Fluxless Feature Research Articles

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.

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.

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 with Sn-rich Sn–Au dual-layer structure

We report our initial result of bonding two 2 in. silicon wafers using Sn-rich Sn–Au dual-layer structure that is produced by electroplating process. No flux is used in the bonding process. Comparing to Au-rich Au80Sn20 eutectic alloy, it is more difficult to achieve fluxless feature using Sn-rich Sn–Au alloys due to tin oxidation. In this initial effort, two samples are produced. The resulting Sn-rich solder joint layer, about 30 μm in thickness, is very uniform over the entire 2-in. sample. The quality of the joint is examined using scanning acoustic microscope (C-SAM) and X-ray micro-imaging technique. Images of these two techniques indicate that the joints are of high quality. Of the two samples, the better one shows nearly perfect joint with only 2% of possible void area. This initial success shows that it is indeed possible to bond entire wafers together with a thin metallic joint of high quality. This fluxless technique can be extended to bonding wafers of different materials for new device and packaging applications. The use of metallic alloy layers of high melting temperature is also possible and is being considered.

A fluxless fabrication process producing Sn-rich Bi–Sn joints with high melting temperature

Abstract A fluxless bonding process to produce tin-rich bismuth–tin solder joints is presented. The process was designed using Sn–Bi multilayer structure deposited in high vacuum to inhibit oxidation and to achieve the fluxless feature. The composition was chosen to be tin-rich, 95.0 wt.% Sn and 5.0 wt.% Bi, to achieve a solidus temperature significantly higher the 139 °C eutectic temperature of the 57Bi–43Sn eutectuic alloy. The resulting joints were imaged with a scanning acoustic microscope (SAM) and shown to be nearly void-free. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy were used to study the microstructure and compositions of the joints. The measured joint thickness was 4.1 μm. SAM and EDX results indicate that the solder joint consists of (β-Sn) matrix embedded with small grains of (Bi) solid solution phase. The melting temperature of the joint was measured to be 210–215 °C, as expected from the designed composition. The fluxless feature of this process gives the packaging community another process to consider for use in devices and packages where the use of flux is prohibited, such as some biomedical, MEMs, fiber optical and photonic devices.

Fluxless InSn bonding process at 140 °C

Device packages usually need more than one soldering operation to complete. For photonic and fiber optic device packaging, indium solder is often used due to its ductility. Since indium has a relatively low melting temperature of 156.6 °C, the subsequent bonding operation requires a process temperature that is lower than 156 °C . In this paper, we report a new bonding process at 140 °C based on an indium–tin multilayer composite. This is a fluxless bonding technique. In fabrication, a chromiurn–tin–indium–gold multilayer composite is deposited on silicon die in one high vacuum cycle to prevent oxidation. Immediately upon deposition, gold and indium react to form a stable AuIn 2 protective outer layer against oxidation. Silicon substrates are deposited with thin chromium–gold layers. The silicon die and substrate are bonded in a hydrogen environment at 140 °C. Scanning acoustic microscopy (SAM) analysis is used to evaluate the joint quality. This bonding technique consistently achieves uniform and void-free joints. Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analyses also are performed on the joint cross sections. The SEM image shows a uniform joint thickness of 5 μm and the joint microstructure. SEM and EDX results indicate the joint consists of In–Sn alloy with embedded AuIn 2 grains. The re-melting temperature of the joint is found to be 125–150 °C, which shows that the joint composition is not exactly eutectic that has a melting temperature of 118 °C, but rather is Sn-rich. The success of this design illustrates the solid-state interdiffusion between Sn and In to form a thin layer of Sn–In eutectic alloy. The fluxless feature of this technology is valuable for bonding and assembling many emerging photonic devices that simply cannot tolerate fluxes.

High temperature silver-indium joints manufactured at low temperature

A fluxless bonding process using indium-silver multilayer composite to produce high temperature joints at relatively low temperature has been developed. The process temperature is 220°C. Joints, which are almost void-free, with melting temperatures of 210°C are first made. After further annealing at 130°C, the re-melting temperature increases to above 750°C. The technique, thus, provides a quantum jump to the post-processing temperature of component fabrication. The joints are examined using a scanning acoustic microscope to confirm the bonding quality. The joint cross-sections are studied using scanning electron microscopy and energy dispersive X-rays to find the microstructure and composition. Upon deposition, In reacts with Ag to form In–AgIn 2 composite. The intermetallic compound AgIn 2 prevents the In layer from oxidation in atmosphere. Thus no flux is needed. Besides the fluxless feature, the low process temperature would significantly reduce the stresses developed due to thermal expansion mismatch comparing to the otherwise high temperature process with temperature exceeding the melting temperature. The multilayer bonding method also facilitates precise control of the alloy composition and the joint thickness. The new technique should find applications in the emergent high temperature electronic devices.

Indium-copper multilayer composites for fluxless oxidation-free bonding

A 200 °C fluxless process is developed to produce InCu joints. The fluxless feature is achieved by preventing indium oxidation during composite fabrication and the subsequent bonding process. Indium and copper are deposited on an object in high vacuum to inhibit indium oxidation. Copper interacts with indium to form a CuIn intermetallic that further protects the inner indium from oxidation. For the specific design of this study, the resulting joints consist of CuIn intermetallic grains surrounded by pure indium as revealed by scanning electron microscopy with energy dispersive X-ray analysis. Scanning acoustic microscope examination indicates that the joints are nearly void free. This technology enables versatile control of alloy composition, thus leading to several remelting temperatures and various physical properties. The CuIn compound was clearly identified in this study even though it was not shown in two published CuIn phase diagrams.

Indium-Copper Multilayer Composite Solder for Fluxless Bonding

ABSTRACTA 200°C fluxless process is developed to produce In-Cu joints. The fluxless feature is achieved by the prevention of indium oxidation during the solder fabrication and the bonding process. Indium and copper are deposited on an object in high vacuum to inhibit indium oxidation. Copper interacts with indium to form CuIn compound that further protects the inner indium from oxidation. For a specific design, the resulting joints consist of mainly CuIn intermetallic grains surrounded by a small amount of pure indium as revealed by SEM with EDX. Scanning acoustic microscope examination indicates that the joints are nearly void-free. This technology enables versatile control of the alloy composition, thus leading to several remelting temperatures and various physical properties.