The results show that the fatigue life and shear strength of SAC-Q with Bi are much higher than SAC305 regardless of the aging and stress conditions. It was also found that increasing stress amplitude leads to a decrease in the fatigue life for both alloys. The aging time has a negative effect on the fatigue life and shear strength of both alloys. The impact of aging on SAC-Q solder joints is significantly less than that for SAC305. Microstructure analysis shows a substantial amount of precipitates coarsening with aging for SAC305 compared with SAC-Q. Hysteresis loop analysis shows that increasing the cycling stress amplitude and aging time leads to an increase in the work per cycle and plastic strain. Common fatigue models, such as Morrow energy model and Coffin–Manson model, fail to predict the life considering the effect of aging; aging affects the constants of both models.

Strain Rate Sensitivity of Mixed SAC-SnBi Solder Joints

Abstract As the electronics industry continues to evolve a concerted effort has developed to implement lower melting point solders. The ability to minimize the thermal exposure that an assembly is subjected to affords significant benefits with respect to both the reliability and the materials that can be used. One of the most popular low melt solder alloys currently being investigated by the industry is the Bi-Sn eutectic system, which has a melting point of 139°C. The BiSn system itself is not particularly novel as it was posited as a SAC alternative during the initial shift from Pb based solders. While a body of knowledge currently exists regarding this system, and the near eutectic variant BiSnAg, there are still concerns regarding its ductility, especially as a function of thermal exposure and strain rate. Bismuth is widely acknowledged as a brittle element and its presence in such quantities raises concerns of not just Cu6Sn5 embrittlement but also solder fragility in high strain rate types of environments. A challenge with regards to near term implementation is that most packages are not available with BiSn solder bumps. Therefore, it will be necessary to use components already balled with SAC 305 solder. This means that the resulting solder interconnect, reflowed below conventional SAC reflow temperatures, will form a type of mixed hybrid microstructure. This non-equilibrium microstructure will be composed of two regions, one Bi-rich region which is well past saturation and a second region which is Bi-deficient. It is of specific industrial interest then to not just investigate the BiSn solder system but also within the context of a realistic mixed interconnect. Recent work by several researchers has shown that this hybrid microstructure is unstable and quite active with respect to the movement and localized concentration of the Bismuth. The degree of mixing of these two regions has been shown to be highly dependent upon reflow temperature and the paste to ball volume ratio. Mixed SAC-BiSn solder joints were formed by placing SAC 305 spheres on BiSn paste deposits for a paste to ball volume ratio of .18. These samples were then reflowed at either 175°C or 200°C. SAC 305 control samples were also made using a conventional Pb-free reflow profile with a peak temperature of 247°C. A 22 mil Cu-OSP pad on a 1.0 mm thick FR4 substrate was used for all samples. A selection of the solder joints were then isothermally aged at 90°C for 200 hours. Using a joint level micromechanical tester, ball shear tests were conducted at a range of strain rates for samples in the as-reflowed and aged state. Using this information, the strain rate sensitivity of the interconnects was mapped and correlated with the observed failure modes. Investigations into the fracture mechanisms were conducted by examining the shear fracture surface with optical and scanning electron microscopy. Additionally, the evolution of the microstructure was characterized. Results showed a clear transition from ductile solder failure to a brittle separation failure at the higher strain rates.

Development and optimization of the laser-assisted bonding process for a flip chip package

In a fine pitch flip chip package, a laser-assisted bonding (LAB) technology has recently been developed to overcome several reliability and throughput issues in the conventional mass reflow (MR) and thermal compression bonding technology. This study investigated the LAB process for a flip chip package with a copper (Cu) pillar bump using numerical heat transfer and thermo-mechanical analysis. During the LAB process, the temperature of the silicon die was uniform across the entire surface and increased to 280 °C within a few seconds; this was high enough to melt the solder. The heat in the die was quickly conducted to the substrate through the Cu pillar bumps. Meanwhile, the substrate temperature was low and remained constant. Therefore, a stable solder interconnection was quickly achieved with minimal stress and thermal damage to the package. The substrate thickness, the number of Cu bumps, and the bonding stage temperature were found to be important factors affecting the heat transfer behavior of the package. The temperature of the die decreased when a thinner substrate, a higher number of Cu bumps, and a lower bonding stage temperature were used. If the temperature of the die was not sufficiently high, insufficient heat was transferred to the solder to melt it, resulting in incomplete solder joint formation. Thermo-mechanical analysis also showed that the LAB process produced lower warpage and thermo-mechanical strain than the conventional MR process. These results indicated that a LAB process using a selective local heating method would be beneficial in reducing thermo-mechanical stress and increasing throughput for the fine pitch flip chip packages.

Void formation in solder joints under power cycling conditions and its effect on reliability

When electronic assemblies are experiencing high current loading conditions, thermo-mechanical mismatch induced from the local Joule heating of the component and electromigration (EM)-induced void formation can lead to the failure of the solder interconnect. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) are adopted to investigate the microstructural evolution of solder joints of shunt components subjected to current stressing. Voids and micro-cracks initiate at the corner of solder meniscus, where the maximal divergence of material flux density is located on the basis of finite element method (FEM) simulation. The EM-induced voids introduce higher accumulated plastic strain in the solder joints, which will result in reduction in the lifetime of the electronic assemblies.