An approach to achieve structural color 3D dynamic moiré images by overlaying a microlens array (MLA) onto the micropattern array (MPA) on polyester film is proposed in this study. Specifically, two types of filling structures within the MPA layer have been employed to produce vivid structural colors in the patterns. Furthermore, by modulating the structural parameters of the MLA layer via the thermal reflow process, the iridescent effect of the optical film is significantly enhanced. Additionally, this work demonstrates both orthogonal and reverse relative motion effects in the resulting iridescent moiré images. The proposed microstructural device offers a low-cost paradigm suitable for applications in green printing, anti-counterfeiting, and related fields.

Purpose This study aims to compare wettability, interfacial morphology and mechanical properties of Sn-3.0Ag-0.5Cu (SAC305) solder on Cu substrate using conventional reflow (CR) and microwave hybrid heating (MHH) methods. Design/methodology/approach The solder paste was reflowed using a conventional reflow oven set to 250°C for 6 min, while MHH was conducted using medium microwave power mode for 2 min with a silicon wafer as a susceptor. Wettability, phase formation, morphology and mechanical properties were analyzed using scanning electron microscopy, X-ray diffraction, ImageJ and Vickers hardness test. Findings MHH significantly improved solder wettability by exhibiting a 42.46% lower wetting angle and a 7.73% higher spreading ratio compared to CR. Phase analysis identified the presence of Cu6Sn5 intermetallic compounds in both heating methods. The morphology showed a slightly thicker Cu6Sn5 layer in the MHH sample relative to the CR sample. Mechanical assessments revealed higher and more consistent Vickers hardness values for MHH reflowed solder, suggesting better mechanical uniformity across the joint. Originality/value This study presents a direct comparative analysis of wettability, phase formation, morphology and Vickers hardness of SAC305/Cu joints reflowed by CR and MHH. The findings highlight the influence of heating profiles on solder joint properties and offer comparative insights to guide optimization of reflow processes for reliable electronic packaging applications.

The mechanical properties of lead-free solder alloys are critical for ensuring the reliability of electronic packaging, with shear strength and hardness being particularly important as electronic devices become smaller and interconnection densities increase. Thermal fluctuations and external mechanical impacts further intensify shear stresses on solder joints, raising concerns about long-term performance. In this study, the shear stress behavior of Sn-Ag-Cu and Sn-3Zn-4Bi solder joints was examined under different reflow temperatures. Sn-Ag-Cu, a widely researched lead-free solder, demonstrated strong resistance to high stress levels, reinforcing its suitability for high-reliability applications; however, its relatively high melting temperature (~221 °C) limits its use in low-temperature reflow processes. By comparison, Sn-3Zn-4Bi solder, with a melting temperature only ~12 °C higher than eutectic SnPb solder, showed potential for low-temperature soldering, while also exhibiting higher microhardness values than Sn-Ag-Cu, suggesting improved structural robustness. Despite these advantages, concerns remain regarding its compatibility with copper substrates, where interfacial reactions may affect joint integrity. Overall, the results suggest that Sn-Ag-Cu is preferable for applications requiring high strength and thermal resistance, whereas Sn-3Zn-4Bi offers notable benefits for low-temperature processing, provided substrate interactions are properly managed.

This paper investigates the climatic reliability of realistic PCBA component geometries soldered using reflow process, from the perspective of the interaction between component design parameters, reflow residue and humidity. The study was carried out using electrochemical characterization, electron microscopy, and X-ray imaging. Custom-designed PCBA test cards with dummy components have been exposed to harsh climate after reflow soldering. A methodology of combining Leakage current and electrochemical impedance measurements has been used to characterize PCBA performance. Changes in flux morphology and any associated negative effects as a function of humidity interaction have been connected to component connector design parameters, to achieve a holistic understanding of flux system-component suitability and humidity interaction.

In this study, an intense pulsed light (IPL) flip-chip bonding process was investigated to enhance the mechanical reliability of solder joints in flip-chip ball grid array (FC-BGA) packages. The process was characterized by using in situ temperature and resistance monitoring systems to provide real-time data during bonding. In addition, a numerical thermal transient simulation model was developed and validated by comparison with in situ monitoring results. The temperature profiles according to IPL parameters (pulse on-time, frequency, and pulse number) were investigated to effectively reduce bonding process time and maximum temperature of the flip-chip bonding process. The microstructure of the solder joint was observed using scanning electron microscope (SEM). The thickness of intermetallic compounds (IMC) was effectively reduced from 6 μm in the conventional reflow process to approximately 800 nm in the IPL flip-chip bonding process, as the process time was significantly shortened from 90 s to 56.4 ms, and the maximum temperature was lowered from 250 to 221.7 °C. Die shear tests demonstrated that the IPL flip-chip bonding process improved die shear force by 30% compared to conventional reflow processes. This study demonstrates that the IPL flip-chip bonding process could produce FC-BGA packages with excellent mechanical reliability.

In this study, an interconnection was formed between a Cu/SnAg pillar bump and an Ni-less surface-treated Cu pad through laser-assisted bonding (LAB), and its bonding characteristics were evaluated. The LAB process influences the bond quality and mechanical strength based on the laser irradiation time and laser power density. The growth of the intermetallic compound (IMC) in the joint cross-section was observed via FE-SEM analysis. Under optimized LAB conditions, minimal IMC growth and high bonding strength were achieved compared to conventional thermo-compression bonding (TCB) and mass reflow (MR) processes. As the laser irradiation time and laser power density increased, solder splashing was observed at bump temperatures above 300 °C. This is hypothesized to be due to the rapid temperature rise causing the flux to vaporize explosively, resulting in simultaneous solder splashing. With increasing laser power density, the failure mode transitioned from the solder to the IMC.

Purpose This study investigates the impact of reflow temperature on the reliability of printed circuit board assembly (PCBA) produced through surface mount technology (SMT). The authors define failure as deviations of the temperature curve from the process window (PW) of key PCB components. This study aims to develop a prognostic and health management system for failure prediction. Design/methodology/approach The study used reflow equipment in a real-world production environment. Key parameters affecting temperature curve deviations were identified, and the eXtreme Gradient Boosting (XGBoost) method was used to construct a failure prediction model. This model allows onsite monitoring and predicts failures based on various feature combinations. Upon detecting potential failures, the model alerts the on-duty engineer, providing failure time and recommending rechecks. Findings The failure prediction system achieved high accuracy (94%), precision (99%) and recall (89%). The PHM system effectively identifies temperature curve deviations, enabling timely interventions. It calculates the PCBA product serial number in the reflow furnace zone, assesses deviations from the PW and recommends rechecks, thus enhancing PCBA production reliability. Originality/value This study integrates the XGBoost method into a PHM system for failure prediction in PCBA production. Combining real-world production data with advanced machine-learning techniques offers a novel approach to addressing reliability concerns in the SMT reflow process. Integrating failure alerts into the Shop Floor system ensures prompt rechecks of high-risk PCBs, enhancing overall system efficiency.

Abstract Sn-Ag-Cu (SAC) solder alloy is the most promising lead-free solder alloy, with Sn as the principal constituent. It offers excellent solderability and mechanical properties and addresses the environmental hazards associated with Pb-Sn solders. Key factors affecting the reliability and solderability of the alloy includes wettability, microstructure evolution, intermetallic compound (IMC) growth at the solder-substrate interface, and mechanical properties. The addition of nanoparticles in low weight fractions reduce surface tension, enhances wettability, refines the microstructure, and improves mechanical properties such as shear strength, tensile strength, and microhardness. The improvement in mechanical properties is achieved by inhibiting IMC growth and strengthening the solder matrix. However, excessive nanoparticle additions can adversely affect the properties of solder joints. Despite advancements in lead-free solders, none of the alloys has fully replaced Sn–Pb solders due to challenges in controlling IMC formation during reflow processes. The present work reviews the effects of nanoparticles on the microstructure, mechanical properties, and reliability of SAC solder alloys. The ongoing research on nanocomposite solders should focus on optimizing nanoparticle additions to enhance reliability under thermal cycling and aging conditions.

Finite-Element Analysis and Multiobjective Optimization of Solder Joint Temperature Difference and Cooling Stress During PCBA Reflow Process

This paper presents a virtual material approach to modeling the viscoelastic properties of underfill materials in flip chip packages. The role of underfill is critical, particularly in addressing the challenges associated with cracks in solder bumps in thermal cycles. To ensure the long-term reliability of the semiconductor packages, it is crucial to understand how different underfill properties interact with the package and influence the stress distribution. A fully cured underfill material exhibits viscoelastic behavior. But in many cases, viscoelasticity is neglected and underfill properties are simplified to linear elastic in the finite element analysis (FEA) to save computational cost. For high accuracy numerical predictions, it is indispensable to include a full characterization of viscoelastic properties for underfill in simulation. This work develops a virtual material approach to parameterize the viscoelasticity of underfill materials. A generalized logistic function is adopted for the parameterization of the frequency-dependent modulus in master curve. The master curve is then calibrated in Prony series for numerical implementation. A three-dimensional FEA model is built for evaluating underfill properties in flip chip packages. Fine details such as solder bumps are only included in the sub-modeling for computational efficiency. The FEA model is validated by warpage measurement on a test vehicle in reflow process. Finally, with the developed virtual material approach and the FEA model, various viscoelastic properties of underfill are compared, and the influence on package stress including fillet top stress, die corner stress, and solder bump creep strain is evaluated. The virtual material approach can be particularly useful when dynamic mechanical analysis (DMA) results are not available or in scenarios where obtaining certain properties is challenging. The findings of this study contribute to the development of guidelines for selecting optimal underfill materials, ultimately enhancing the reliability of electronic devices.

Indium and indium-containing alloys such as indium silver (InAg) alloys have generally been paid considerable attention as thermal interface materials for usage in thermal management of microprocessors such as CPU, GPU and AI accelerator. This is, especially true for high-performance computing electronics with high power density. Reflow process parameters of InAg preforms with differing compositions of Ag ranging from 3 to 10 wt% as well as various thicknesses were extensively studied bonding Cu to Cu to achieve optimum properties and performance of the InAg soldered joint using formic acid vacuum reflow. Basic characterization was also completed including basic physical properties such as density, modulus and phase transition. Moreover, X-ray scanning and confocal scanning acoustic microscopy (C-SAM) were used to identify voids and delamination of the InAg soldered joints, respectively. Laser Flash Analysis (LFA) was employed to measure thermal conductivity of the soldered joint bonding Cu to Cu after the reflow process in the determined optimum conditions. Thermal cycling testing (TCT) was conducted with a profile of -40°C to 125°C, 30 min of dwell time and highly accelerated stress testing (HAST) with a profile of 110°C/85%RH/0.22 MPa/264 hr, to evaluate reliability of the soldered joints, the initial characterization testing was repeated at several intervals. To further understand the change of microstructure after the given number of thermal cycling, cross-sectional samples were prepared and scanning electron microscopy (SEM) was used to characterize morphologies of the soldered joints. In this presentation, the influence of the different compositions of InAg alloys and bond line thickness (BLT) on the voiding performance, delamination and thermal conductivity of the soldered joints, as well as the results of reliability are discussed.

Fused Silica Micro Shell Resonators by a Wafer-Level Thermal Reflow Process

Immersion tin is a final finish highly accepted in the automotive industry and finds increasingly application in the package substrate industry. As due to the immersion reaction the plating thickness of tin is limited, there are concerns that due to the formation of the intermetallic phase during the reflow process, the solderability of the tin layer might be affected. During the assembly process and the formation of the intermetallic phase the free tin is consumed and only islands of free tin remain after the second reflow cycle. Additionally, an oxide layer is formed, and the homogeneity and thickness of the oxide layer is impacted by the reflow conditions and the overall process cleanliness. The interaction of intermetallic phase formation, the amount of remaining free tin and the tin oxide layer will define the soldering performance of the tin layer and the risk of the occurrence of soldering defects. In this paper different failure modes and connected failure mechanisms will be explained. The tin oxide layer is identified to play a crucial role in this interaction and factors are investigated, which impact the oxide layer formation and homogeneity. Various methods are presented to determine the oxide layer thickness and different approaches are studied to modify it. Results are presented that show that with a dedicated solution to modify the oxide layer the risk for solderability defects on immersion tin can be reduced.

Purpose There are three major processes in the surface mounting technology (SMT) manufacturing line, namely, printing, mounting and reflowing. During the reflow process, the printed solder pastes are melted into liquid and cooled down into solid again, forming the solder joint. During this process, because of the combined force of the liquid solder, the components will move from the placed location to the final location. This is known as the self-alignment performance in the reflow process. From the comparable studies, it is known that the reflow process with a longer time above the liquidus (TAL) results in better self-alignment performance, as well as higher peak temperatures of the reflow profiles. The TAL and the peak temperatures are influenced by multiple factors, but the experimental designs from the comparable studies kept the same ramping and cooling slopes to modify the TAL and the peak temperature. The purpose of this study is to study on the multiple factors influencing the TAL and the peak temperatures (ramping slope, cooling slope and peak temperature) by conducting designed experiments for each of the factors. Design/methodology/approach In this study, the TAL-influencing features are studied independently, including ramping slope, cooling slope and peak temperature; designed and conducted an experiment to reveal the relationship between the self-alignment and each of the three features; the statistical-based model for the combination of the three features for optimal self-alignment performance; and proposed a statistical-based simulation model for the offset after reflow. Findings The authors conducted a case study validating the mounter optimization model previously proposed. As a result, the optimized reflow profile improved the self-alignment performance by at least 10%, and the simulation within the error of 15 µm. Research limitations/implications This research is statistical-based, which is limited to component types and sizes used in the design of experiments (DOE). The model proposed in this study can suggest a new reflow profile that can increase the self-alignment performance, which is critical to the solder joint quality, especially long-term quality. Practical implications This study can suggest a new reflow profile that can increase the self-alignment performance, which is critical to the solder joint quality, especially long-term quality. Social implications This study can suggest a new reflow profile that can increase the self-alignment performance, which is critical to the solder joint quality, especially long-term quality. Originality/value The proposed statistical-based soldering reflow target profile optimization model offers a novel and practical approach. The soldering profile provided by the manufacturer contains recommended and acceptable ranges in the critical features. This study provides the optimal setting within the ranges.

Soldering reflow process optimization based on simulation

ABSTRACT Miniaturization has been a pivotal trend in the electronics industry since Gordon Moore's prediction of doubling microchip transistor counts every two years. While this trend has driven the development of smaller, lighter electronic devices, recent advancements in semiconductor technology are pushing towards larger and more complex integrated circuit chips. This evolution, necessitated by the need for higher performing semiconductors and enhanced manufacturing processes, introduces significant challenges during reflow attachment to printed circuit boards, particularly concerning component warpage and boards that heat unevenly. Component warpage is a growing challenge when it comes to printed circuit board assembly. Warpage can appear in a “smile, frown, M or W“ shape. Warpage complexity typically increases as component dimensions increase. Warpage during reflow can directly lead to defects such as head-in-pillow (HiP), non-wet opens and bridging. One of the best ways to control, or reduce component warpage is by lowering the peak reflow temperature. Checking the moiré shadow diagram for each component can help ensure that the reflow peak temperature is compatible with the component warpage requirement. The warpage issue extends beyond component size alone; modern circuit boards are also growing in complexity. High-performance IC components require thicker PWB copper traces and take up more board real estate, contributing to densely populated boards with varied component types and shapes. During reflow, these boards often exhibit uneven heating, with standard SAC305 profiles showing temperature differentials ranging from 40-60°C. This temperature variance forces a compromise: while ensuring components with substantial thermal mass reach 240°C, smaller components may inadvertently reach temperatures as high as 300°C. A novel mixed solder powder alloy, referred to as DFLT, has emerged as a crucial solution to these manufacturing complexities. Leveraging advanced alloy technology, DFLT mitigates challenges associated with uneven heating due to varying thermal mass across components. This innovative alloy not only widens the reflow process window, but also can effectively reduce warpage during reflow processes. Moreover, its peak reflow temperature can be 20-40°C lower than SAC305, ensuring quality solder joints without compromising component integrity. In summary, DFLT represents a transformative solution for modern electronics manufacturing, adeptly addressing the intricate demands posed by larger, more complex IC components and high-density circuit boards.

Mitigating solder voids in quad flat no-lead components: A vacuum reflow approach

Perfectly vertical grating couplers (VGCs) are essential for the development of high-density, low-cost, and scalable photonic devices. This paper introduces an integrated microlens-assisted grating coupler (GC) designed to enhance coupling efficiency under vertical incidence conditions. The microlens, fabricated through a thermal reflow process, is integrated onto a standard 220 nm silicon-on-insulator (SOI) grating coupler. This hybrid integration approach provides greater flexibility in manipulating the transmission angle of vertically incident light, aligning it with the optimal coupling angle of the underlying grating, thereby effectively improving the overall coupling efficiency (CE) of the device. Our proposed microlens-assisted grating coupler achieves 2 dB higher coupling efficiency under the TE mode in the C-band compared to a bare grating coupler, significantly reducing alignment complexity and enabling wavelength division multiplexing applications. Furthermore, this methodology is applicable to polarization-insensitive vertical grating couplers, providing an increase of 1.3 dB and 1.4 dB for TE and TM modes, respectively. Compared to traditional nanostructures, our method offers simpler fabrication processes and more flexible optical characteristics, highlighting its significant potential for applications in ultra-high-capacity optical communications, high-performance optical detection, real-time biochemical sensing, and other related fields.

Fabrication and Reflow of Indium Bumps for Active-Matrix Micro-LED Display of 3175 PPI

SnPbInBiSb high-entropy solder joints with inhibited interfacial IMC growth and high shear strength