Fluxless Process Research Articles

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.

Influence of Ni/Zn double coating on the steel on penetration welding-brazing by CMT arc-laser hybrid heat source

To improve interfacial inhomogeneous temperature distribution and weld appearance of butt welding-brazing of steel/aluminum dissimilar metals, a hybrid heat source method combining laser penetration welding with CMT arc was developed. The Ni/Zn double coating was prepared on the surface of steel to improve the interfacial reaction and achieve fluxless process. The welding characteristics of bare steel/aluminum alloy, Ni coated steel/aluminum alloy, and Ni/Zn double coated steel/aluminum alloy were comparatively studied. Transmission electron microscope (TEM), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS) were used to analyze the interfacial microstructures of the joint. The results showed that the fluxless process were implemented by preparing Ni/Zn double coating on the surface of steel. The interfacial reaction between liquid Al and solid steel was obstructed to some extent by Ni/Zn double coating. The thickness of intermetallic compounds (IMCs) was reduced significantly. The interfacial reaction layer consisted of Fe2Al5 near the steel side and Fe4Al13 near the aluminum side. No continuous Ni-Al IMCs were found at the Fe/Al butt interface. The maximum tensile strength of Ni coated steel/aluminum joint was up to 93.4 MPa, which increased by 12% than that of the bare steel/aluminum joint. The maximum tensile strength of Ni/Zn double coated steel/aluminum joint was up to 112.6 MPa, which increased by 35% than that of the bare steel/aluminum joint. On this basis, the fluxless wetting and interfacial reaction mechanisms were revealed.

Characterization of tin-oxides and tin-formate crystals obtained from SnAgCu solder alloy under formic acid vapor

Crystals of tin-oxides and tin-formates are grown from Sn–Ag–Cu alloy under formic acid vapour as used in the fluxless soldering process.

Bonding of Si chips to low carbon steel boards using electroplated Sn solder

PurposeThe purpose of this research was to develop a new process to bond silicon (Si) chips to low carbon steel substrates using pure tin (Sn) without any flux.Design/methodology/approachIron (Fe) substrates were first electroplated with a Sn layer, followed by a thin silver (Ag) layer that inhibits Sn oxidation thereafter. It is this Ag capping layer that makes the fluxless feature possible. Fluxless processes are more environmentally friendly and more likely to produce joints without voids. The Si chips were deposited with Cr/Au dual layer structure. The bonding process was performed at 240°C in vacuum. The Sn joint thickness was controlled by spacers during the bonding. Scanning electron microscopy images on cross sections exhibited quality joints without visible voids. Energy dispersive X-ray spectroscopy analysis was used to detect joint compositions.FindingsIt was revealed that the Sn layer was bonded to a Si chip at the Cr–Sn interface and to the Fe substrate by forming an FeSn2 intermetallic compound (IMC). The IMC is only 1.1 to 1.5 µm in thickness. Thin IMC is highly preferred because IMC deforms a little in accommodating the coefficient of thermal expansion (CTE) mismatch between Si and Fe. Shear test results showed that the fracture forces of the samples passed the military criteria by a wide margin.Originality/valueThis new fluxless bonding process on Fe should make Fe or low carbon steel a more likely choice of materials in optical modules and electronic packages.

Investigation of Flux-Less Soldering of Metal Matrix Composite and Ceramics by Use of Zn Solder

The work was dealing with direct soldering of Al2O3 ceramics and a metal/ceramic composite (MMC) with Al matrix. The joints were fabricated by application of flux-less process by use of a hot plate at the assistance of ultrasound with frequency of 40 kHz. The experiments were performed by use of Zn6Al6Ag solder. Wetting was attained on both substrates. The joints of a good quality were fabricated. The soldering temperature was 420 °C. The microstructural studies of interface have revealed a transient zone of Al dissolving on the side of MMC and formation of Zn oxides on the side of ceramics, which were responsible for bond formation.

The Thermal-Aging Effect on the Microstructure Evolution and Shear Strength of the Sn-Rich Au-Sn Soldering between Altic and Si Substrate in Microelectronics

The Au-Sn soldering alloys are commonly used in microsoldering process for microelectronic industry due to fluxless process and relatively low melting temperature with good eutectic microstructures. This study investigated the microstructures of Au-Sn soldering between AlTiC and Si substrates with Ti/Pt/Au under bump metallization (UBM). The microstructures of the solder samples under three conditions: before bonding, after bonding and after thermal-cycle aging, were investigated. The shear strength values of pre-aging and post-aging soldering were compared. The thermal-cycling temperatures were ranged from -40 to 125 °C for 300 cycles. The intermetallic compounds (IMCs) of the AuSn solders consist of AuSn, AuSn2, and AuSn4. After thermal-cycle aging, the bonding strength was increased due to the improved IMC bonding between solders and UBM; the shear surfaces were rougher due to the growth of AuSn and AuSn2.

ULTRASONIC SOLDERING OF Cu AND Al2O3 CERAMICS BY USE OF Bi-La AND Bi-Ag-La SOLDERS

This work deals with the effect of solder alloying with a small amount of lanthanum on joint formation with metallic and ceramic substrate. The Bi-Ag – based solder with 2 wt.% lanthanum addition and Bi solder with 2 wt.% lanthanum addition were studied. Soldering was performed by a<br />fluxless process on the air, by activation with a power ultrasound. It was found out that, during the process of ultrasonic soldering, lanthanum is distributed on the boundary, both with the copper and the ceramic substrate, which enhances the joint formation. The bond with Al<sub>2</sub>O<sub>3 </sub>ceramics is of an adhesive character, without the formation of a new contact interlayer.

Thermal Cycling Reliability Study of Ag–In Joints Between Si Chips and Cu Substrates Made by Fluxless Processes

The purpose of this research is to assess the reliability of Ag-In joints in thermal cycling (TC) environment. Si chips and Cu substrates were bonded using silver (Ag) and indium (In) multilayer structure without applying any flux. After bonding, the samples were annealed in air at 250 °C for 190 h to convert the joint into an alloy of small intermetallic grains and solid solution (Ag). The resulting joint has a melting temperature higher than 800 °C. Si-Cu pair was chosen because of the large coefficient of thermal expansion mismatch, i.e., 2.7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> /°C of Si versus 17 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> /°C of Cu. Two TC tests were performed. All 10 samples passed 100 cycles of initial TC test between -40 °C and 85 °C. They were then subjected to 5000 cycles of TC test between -40 °C and 200 °C. Seven of ten samples survived beyond 5000 cycles. Three samples broke at 850, 2600, and 3000 cycles, respectively. The early failure was probably caused by imperfections and defects in the joints. Based upon these results, it seems that our Ag-In joints compare favorably with sintered silver joints. The Ag-In joints not only have high-melting temperature but also survive harsh TC environment.

In Situ Measurement of the Thermal Contact Resistance of an Al Lap Joint During Braze Processing

A technique was developed, using a laser flash thermal diffusivity apparatus, to measure the thermal contact resistance, Rc, of an Al lap joint during braze processing. The method required a determination of the temperature dependence of the thermal diffusivity of the individual Al braze sheets, as well as a two-layer lap joint made from these sheets, under identical conditions. A two-layer modeling method was then used to analyze the above data to determine Rc for the lap joint as a function of processing temperature. This in situ analysis of Rc for a developing joint during brazing was able to distinguish both solid-state and liquid-phase reactions occurring at the faying surfaces of the joint. Consequently, it represents a powerful, new experimental tool that can be used to investigate the mechanisms of braze joint formation and the thermal properties of a joint as a function of processing condition. In the particular case of this study, the technique demonstrates that the use of a Ni-based fluxless brazing process to join aluminum (Al) sheets, reduced the Rc of the lap joint by 100-folds from 1.3 × 10−4 m2 K/W before brazing to 1.35 × 10−6 m2 K/W after brazing.

Fluxless Tin and␣Silver-Indium Bonding Processes for␣Silicon onto␣Aluminum

The high thermal conductivity, light weight, and low cost of aluminum (Al) make it a promising material for use in high-power electronic packaging. The challenges are its high coefficient of thermal expansion (CTE) of 23 × 10−6/°C and difficulty in soldering. In this research, we surmounted these challenges by bonding large Si chips to Al boards using fluxless Sn and Ag-In processes, respectively. Despite the large CTE mismatch, the bonded structures were strong as determined by fracture force measured by shear test machine. The reference is the fracture force specified in MIL-STD-883H method 2019.8. The microstructure and composition of the joints were examined using scanning electron microscopy (SEM) and energy-dispersive x-ray (EDX) analysis. The resulting Sn joint is almost pure Sn with thin intermetallic layer. The Ag-In joint consists of Ag/(Ag)/Ag2In/(Ag)/Ag that has a melting temperature higher than 695°C even though the bonding process was performed at 180°C. These bonding processes are entirely fluxless. The fluxless feature greatly helps reduce voids in the joints, which in turn increases the joint strength. These preliminary but encouraging results should open up new applications of Al boards in electronic packaging where Al was avoided because of its high CTE and difficulty in bonding.

Magnesium Recycling: To the Grave and Beyond

Demands for the reduction of carbon emissions imposed by the legislative bodies mean that magnesium and its alloys receive continued interest from the manufacturing industries. The use of alternative energy sources such as hybrid-, battery-, or fuel-cell-powered cars alleviates some of the environmental impact of using fossil fuels, but these technologies continue to require lighter materials to maintain fuel efficiency. Many of the lightweight materials such as fiber-reinforced polymers cannot be reused or recycled in the conventional ways. High-grade magnesium and magnesium alloy scrap can be easily recycled where the alloy composition can be controlled, such as defected parts in a die casting operation. However, where the alloys are mixed or the alloys contain impurity elements or oxide particles, the alloys cannot be easily recycled. As the use of the magnesium alloys increases the recycling of magnesium alloys becomes both environmentally and economically important. Moreover, various countries in the world regulate the end of life of the vehicles and the recycling of the parts. The European Union regulates the minimum amount for reuse and the recycling rates for automotive components. Canada, Japan, China, and some states in the United States also regulate the end of life of motor vehicles in order to recover, reuse, and recycle some of the materials. The production of primary magnesium is an energy-intensive process with a large carbon footprint. This is especially the case with the conventional pigeon process or the electrolytic reduction of magnesium chloride. However, the use of secondary magnesium is much less energy intensive compared with the primary production. At present, the aluminum and magnesium components are separated from heavy metals such as steel. The recovered magnesium is used in secondary aluminum processing to regulate the concentration of magnesium. However, with the increased use of magnesium, a commercially viable method to separate magnesium and aluminum is essential. A new system to separate magnesium has been developed in Germany that distinguishes between magnesium and aluminum through their coefficients of friction. Thus, a commercially viable and environmentally sound recycling process can lead to increased use of magnesium alloys. Recently, many different methods of recycling have been investigated to determine their viability. Several different technologies can be used to refine magnesium scrap, which can be broadly categorized as flux based and fluxless technologies. Flux refining technologies use a flux to protect the magnesium and another flux to clean the magnesium. An example of a refining process that uses fluxes to refine magnesium is described in the article titled ‘‘An Environmentally Friendly Process Involving Refining and Membrane Based Electrolysis for Magnesium Recovery from Partially Oxidized Scrap Alloy’’ by Xiaofei Guan, Uday B. Pal, and Adam C. Powell. They describe refinement of partially oxidized Mg-Al alloy to recover magnesium using a flux containing MgF2 and CaF2. The second mode for refining magnesium, the fluxless process, includes salt furnace technologies, gas filtering vacuum distillation, and hydrometallurgy. The methodology described by Fabrizio D’Errico, Gerardo Garces Plaza, Markus Hofer, and Shae K. Kim would be an example of a flux-free method. In their article titled ‘‘Final Assessment of Pre-industrial Solid-state Route for High Performance Mg-System Alloys Production: Toward Conclusion of the Green Metallurgy EU Project,’’ use of magnesium chips in the production of new alloys containing CaO has been discussed alongside commercialization of such a process. The third contribution is entitled ‘‘Life Cycle Assessment of Recycling of Magnesium Vehicle Components’’ by Simone Ehrenberger and Horst E. Friedrich. It looks at the life-cycle assessment of using magnesium alloys in automotive components. Chamini L. Mendis and Alok Singh are the guest editors for the Magnesium Committee of the TMS Light Metals Division and coordinators of the topic ‘‘Magnesium Recycling: To the Grave and Beyond’’ in this issue. JOM

Kinetics of Dissolution and Isothermal Solidification for Gold-Enriched Solid–Liquid Interdiffusion (SLID) Bonding

This paper presents the development and characterization of a fluxless die-attach soldering process based on gold-enriched solid–liquid interdiffusion (SLID). Eutectic Au-Sn and pure Au were deposited by jet vapor deposition (JVD) onto two substrates, assembled in a sandwiched structure, and processed in a vacuum furnace using different temperatures and times. Microstructural characterization, based on scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) analysis, revealed the formation of sound joints governed by the interdiffusion of the main constituents. Kinetic studies for the dissolution and the isothermal solidification stages were conducted. Differential scanning calorimetry (DSC) revealed a solder joint that is thermally stable up to 498°C, thus demonstrating the effectiveness of using the SLID process for the production of joints which require a lower processing temperature compared with their remelting point. Based on these findings, the recommended final bonding parameters are processing temperature and time of 340°C (310°C < TP < 340°C) and 5 min, respectively.

Fluxless tin bonding of silicon chips to iron substrates

A fluxless process of bonding silicon (Si) chips to iron (Fe) substrates was successfully developed. Iron substrates are plated with Ni, followed by tin (Sn). Si chips are deposited with thin chromium (Cr) and gold (Au) using E-beam evaporation in high vacuum. The Si chip is placed over the Fe substrate and bonded at 240 °C. The resulting joint is very uniform and contains few voids as examined by scanning electron microscopy. The reactions between Fe/Ni and Sn and between Si/Cr/Au and Sn during the soldering process are studied. The Si/Cr-Sn interface is free from intermetallic compound (IMC), i.e., IMC-free. Three-dimensional Ni3Sn4 microstructure is revealed by etching Sn away from the samples. The Fe/Ni–Sn interface forms rod-shaped or polygonal-shaped Ni3Sn4 microstructure. This new fluxless bonding process should be valuable in expanding the use of iron to electronic packaging applications such as substrates, base plates, thermal blocks, and chip-attachable casings.

Low-Temperature Bonding to 304 Stainless Steel for High-Temperature Electronic Packaging

In this paper, silicon (Si) chips were bonded to 304 stainless steel (304 SS) substrates using silver-indium (Ag-In) binary system without any use of flux. 304 SS substrates were also bonded to 304 SS substrates to develop a low-temperature fluxless process to seal two 304 SS parts together. In the bonding design, Ag and In were deposited separately in a layered structure. Various processes and solutions were experimented to plate Ag on 304 SS. So far, we have not found a process that could plate Ag directly on 304 SS without an intermediate layer. At present, the most successful intermediate layer seems to be nickel (Ni). Thus, Ni was plated on 304 SS, followed by Ag. The resulting 304 304 SS substrates were annealed to increase Ag grain size if grain growth is needed for successful bonding. Nearly joints were produced on Si to 304 SS bonding and 304 SS to 304 SS bonding. The resulting joints consist of Ag, Ag-rich solid solution (Ag), Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> In, and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> In. The joints were fabricated at only 190°C. Yet, the resulting joints exceed 660°C in melting temperature. This new bonding technique should be valuable for packaging electronic devices that need a high operating temperature. It is also useful for bonding 340 SS parts together at low temperature.

Optimization of a Flux-Less Metal Thermal Interface Material by Reflow in a Vacuum Furnace

This paper describes the work performed with a pure metal thermal interface material (TIM) for the sole purpose to improve the transfer of heat from the die to the metal cover case. A flux-less reflow process is employed in order to reflow the indium TIM material. This operation is performed in a vacuum furnace utilizing heat, vacuum, and pressure in a specific sequence in order to wet the metal lid and the backside of the flip chip die. The initial objective was to demonstrate minimal voiding of the TIM and subsequently limited flow out of molten solder from and along the sides of the die. A series of experiments were employed where acceptance criteria is evaluated by a) X-Ray, b) scanning acoustical microscopy (SAM), and c) cross-section. Acceptance criteria consists of 1) indium wetting of both lid to indium interface and indium to silicon interface die, 2) indium bond line (BLT) thickness, 3) lid tilt, and 4) lid shear strength. Acceptance is determined after a subsequent 4X ball grid array (BGA) reflow in a conventional belt reflow furnace with minimal voiding, no popcorn or blistering of the laminate substrate, and TIM thickness and solder flow out at sides of the die within the acceptable limits of the above mentioned criteria.

Gold in Flux-less Bonding: Noble or not Noble

ABSTRACTThis work highlights the solder joints reliability issues emerged during the development of a novel compliant contacting technology. The peculiar process in this technology is a mechanical lifting procedure in which a pulling force is exerted onto 63Sn-37Pb (eutectic) solder joints (realized by a flux-less thermo compression process), releasing metal traces from the substrate, to form free standing vertical structures. Since joints mechanical characteristics are critical for the successful fabrication of contacts, different bonding conditions (inert or forming atmosphere, temperature rates) and surface finishing (electroplated gold and preformed solder) have been tested. SEM and EDX analyses have been performed on failing joints to investigate failure causes and classify defect typologies. A constantly higher failure rate (percent number of failing joints) has been observed on gold finished surfaces. Analyses proved that such unusual rate was due to contamination of gold surface left by additives in the plating bath and to the embrittlement caused by gold diffusion into molten solder. Plating additives contamination reduces the wettability of gold surfaces. Concentration values of 3 wt.% for gold, considered safe for surface mount applications, caused embrittlement in solder bumps of 20-40 μm diameters.

Fluxless Bonding of Large Silicon Chips to Ceramic Packages Using Electroplated Eutectic Au/Sn/Au Structures

A fluxless process of bonding large silicon chips to ceramic packages has been developed using a Au-Sn eutectic solder. The solder was initially electroplated in the form of a Au/Sn/Au multilayer structure on a ceramic package and reflowed at 430°C for 10 min to achieve a uniform eutectic 80Au-20Sn composition. A 9 mm × 9 mm silicon chip deposited with Cr/Au dual layers was then bonded to the ceramic package at 320°C for 3 min. The reflow and bonding processes were performed in a 50-mTorr vacuum to suppress oxidation. Therefore, no flux was used. Even without any flux, high-quality joints were produced. Microstructure and composition of the joints were studied using scanning electron microscopy with energy-dispersive x-ray spectro- scopy. Scanning acoustic microscopy was used to verify the joint quality over the entire bonding area. To employ the x-ray diffraction method, samples were made by reflowing the Au/Sn/Au structure plated on a package. This was followed by a bonding process, without a Si chip, so that x-rays could scan the solder surface. Joints exhibited a typical eutectic structure and consisted of (Au,Ni)Sn and (Au,Ni)5Sn phases. This novel fluxless bonding method can be applied to packaging of a variety of devices on ceramic packages. Its fluxless nature is particularly valuable for packaging devices that cannot be exposed to flux such as sensors, optical devices, medical devices, and laser diodes.

Fluxless Bonding of Silicon to Ag–Copper Using In–Ag With Two UBM Designs

A fluxless process of bonding silicon to Ag-cladded copper using electroplated In-Ag multilayer structure is developed. The Ag cladding on the copper substrate is a stress buffer to deal with the large mismatch in coefficient of thermal expansion (CTE) between semiconductors such as Si (3 ppm/degC) and Cu (17 ppm/degC). To manufacture Ag on copper substrate, two techniques are developed. The first is an electroplating process to fabricate a thick Ag layer. The second technique is a novel laminating process that bonds Ag foil directly on Cu substrate. On Si chips, two underbump metallurgy (UBM) structures are designed, Si/Cr/Au and Si/Cr/Ni/Au. To produce a solder layer, Si chips are electroplated with In followed by thin Ag. The thin Ag cap layer prevents oxidation of the inner In region. To achieve a fluxless feature, the bonding process is performed in a vacuum environment (50 mtorrs) to suppress indium oxidation. Compared to bonding in air, the oxygen content is reduced by a factor of 15 200. Using Cr/Au UBM structure, the silicon chip was detached from Cu substrate. The broken interface lies between Si/Cr and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> In IMC on Cu substrate. Using a new UBM design of Si/Cr/Ni/Au, high-quality joints are produced that comprise of three distinct layers of In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub> Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> In , and Ag. Microstructure and composition of the joints are studied using a scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX).

Fluxless Wafer Bonding in Vacuum Using Electroplated Sn–Ag Layers

A wafer-to-wafer bonding process using Sn-Ag solder without any flux is successfully developed. This fluxless or flux-free feature makes void-free and uniform bonding layers possible. This is in contrast to the fluxing process employed in nearly all soldering processes adapted in the electronic industry. With the use of flux, the flux or flux residues are easily trapped in the solder joint, resulting in voids and uneven solder layers. This is particularly true if the bonding area is large, such as the entire wafer. Thus, void-free wafer bonding using solders has never been reported. It is thus clear that the key to achieve void-free wafer soldering is to eliminate flux completely. The new fluxless process is performed in a vacuum furnace built in house to inhibit solder oxidation. To prevent oxidation during solder manufacturing, a thin Ag capping layer is plated over the Sn layer right after the Sn layer is plated over an entire 2-in silicon wafer having Cr/Au under bump metallurgy (UBM). This outer Ag layer is critical in preventing the inner Sn layer from oxidation when the wafer is exposed to air. The Si wafer with Cr/Au/Sn/Ag structure is bonded with another Si wafer with Cr/Au at 240degC in the vacuum furnace. To evaluate the joint quality and study the microstructure and composition, scanning acoustic microscopy (SAM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) are used. A solder joint with only 1% void area is accomplished. The initial success of this process illustrates that it is indeed possible to bond entire wafers together with a thin metallic joint of high quality. This fluxless bonding technique can be extended to bonding wafers of different materials for new device and packaging applications.

Fluxless flip–chip Sn [sbnd]Au solder interconnect on thin Si wafers and Cu laminated polyimide films

Initial success of new fluxless flip–chip interconnect technique is reported. Non-eutectic Sn-rich Sn Au solder bumps are used to connect thin silicon chip to glass substrate and Cu laminated polyimide film, respectively. Fluxless process development on tin-rich alloys is challenging because tin atoms get oxidized easily. The solder bumps of Sn Au multilayer composite with overall composition of 95 at.% Sn and 5 at.% Au are fabricated by vacuum deposition on thin silicon wafers to prevent solder oxidation. The bonding is performed at 250 °C in hydrogen environment without using any flux. The diameter of the bump is 200 μm and height is 20 μm. Nearly void-free solder joints with randomly distributed AuSn 4 intermetallic compound (IMC) are produced using this fluxless bonding process. Continuous layers of Cu 3Sn and Cu 6Sn 5 instead of commonly known scallop-type Cu 6Sn 5 are formed between laminated Cu layer and the Sn-rich non-eutectic Au Sn solder. The re-melting temperature of solder joints is measured to be 220 °C. The bonding process is entirely flux-free and thus is particularly valuable for many devices that cannot take any flux, such as biomedical, photonic, MEMS, and sensor devices.