Soldering Time Research Articles

Influence of the Structure and Mechanisms of Intermetallic Phase Formation on the Strength Properties of a Newly Developed Solder Joint

The current development of soldering materials focuses on the properties of the solder itself, while the reliability of the joints formed after soldering is evaluated to a lesser extent. It is essential to understand the relationship between the structure and the strength of the solder joint obtained. This article shows that the properties of the material used for soldering only to a certain extent largely translate into the mechanical properties of the joint. The aim of this article is to emphasize the importance of the problem of the selection of the chemical composition of the solder with the simultaneous selection of the parameters of the soldering process, including the width of the solder gap for the selected strength properties of the connection. The purpose of this article is to emphasize that the selection of a chemical composition solder with a simultaneous selection of parameters of the soldering process, including the dimensions of the gaps of the soldered materials, are affected properties of the soldered joint. In this article, the main importance was focused on the chemical composition of a tin-based solder. The influence was analyzed, and the most favorable content of elements, such as Al and Cu, which create intermetallic phases, strengthening the soldered joint, was determined. The properties of the newly developed solder joint for alternator applications due to the specified conditions of the soldering process, including the width of the solder gap permissible for alternators, ensured the correct connections, the strength properties of which differed despite the use of the same soldering material and substrate material, as well as the soldering time and tip temperature. This article presents a change in the cracking model of a solder joint made using a newly developed material due to the width of the permissible solder gap in the production process of alternators.

Ultrasonic dissolution of solid Al in liquid Sn during soldering: Modeling and equation, trend prediction, accelerating effect

Soldering of ceramics/metals using an inactive commercial solder with the advantage of low cost has wide application prospects. The dissolution behavior of base metal could not be quantified, which has been a basic issue for the joining design. This work investigated the dissolution of the solid Al in liquid Sn with and without the ultrasound. The physical model for the dissolving process was established based on the experiments. The relationship equation of the average concentration of Al (Ca) and soldering time was derived by using the Nernst-Brunner principle. The key parameters of limiting solubility (C0L) and dissolution rate constant (K) were obtained through dissolving experiments of liquid Sn to solid Al. The calculation results show that the effect of soldering temperature, and thickness Sn layer on the Al concentration (Ca) and dissolution rate (dCa/dt) under ultrasound is weaker than that without ultrasound. Compared to the condition without ultrasound, the value K, the maximum value of Ca and dCa/dt was increased by 5.8 times, 4.5 times, and 52 times at 250 °C under the ultrasound action, respectively, and the activation energy of dissolution was reduced by 41 %. The mechanism of ultrasonically accelerating dissolution of the solid Al by the liquid Sn has been revealed by using the bubble dynamics principle. It will provide a guideline for the design of soldering ceramics/metals using an inactive commercial solder.

Effects of Ni addition on wettability and interfacial microstructure of Sn-0.7Cu-xNi solder alloy

Purpose This paper aims to study the effect of different Ni content on the microstructure and properties of Sn-0.7Cu alloy. Then, the spreading area, wetting angle, interface layer thickness and microstructure of the soldering interface was observed and analyzed at different soldering temperatures and times. Design/methodology/approach Sn-0.7Cu-xNi solder alloy was prepared by a high-frequency induction melting furnace. Then Sn-0.7Cu-xNi alloy was soldered on a Cu substrate at different soldering temperatures and times. Findings It was found that Ni made the intermetallic compounds in the Sn-0.7Cu solder alloy gradually aggregate and coarsen, and the microstructure was refined. The phase compositions of the solder alloy are mainly composed of the ß-Sn phase and a few intermetallic compounds, Cu6Sn5 + (Cu, Ni)6Sn5. The maximum value of 12.1 HV is reached when the Ni content is 0.1 Wt.%. When the Ni content is 0.5 Wt.%, the wettability of the solder alloy increases by about 15%, the interface thickness increases by about 8.9% and the scallop-like structure is the most refined. When the soldering time is 10 min and the soldering temperature is 280 °C, the wettability of Sn-0.7Cu-0.2Ni is the best. Originality/value It is groundbreaking to combine the change in soldering interface with the soldering industry. The effects of different soldering temperatures and times on the Sn-0.7Cu-xNi alloy were studied. Under the same conditions, Sn-0.7Cu-0.2Ni exhibits better wettability and more stable solder joint stability.

Low temperature soldering technology based on superhydrophobic copper microlayer

Cu–Cu soldering is realized under certain pressure and low temperature conditions by using a surface silver film to modify the copper microlayer structure, thus solving the problems of high thermal stress and signal delay aggravation caused by high temperature in the traditional reflow soldering process. The copper microlayer modified with silver film is obtained by electrodeposition. The surface substructure of the Cu microlayer is a nano cone-shaped protrusion. The diameter of the bottom of the cone is 500 nm∼1 μm, and the height of the cone is 1∼2 μm. The thickness of the silver film is about 320 nm, and the modification of the copper layer with silver film can effectively prevent the oxidation of the copper layer. Two silver-modified copper microlayers are placed in face-to-face contact as a soldering couple. A certain pressure and low temperature are applied to the contact area to realize the soldering and interconnection.The morphology of the soldered interface and the average shear strength of the soldered joints are analyzed by scanning electron microscopy, transmission electron microscopy and solder joint tester. It is found that under the optimal soldering parameters of soldering temperature 220 °C, soldering pressure 20 MPa and soldering time 20 min, the nano-conical projections of the Cu micrometer layer are inserted into each other to produce a physical blocking effect. The highly surface-meltable silver film effectively connects the surrounding copper layer as an intermediate buffer layer. The average shear strength of soldering joints is significantly increased. Heat treatment experiments have shown that the average shear strength can be effectively increased by heat treatment for an appropriate period of time. Prolonged exposure to heat has little effect on the average shear strength. With the special morphology of the copper microlayer structure and the nano-size effect of the silver layer, soldering can be done at low temperatures. The quality of the soldering interface is good and small soldering dimensions can be obtained.

Effect of Microwave Hybrid Heating on Mechanical Properties and Microstructure of Sn3.0Ag0.5Cu/Cu Solder Joints

Microwave hybrid heating (MHH) has become soldering’s alternative method for lead-free solder alloys due to its benefits towards modern microtechnology, such as shorter processing time, lower energy consumption and lower defect rate. Nonetheless, it still requires susceptors to improve its heating performance, such as SiC, which is known for its high loss factor under low microwave frequencies. In this study, the effect of microwave hybrid heating on mechanical properties, as well as the microstructure of solder joint between Sn3.0Ag0.5Cu (SAC305) solder alloy and Cu substrate was investigated. Solder joint was created using MHH with different soldering parameters (amount of SiC in a range of 3-7g and exposure time in a range of 7-10min) between SAC305 solder alloy (in the form of wire and paste) and Cu substrate. Then, a lap shear test was carried out following a standard of ASTM D1002 to determine solder joint strength. Characterization was made using an optical microscope and scanning electron microscopy. Results showed that solder wire produced the highest solder joint strength with the value of 115.45 MPa when using 3.05g of SiC for 8.92min soldering time. Meanwhile, the solder paste produced 109.76MPa solder joint strength when using 3.03 g of SiC for 9.39 min soldering time. The intermetallic compound (IMC) form was scallop-like Cu6Sn5, both solder/substrate joints with a thickness of 2.87 μm for solder wire and 3.62 μm for solder paste. Nonetheless, an excessive amount of SiC would generate more heat in MHH and increase the IMC thickness as well as reduce shear strength, which eventually decreases the solder joint stability.

Formation of ductile close-packed hexagonal solid solution on improving shear strength of Au–Ge/Cu soldering joint

The morphology, chemical composition, mechanical properties of the interface reaction products and their effects on the shear strength of the Au–Ge/Cu soldering joint were systematically investigated. The results showed that a close-packed hexagonal (HCP) structure solid solution phase was formed at the interface after soldering at 400 °C for 5–60 min. The composition of the HCP phase varied among 78–46 at.% Cu, 9–42 at.% Au and ~12 at.% Ge. The Young’s modulus and hardness of HCP phase were 105–112 GPa and 4.3–4.7 GPa, respectively. The shear strength of the 5 min soldering joint was 57 MPa; however, it increased to 68 MPa when the soldering time was prolonged to 60 min. The increasing shear strength can be ascribed to the formation of HCP ductile phase and its effect on depleting the brittle (Ge) phase. It suggested that a HCP structured solid solution formed at the interface could enhance the shear strength of the solder joint.

The Research on Characteristics of SAC305/Sn-37Pb/ENIG Solder Joint

The reliability of the mix alloy of lead and lead-free components with SnPb paste is an important issue for military electronics. With the increase of density in PCB assembly, the traditional HASL pad has been gradually replaced by the ENIG pad for its better flatness and solderability. But the relationship between the interfacial morphology, microstructure and reliability of the alloy of lead and lead-free components still needs to be further studied. In this paper, the microstructure on the interface of SAC305/ Sn-37Pb/ ENIG solder joint is compared with that of typical SAC305/ Sn-37Pb3/ Cu solder joint, Sn63Pb37 / ENIG solder joint. It is found that the IMC layer on the interface of SAC305/Sn-37Pb/ENIG solder joint is more closer to compound (Cu, Ni) 6Sn5, and limited reflow soldering times has little effect on the strength of SAC305/Sn-37Pb/ENIG solder joint. Based on the case study, the thickness of gold layers has a significant effect on black pad problem in ENIG pad.

Intermetallic compound evolution and mechanical properties of Cu/Sn58Bi/Cu 3D structures blended with B4C nanoparticles during isothermal aging

The Cu/Sn58Bi/Cu and Cu/Sn58Bi-0.6B4C/Cu joints were prepared by transient liquid phase (TLP) bonding technology to verify the reliability of low-temperature connection joints. Intermetallic compound (IMC) evolution at the interface under different isothermal aging times was analyzed, and the mechanical properties of the joints were tested. The results show that two kinds of IMC, lamellar Cu3Sn and fan-shaped Cu6Sn5, are formed on both sides of the joint. With increasing soldering time, the thickness for both kinds of IMC become thicker, and it is significantly thicker for the IMC near the cold end. Furthermore, the IMC growth rate of the Cu/Sn58Bi-0.6B4C/Cu solder joint is significantly lower. When the isothermal aging time reaches 36 h, the Cu/Sn58Bi/Cu joint has been almost composed of IMC, while it needs a longer time for the Cu/Sn58Bi-0.6B4C/Cu joint. The growth of Cu3Sn IMC is in line with parabolic law, being mainly controlled by volume diffusion near the hot end, while the grain boundary diffusion significantly affects it at the cold end. The joints shear strength decreases gradually with extending bonding time, but it is always higher for the Cu/Sn58Bi-0.6B4C/Cu joint. When the heating time is not long, the fracture mainly appears at the Bi-rich phase in the solder matrix. With increasing bonding time, the location of fracture gradually transformed from the Cu6Sn5 grains to the junction of the two IMC and Cu3Sn grains, along with the intercrystalline fracture and transcrystalline fracture. In summary, B4C nanoparticles can significantly improve the joint reliability and enhance its mechanical properties.

Microstructure transformation and mechanical properties of Al alloy joints soldered with Ni-Cu foam/Sn-3.0Ag-0.5Cu (SAC305) composite solder

In this study, Ni-Cu/SAC composite solders with different Cu contents were used as an interlayer to solder Al alloy joints at 250 ℃ for different time. The results showed that the formation and evolution of the microstructure were affected by the Cu contents. In the Ni18Cu/SAC joint, the Sn solder was completely consumed to form a solid-phase joint in 5 min. And (Cu,Ni)6Sn5 was the initial phase formed from the reaction between Sn solder and the Ni18Cu skeleton. Due to the higher Ni concentration than (Cu,Ni)6Sn5 phase in the Ni18Cu skeleton, the Ni atoms diffused from Ni18Cu skeleton to the (Cu,Ni)6Sn5 phase and replaced the Cu atoms in it. As a result, the content of Ni atoms in (Cu,Ni)6Sn5 phase increased continuously, and transformed into (Ni,Cu)3Sn4 at 10 min. Phase transition also occurred in the Ni38Cu/SAC joints. However, the initial phase formed from the reaction between Sn solder and the Ni38Cu skeleton was (Ni,Cu)3Sn4. Therefore, the (Ni,Cu)3Sn4 phase was transformed into (Cu,Ni)6Sn5 at 10 min because the Cu atoms diffused from Ni38Cu skeleton to (Ni,Cu)3Sn4. On increasing the soldering time, the shear strength of the joints first increased and then decreased, the Ni18Cu/SAC joints soldered for 10 min exhibited the highest shear strength of 58.3 MPa, the shearing failure mainly happened in the composite solder layers.

Microstructure and mechanical performance of dissimilar material joints of 2024Al and SiO2 glass by ultrasonic assisted soldering with Cu interlayer

Ultrasonic assisted soldering was used to join SiO2 glass and 2024Al with Sn solder. The soldering temperature and pressure were set at 300 °C and 0.2 MPa respectively. The Sn solder had good wettability on the SiO2 glass surface by ultrasonic cavitation. At the same time, ultrasonic accelerated the dissolution of 2024Al in molten Sn solder. With the extension of ultrasonic time, Sn solder had better wettability on the surface of base material. The shear strength of the SiO2/Sn/2024Al joint was stabilized at 14–17 MPa by ultrasonic wetting for 5 min before soldering. Due to the large residual stress in the joint, the brittle fracture of the glass occurred during the shear process. To improve the load capacity of the joint, Cu interlayer was added to the joint to relieve the thermal stress. Cu foil effectively relieved the residual stress of the joint during the cooling process. Molten Sn solder reacted with Cu foil and formed intermetallic compounds (IMCs). IMCs effectively block the deformation of filler metal and reduce the generation of stress. When the ultrasonic assisted soldering time was 5 s, the SiO2/Sn/Cu/Sn/2024Al joint reached the highest shear strength about 28 MPa.

Experimental identification and prioritization of design and process parameters on hole fill in mini wave soldering

Mini wave soldering is a common method for soldering through-hole components on printed circuit boards (PCB). With PCB design becoming increasingly complex and copper layer thickness rising, the vertical hole fill of through-hole solder joints becomes a serious problem during manufacturing. At the same time, the process window is limited by limited material heat resistance of the PCB and the through-hole components. Especially critical is latent damage inside the PCB and the through-hole component which must not exceed its specific maximum core temperature because this would compromise reliability over lifetime. The choice of soldering parameters is critical to reaching sufficient vertical fill to achieve a reliable connection according to IPC-A-610. The dedicated description of the interaction between PCB design and vertical fill has the potential to considerably improve the understanding of the basics of design for manufacturing (DfM). In this paper, the essential process parameters of the selective wave soldering process are examined by design of experiment (DoE) with respect to hole fill, which determines mechanical and thermal reliability as well as current carrying capacity. The results show the individual effects of the copper layer design parameters including layer count, layer thickness, hole copper plating attachment and solder alloy on vertical hole fill, in combination with the solder process parameters. The copper layer design parameters, solder bath temperature and preheating temperatures show the strongest effects. The soldering time has the lowest impact within the tested process parameters. For a given solder joint with its defined heat capacity, the benefit of solder time diminishes the longer the contact time is. Component surfaces and connector pin material and machine parameters like pump power as well as environmental conditions show little to no impact within a reasonable operation window.

Trustworthiness of machine learning models in manufacturing applications using the example of electronics manufacturing processes

The design phase of products decides about a large portion of their costs. Both applied manufacturing technologies and systems are ultimately defined by the products that have to be assembled. Due to global trends towards e-mobility and regenerative energy, electronic printed circuit boards with high copper content and mixed SMT/THT assembly is the predominant assembly scenario. This leads to thermally challenging soldering operations in THT-soldering as high copper masses dissipate the required heat and no reliable manufacturability check is available. Consequently, in electronic manufacturing complex thermal processes have still to be defined in iterative experimental steps. Reliable models have great potential to reduce time-to-market and scrap during new product introduction. Future manufacturing systems require infrastructure that allows manufacturability checks in the early design phase and automatically provides process programs on the shop floor once the design is finished. However, the lack of availability, long-term experience and consequently trust in automatically, by artificial intelligence generated and optimized process programs, blocks the realization and hinders further productivity gain. In this paper, an ML audit methodology is suggested that mitigates this lack of trust by providing physically plausible prediction series and quantitative risk assessment to the user. This complements typical evaluation scores and adds the model behavior in the process context and transmits a sense of transparency. A neural network is trained with an augmented dataset, exemplary for the prediction of the hole fill in mini wave soldering process. The model is then used to predict series of predictions for increasing soldering time and temperatures comparable to design of experiments. The resulting time versus hole fill plots helps the operator understanding the behavior of the model and thus, demonstrate the potential benefit of this approach to the entire manufacturing site.

Low-Temperature Sealing of Titanium for Hermetic Implant Packages

Implantable medical electronics are almost exclusively packaged in titanium shells to ensure reliability and longevity. Additional silicone rubber encapsulation improves structural biocompatibility and limits the diffusion of ion from the body to package materials. These capsules must be hermetically sealed and provide electrical feedthroughs that are insulated against each other by connecting ceramic parts. In this study, a novel RoHS compliant low temperature soldering process is presented, allowing the required assembly. Tungsten/titanium as adhesion material and platinum as wetting layer were deposited by physical vapor deposition as thin-films and were structured by a rapid prototyping laser process. Different devices were soldered with a tin-silver alloy below 300 °C resulting in low thermal stress. The platinum acts as an excellent solder pad. A 50 nm tungsten/titanium adhesion promoter efficiently strengthens the bond to the titanium substrate. The results show that the platinum layer must be thicker than 300 nm to guarantee high bonding strength. Analyses of cross-sections of the joint show occurrence of diffusion of platinum and silver during the soldering process. At soldering times above 30 seconds, a large intermetallic layer forms at the soldering contact, which is prone to crack formation. Likewise, intermixing of tungsten and platinum were observed but without detrimental effects. Limitations in solder times are therefore advisable, for example by introduction of automatic processes. Soaking tests were performed with sealed capsules in water at 60 °C for one year without measurable increase of humidity inside the package. In the used accelerated aging model, this would correspond to a hermetic sealing of about 5 years at 37 °C.

Study on the microstructure and mechanical property of Cu-foam modified Sn3.0Ag0.5Cu solder joints by ultrasonic-assisted soldering

Cu-foam sheets with the thicknesses of 0.15 mm and 0.5 mm were utilized as strengthening structure to improve the performances of the Sn3.0Ag0.5Cu (SAC305)/Cu solder joints. The ultrasonic-assisted soldering was applied to manufacture the solder joints. The effects of Cu-foam and ultrasound-assisted soldering time on microstructure and mechanical property of solder joints were researched. Experimental results showed that excellent metallurgical bonding was generated between the copper substrate and the Cu-foam/SAC305 composite solder layer. The average thickness of interfacial intermetallic compound (IMC) layer decreased with the increase of ultrasonically soldering time, and a thinner interfacial IMC layer was found in the joints with ultrasonic treatment in narrow channels. With the ultrasonic soldering time prolonged, the Cu 6 Sn 5 grains became more refined. Some round-shape Cu 6 Sn 5 grains were transformed into hexagonal prismatic with the increase of the ultrasonic soldering time. The shear test was conducted for the solder joints under different soldering conditions. It was found that the shear strength of the Cu-foam/SAC305 composite solder joints were higher than that of SAC305 solder joints. The grain refinement caused by ultrasonic cavitation significantly improved the shear strength of the solder joints. Besides, the fracture behavior of solder joints without ultrasonic vibration was classified to be ductile-brittle mixed type while that with ultrasonic vibration was changed to be ductile type.

Ultrasonic-assisted soldering of aluminium and copper with Sn–Zn–Bi–Ag solder

The effects of ultrasonic-assisted soldering parameters on the interfacial microstructural evolution and shear strength of designed Al/Sn–9.77Zn–11.36Bi–1.46Ag/Cu solder joints were investigated. The ultrasonic-assisted soldering process promoted the dissolution of Al substrate and roughed Al/solder interface, which improved the pinning effect of Sn–9.77Zn–11.36Bi–1.46Ag on Al substrate. With an increasing ultrasonic soldering temperature, time and power, more Al atoms dissolved from Al substrate and migrated across solder to the opposite Cu substrate to form Al-rich (Zn) solid solutions. Consequently, Al4.2Cu3.2Zn0.7 formed between ϵ-AgZn3 and Cu5Zn8 layers at Cu substrate, resulting in thinner Cu5Zn8 and higher shear strength of solder joints. The highest shear strength of solder joints (38.6 MPa) was achieved after ultrasonic-assisted soldering at 300°C for 10 s under 9 W.

Thin Film Metallization Stacks Serve as Reliable Conductors on Ceramic-Based Substrates for Active Implants

Substrates and packages of active implantable medical devices are often fabricated from ceramics, such as alumina. Screen-printed PtAu paste is the state-of-the-art metallization for functional structures. Due to solid-state and liquid diffusion of Au during thermal exposure, solder times are limited. Otherwise, metal structures tend to delaminate. Moreover, it was shown that PtAu with solder fails after 37.4 years. We established a thin film metallization on the alumina process to overcome these disadvantages. We used sputtered platinum with an underlying adhesion layer made of tungsten–titanium to increase the adhesion strength of the alumina substrate. We avoided using gold in this work due to its high diffusion tendency. All used materials provided relatively low diffusion properties, which increases independence from joining techniques and mechanical longevity during use. Utilizing the Design of Experiment (DoE) methodology, we derived an optimal Pt thickness of 500 nm with 43 nm of WTi as an adhesion-promoting layer. After accelerated aging at 150 °C, corresponding to 125 years at body temperature (37 °C), the contact pad adhesion strength was 32.75 ± 7.08 MPa. This exceeded the safety limit of 17 MPa by far, set as a recommendation for robust screen-printing metallization processes. Soldering times of up to 120 s did not influence the adhesive strength. The new process reduced the minimum track distance to 50% of screen-printing values and is capable to be transferred into rapid prototyping techniques. It helps to make the assembly process independent of the manufacturing person in order to increase the yield of device fabrication and—most important in implantable device manufacturing—to make it more robust and thereby safer for the patient.

Diffusion reaction-induced microstructure and strength evolution of Cu joints bonded with Sn-based solder containing Ni-foam

Sn based composite solder, reinforced with low porosity Ni-foam, was used to fabricate high melting point Cu interconnects by soldering at 260 °C. The effects of soldering time on the microstructure and mechanical properties of the joints were investigated. The Sn matrix was rapidly consumed by the formation of (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 phases in under 10 min. With increasing soldering time, the (Cu,Ni)6Sn5 phase was largely transformed into the (Ni,Cu)3Sn4 phase, accompanied by the refinement in the size of the phases. The average shear strength of the joint soldered for 60 min was up to 76.9 MPa, which is significantly stronger than commonly reported for Cu joints soldered with Sn based alloys.

Improving the Efficiency of Induction Heating in the Air Gap of the Magnetic Circuit

Improving the efficiency of induction heating of parts in the air gap of the magnetic circuit is associated with the use of surface and edge effects. Through modeling in ANSYS Electromagnetics Suite 19.2 and experimental studies identified patterns of edge effect in the heated parts. To ensure the uniformity of induction heating of small parts and reduce the soldering time, the electrical switch of soldered parts is used, which with the help of device controller forms a secondary circuit with low electrical resistance and high density of eddy currents.

Microstructure and mechanical properties of Cu joints soldered with a Sn-based composite solder, reinforced by metal foam

In this study, Ni foam, Cu coated Ni foam and Cu-Ni alloy foams were used as strengthening phases for pure Sn solder. Cu-Cu joints were fabricated by soldering with these Sn-based composite solders at 260 °C for different times. The tensile strength of pure Sn solder was improved significantly by the addition of metal foams, and the Cu-Ni alloy/Sn composite solder exhibited the highest tensile strength of 50.32 MPa. The skeleton networks of the foams were gradually dissolved into the soldering seam with increasing soldering time, accompanied by the massive formation of (Cu,Ni)6Sn5 phase in the joint. The dissolution rates of Ni foam, Cu coated Ni foam and Cu-Ni alloy foams into the Sn matrix increased successively during soldering. An increased dissolution rate of the metal foam leads to an increase in the Ni content in the soldering seam, which was found to be beneficial in refining the (Cu,Ni)6Sn5 phase and inhibiting the formation of the Cu3Sn IMC layer on the Cu substrate surface. The average shear strength of the Cu joints was improved with increasing soldering time, and a shear strength of 61.2 MPa was obtained for Cu joints soldered with Cu-Ni alloy/Sn composite solder for 60 min.

Bonding and strengthening mechanism of ultrasonically soldered 7075 Al joint using Ni-foam/Sn composite solder foil

Ni-foam reinforced Sn-based composite solder was employed to join ultrafine grain 7075 Al alloy with the assistance of ultrasound. The solder seam was mainly composed of Ni skeletons, Sn-based solder, α-Al phase and very fine (hundreds of nanometers to several microns diameter) Ni3Sn4 particles. Discontinuous networks surrounded by the fine Ni3Sn4 and α-Al particles were in-situ formed in the solder seam, the diameter of which was decreased with increasing soldering time. Smooth transition of the lattice from Al substrate to Sn-based solder was achieved by the formation of an amorphous Al2O3 interlayer at the interface. The microstructure evolution mechanism and the amorphous Al2O3 interlayer formation mechanism are discussed in detail. The shear strength of Al/Ni-Sn/Al joints increased with prolonging soldering time. The Al/Ni-Sn/Al joint ultrasonically soldered for 60 s exhibits a shear strength of 71.4 MPa, which was approximately 37% higher than that soldered with pure Sn solder.