Search Results (113)

Sn-3Ag-0.5Cu-xBi alloys (SAC305-xBi) represent promising lead-free alternatives for low-temperature soldering. Low Bi concentrations can strengthen SAC-based solders through solid-solution strengthening, refining β–Sn grains and transforming needle-like Ag₃Sn phases into equiaxed morphologies. However, excessive Bi alloying may induce precipitation of brittle Bi particles, cause microstructural instability and interfacial degradation, thereby weakening the solder joint performance. As such, the concentration of Bi in the SAC305 alloys should be carefully controlled. In this work, the microstructure and corrosion behavior of Sn-3Ag-0.5Cu-xBi solder alloys (SAC305-xBi, where x = 0, 1, 2 and 4 wt. %) were investigated. Attention has been paid to the influence of low Bi concentration on the microstructure, morphology, and chemical composition of the phases present in the solder alloys before and after corrosion exposure. The alloys were prepared by induction melting of Sn, Ag, Cu and Bi lumps under Ar gas. The microstructure of the SAC305 and SAC305-1Bi alloys represented a hypoeutectic microstructure with dendritic (Sn) grains and the ternary eutectic, consisting of (Sn), Cu₆Sn₅ and Ag₃Sn, located in inter-dendritic regions. In the SAC305-2Bi and SAC305-4Bi alloys, a segregation of (Bi) particles was observed in addition to dendritic (Sn) and ternary eutectic. The (Bi) particles were located at the (Sn)Ag₃Sn interface in the inter-dendritic spaces of the (Sn) solid solution. The corrosion resistance of the as-cast alloys was studied in aqueous NaCl electrolyte (3.5 wt. %) using electrochemical methods. Open circuit potentials of the alloys were found to increase with increasing concentration of Bi. The highest corrosion current was found for the SAC305-1Bi alloy. It was observed that micro-galvanic cells at the Sn-Ag₃Sn interface were the initiating factors of corrosion in the SAC305-1Bi alloy. The corrosion activity of the SAC305-1Bi alloy is related to the high density of fine Ag₃Sn particles. The higher fraction of Ag₃Sn particles provided a dense network of local galvanic interaction sites, leading to the acceleration of the corrosion rate. The presence of discrete Bi precipitates in the SAC305-2Bi and SAC305-4Bi alloys, on the other hand, partially reduced the risk of galvanic corrosion. Since Bi has a higher standard electrode potential compared to Sn, the Bi/Ag₃Sn and Bi/Cu₆Sn₅ couples were less prone to corrosion. The corrosion mechanism of the SAC305-xBi alloys is discussed, and results are compared to previously studied SAC-Bi alloys. Full article

Cu is widely employed in power device packaging materials owing to its excellent electrical and thermal conductivity, coupled with economic viability. Sintered Cu currently stands as one of the representative interconnect materials in power device packaging. However, it is prone to oxidation during bonding, requires extended bonding times, and needs considerable pressure. Transient liquid phase bonding (TLPB) technology is regarded as a viable solution for power device packaging, enabling high-melting-point, high-strength, and thermally stable connections at low temperatures. Cu and Sn are widely employed metallic materials in common TLP systems. The Sn-coated Cu particle increases the effective reaction area between Cu and Sn, accelerating the formation of intermetallic compounds (IMCs) and reducing bonding time. Sn-coated Cu particles were produced in this study by chemically plating Sn onto micron-sized Cu powder surfaces. The effects of flux content, bonding time, and applied pressure on joint shear strength were investigated. Results indicate that as flux content increases, the shear strength of the solder joints initially increases and then decreases. The shear strength of the solder joint gradually decreased with increasing bonding time, but no significant change was observed when the time exceeded 20 min. Increasing the applied pressure significantly enhanced the shear strength of the solder joint. The shear strength of the solder joint at 10 MPa is 90.2% higher than at 5 MPa. Full article

The phase transformation characteristics of Sn-Pb-Bi ternary alloys with four representative Bi/Pb mass fraction ratios (0, 0.14, 0.33, and 0.60) were systematically investigated using the deep potential molecular dynamics (DeePMD) method over a temperature range of 300–600 K. A high-precision machine-learned interatomic potential was achieved using large-scale ab initio molecular dynamics (AIMD) datasets, reaching chemical accuracy (energy error <5 meV/atom, force error <100 meV/Å). Complete solid–liquid–solid heating–cooling cycle simulations were performed to accurately determine the melting temperature $T_m$, solidification temperature $T_s$, and undercooling $\Delta T$. The microscopic mechanisms through which Bi regulates phase transitions were revealed through radial distribution function (RDF), mean square displacement (MSD), self-diffusion coefficient, and viscosity analyses. Our results show that increasing the Bi/Pb ratio monotonically lowers $T_m$ from 475 K to 450 K, while $\Delta T$ reaches a maximum of ~48 K at Bi/Pb = 0.14. Bi addition disrupts short-range order, enhances chemical homogeneity, suppresses atomic diffusion, and optimizes liquid viscosity, with the optimal composition found to be Bi/Pb ≈ 0.14, balancing a low melting point, controlled undercooling, and improved flowability. This study provides an atomic-scale theoretical foundation for the precise composition design of low-melting-point Sn-Pb-Bi solders for photovoltaic and electronic packaging applications. Full article

Low-Temperature Co-Fired Ceramic (LTCC)-based mechanical sensors are inherently limited by the thickness and rigidity of multilayer ceramic stacks, which restrict miniaturization and mechanical compliance. To overcome these constraints, this work presents a hybrid LTCC/Kapton^® platform enabling high-sensitivity mechanical sensing through mechanically tunable RF passive components. The proposed approach integrates a flexible polyimide membrane, bonded onto an LTCC substrate at low temperatures using selectively electroplated indium pillars that simultaneously define the air gap and provide mechanical fixation. Inductance tuning is achieved via metal-shielding proximity effects, whereas capacitance tuning relies on force-controlled air-gap modulation in a metal–insulator–metal configuration. The fabrication process ensures precise gap control, high compliance, and structural robustness without requiring deformable ceramic membranes. Experimental characterization, including three-dimensional surface profiling and impedance measurements, demonstrates a 48% inductance tuning range with a sensitivity of 0.715 nH/mN and a 36% capacitance tuning range with a sensitivity of 47.3 fF/mN at 1 MHz. The proposed hybrid platform provides a compact and scalable solution for high-sensitivity sensors and mechanically reconfigurable RF components suitable for harsh-environment and adaptive electronics applications. Full article

Bi-doped low-silver Sn-Ag-Cu solders are increasingly gaining attention in advanced electronic packaging due to their cost-effectiveness and enhanced mechanical properties. However, the thermo-mechanical reliability mechanisms of such modified solders, particularly Sn-0.3Ag-0.7Cu-3Bi (SAC0307-3Bi) within Ball Grid Array (BGA) assemblies, remain insufficiently understood. To address this gap, this research proposes a comprehensive assessment framework integrating constitutive parameter calibration with finite element analysis (FEA) to accurately characterize the mechanical behavior and fatigue durability of SAC0307-3Bi solder joints under cyclic thermal loads. The Anand viscoplastic parameters were first calibrated via the Norton creep law and virtual tensile tests. Subsequently, a 3D quarter-symmetry model was constructed to replicate thermal cycling conditions between 25 °C and 125 °C. Simulation data reveal a strong correlation between stress concentration and the Distance to Neutral Point (DNP), pinpointing the chip-side interface of the corner joint as the critical failure site. Moreover, creep strain was observed to accrue in a “step-wise” pattern, predominantly during the heating and cooling ramps, reflecting distinct temperature sensitivity. Utilizing the Syed model, the fatigue life was estimated at approximately 2239 cycles. These insights serve as a crucial benchmark for designing robust packages using Bi-doped, low-silver lead-free solders. Full article

The growing demand for high-efficiency photovoltaic (PV) technologies has intensified interest in advanced cell architectures, including hybrid passivated back contact (HPBC) solar cells. Conventional solder-based interconnection processes require high thermal budgets, which can induce thermomechanical stress and lead to performance degradation in thin back-contact cell structures. In this study, electrically conductive adhesive (ECA) interconnection is investigated as a low-thermal-budget, solder-free alternative for HPBC solar cells. The curing behavior of an acrylic-based, silver-filled ECA is systematically examined by controlling the upper lamp temperature and the welding time during the interconnection process. Electrical performance is evaluated through current–voltage characterization, fill factor, and series resistance analysis, while interfacial microstructural evolution is examined using scanning electron microscopy. The results identify a well-defined processing window in which adequate curing enables stable electrical contact formation. In contrast, both insufficient curing and excessive curing result in degraded electrical performance. To assess practical applicability, HPBC modules with an industry-relevant size of ~1000 × 1160 mm² are fabricated and evaluated using electroluminescence imaging and I–V measurements. By identifying a robust curing window and demonstrating its successful transfer from string-level interconnections to full-size HPBC modules, this study establishes a practical, low-thermal-budget, solder-free interconnection strategy for advanced back-contact PV architectures. Full article

This study systematically investigates the influence of Bi content on the electromigration (EM) lifetime of low-temperature Cu/Sn-xBi-1Ag (600 μm)/Cu interconnects, where x = 57, 47 and 40 wt.%. The intrinsically higher product of diffusivity and effective charge number (DZ*) for Bi compared to Sn drives pronounced preferential migration of Bi atoms towards the anode, resulting in progressive β-Sn/Bi phase separation and linear thickening of a Bi-rich layer at the anode. Reducing the Bi content suppresses the EM-induced atomic flux (J_EM) through three principal mechanisms: (i) a decrease in the atomic concentration of mobile Bi atoms; (ii) a reduction in electrical resistivity that weakens the electron wind force; and (iii) an increase in lattice diffusion distance that lowers the effective diffusion coefficient (D_eff). The suppression of J_EM directly governs the thickening kinetics of anodic Bi layer, as evidenced by the close agreement between the calculated (1:0.40:0.23) and measured (1:0.45:0.26) anodic Bi layer growth rate ratios. Consequently, the EM lifetime is significantly extended from 62.3 h (Sn-57Bi-1Ag) to 164.9 h (Sn-47Bi-1Ag) and 414.1 h (Sn-40Bi-1Ag), representing 2.6-fold and 6.6-fold improvements, respectively. This study highlights that reducing the Bi content is an effective strategy for enhancing the EM reliability of Sn-Bi-Ag solder interconnects. Full article

Electromigration (EM) presents a major reliability challenge in advanced electronic packaging as device scaling and rising power demands lead to higher current densities in solder joints. While eutectic Sn-58Bi solder is widely adopted as a low-temperature alternative for its energy efficiency and compatibility with heat-sensitive substrates, its heterogeneous microstructure renders it vulnerable to EM-induced degradation. This review summarizes recent progress in understanding the EM behavior of Sn-Bi solder joints. We first introduce lifetime prediction models based on Black’s law, emphasizing the influences of current density, Joule heating, and thermomigration. Subsequently, the microstructural mechanisms accelerating degradation, including phase segregation and the coarsening of intermetallic compounds (IMCs), are examined. Various alloying strategies are evaluated for their effectiveness in strengthening the solder matrix and suppressing atomic diffusion to improve EM resistance. The critical role of substrate metallization is also discussed, comparing how different surface finishes affect interfacial reactions and joint lifetimes. Additionally, operational methods such as current polarity reversal are explored as potential pathways to mitigate degradation. Finally, we conclude that the EM reliability of Sn-Bi solder joints depends on the combined effects of alloy chemistry, interfacial reactions, and operating conditions, and we suggest future research directions in advanced modeling and material design for next-generation electronic applications. Full article

Characterized by its low melting temperature of 138 °C, the eutectic Sn-58Bi solder expands the melting temperature range of interconnect joints in electronic packaging, making it widely used in multi-level packaging processes. However, its reliability at higher current densities poses a challenge. This paper employs a hybrid process combining laser soldering and hot-air reflow to fabricate Cu/Sn-58Bi/Cu solder joints in ball grid array (BGA) structures. Through mechanical testing under current loading, the effects of increasing current density (0 A/cm², 0.85 × 10³ A/cm², 1.70 × 10³ A/cm², 2.55 × 10³ A/cm², 3.40 × 10³ A/cm², 4.25 × 10³ A/cm²) were studied systematically. Results indicate that the shear strength decreases markedly with increasing current density, exhibiting a reduction of approximately 5.63% to 95.75%. This degradation is initiated by the overall temperature increase and material softening due to Joule heating. It is further exacerbated by the loss of the non-thermal electron wind’s strengthening contribution, which weakens as the dominant thermal impact escalates with current density. Fracture mode transitions from ductile failure within the solder matrix to a ductile-brittle mixture at the solder/IMC interface, with the transition initiating at 3.40 × 10³ A/cm². Finite element simulations reveal that current crowding in Sn-rich regions and at the solder/IMC interface induces localized Joule heating and thermomechanical strain, which jointly drive the degradation in shear strength and the shift in fracture path. Full article

Anisotropic conductive films (ACFs) are widely used for circuit interconnection due to their easy use, low temperature bonding, higher precision than soldering and eco-friendliness. However, current ACFs are generally prepared by randomly distributing conductive particles into suitable resins. The ACFs prepared by this approach have risks to result in shortcut when applied for high precision bonding (<100 μm). In order to alleviate this problem, we designed and prepared a new kind of ACFs with conducting particles well aligned in adhesive film, which is named as array-patterned ACFs (A-ACFs). A template with 12 μm periodic microcavities was prepared and used to load 5.4 μm silver-coated polystyrene particles. Through a series of process optimizations including particles-filling cycles and particles-transferring-pressure/temperature into the used polyurethane (PU) adhesive, well-aligned particles with a spacing of 6.6 μm in the PU film was obtained. Such prepared A-ACFs were used to bond two flexible printed circuits (FPC) not only with a spacing of 200 μm (FPC-200) but also with 40 μm (FPC-40). The bonding conditions including temperature and pressure for the FPC-200 connections were investigated in detail. The connecting resistance, insulation resistance, peeling force, and the particles’ morphologies between the bonded FPCs were investigated. The reliability of the two bonded FPCs were tested under 85 °C and 85% relative humidity. Results showed that the new kinds of A-ACFs are suitable for achieving high precision circuits bonding and show better accuracy than those of traditional ACFs (T-ACFs). Thus, this study might have new insight for designing A-ACFs and great potential for applications in high-precision devices. Full article

Power modules in silicon carbide (SiC)-based modular multilevel converters (MMCs) suffer from notably severe thermal imbalance and localized overheating. This paper puts forward an asymmetric SiC power module with direct integration of a vapor chamber (VC), designed to balance the thermal distribution inside MMC SMs. Specifically, the chips on the lower side of the HBSM are soldered onto a VC, which is additionally mounted on the direct bonding copper (DBC). Endowed with merits such as favorable temperature uniformity, exceptional thermal conductivity, compact size, flexible design, high integration level, and reasonable cost, the VC serves as an outstanding heat diffuser significantly expanding the effective thermal conduction area and reducing thermal resistance. Moreover, in this structure, the VC also functions as a conductor for device current. Finite element method (FEM) simulation results reveal that the proposed structure can notably reduce the hotspot temperature (from 109 $°\mathrm{C}$ to 71.8 $°\mathrm{C}$), the maximum temperature difference among chips (from 45 $°\mathrm{C}$ to 13.89 $°\mathrm{C}$), and the low-frequency temperature swing (TSL) (from 68 $°\mathrm{C}$ to 38 $°\mathrm{C}$). Consequently, the issues of localized overheating and thermal imbalance in SiC-MMC SMs are effectively addressed. Lifetime analysis further indicates that the proposed structure can reduce the annual damage rate of the chip solder layer by 92.6%. Full article

The Bi-Sn, Bi-Sn-Ag, and Bi-Sn-Sb solder alloy systems represent lead-free, environmentally friendly alternatives for reliable electronic assembly. These alloys comply with increasingly strict environmental and health regulations, while offering low melting points suitable for soldering temperature-sensitive components. Microstructural analysis revealed distinct phase segregation in all alloys, with Sb promoting coarse Sn₂Sb₃ intermetallic compounds and Ag inducing fine needle-like Ag₃Sn precipitates. Eutectic refinement and compositional contrast were confirmed by SEM-BSE and EDS mapping. Vickers microhardness measurements revealed increased hardness in Sb- and Ag-modified Bi–Sn alloys, with Ag₃Sn dispersion yielding the highest strengthening effect, indicating enhanced mechanical potential. This study also reports the thermal and electrical conductivities of Bi60Sn40, Bi60Sn35Ag5, and Bi60Sn35Sb5 alloys over the 25–140 °C range. Bi60Sn40 showed an increase in thermal conductivity across the full temperature range from 16.93 to 26.93 W/m·K, while Bi60Sn35Ag5 reached 18.28 W/m·K at 25 °C, and Bi60Sn35Sb5 exhibited 13.90 W/m·K. These findings underline the critical influence of alloying elements on microstructure, phase stability, and thermophysical behavior, supporting their application in low-temperature soldering technologies. Full article

This study presents a novel approach to manufacture a rigid printed circuit board (PCB) using sustainable polymers. Current PCBs use a fossil-fuel-based substrate, like FR4. This presents recycling challenges due to its composite nature. Replacing the substrate with an environmentally friendly alternative leads to a reduction in negative impacts. Polylactic acid (PLA) and Polyhydroxybutyrate (PHB) biopolymers are used in this study. These two biopolymers have low melting points (130–180 °C, and 170–180 °C, respectively) and cannot withstand the high temperature soldering process (up to 260 °C for standard SAC (SnAgCu, tin/silver/copper) lead free solder processes). Our approach for replacing the PCB substrate is applying the PLA/PHB carrier substrate at the end of the PCB manufacturing process using injection molding technology. This approach involves all the standard PCB processes, including wet etching of the Cu conductors, and component assembly with SAC solder on a thin flexible polyimide (PI) foil with patterned Cu conductors and then overmolding the biopolymer onto the foil to create a rigid base. This study demonstrates the functionality of two test circuits fabricated using this method. In addition, we evaluated the adhesion between the biopolymer and PI to achieve a durable PCB. Moreover, we performed two different end-of-life approaches (debonding and composting) as a part of the end-of-life consideration. By incorporating biodegradable materials into PCB standard manufacturing, the CO₂ emissions and energy consumption are significantly reduced, and installation costs are lowered. Full article

Transient liquid-phase bonding (TLPB) enables the low-temperature fabrication of encapsulated solder joints with high-temperature resistance and electromigration resilience; yet, Ni-Sn TLPB joints suffer from brittle fracture due to intermetallic compounds (IMCs). This study investigates the Co, Cu, and Pt alloying effects on Ni₃Sn via formation energy, molecular dynamics, and first-principles calculations. Occupancy models of Ni_6−xM_xSn₂ (M = Co, Cu, and Pt) were established, with the lattice parameters, B/G ratios, fracture toughness (K_IC), and stress–strain behaviors analyzed. The results reveal that Co enhances fracture toughness and reduces Ni₃Sn anisotropy, mitigating microcrack risks, while Cu/Pt introduce antibonding interactions (Cu–Sn and Pt–Sn), weakening the bonding strength. The classical B/G brittleness criterion proves inapplicable in Ni–M–Sn systems due to mixed bonding (metallic/covalent) and the hexagonal structure’s limited slip systems. The Ni_6−xCo_xSn₂ formation improves toughness with a low Co content, supported by an electronic structure analysis (density of states and Bader charges). The thermodynamic stability and reduced molar shrinkage (Ni + Sn → Ni₃Sn) confirm Co’s efficacy in optimizing Ni–Sn solder joints. Full article

Effective thermal management is a critical challenge in achieving high-power output for semiconductor laser devices. A key factor in laser device packaging is the bonding between the laser device on a GaAs substrate and a heat spreader, typically composed of high thermal conductivity materials such as SiC. Conventional soldering methods introduce thick bonding layers with relatively low thermal conductivity, resulting in high thermal resistance at the interface. In this study, we demonstrate the room temperature bonding of GaAs and SiC via a 30 nm thick Au layer, eliminating the need for a thermal reaction bonding layer or vacuum process. Using surface-activated bonding (SAB), GaAs and SiC were successfully bonded, with a strength comparable to bulk fracture. A uniform and ultrathin Au bonding interface significantly reduces thermal resistance compared to conventional soldering methods. These results highlight the potential of SAB with thin Au films as a promising approach for improving thermal management in high-power semiconductor laser devices. Full article