Laser-assisted Bonding Research Articles

Recent Advences in Laser and Laser-Assisted Bonding (LAB) Technologies for Micro-Soldering and Applications to Mini-LED

Advancements in laser soldering and laser-assisted bonding (LAB) have revolutionized electronic manufacturing by enhancing solder joint formation with precise control, reduced thermal impact, and improved reliability. These technologies are crucial for high-density assemblies in advanced electronics, supporting miniaturization and increasing functionality. This paper introduces recent advancements in laser and LAB technologies for micro-soldering. The paper describes Nd:YAG and diode lasers, their micro-soldering results, the microstructure of the solder joint, and the author's LAB application to mini-LEDs using type-8 SAC305 solder paste. Future advancements in laser technology will further improve soldering and bonding techniques to ensure the reliability and performance of smaller, more complex electronic devices.

Brittle Fracture Behavior of Sn-Ag-Cu Solder Joints with Ni-Less Surface Finish via Laser-Assisted Bonding.

In this study, we investigated the brittle fracture behavior of Sn-3.0Ag-0.5Cu (SAC305) solder joints with a Direct Electroless Gold (DEG) surface finish, formed using laser-assisted bonding (LAB) and mass reflow (MR) techniques. Commercial SAC305 solder balls were used to ensure consistency. LAB increases void fractions and coarsens the primary β-Sn phase with higher laser power, resulting in a larger eutectic network area fraction. In contrast, MR produces solder joints with minimal voids and a thicker intermetallic compound (IMC) layer. LAB-formed joints exhibit higher high-speed shear strength and lower brittle fracture rates compared to MR. The key factor in the reduced brittle fracture in LAB joints is the thinner IMC layer at the joint interface. This study highlights the potential of LAB in enhancing the mechanical reliability of solder joints in advanced electronic packaging applications.

Process Window of Mini-LED Display Panel Packaging using Laser Assisted Bonding Technology

For high-resolution display panels using mini-LEDs, high alignment accuracy and high yield of LED transfer and bonding technology are very important parameters of packaging technologies. These parameters of packaging technologies are highly dependent on the interconnection material and bonding process. The use of laser assisted bonding (LAB) technology and epoxy-based interconnection materials has been introduced in mini-LED display panel packaging to improve bonding accuracy and yield. The bonding process of the entire display panel was performed by a tiling SITRAB process because the laser irradiation area is smaller than the LED bonding area on the substrate. For the SITRAB process, an epoxy-based SITRAB material to remove oxide on the surface of solder bump has been developed as a solvent free material. For good wetting of the solder bump during the SITRAB process, the temperature of the solder bumps must be higher than the melting temperature of solder. In general, the stage temperature of the laser process is determined to be around 100C in order to minimize the laser power. However, if the stage temperature is high, the initial properties of the SITRAB material (such as low viscosity and deoxidation function) may be lost due to the high temperature. In this study, the process window of the SITRAB material was determined for a stable SITRAB process. The process window was identified according to three different pot lifes: room temperature pot life (RPL), stage pot life (SPL), and laser pot life (LPL).

Anisotropic Solder Paste (ASP) Material Solution for Laser Assisted Bonding (LAB) Process

Anisotropic conductive paste (ACP) or anisotropic conductive film (ACF) consisting of a polymer matrix and a metal-coated polymer bead are introduced as interconnection materials in mini-LED display panel packaging through a thermal compression bonding process. However, this packaging solution has been leading to important problems such as alignment difficulties due to high bonding temperature and pressure, and its repair issues for bad LED devices after the bonding process. Recently, laser-assisted bonding (LAB) process for mini-LED display packaging has been introduced to solve these problems[Ref. 1-2]. This study introduces, an anisotropic solder paste (ASP) using conductive solder particles and applies it to the mini-LED packaging through the LAB process.

Solder Flux Evolution for Heterogeneous Integration

Advanced packaging techniques continue to evolve within the semiconductor industry. Within advanced packaging, heterogeneous integration has become a solution for those wishing to integrate distinct and complex dies into smaller packages. Many application trends are emerging to meet the challenges of heterogeneous integration which are causing the assembly process to become more intricate, with these trends requiring soldering material technology, especially that of solder flux, to be up to date and cutting-edge. In this paper, three assembly processes that necessitate advanced solder flux will be discussed: flip-chip assembly, BGA ball-attach assembly, and assemblies which have advanced to necessitate fluxless component attach. First, flip-chip assembly methods will be discussed and how different assembly technologies, such as standard mass reflow, thermocompression bonding (TCB), and laser assisted bonding (LAB), require differently formulated fluxes. This paper will then discuss the ball grid array assembly process, first outlining common process steps which negatively affect the solderability of ball-grid array pads, such as flip-chip reflow and molding compound applications, then discuss advancements in ball-attach fluxes which help eliminate these extra process steps, or ‘prefluxing’ steps. Finally, this paper will then discuss assembly trends which have led to fluxless component attach, and will discuss materials designed for fluxless processes, such as formic acid reflow, and how these fluxless processes tie into flip-chip assembly.

Reconfigurable Vacuum Sample Holder for Through-Silicon Microscopy and Laser-Assisted Bonding

Reconfigurable Vacuum Sample Holder for Through-Silicon Microscopy and Laser-Assisted Bonding

Interfacial Properties of Sn-Cu-xCr Alloy using Laser-Assisted Bonding

Conventional hot air reflow soldering is one of major bonding technology when bonding is proceeded on automotive electronics applications. However, here we focus on modifying the bonding process and composition of solder to enhance mechanical and thermal properties. This study experimentally investigated the mechanism of how laser-assisted bonding (LAB) form a small size of initial intermetallic compounds (IMC), and 0.2 wt% Cr element was added to Sn-0.7Cu eutectic solder to suppress the IMC growth. As a result, Sn-0.7Cu and Sn-0.7Cu-0.2Cr initial grain refinement was achieved by LAB. Isothermal aging was conducted to observe the mechanical and thermal properties depending on the IMC growth under 100, 125, 150 ℃ respectively. Sn-0.7Cu-0.2Cr tended to inhibit the growth of IMC at solder matrix and interface compared to Sn-0.7Cu throughout all aging temperature and time. The shear test was proceeded and Sn-0.7Cu-0.2Cr shear force was achieved 18% higher than Sn-0.7Cu. Even after isothermal aging, Sn-0.7Cu-0.2Cr showed lower reduction rate of shear force compared to Sn-0.7Cu.

Mechanical Reliability Assessment of a Flexible Package Fabricated Using Laser-Assisted Bonding

The aim of this study was to develop a flexible package technology using laser-assisted bonding (LAB) technology and an anisotropic solder paste (ASP) material ultimately to reduce the bonding temperature and enhance the flexibility and reliability of flexible devices. The heat transfer phenomena during the LAB process, mechanical deformation, and the flexibility of a flexible package were analyzed by experimental and numerical simulation methods. The flexible package was fabricated with a silicon chip and a polyimide (PI) substrate. When the laser beam was irradiated onto the flexible package, the temperatures of the solder increased very rapidly to 220 °C, high enough to melt the ASP solder, within 2.4 s. After the completion of irradiation, the temperature of the flexible package decreased quickly. It was found that the solder powder in ASP was completely melted and formed stable interconnections between the silicon chip and the copper pads, without thermal damage to the PI substrate. After the LAB process, the flexible package showed warpage of 80 μm, which was very small compared to the size of the flexible package. The stress of each component in the flexible package generated during the LAB process was also found to be very low. The flexible device was bent up to 7 mm without failure, and the flexibility can be improved further by reducing the thickness of the silicon chip. The bonding strength and environmental reliability tests also showed the excellent mechanical endurance of the flexible package.

Characterization of laser beams: theory and application in laser-assisted bonding process

The International Organization for Standardization (ISO), in collaboration with the academic community, has developed and published a comprehensive system of standards for the characterization of optical components and laser beams. Our study offers a pedagogical introduction to the theory and application of laser beam characterization in accordance with ISO 13694:2018 and ISO 11145:2018. The characterizing parameters of each ISO standard are presented, defined mathematically, and explained in detail through the use of a minimum working example. A laser beam applied to an actual industrial process [laser-assisted bonding (LAB) process in the semiconductor packaging industry] is thoroughly analyzed to help the reader understand and interpret the characterizing results. Background noise correction and unit conversion methods are also discussed. We target researchers new to the field of laser beam analysis who want the acquire background knowledge, as well as more experienced researchers who want to implement their own laser beam analysis software and propose new definitions for their specific application. Although our study deals with only two ISO standards, the methods covered in our study can be easily applied to other norms that fit the reader’s application. Our study should not be treated as a full review of the LAB process nor of the beam profiling instruments and measurement principles. These are only mentioned to enhance the reader’s learning experience.

Mechanical properties and microstructural evolution of solder alloys fabricated using laser-assisted bonding

The mass reflow (MR) process is widely used in electronic packaging interconnection. However, the conventional MR process can cause thermal damage to components and requires a long process time. The laser-assisted bonding (LAB) process is considered as an alternative soldering process for overcoming these limitations due to its extremely fast process time and high thermal selectivity. The LAB process causes low thermal damage; hence, it can be used to fabricate flexible and stretchable applications. This study investigates the mechanical properties, microstructural variation, and intermetallic reaction between the SAC 305 solder and Cu electrodes. The mechanical properties are investigated using shear tests, while the microstructure is analyzed using scanning electron microscopy and X-ray scans. Furthermore, the reliability of the LAB- and MR-produced SAC 305 solders is assessed using a high-temperature storage test. As a result, the intermetallic compound layer is found to be considerably thinner, and the volume of voids in the solder matrix is lower in the LAB-produced solder than that in the MR-produced solder. The mechanical properties of the LAB-produced solders are enhanced by these microstructural advantages.

Laser-Assisted Bonding (LAB), Its Bonding Materials, and Their Applications

Laser-Assisted Bonding (LAB), Its Bonding Materials, and Their Applications

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.

Accurate Temperature Monitoring in Laser-Assisted Polymer Bonding for MEMS Packaging Using an Embedded Microsensor Array

This paper presents an experimental study of an accurate temperature-monitoring method using an embedded thin-film microsensor array for laser-assisted polymer bonding for MEMS packaging. The work is carried out using a fiber-coupled diode laser system and benzocyclobutene polymer as the bonding material. Beam-forming optical elements are used to generate top-hat and frame-shaped beam profiles. Platinum-based sensor arrays are fabricated using sputtering and ion-beam etching methods. Peripheral sensors are embedded at the interface between the polymer sealing ring on the top (capping) substrate and the sensor substrate in the bonding process. The embedded peripheral sensors allow precise monitoring of the temperature profile of the polymer track in the laser-assisted thermal curing process for substrate bonding. The sensor at the center of the array can monitor the temperature that would be experienced by a MEMS device in a manufacturing environment. Results show that accurate temperature monitoring can be obtained using the embedded sensor array. A lower temperature than that required for bonding is seen at the center of the bottom (device) substrate. This is a highly desirable effect for packaging of temperature-sensitive devices. In addition, the effects of substrate material and arrangement of heat dissipation on the resultant temperature profiles have been investigated.