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The reduction behavior of the surface oxide on Cu particles under a reducing gas atmosphere was investigated for a pressureless sinter joining. We conducted x-ray thermodiffraction analysis and simultaneous thermogravimetry, differential thermal analysis, and mass spectroscopy (TG–DTA–MS) under a reducing atmosphere to investigate the reduction and subsequent sintering behaviors of copper particles at different oxygen concentrations. The shear strength of the pressureless sinter joint decreased with increasing oxygen concentration. The thermodiffraction results revealed that the reduction onset of Cu2O started at the same temperature (220°C), whereas the reaction markedly persisted at higher oxygen concentrations. Cu sintering progressed significantly after the reduction due to generation of Cu nanoparticles. The TG–DTA–MS results indicated that H2O formation temperature associated with the reduction depends on the oxygen concentration, consistent with the thermodiffraction results. The surface oxides were found to play an important role in pressureless sinter joining via nanoparticle formation, while the presence of a large amount of oxide delayed the reduction and subsequent sintering.

Pressureless sinter joining of bare Cu substrates using submicron Cu particles was successfully achieved at 250–300 °C in N2–3%H2 forming gas atmosphere via the in-situ generation of Cu nanoparticles reduced from a thin oxide layer present on the surface of Cu particles. The joining strength of the Cu joints at 300 °C reached 26.2 MPa in the forming gas atmosphere, while it was 7.4 MPa under N2 gas atmosphere. Thermogravimetry–differential thermal analysis and X-ray diffraction showed that the natural oxide layer (Cu2O) started to reduce to Cu at approximately 220 °C only under the N2–H2 gas atmosphere. Cu sintering progressed rapidly above the reduction temperature, and the joining strength improved with increasing temperature. Transmission electron microscopy demonstrated the generation of Cu nanoparticles around the submicron Cu particles and the subsequent promotion of sintering behavior. These results suggest that reduction of the initially present oxide layer on submicron Cu particles is crucial for pressureless sinter joining. Based on the reduction and sintering behavior of submicron Cu particles, pressureless sinter joining was realized at 250 °C by increasing the holding time, and the joining strength was > 18 MPa.

Thermal stability and characteristic properties of the microstructure of sintered Ag layers were examined with morphological, crystallographic and mechanical analyses. The sintered Ag layer was prepared by a pressureless sintering procedure with an Ag paste including Ag nanoparticles having a diameter distribution of 50–300 nm. The microstructure of the sintered Ag had a porosity of 8% and was morphologically and crystallographically dense. Cross-sectional SEM images showed that pores having a diameter of 0.2–1.0 μm were homogenously located in an Ag matrix. The dense microstructure remained intact after storage at 250 °C and for 1000 h. Shear strength of the sintered Ag layers was over 40 MPa and elastic modulus of the sintered Ag layer was 14 GPa before and after the storage tests at 250 °C and for 1000 h.

We conduct high-resolution microscale tensile testing of galvannealed steel/Al–Mg–Si alloy dissimilar joints and find that the interfacial reaction layer is sufficiently strong to induce microscale aluminum fractures. The interfacial reaction layer with the 5-µm-thickness generated by friction stir spot welding contains Fe–Al intermetallic compounds (IMCs) with grain diameters of 10 nm to 2 µm, Zn–Mg compounds present around the Fe–Al IMC, and microscale defects within the layer. The microtensile test demonstrates the slip fracture of aluminum regardless of defects present within the reaction layer. The maximum stress withstood by the specimen is 309 MPa.

Micron-scale Ag2O paste facilitates direct sintering bonding to silicon and silicon ceramic substrates because of in-situ formation of Ag during the decomposition of Ag2O. However, it remains difficult to obtain a high-strength joint at low bonding temperatures using the conventional process (even at 300 °C) despite their good performance at temperatures above the thermal decomposition temperature. In the present study, the dominant factors for low-temperature direct sintering bonding of Ag to Si and SiC substrates using Ag2O decomposition were investigated by controlling the preheating temperature and evaluating the bondability. The influence of preheating temperature was evaluated using thermal analysis and the assessment of joint strength. Lower preheating temperatures led to higher joint strength at lower bonding temperatures. It was observed that Ag nanoparticles were generated during the preheating process at high temperatures, indicated from the occurrence of the exothermic Ag2O decomposition reaction. However, the initial formation of Ag nanoparticles during the preheating process did not contribute to the sintering process. We propose that lowering the preheating temperature plays an important role in precisely controlling the amount of organic solvent required for the decomposition reaction of Ag2O. Finally, we confirmed that the low-temperature preheating process realized a sound joint for the Ag sintering bonding to SiC, even at 300 °C.

We successfully used CoCrW alloy as the feedstock for directly shaping wear-resistant products by additive manufacturing. We present a guideline for forming substantive, high-density, crack-free CoCrW alloys via laser metal deposition (LMD) and evaluated the relationship between the microstructure and mechanical properties. CoCrW alloy with a scale of several tens of cubic centimeters has not been previously reported because the brittle material undergoes cracking due to residual stresses. Densification and crack suppression were achieved by controlling the oxygen concentration in an Ar atmosphere and preheating the base plate to 400 °C. The tensile strength (MPa) of the LMDed CoCrW alloy was 1492 ± 141 along the scan direction, 1464 ± 234 along the overlap direction, and 1359 ± 72 along the build direction. The overlap direction had almost the same strength as the scan direction. In the microtensile test for the overlap direction, the fracture ratio in the overlap region was only 17.4%, and it was found that softening in this region did not contribute to the decrease in strength.

Transient liquid-phase sintering (TLPS) using a Cu-solder-resin composite for the die-attach application of high-temperature silicon carbide (SiC) power modules was evaluated with the goal of controlling the joint stiffness. The Cu-solder-resin composite mainly contains Cu particles, Sn–3Ag–0.5Cu solder particles, and polyimide-type thermosetting resin. Microstructural observations, shear strength tests, and thermal cycling tests of the SiC die-attached specimens bonded through the pressureless reflow process at 250 °C for 1 min were carried out after preheating at 100 °C for 60 min in a nitrogen atmosphere. A skeleton-shaped microstructure, consisting of Cu and Cu–Sn intermetallic compounds (IMCs) partially filled with polyimide resin, was observed in the joints, which exhibited shear strengths exceeding 12 MPa. The thermal cycling tests revealed that the TLPS specimens showed a superior reliability through 1200 thermal cycles in the temperature range from −55 °C to 175 °C. These experimental results were supported by mechanical finite-element (FE) simulations of simplified models that consist of the skeleton-shaped microstructure between the SiC chip and the substrate against an external force to mimic the shear test condition. The FE analyses proved that the stiffness of the composite joints strongly depends on the interconnection density that results from the formation of IMCs between the particles.

In this study, the synergistic effects of a hybrid surface treatment involving hydrochloric acid (HCl) immersion and silanization on friction stir spot welding (FSSW) of aluminum and carbon fiber reinforced thermoplastic (CFRTP) were investigated. Maximum tensile shear strength of 10.2 kN and maximum cross-tension strength of 1.92 kN were achieved. An unloading test and a miniature tensile test revealed that the fracture behavior changed from interfacial to CFRTP fracture due to increased interfacial strength with the hybrid treatment and utilization of the CFRTP strength enhanced the joint strength. The nanostructures at the interface with HCl immersion treatment indicated that the interfacial properties were improved mechanically in addition to the chemical enhancement by silanization. Owing to the hybrid treatment, aluminum and CFRTP were efficiently joined by the adhesion behavior of spreading melted plastic resin during the FSSW process.

Yoshida M., Nishibata I., Matsuda T., et al., Journal of Applied Physics, 132(7), 075101 (2022) https://doi.org/10.1063/5.0083511, This study aims to investigate the influence of the pulse duration on the mechanical properties and dislocation density of an aluminum alloy treated using dry laser peening (DLP), which is a laser peening technique that uses ultrashort pulsed laser-driven shock wave to eliminate the need for a sacrificial overlay under atmospheric conditions. The results of the micro-Vickers hardness test, residual stress measurement, and dislocation density measurement demonstrate that over a pulse duration range of 180 fs to 10 ps, the maximum peening effects are achieved with a pulse duration of 1 ps. Moreover, the most significant DLP effects are obtained by choosing a pulse duration that achieves a laser intensity that simultaneously generates the strongest shock pressure, suppresses optical nonlinear effects, and realizes the least thermal effects, which weaken the shock effects. Shock temperature calculations based on thermodynamic equations also suggest that a laser intensity driving a shock pressure less than 80 GPa, as in the case of a pulse duration of 1 ps in this study, maintains the solid state of the material throughout the process, resulting in significant DLP effects.

Joining of 5083 and 6063 aluminum alloys to 304 stainless steel was successfully achieved by high-frequency linear friction welding working at 245 Hz with varied friction pressure and time, which affect the heat input. Increasing the friction pressure and friction time resulted in the softening of aluminum alloys, which facilitated plastic flow and subsequent squeezing behavior. The morphology of the intermetallic compound layer, dependent on the plastic flow, determined the performance of 5083 aluminum alloy/steel joints. Excessive friction pressure and time led to inefficient welding in 6063 aluminum alloy/steel joints because the softening and squeezing behavior of the heat-treatable alloy was susceptible to the heat input. Highly efficient joints were obtained in a short duration, i.e., 1 s, through the formation of a thin intermetallic compound layer at the interface below 500 and 50 nm for 5083 and 6063 aluminum alloy/steel joints by controlling the appropriate friction pressure. High-frequency linear friction welding was demonstrated to be effective for the sound joining of various materials by controlling the friction pressure.

We succeeded in improving the fatigue properties of friction stir welded 7075-T73 aluminum alloy, e.g., prolonging the fatigue life at lower stress amplitude as compared to the base metal, using femtosecond laser peening, which was performed in air without any overlays such as a protective coating and plasma confinement medium, which are necessary for conventional nanosecond laser peening. The results of the plane bending fatigue tests with a stress ratio of R= -1 demonstrated that femtosecond laser peening enhanced the fatigue strength of the friction stir welded specimen at 106 cycles by approximately 15 MPa compared with that of the base metal, and prolonged the fatigue life of the friction stir welded specimen under cyclic loading at a stress amplitude of 200 MPa by approximately 3.7 times compared with that of the base metal. We suggest that dislocation generation, which occurs behind the shock front under femtosecond-laser-driven shock compression owing to its large shock amplitude, induces surface hardening and compressive residual stresses in a near-surface volume. They prevent the initiation of fatigue cracks on the surface of the specimen, resulting in improved fatigue properties.

Thin metallic Cu and Ni meshes were successfully embedded within SAC305 solder joints. Defects (residues) within the joint were removed by a vacuum step in the bonding sequence. The metallic mesh inlets were metallurgically bonded by the formation of scallop-like Cu6Sn5 and faceted Ni3Sn4 intermetallic compounds (IMCs) at the Cu mesh-SAC305 solder (CMS) and Ni mesh-SAC305 solder (NMS) interfaces, respectively. After process optimization, the averaged shear strengths were found to be 44.1 (reference), 44.7 (+1%), and 51.4 MPa (+16%) for SAC305, NMS, and CMS composite joints, respectively. The solder-substrate-mesh interaction resulted in a characteristic fracture pattern, with the cracking path partly within the solder and partly at the solder-mesh interface. Finite element (FE) simulations suggested a mesh-induced stiffening effect of the solder joint by 5%. The additional reinforcement (11%) introduced by the Cu mesh inlets was attributed to a locally enhanced fracture resistance. Keywords: Solder-mesh composite joints, Microstructure, Shear strength, Finite element modeling, Fracture

The relationship between the intermetallic compound (IMC) layer thickness including the deviation and joint strength was evaluated for the dissimilar joints between aluminum alloys and austenitic stainless steel. To evaluate the interface with various IMC layer thicknesses, the joints by friction stir welding were solution-heat-treated and artificially aged. The transitions of joint strength and fracture position were characterized for the IMC layer thickness ranging from 0 to 2.5 µm. Further, we demonstrated that the fracture position can be determined by the percentage of the joining area via the IMC layer thickness below a threshold value, which was 0.4 µm.

Double-pass process was newly performed to a friction stir welded (FSWed) A3003 aluminium alloy/SUS304 stainless steel dissimilar lap joint in order to improve the asymmetry of the FSWed joint. A sound symmetrical joint was produced by running the tool again at the bead which ˆrstly produced by FSW. Tensile strength of specimens was improved by leaving the tool distance from 2 mm to 5 mm where both edges of the stirred zone were advancing side (AS-AS joint). Micro-tensile test revealed that most of the stirred zone in the AS-AS joint with the tool distance of 5 mm fractured at A3003 base metal.

The purpose of the present study was to verify the effectiveness of dry laser peening (DryLP), which is the peening technique without a sacrificial overlay under atmospheric conditions using femtosecond laser pulses on the mechanical properties such as hardness, residual stress, and fatigue performance. After DryLP treatment of the laser-welded 2024 aluminum alloy, the softened weld metal recovered to the original hardness of base metal, while residual tensile stress in the weld metal and heat-affected zone changed to compressive stresses. The fatigue life almost doubled at a stress amplitude of 180 MPa and increased by a factor of more than 50 at 120 MPa. As a result, DryLP was found to be more effective for improving the fatigue performance of laser-welded aluminum specimens with welding defects at lower stress amplitudes., 本研究の目的は、ドライレーザーピーニング(DryLP)の有効性を検証することである。DryLPは、大気条件下でフェムト秒レーザーパルスを使用して、加工合金中の硬度,残留応力,疲労性能などの機械的特性を改善するピーニング技術である。レーザー溶接された2024アルミニウム合金にDryLP処理を施すと軟化した溶接金属は母材の元の硬度に回復し、溶接金属と熱影響部の残留引張応力は圧縮応力に変わった。疲労寿命は180MPaの引張圧縮試験でほぼ2倍になり、120MPaでは50倍以上増加した。以上から、DryLPは低応力振幅での溶接欠陥のあるレーザー溶接アルミニウム試験片の疲労性能を改善するのにより効果的であることを明らかにした。

Bonding between AlN and metals conventionally requires a surface modification process at high temperature such as metallization. The AlN-to-metal direct bonding process by sintering of Ag nanoparticles derived from in situ reduction of Ag2O microparticles mixed with diethylene glycol is examined. Bonding was conducted at 300–500 °C after a preheating process at 100 °C, and the shear strength exceeded 20 MPa for the joint bonded at 500 °C. The role of preheating in the direct bonding of AlN and Ag was identified. The Ag nanoparticles generated during preheating at 100 °C cover the AlN surface, and they are converted into a thin Ag film. The thin film promotes the formation of a bonding layer, owing to the effective adhesion of the sintered Ag to the film, and contributes to the bonding of Ag and AlN. No interfacial reaction layer is observed. The same bonding process can be applied to other ceramics with poor wettability.

High-frequency linear friction welding (HFLFW) with a 250 Hz oscillation frequency was applied on similar joining of AA5052 and AA6063, i.e., work- and precipitation-strengthened Al alloys, with 11 × 11 mm2 friction surface. Sound joints were achieved under a 30 MPa friction pressure and with processing times longer than 1 s. The thermomechanically affected zone (TMAZ) of AA6063 joints suffered from softening at a distance of 2–3 mm from the joined interface in contrast to the TMAZ of AA5052 joints. The pressure reduction to 7 MPa narrowed down the thickness of the softened TMAZ to 1 mm from the interface. A thermal finite element (FE) model was calibrated against in situ temperature measurements. Significant temperature differences of up to 300 °C at the welding interface were assigned to the process parameters as well as to the different thermal and mechanical properties of the Al alloys. Linear hardening and softening effects in AA5052 and AA6063, respectively, were quantified in hardness vs. temperature diagrams, and a pressure-hardening phenomenon was identified. A user subroutine was fed with these correlations, which visualized the hardness distribution over the near-interface volume for a low- and a high-pressure process.

This study introduces a novel concept that uses composite structures of thin metallic meshes and Sn3.0Ag0.5Cu (SAC305) solder alloys to interconnect semiconductor chips to DBC substrates. The feasibility was proven by bonding Cu-to-Cu substrates. The averaged shear strength was measured as 44.1, 44.7 and 51.4 MPa for samples bonded with SAC305 (reference), Ni mesh/SAC305 and Cu mesh/SAC305 composites, respectively. The solder-mesh joints revealed a specific fracture behavior where crack propagation occurred partly within the solder and partly at the solder-mesh interface. Microstructural analyses confirmed that the metallic meshes were bonded by the formation of intermetallic compounds (IMC) while almost no (larger) defects were found. The solder-mesh concept was subsequently applied on Si-to-DBC assemblies. A very good resistance of Cu mesh/SAC305 composite joints against cyclic temperature between 80 and 200 °C was observed when the bonded area only reduced by 4.2 % after 8000 cycles. Thermal finite element (FE) simulations indicated that in particular Cu mesh/SAC305 composites can significantly reduce the thermal resistance of the interconnections which is equivalent to a better heat dissipation through the bonding layer. Thus, solder-mesh composite joints seem to be an attractive solution for high-temperature applications up to 200 °C.

We developed High-Frequency Linear Friction Welding (HFLFW) operating at 250 Hz, which is larger than conventional LFW methods, to realize welding aluminum alloys. In order to establish the fundamental HFLFW process for the welding of Al alloys, the relationship between the mechanical properties and microstructure of the HFLFW joint was investigated for both 6063 and 5052 Al alloys, which have different strengthening mechanisms. Sound welds were achieved for both 6063 and 5052 Al alloys. The change in the shape of flash with increasing process time indicated that the material flow in the joint was largely affected by the material properties such as proof stress at high temperature and thermal conductivity. Further, the hardness increased and refined grains were observed at the interface of the 6063 Al alloy joints welded in the low-heat-input condition, which led to a higher joint strength at a shorter process time. The size of the refined grains was about 100 nm, as observed by transmission electron microscopy. Our findings reveal that a high frequency is effective and HFLFW is a promising process for the welding of precipitation-hardening aluminum alloys with a high joint strength owing to the less heat effect during the process. Keywords: Linear friction welding, Aluminum alloy, Precipitation hardening, Frequency, Microstructure, Mechanical properties

A bonding method utilizing redox reactions of metallic oxide microparticles achieves metal-to-metal bonding in air, which can be alternative to lead-rich high-melting point solder. However, it is known that the degree of the reduction of metallic oxide microparticles have an influence on the joint strength using this bonding method. In this paper, the reduction behavior of CuO paste and its effect on Cu-to-Cu joints were investigated through simultaneous microstructure-related x-ray diffraction and differential scanning calorimetry measurements. The CuO microparticles in the paste were gradually reduced to submicron Cu2O particles at 210–250°C. Subsequently, Cu nanoparticles were generated instantaneously at 300–315°C. There was a marked difference in the strengths of the joints formed at 300°C and 350°C. Thus, the Cu nanoparticles play a critical role in sintering-based bonding using CuO paste. Furthermore, once the Cu nanoparticles have formed, the joint strength increases with higher bonding temperature (from 350°C to 500°C) and pressure (5–15 MPa), which can exceed the strength of Pb-5Sn solder at higher temperature and pressure.

Metallization-free Ag sinter bonding to Si and SiO2 substrates was achieved via the thermal decomposition of Ag oxalate. Ag oxalate generated atomic-scale Ag through decomposition at approximately 200 °C and transformed into sintered Ag. Ag derived from Ag oxalate successfully bonded to Si substrates through close contact formation with Si surface oxide. Similarly, amorphous and α-quartz SiO2 substrates were directly bonded with Ag. These results indicate that the atomic-scale Ag generated on SiO2 contributed to interface formation over a large area. Moreover, the joints of the bare Si chip and metal substrate showed a maximum shear strength of 29 MPa at a bonding temperature of 250 °C and a bonding pressure of 5 MPa. The in-situ generation of Ag through the decomposition of Ag precursors can facilitate the direct bonding of electronic materials, making the proposed method a promising one for electronics applications.

Metal-to-ceramics direct sintering was carried out with Al2O3 and Cu / 3 μm Ni / 1 μm Au substrates. The bonding paste consisted of micron-sized Ag2O particles and a reducing solvent that provokes Ag2O-to-Ag reduction during processing accompanied by the formation of Ag nano particles. Five different sets of process parameters resulted in different joint microstructure and strength. The experimental data was used to calibrate an elasto-visco-plastic finite element model of the sintered assembly which yielded a quantitative damage function and criterion to predict the strength of direct-sintered joints. The developed ductile damage formulation introduced a parameter ξ, i.e. the product of equivalent creep strain and stress triaxiality, that controls the tolerable plastic strain at fracture. An extended numerical parameter study subsequently revealed the complex interaction between the joint strength and microstructural joint features. Thinner joints were found to provide a slightly higher strength while the amount of sinter material and costs is significantly reduced. Moreover, it is recommended to aim at a higher level of densification at the edges and corners of sintered joints since these areas apparently contribute more to the overall mechanical strength. The developed concept is capable of tailoring the microstructure of direct-sintered joints according to the requirements or vice versa.

Herein, we investigated the fracture behavior of thermally aged Ag–Cu composite sinter joints through nano-X-ray computed tomography (nano-CT) combined with microscale tensile testing. Nano-CT results showed that the joint comprised Ag/oxidized Cu flake sintered layer and Cu oxide, which was formed on the Cu substrate; further, pores were present at the interface between the substrate and Cu oxide layer. The tensile test showed that the stress increased with plastic deformation up to a maximum of 308 MPa, before a subsequent steep decrease due to crack initiation inside the sintered layer. The nano-CT and in situ observation results suggest that the microscale joint fractured through the interface between the oxidized Cu flake and Ag. Results obtained herein suggest that microscale tensile testing coupled with nano-CT is an effective approach to describe the intrinsic fracture behavior of joints.

In the present study, low temperature bonding of gold-to-alumina was investigated by the in situ generation of silver via a redox reaction between silver oxide microparticles and diethylene glycol. The shear strength of gold-to-alumina joints with a silver layer bonded at 400 °C was comparable to that of conventional methods that require a high bonding temperature. Microstructural observations revealed that the adhesion of silver nanoparticles, which are formed by redox reaction, on an alumina surface improves the strength leading to the fracture of alumina. High-resolution transmission electron microscopy and energy dispersive spectroscopy analysis showed the direct bonding between silver and alumina at the interface without diffusion or formation of a reaction layer. Additionally, x-ray photoelectron spectroscopy analysis at the interface showed that the bonding of silver and alumina could be attributed to the silver-oxygen bond, which revealed that silver ions generated during the redox reaction could contribute to the bond formation. Further, molecular dynamics simulation confirmed that the presence of silver ions promotes the formation of an initial silver layer, which plays a role in bonding silver nanoparticles to an alumina surface. As mentioned above, it was found that a bonding method utilizing the reduction of silver oxide by reducing agent involves a significant, unique process that realizes metal-to-alumina bonding at a low temperature.

Al2O3-to-Cu/3 μm Ni/1 μm Au assemblies were direct-sintered with a paste consisting of micron-sized Ag2O particles and a reducing solvent that provokes Ag2O-to-Ag reduction during processing accompanied by the formation of Ag nanoparticles. The variation of process temperature, pressure and time resulted in different porosity and shear strength. This experimental data set was used to calibrate a finite element model of the assembly considering the elasto-visco-plastic joint properties and the joint porosity. Local analyses within the sintered Ag layer yielded a damage function D f and a damage criterion that could assess the joint strength as a function of geometry, microstructure and process conditions. The developed damage model introduced a parameter ζ, i.e. the product of equivalent creep strain e cr,eq and stress triaxiality η, that balanced the tolerable equivalent plastic strain e pl,eq at fracture. A Fortran subroutine (UVARM) visualized the damage distribution over the sintered layer. The predominant crack initiation and propagation were identified in good agreement with the experimental fracture behavior. A numerical parameter study subsequently revealed the complex interaction between the ceramic–metal interfacial porosity, the total bulk porosity and the shear strength.

Al-rich Zn/Al/Zn clad solder were developed as Pb-free solder for a die-attachment. The Zn/Al/Zn clad solder was produced by clad rolling of Zn and Al strips in order to prevent Al from oxidation and improve wettability. The Zn/Al/Zn clad solder was melted at 382°C after solid-state interdiffusion of the Zn and Al layers. Bonding was successfully achieved with bonding pressure of a few kilopascals. Thermal cycle life of Invar-to-Cu substrate joint using the Zn/Al/Zn clad solder was longer than that of Pb-Sn-Ag solder. No Kirkendall voids were observed in the vicinity of the bonded interface after ageing at 250 °C for 1000 h.

We propose a room-temperature multiple-reaction bonding method for aluminum, by combining the thermite reaction of Al-CuO powder and self-propagating formation reaction of Ti-B powder with no external heat source. The joint microstructure consists in various composites (Al2O3, TiB2 and Al-Cu intermetallic), and their distribution affects the joint bonding properties. The multiple reaction leads to a compact joint layer with a very low porosity, in contrast to joints formed only by a formation reaction. This observation is explained by the inflow of molten aluminum metal into the joint layer during the joint formation, a process which is central in the formation of a sound joint. Furthermore, bonding between different composites in the joint is achieved by lattice defects at their interface. Analysis of shear tests show that fracture occurs in regions at the metal base/joint interface where Al2O3 particles were segregated. It is concluded that appropriate reactant design plays a significant role in controlling the microstructure, and thus, the bonding properties of the joint. Keywords: Self-propagating high-temperature synthesis, Room-temperature bonding, Composite structure, Aluminum alloys, Interface structure

Solder joints between leads and printed circuit boards in thin small outline packages were produced with conventional Sn1.0Ag0.7Cu (SAC107) and Sn3.0Ag0.7Cu (SAC305) solders as well as various solder alloys with gradually increasing amounts of Bi (up to 3.0 wt.%) and In (up to 1.0 wt.%) within the SAC107 base solder. The reliability of soldered leads in temperature cycle (TC) tests improved most with solder alloys containing both Bi (1.6 wt.%) and In (0.5 wt.%). Microindentation and electron probe microanalysis mappings revealed that the effect originates from a combination of solution and precipitation strengthening of the initial SAC alloy. The distribution of inelastic strain accumulation (ISA), as a measure for degradation, was determined in the solder joints by finite element calculations. It was shown that defects in the solder proximal to the lead ( 60–75 μm), which was underpinned by similar cracking characteristics along the lead–solder interface. The ISA was confirmed to be lower in SAC+Bi/In alloys owing to their enhanced elasto-plastic properties. Moreover, the addition of a thin Cu coating on the leads could improve the joint reliability, as suggested by the calculation of the ISA and the acceleration factor.