Tin Solder Research Articles

First-principles calculation of the effects of In doping on η-Cu6Sn5 structure, elastic anisotropy, electronic properties and fracture toughness

During the welding and service stages, the anisotropy of Cu6Sn5 between tin solder / Cu solder joints will lead to abnormal grain growth and fracture of Cu6Sn5, which will eventually lead to solder joint failure, so it is very important to study the alloying element indium (In) to reduce the anisotropy of Cu6Sn5. The effects of indium doping on the crystal structure, electronic structure and mechanical properties of η-Cu6Sn5 have been studied systematically by means of first-principles density functional theory. Through calculation, it is found that with the change of indium doping position, the crystal structure is slightly deformed, but remains stable. Indium doping changes the electronic structure of η-Cu6Sn5, and a new bonding peak is formed by the hybridization of Cu-d and In-s around -5.9eV. In addition, indium doping can significantly increase the mechanical properties of η-Cu6Sn5, including elastic modulus and hardness. Indium significantly reduces the anisotropy of η-Cu6Sn5, and the anisotropy decreases by nearly 80% at most. At the same time, the addition of indium can increase the fracture toughness of η-Cu6Sn5.

Site-selective core/shell deposition of tin on multi-segment nanowires for magnetic assembly and soldered interconnection

The field of nanotechnology continues to grow with the ongoing discovery and characterization of novel nanomaterials with unconventional size-dependent properties; however, the ability to apply modern manufacturing strategies for practical device design of these nanoscale structures is significantly limited by their small size. Although interconnection has been previously demonstrated between nanoscale components, such approaches often require the use of expensive oxidation-resistant noble metal materials and time-consuming or untargeted strategies for welded interconnection such as laser ablation or plasmonic resonance across randomly oriented component networks. In this work, a three-segment gold–nickel–gold nanowire structure is synthesized using templated electrodeposition and modified via monolayer-directed aqueous chemical reduction of tin solder selectively on the gold segments. This core/shell nanowire structure is capable of directed magnetic assembly tip-to-tip and along substrate pads in network orientation. Upon infrared heating in a flux vapor atmosphere, the solder payload melts and establishes robust and highly conductive wire–wire joints. The targeted solder deposition strategy has been applied to various other multi-segment gold/nickel nanowire configurations and other metallic systems to demonstrate the capability of the approach. This core/shell technique of pre-loading magnetically active nanowires with solder material simplifies the associated challenges of size-dependent component orientation in the manufacture of nanoscale electronic devices.

Study on the reliability of nano-silver-coated tin solder joints for flip chips

Abstract The nano-silver-coated tin (Sn@Ag) paste was modeled using the method of the Anand unified viscoplastic constitutive model and elastic model. After thermal cyclic loading, the plastic strain of the solder joints increased cumulatively, while the maximum equivalent stress remained basically stable. The simulation results show that the maximum displacement occurs at the solder joint at the farthest end from the center of the chip, which is a dangerous solder joint. Using the finite element method and EPRI (the American Electric Power Research Institute) estimation method, the fatigue life of the nano-silver-coated tin solder joint is predicted to be 616 weeks, which is significantly higher than that of the nano-silver solder joint. It is indicated that adding a certain amount of nano-tin to the nano-silver paste to form a core–shell structure can improve the shear strength of the solder joint and reduce the plastic strain, thereby significantly improving the reliability of the solder joint. It is proved by experiments that the nano-silver paste is feasible for flip–chip interconnection. The research on this topic provides experimental reference and theoretical basis for the application of a new generation of interconnection materials in power devices and promotes the development and application of microelectronic packaging technology.

A human-centric system combining smartwatch and LiDAR data to assess the risk of musculoskeletal disorders and improve ergonomics of Industry 5.0 manufacturing workers

More than one in four workers reportedly suffer from back pain worldwide, leading to 264 million work days lost yearly. In the U.S. alone, it causes $50 billion in healthcare costs every year, up to $100 billion if including decreased productivity and lost wages. The upcoming Industry 5.0 revolution will introduce human-centric manufacturing systems where workers’ well-being comes first while safeguarding their rights: privacy, autonomy, and human dignity. This paper presents a privacy-preserving system based on artificial intelligence that tracks the posture of assembly/disassembly line workers while performing typical standardized tasks on repeat: connecting and separating parts; screwing and unscrewing using an electric screwdriver; tin soldering and unsoldering. The proposed solution assesses the upper-body (dominant arm, dominant shoulder, and trunk) and lower-body (legs) postures according to the ISO 11226 European standard, based on inertial data recorded by a smartwatch and using Laser imaging Detection and Ranging (LiDAR), respectively. These techniques preserve privacy as they collect data that cannot reveal the worker’s identity or sensitive information. Experiments showed that the system recognized a worker’s posture with a mean accuracy close to 98%. The system could help detect bad posture habits and reduce the chances of musculoskeletal disorders while preserving the workers’ privacy, in compliance with the upcoming Industry 5.0 paradigm.

Immersion Tin Plating Enabling Reliable Wettable Flanks on QFN Packages

The automotive market increasingly uses bottom-termination IC packages, like Quad-Flat No leads (QFN) packages, for miniaturization, cost reduction and performance. QFN lead frames are electroplated with 8?12 um of tin for PCB soldering, but subsequent singulation exposes the copper-alloy lead frame at the cut edges. On these exposed lead flanks a solder fillet does not form, making Automatic Optical Inspection (AOI) of solder joints on lead-less packages not reliable. A solder fillet on lead flanks also improves the reliability of solder joints. Therefore, the automotive industry strives for a tin plated wettable flank on lead-less packages. Several approaches, e.g. dimpled or step-cut leads, to create a wettable flank are in use or development. Disadvantage of these methods is that only a part of the flank is tin-plated. Immersion tin plating processes are able to fully plate lead flanks. However, the maximum ~1.5um of standard self-limiting displacement immersion tin processes is too thin to comply with the durability requirements of automotive tin solder finishes. This contribution will discuss an immersion tin-plating process, machine concept and test results for a >2 um tin plated wettable flanks. The immersion tin plating process consist of the following process steps: 1. A high-pressure spray rinse prevents incomplete tin coverage by removing copper burrs formed during singulation. 2. A selective descaling process removes the natural copper oxide film on the exposed copper leads. 3. A selective activation process creates a high tin nucleation density to obtain a fine-grained tin layer. 4. The autocatalytic immersion tin process plates a 2 ? 3um tin layer in 45-60 minutes at 70-75oC. 5. An anti-tarnish process protects the tin layer against oxidation. Finally, rinsing and drying remove all traces of the used chemicals from the packages with tin plated flanks. A machine concept for the immersion tin-plating process is currently under development. The lead-less packages are processed directly on Standard singulation tape rings without repacking or manipulation of the packages. This enables a throughput of 20 tape rings per hour with the full package traceability required for the automotive market. Different types of QFN packages mounted on singulation tape were tin plated and subjected to standard (JEDEC J-STD-002E) solderability tests for component lead durability. Fine-grained 2-3 um thick pure tin deposit of good visual quality were obtained on the exposed QFN copper lead sidewalls with the following properties: • 0.5-1.5um residual tin layer after intermetallic formation in 8 h steam aging and 16 h oven bake. • 100% solder wetting after 6h dry bake at 150C • 100% solder wetting after 8h steam aging (93C and 80-90% RH). • 100% solder fillet height after ceramic glass test.

The design of low-temperature solder alloys and the comparison of mechanical performance of solder joints on ENIG and ENEPIG interface

In recent years, in order to adapt to the trend of low-temperature assembly and integration, low-temperature hybrid solders with multiple elements added to the tin solder matrix have been investigated. In this regard, the morphology and size of intermetallic compounds (IMCs) are altered by adding some alloying elements, and the strength of solder joints is improved to some extent. Different pad surface treatments affect the growth of IMC, which leads to various failure modes in interconnect solder joints. Therefore, it is necessary to study the interfacial reaction, microstructure and mechanical properties of hybrid solder and different metal pads to explore the growth mechanism of IMC and the failure modes of solder joints. In this paper, mixed Sn–3Ag-0.5Cu/Sn–58Bi low-temperature solder joints on Au/Ni/Cu (ENIG) and Au/Pd/Ni/Cu (ENEPIG) substrates are investigated. The results show that the addition of palladium element can change the IMC morphology, refine the grain size of the hybrid solder joints, and affect the fracture failure mode of the solder joints under shear force. The average shear strength is significantly higher than that of solder joints with ENIG surfaces under different process parameters. Thermodynamic calculations of enthalpy change and the effect of Jackson's parameter α value on IMC morphology are also investigated in this paper. The addition of palladium elements increases the α value and is more favorable to the formation of columnar grains. This provides guidance for the grain growth of IMC in hybrid solder joints with added alloying elements.

Kinetics of indium and indium–tin soldering materials in vacuum volatilization

Vacuum volatilization is a clean and efficient method for metal purification, alloy separation, and comprehensive recovery of secondary resources. The evaporation kinetics of In and In–Sn alloys were investigated to better understand the evaporation mechanism during the vacuum volatilization of In. The evaporation rates of the In and In–Sn alloys at 1173–1373 K and 5 Pa increased significantly with increasing temperature in accordance with the equation ω = e(a+b∙T). The evaporation rates at 1323 K and 5 Pa decreased with increasing crucible depth in accordance with the equation ω = a2+(a1-a2)/(1+e(h-b)/c). For In and In–Sn, the total mass-transfer coefficients of In were obtained at different temperatures and crucible depths. A theoretical kinetic model of In was established and limiting links were obtained vacuum volatilization of In and In–Sn.

Electrodeposition of Multi-Segment Nanowires and Site-Selective Solder Deposition for Magnetic Assembly and Nanoscale Joining

Electronic device packaging involves complex wiring assemblies for transport of electrical and thermal energy, enabling increasingly sophisticated circuit designs in an industry favoring aggressive device miniaturization. Nanoscale miniaturization of the wire morphology has become increasingly sophisticated in recent years with regards to synthesis; however, the ability to arrange and manufacture these small-scale materials to fully exploit their novel properties has been quickly outpaced. The purpose of this investigation is the establishment of a synthesis technique for a core/shell nanowire structure with tunable compositional templating via electrodeposition which can pass through this bottleneck via directed matrix assembly. A gold-nickel-gold nanowire structure was utilized as the core, synthesized via electrochemical deposition into an anodic aluminum oxide (AAO) template; the nickel segment was chosen to act as a magnetic vehicle for directed assembly, while the bordering gold tips were coated with a solder shell. Gold-selective deposition of this tin solder shell was achieved by establishing a chemisorbed nickel-protective monolayer via immersion in azelaic acid solution. Next, the protected nanowires were dispersed in an aqueous reaction vessel with excess sodium borohydride reducing agent. Controlled addition of tin sulfate and sodium dodecyl sulfate solution to the vessel resulted in the formation of tin seeds which deposited along the gold tips of the nanowires, growing into a “dumbbell” shape nanowire with tin deposited on both gold tips. Composition and confirmation of the solder deposition was performed with scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopic (EDS) analysis. Crystal structure of the core/shell nanowires was performed with x-ray diffraction (XRD) to identify intermetallic formation between the tin and gold. The core/shell nanowires were arranged magnetically across a conductive interdigitated electrode and melted by an infrared-driven reflow procedure under flux atmosphere, establishing ohmic contact tip-to-tip between nanowires and in wire-to-pad morphologies. This work establishes an effective and scalable method for site-specific nanowire functionalization while simultaneously addressing miniaturization-associated assembly issues in its desired applications.

Effects of suppressors on the incorporation of impurities and microstructural evolution of electrodeposited Cu solder joints

BackgroundCopper (Cu) electrodeposition is a cost-effective and scalable technology commonly used to fabricate interconnects and heat sinks in microelectronic devices. Adding organic functional additives, such as suppressors, to plating solutions is crucial to achieve conformal microvia filling by Cu electroplating. However, codeposition of organic additives is inevitable and leads to reliability concerns regarding impurity residues in the Cu electrodeposited layer. MethodsIn this study, polyethylene glycol (PEG) at 8,000 g mol−1 and a triblock copolymer EPE (polyethylene oxide-polypropylene oxide-polyethylene oxide) at 2,900 g mol−1 are applied as suppressors and individually added to plating solutions to compare their suppression strength toward Cu deposition using galvanostatic measurements. Their adsorption/desorption behaviors on the Cu surface are theoretically analyzed based on the electronic distribution properties of the two suppressor molecules calculated by quantum chemical calculations. The impurity residues in the Cu layers are measured by a time-of-flight secondary ion mass spectrometer, and their effects on the joining reactions between tin (Sn) solder and Cu are investigated by observing the microstructural evolution in the Sn/Cu joints subjected to isothermal aging. Significant findingsEPE2900 outperforms PEG8000 in the suppression strength toward Cu deposition. Unlike PEG8000, which causes a high level of impurity incorporation, the impurity residues in the Cu layer constructed by EPE2900 are negligible. As a result, the Sn/Cu solder joint constructed by EPE2900 exhibits high microstructural stability free of the propagation of detrimental voids during isothermal aging at 200 °C. The better suppression efficacy and negligible impurity residues of EPE2900 can be rationalized based on quantum chemical calculations.

Research on microstructure and shear strength of Al alloy jointed by sputtered Cu thin film deposited through HiPIMS and DCMS techniques

Transient liquid phase bonding (TLP bonding) in jointing technology is a promising method for achieving precision joining of aluminum alloys and creating low-cost products for use in the electric vehicles (EVs) industry, which can contribute to a low-carbon economy. This research is primarily focused on the jointing technology of aluminum alloys and is divided into two parts. The first part involves vacuum soldering between A6063 aluminum alloy with copper layers of 1, 3 and 5 μm thickness, and tin solder under thermal treatment. The second part is Cu-Al eutectic bonding. In this study, we used Ar ions to bombard the surface of A6063 aluminum alloy to remove the oxide layer and deposited a copper layer using direct-current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS). We then analyzed the joint interface of the samples using Scanning Electron Microscope (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), and Scanning Acoustic Tomography (SAT) to calculate the joint ratio of the entire joint surface. Finally, we measured the shear strength to evaluate the bonding effect. The results indicate that the eutectic bonding method exhibits better bonding performance for aluminum alloys compared to the soldering method. Furthermore, the shear strength increased with increasing thickness of the sputtered copper layer. When comparing two sputtering technologies, the copper films sputtered by HiPIMS system exhibit better quality those produced by the DCMS system. As a result, aluminum alloy bonding completed with HiPIMS has higher bonding strength, demonstrating that HiPIMS is a powerful coating tool.

ПОЛИАРИЛЕНЭФИРКЕТОНЫ В КАЧЕСТВЕ ПРОВОДНИКА В ТОКОСЪЕМНОЙ СИСТЕМЕ СОЛНЕЧНОГО ЭЛЕМЕНТА

The paper investigates the thermomechanical and electrical properties of a number of thermoplastic copolymers poly(arylene ether ketones) (co-PAEK) containing fluorene groups in the main chain of the macromolecule. Co-PAEKs are being studied for the purpose of their use in the contact system of a solar cell with a transparent conductive oxide (TCO) electrode and a multiwire contact grid. Polymer films are used as conductive adhesives to form an electrical contact between metal wire and TCO. The formation of an electrically conductive contact occurs during lamination at an excess pressure of 1 atm and a temperature of maximum deformation of co-PAEK, while the copolymer switches from the initial dielectric to a stable highly conductive state. Co-PAEKs with the content of fluorene groups ( q ) 5, 10, 15, 20 and 25 mol.% are synthesized, having the same molecular weight and the reduced viscosity of ~0.38 dl/g. To select the optimal contact system formation temperature for each coPAEK, the glass transition temperature ( T g) and the peak strain temperature ( T d) were determined from thermo mechanical curves. It was found that with increasing q , the temperature T g increases from 162 to 185°С, and T d from 203 to 223°С. The specific contact resistances (ρС) in the metal wire/co-PAEK/TCO (ITO - In2O3:Sn) system were determined for a round copper wire coated with indium:tin (In:Sn) solder, as well as for tin-plated wire, depending on the content of fluorene groups in the copolymer. The specific contact resistance ρС for wire with (In:Sn) solder varies in the range of 0.56-0.83 mΩ cm2, for tin-plated wire - 0.84-1.43 mΩ cm2, and co-PAEK with q = 5 mol % showed the lowest ρС in both cases. The ρC of wire/co-PAEK/ITO contacts has been studied as a function of the resistivity of ITO grown by ultrasonic spray pyrolysis on silicon substrates. The temperature dependence of the resistivity of the wire/co-PAEK/ITO contact was studied in the temperature range of 5-85°C, and a conclusion was made about the metallic type of dependence. According to the results of microscopic studies of the contact area, the typical minimum co-PAEK layer thickness between the wire and ITO after the formation of the contact was determined, which is in the range of 0.5-2.4 μm.

Passive layer investigation on tin and tin solder alloys

AbstractThe present work investigates the passive film formation and properties on tin and tin solder alloys in weakly acidic nitrate solution by using conventional electrochemical methods as well as electrochemical impedance spectroscopy and Mott–Schottky analysis. It is observed that the influence of the main alloying elements, antimony and copper, positively influences the formation and properties of native oxide films on the respective alloys. If the oxide film is anodically formed, the process can be interpreted as a high‐field mechanism. In this case, antimony, as well as copper, increases the electron conductivity of the anodic formed layers analogous to the transparent conducting oxide layers (e.g., ATO layers).

High-performance electrically conductive adhesives with aluminum-doped zinc oxide (AZO) for various flexible electronic devices

Electrically conductive adhesives (ECAs) are a kind of attractive alternative to tin solders in microelectronics. However, fabricating high-performance, low-cost ECAs remains challenging due to the need for high-content metal fillers. This work reported a facile preparation of cost-effective ECAs composed of aluminum-doped zinc oxide (AZO) nanoparticles and silver (Ag) micro-flakes. The AZO nanoparticles could serve as a “bridge” to connect neighboring Ag micro-flakes to significantly improve the ECAs′ conductivity (from 1638.8 × 10-5 Ω⋅cm to 8.5 × 10-5 Ω⋅cm). After the aging and bonding performance tests, prototyped applications of the as-prepared ECAs for three flexible electronic devices were demonstrated sequentially: (Ⅰ) The resistance of flexible conductive film showed excellent dynamic stability after being folded for 1000 cycles, bent/released within 6 mm radius for more than 2000 cycles, or applied with 1000 kPa pressure; (Ⅱ) The coil antenna achieved fast transmission of radio frequency signal for near field communication (NFC) system; (III) The flexible heater performed efficient thermal management at 0.4–1.4 V input voltages. Therefore, the fabricated ECAs are expected to provide new opportunities for the development of novel flexible electronic devices for future applications.

High performance of Bi2Te3-based thermoelectric generator owing to pressure in fabrication process

A Bi2Te3-based thermoelectric generator (TEG) is known to be the leading technology in low-temperature heat energy recovery. In its fabricating process, the thermoelectric (TE) materials should be heated over the melting temperature of tin solder, but the unmatched thermal expansion between p-type and n-type TE materials will lead to considerable interfacial resistivity, resulting in the sharp decrease of the output power and conversion efficiency. Here, we introduce the pressure to suppress interfacial resistance of Bi2Te3-based TEGs. The theoretical model governing the pressure and interfacial resistivity is built based on the equations of thermal-electric-elastic coupling, and the explicit expressions for maximum output power and conversion efficiency are derived when considering interfacial resistivity. With the guidance of theoretical and simulation results, the average interfacial resistivity of 10 μΩ·cm2 is measured in a Bi2Te3-based TEG, while the conversion efficiency is increased by 44% from the commercial devices. Besides, the stress caused by suitable pressure force is less than the allowable stress of Bi2Te3-based TE materials. These findings provide strong support for the fabrication of high-performance TEGs.

Study of Wettability and Solderability of SiC Ceramics with Ni by Use of Sn-Sb-Ti Solder by Heating with Electron Beam in Vacuum.

The aim of this research was to study the wettability and solderability of SiC ceramics by the use of an active solder of the type Sn5Sb3Ti in a vacuum by electron beam heating. This solder exerts a narrow melting interval, and only one thermal effect, a peritectic reaction, was observed. The liquidus temperature of the solder is approximately 243 °C. The solder consists of a tin matrix where the Ti6(Sb,Sn)5 and TiSbSn phases are precipitated. The solder wettability on a SiC substrate decreases with decreasing soldering temperature. The best wetting angle of 33° was obtained in a vacuum at the temperature of 950 °C. The bond between the SiC ceramics and the solder was formed due to the interaction of Ti and Ni with silicon contained in the SiC ceramics. The formation of new TiSi2 and Ti3Ni5Si6 phases, which form the reaction layer and thus ensure the bond formation, was observed. The bond with Ni is formed due to the solubility of Ni in the tin solder. Two phases, namely the Ni3Sn2 and Ni3Sn phases, were identified in the transition zone of the Ni/Sn5Sb3Ti joint. The highest shear strength, around 40 MPa, was attained at the soldering temperature of 850 °C.

Interface Reaction between Tin Solder and Nanocrystalline Ni and Ni-Mo Coatings Obtained by Electrodeposition

Interface Reaction between Tin Solder and Nanocrystalline Ni and Ni-Mo Coatings Obtained by Electrodeposition

Tin Whisker Growth Suppression Using NiO Sublayers Fabricated by Dip Coating

Whiskers are small crystalline growths, which can grow from certain metals or alloys. Reaching up to several millimeters long, whiskers have the potential to cause device failures due to short circuits and contamination by debris. Tin (Sn) is one such metal that is particularly prone to whisker development. Until the 2006 RoHS Initiative, lead (Pb) was added to tin in small amounts (up to 2%) to greatly reduce the growth of whiskers. Since then, however, industry has switched to lead-free tin solders and coatings, and the issue of whisker growth on tin has attracted new interest. A reactive-sputtering-deposited nickel oxide sublayer was shown recently to strongly suppress the growth of whiskers from an overlaying tin layer. This paper reports on using nickel oxide films, obtained by a sol–gel dip coating method, as whisker suppressing sublayers. The proposed method is simple, low-cost, and can easily be scaled up for manufacturing purposes. The properties of the sol–gel deposited nickel oxide film were examined using SEM, EDS, and Raman spectroscopy. Samples containing the nickel oxide sublayer were observed through SEM periodically over several months to examine the surfaces for whisker development, and the results show that such layers can be very effective in suppressing whisker growth.

International Conference on Electronic and Advanced Materials 2021

The International Conference on Electronic and Advanced Materials (ICEAM) 2021 was held online on the 2nd November 2021 streaming from Digital Management and Development Centre, Universiti Malaysia Perlis (UniMAP). This conference was organized by the Center of Excellence Geopolymer & Green Technology (CEGeoGTech), UniMAP, with Tin Solder Technology Research Group (TSTRG) MALAYSIA and The Electronics Packaging Research Society (EPRS) as co-organizer. The conference was also sponsored by the Tin Industry (R&D) Board. The conference was held virtually in order to follow the standard operation procedure imposed by the Malaysian government due to the COVID-19 movement control order and travel restriction.The conference serves as the best platform for sharing and exchanging ideas on Advanced Materials topics such as electronic materials, ceramic materials and geopolymer, bio-materials, polymers and rubber, and composites. The conference aimed to provide a platform for scientists, researchers, and students in the related field to discuss current research progress, challenges, and practical solutions that could be adopted to keep abreast in this challenging world. The program provides networking and cooperation amongst academicians, scientists, researchers, and engineers from the materials sciences and engineering fields.List of Organizing committee is available in this pdf.

Investigation the Ultrasonic Vibration on Tin Soldering Welding of Copper Wires and Plates

Ultrasonic-assisted soldering welding is widely applied for joining difficult materials. The cavitation phenomenon in liquid always occurs during the ultrasonic excitation. Base metals are striked by ultrasonic cavitation, creating erosion on the surface. The soft solder materials are penetrated on the rough surface, generated inter-metallic compounds. This work expresses the design of ultrasonic soldering machine using 20 kHz source and steel sonotrode. The curvature of reflecting plates with specific radius and their location are also condidered. The major technological parameters of ultrasonic soldering welding such as ultrasonic exciting time, power and curvature radii of reflecting plate are discussed. Tin soldering material is utilized for joining copper wires and plates are investigated. SEM images on the surface of tin soldering on cooper plates and tensile strength are investigated.

Drop-on-demand printing of recyclable circuits by partially embedding molten metal droplets in plastic substrates

Recyclable electronics are important in green manufacturing and sustainable development, but in infancy, printing strategies still need more exploration and improvement. Metal drop-on-demand (DOD) printing is an ideal candidate for fabricating recyclable circuits, owing to its advantage of inherent direct recyclable feedstocks. Liquid metals with melting points at room temperature have been utilized to print circuits. However, developing liquid metal-based composites or encapsulation is still needed to maintain shapes of printed patterns and improve the adhesion between liquid metals and substrates, restricting the directly recycling of printed circuits. Herein, tin solder is utilized to directly print recyclable circuits via metal drop-on-demand printing method. The molten tin solder droplets directly melt plastic substrates and partially embed the substrate to obtain good adhesion. Molten droplets solidify timely to maintain the shapes of printed 2D conductive patterns and 3D microstructures without encapsulation. Moreover, the electrical conductivity of printed lines is close to the homogeneous bulk metal due to the direct metallurgical bonding between droplets. The conductive traces can be directly recycled by mechanically peeling from the substrate and remelting in the crucible. To validate this concept, a 555 circuit is successfully printed and recycled to fabricate a new 3D cross circuit. This work opens a new direction to directly fabricate recyclable circuits by using metal drop-on-demand printing.