Sn-Bi Solder Research Articles

Electrochemical Migration Study on Sn-58Bi Lead-Free Solder Alloy Under Dust Contamination.

With the development of electronic packaging technology toward miniaturization, integration, and high reliability, the diameter and pitch of solder joints continue to shrink. Adjacent solder joints are highly susceptible to electrochemical migration (ECM) due to the synergistic effects of high-density electric fields, water vapor, and contaminants. Dust has become one of the non-negligible causal factors in ECM studies due to air pollution. In this study, 0.2 mM/L NaCl and Na2SO4 solutions were used to simulate soluble salt in dust, and the failure mechanism of an Sn-58Bi solder ECM in the soluble salt in dust was analyzed by a water-droplet experimental method. It was shown that the mean failure time of the ECM of an Sn-58Bi solder in an NaCl solution (53 s) was longer than that in an Na2SO4 solution (32 s) due to the difference in the anodic dissolution characteristics in the two soluble salt solutions. XPS analysis revealed that the dendrites produced by the ECM process were mainly composed of Sn, SnO, and SnO2, and there were precipitation products-Sn(OH)2 and Na2SO4-attached to the dendrites. The corrosion potential in the NaCl solution (-0.351 V) was higher than that in the Na2SO4 solution (-0.360 V), as shown by a polarization test, indicating that the Sn-58Bi solder had better corrosion resistance in the NaCl solution. Therefore, an Sn-58Bi solder has better resistance to electrochemical migration in an NaCl solution compared to an Na2SO4 solution.

Synergistic effect of thermomigration and electric current stressing on damping capacity of Sn58Bi solder

In order to evaluate the vibration resistance of Sn58Bi solder in serving electronic devices, the damping capacities of Sn58Bi solders after thermomigration (TM) test at a temperature gradient of 2000 °C/cm for different time (0, 120, 360, 720, and 1440 h) were characterized under electric current stressing (0, 4.0, and 8.0 A). The results indicate that the phase segregation of TM-tested Sn58Bi solders determines the damping performance of solders. The Bi-rich layer thickens with prolonged TM time and migrates in the direction from high temperature to low temperature. Both the critical strains (the values of dislocation getting rid of pining points) of strain-amplitude-related damping capacity curves increases with prolonged TM time, while decreases with increasing electric current. Moreover, both strain-amplitude-related and temperature-related damping capacity shows a general decreasing trend with prolonged TM time, while increases exponentially with increasing electric current. In addition, the damping mechanism changes from dislocation motion to phase boundary sliding with increasing temperature, and the transition temperature decreases with increasing current but generally increases with TM time.

The Interfacial Reaction between Amorphous Ni-W-P Coating and Sn-58Bi Solder

With the rapid development of the advanced electronic packaging field, the requirements for the connection between solder and Cu substrate are becoming increasingly stringent. Currently, the commonly used Ni-P diffusion barrier layer in the industry lacks long-term reliability, and its resistivity is higher than that of other substrates. This paper introduces the highly conductive metal element W to modify the binary Ni-P coating and prepares a ternary Ni-W-P coating through electrodeposition to improve this situation. The key parameters for the electrodeposition of ternary Ni-W-P are determined. The isothermal aging reaction of Ni-W-P with Sn-Bi solder at 100 °C was studied, and the results showed that, compared to the conventional Ni-P coating, the Ni-W-P barrier coating with higher W content has a much longer lifespan as a barrier layer and exhibits significantly better electrical conductivity. Additionally, the reaction mechanism between Ni-W-P and the Sn-Bi solder is proposed. This research presents a promising advancement in the development of barrier layers for electronic packaging, potentially leading to more reliable and efficient electronic devices. Introducing tungsten into the Ni-P matrix not only extends the lifespan of the coating but also enhances its electrical performance, making it a valuable innovation for applications requiring high conductivity and durability. This study could guide further investigations into the application of ternary coatings in various electronic components, paving the way for improved designs and materials in the semiconductor industry.

High-reliability Sn37Pb/Sn58Bi composite solder joints by solid-liquid low-temperature soldering

As the feature sizes of integrated circuits approach their physical limits, 3D packaging has emerged as a crucial technology for enhancing integration and extending Moore's Law. To meet the future requirements for low-temperature solder and interconnect processes in aerospace 3D packaging, this study combined Sn37Pb solder balls with Sn58Bi solder paste to prepare uniform composite solder joints under reflow conditions of 180°C for 10 minutes (CS-180–10) and 190°C for 1 minute (CS-190–1). The Bi atoms dissolved in the Pb phase strengthened the composite solder and made the crystal structure more ordered and predictable. The composite solder joints formed under both process conditions exhibited shear strengths higher than those of Sn37Pb solder joints prepared at 220°C for 1 minute, both before and after aging at 100°C. The fracture mode of the composite solder joints transitioned from ductile to mixed ductile-brittle with aging. Additionally, the composite solder demonstrated better creep resistance than SnPb solder at all tested strain rates in nanoindentation tests. Compared to CS-180–10, the lower heat input CS-190–1 featured finer Pb phases and Pb grains, better wettability, and better thermal stability. CS-190–1 showed only a 5.9 % decrease in shear strength after 42 days of aging at 100°C. This study offers a new solution for low-temperature, high-reliability interconnections in future aerospace integrated circuit 3D packaging.

Effect of Mo and ZrO2 nanoparticles addition on interfacial properties and shear strength of Sn58Bi/Cu solder joint

The influence of Mo and ZrO2 nanoparticles addition on the interfacial properties and shear strength of Sn58Bi solder joint was investigated. The interfacial microstructures of Sn58Bi/Cu, Sn58Bi + Mo/Cu and Sn58Bi + ZrO2/Cu solder joints were analysed using a scanning electron microscope (SEM) coupled with energy dispersive X-ray (EDX) and the X-ray diffraction (XRD). Intermetallic compounds (IMCs) of MoSn2 are detected in the Sn58Bi + Mo/Cu solder joint, while SnZr, Zr5Sn3, ZrCu and ZrSn2 are detected in Sn58Bi + ZrO2/Cu solder joint. IMC layers for both composite solders comprise of Cu6Sn5 and Cu3Sn. The SEM images of these layers were used to measure the IMC layer’s thickness. The average IMC layer’s thickness is 1.4431 µm for Sn58Bi + Mo/Cu and 0.9112 µm for Sn58Bi + ZrO2/Cu solder joints. Shear strength of the solder joints was investigated via the single shear lap test method. The average maximum load and shear stress of the Sn58Bi + Mo/Cu and Sn58Bi + ZrO2/Cu solder joints are increased by 33% and 69%, respectively, as compared to those of the Sn58Bi/Cu solder joint. By comparing both composite solder joints, the latter prevails better as adding smaller sized ZrO2 nanoparticles improves the interfacial properties granting a stronger solder joint.

Investigation of the mechanical properties of lead-free Sn-58Bi solder alloy with cobalt addition through flux doping

PurposeThis study aims to investigate the effect of incorporating cobalt (Co) into Sn-58Bi alloy on its phase composition, tensile properties, hardness and thermal aging performances. The fracture morphologies of tensile-tested solders are also investigated to correlate the microstructural changes with tensile properties of the solder alloys. Then, the thermal aging performances of the solder alloys are investigated in terms of their intermetallic compound (IMC) layer morphology and thickness.Design/methodology/approachThe Sn-58Bi and Sn-58Bi-xCo, where x = 1.0, 1.5 and 2.0 Wt.%, were prepared using the flux doping technique. X-ray diffraction (XRD) is used to study the phase composition of the solder alloys, whereas scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) are used to investigate the microstructure, fractography and compositions of the solders. Tensile properties such as ultimate tensile strength (UTS), Young’s modulus and elongation are tested using the tensile test, whereas the microhardness value is gained from the micro-Vickers hardness test. The morphology and thickness of the IMC layer at the solder’s joints are investigated by varying the thermally aging duration up to 56 days at 80°C.FindingsXRD analysis shows the presence of Co3Sn2 phase and confirms that Co was successfully incorporated via the flux doping technique. The microstructure of all Sn-58Bi-xCo solders did not differ significantly from Sn-58Bi solders. Sn-58Bi-2.0Co solder exhibited optimum properties among all compositions, with the highest UTS (87.89 ± 2.55 MPa) at 0.01 s−1 strain rate and the lowest IMC layer thickness at the interface after being thermally aged for 56 days (3.84 ± 0.67 µm).Originality/valueThe originality and value of this research lie in its novel exploration of the flux doping technique to introduce minor alloying of Co into Sn-58Bi solder alloys, providing new insights into enhancing the properties and performance of these solders. This new Sn-Bi-Co alloy has the potential to replace lead-containing solder alloy in low-temperature soldering.

Effects of TiC nanoparticle addition on microstructures and mechanical properties of Sn-58Bi solder joints

The Sn58Bi solder has gained significant attention in the electronic packaging industry owing to its low melting temperature and excellent mechanical properties. However, the brittleness of Sn58Bi solder presents challenges in practical applications. This study investigates the effect of adding titanium carbide (TiC) nanoparticles on the mechanical properties of Sn58Bi solder joints. The study examines the melting behavior, spreadability, microstructure, and mechanical properties of Sn58Bi-xTiC (x = 0, 0.05, 0.1, 0.2 wt%) composite solders. The results indicate that adding TiC nanoparticles has merely a minor effect on the melting point of the Sn58Bi solder. However, the presence of TiC nanoparticles refined the solder’s microstructure, reduced the thickness of the intermetallic compounds, and enhanced the spreadability and mechanical properties. The Sn58Bi-0.1TiC composite solder exhibits the most favorable properties, including a 28.6 % increase in fracture energy compared with that of Sn58Bi solder. Consequently, incorporating TiC nanoparticles enhances the performance of the Sn58Bi solder by improving its spreadability and reducing its brittleness. These results hold great potential for advancing the development of lead-free solders with improved reliability and performance in electronic applications. The findings of this study could pave the way for the practical and widespread use of Sn58Bi-xTiC composite solders, offering a viable alternative to lead-containing solders while maintaining excellent mechanical characteristics.

Ni and Ni–P substrates inhibit Bi phase segregation and IMC overgrowth during the soldering process of Sn–Bi solder

In this paper, presents surface modification of Cu substrates by electroplating or chemical plating to obtain high-quality Ni and Ni–P coatings. The Sn58Bi solder reactions with Cu, Ni, and Ni–P UBMs at 180 °C, 250 °C, and 320 °C, respectively. Research has found that Ni and Ni–P substrates can inhibit Bi phase segregation and interfacial IMC overgrowth in Sn–Bi solder joints. Bi phase segregation occurs during the transition from Cu6Sn5 to Cu3Sn. There is Bi segregation at the interface of Sn58Bi/Cu solder joints to Cu3Sn/Cu interface, and IMC grows rapidly. However, Ni/Cu and Ni–P/Cu substrates can not only suppress the Bi phase segregation behavior of Sn58Bi solder joints during soldering, but also slow down the excessive growth of interfacial IMC; Even under the harsh soldering process of up to 320 °C for 600 s, it still has an effective blocking effect. In addition, at 320 °C, when Sn58Bi solder react with Cu, the interface directly crosses Cu6Sn5 to generate Cu3Sn, which exhibits the morphology characteristics of Cu6Sn5 due to high thermal conditions. During the soldering, Ni atoms continuously diffuse towards the interface, and Ni3Sn4 (Ni/Cu) grows from needle like in the early stage of soldering to rod like, and finally to block like; The block like appearance of Ni3Sn4 (Ni–P/Cu) occurs earlier than that of Ni3Sn4 (Ni/Cu), because the presence of columnar Ni3P layers provides a wider diffusion channel for Ni atoms, but the overall growth size is smaller than that on Ni/Cu. A higher undercooling can promote the growth of interfacial IMC, including Cu6Sn5 and Ni3Sn4. At the same time, the Bi phase inside the Sn58Bi solder joint is coarsened, and the substrate Ni/Ni–P can eliminate interfacial brittleness after eliminating Bi segregation.

Effect of electromigration on microstructure and properties of CeO2 nanopartical-reinforced Sn58Bi/Cu solder joints

To mitigate the decrease in mechanical performance of Sn58Bi/Cu solder joints resulting from electromigration-induced damage. The CeO2 nanoparticles were incorporated into Sn58Bi solder by a melt-casting method, and their effects on the microstructure and properties of Sn58Bi/Cu solder joints under electromigration were investigated. The study results demonstrate that the addition of 0.125 ~ 0.5 wt% CeO2 nanoparticles refines the eutectic microstructure of Sn58Bi solder alloy. At an addition amount of 0.5 wt%, the composite solder alloy exhibits the maximum tensile strength of 68.9 MPa, which is 37% higher than that of the base solder. CeO2 nanoparticle-reinforced Sn58Bi solder can achieve excellent solderbility with Cu substrates and the joints can significantly inhibit the growth of the anodic Bi-rich layer, which is responsible for electromigration. With the extension of current stressing time, Bi-rich and Sn-rich layer are respectively formed on the anode and cathode in the joints. The intermetallic compound (IMC) layer grows asymmetrically, transitioning from a fan-shaped morphology to a flattened structure at the anode and to a thickened mountain-like morphology at the cathode. Adding the CeO2 nanoparticles helps to mitigate the decrease in mechanical performance caused by electromigration damage during current application to some extent. Over the current stressing period of 288 ~ 480 h, the fracture position shifts from the anodic IMC/Bi-rich interface to the cathodic Sn-rich/IMC interface. The fracture mechanism transitions from a brittle fracture characterized by plate-like cleavage to a ductile–brittle mixed fracture with fine dimples and cleavage.

Investigation on the Thermal and Wettability Properties Aided with Mechanical Test Simulation of Tin (Sn) - Bismuth (Bi) Solder Alloy at Low Reflow Temperatures

The current study proposes to investigate the thermal, wettability and mechanical properties of a low temperature SnBi solder. The main aim is to investigate the performance of the SnBi solder alloy with different Bi composition. The study also establishes the relationship between melting temperature, spreading area and tensile stress of the SnBi with different Bi composition at different low reflow temperatures. The thermal and wettability tests are conducted experimentally, while the mechanical test will be analysed via finite element analyses (FEA). The single shear lap test method was adopted for the simulation. The thermal properties of the SnBi solder are investigated using the differential scanning calorimeter (DSC). The reflow temperature selected ranges from 160 °C to 220 °C to accommodate the purpose of low temperature soldering. Wetting test results showed that spreading area of Sn48Bi solder alloy increased to 28.1 and 42.88 at 180 °C and 210 °C respectively. The increase in the Bi composition reduced the tensile strength regardless of the increase of the reflow temperature. The preliminary results commend the characteristics of the SnBi solder as a possible alternative to the Pb solder.

Effect of (111) oriented nanotwinned Cu substrate on the electromigration of Sn58Bi solder joint at high current density

Effect of (111) oriented nanotwinned Cu substrate on the electromigration of Sn58Bi solder joint at high current density

Research Overview on the Electromigration Reliability of SnBi Solder Alloy.

Due to the continuous miniaturization and high current-carrying demands in the field of integrated circuits, as well as the desire to save space and improve computational capabilities, there is a constant drive to reduce the size of integrated circuits. However, highly integrated circuits also bring about challenges such as high current density and excessive Joule heating, leading to a series of reliability issues caused by electromigration. Therefore, the service reliability of integrated circuits has always been a concern. Sn-based solders are widely recognized in the industry due to their availability, minimal technical issues during operation, and good compatibility with traditional solders. However, solders that are mostly Sn-based, such as SAC305 and SnZn, have a high melting point for sophisticated electronic circuits. When Bi is added, the melting point of the solder decreases but may also lead to problems related to electromigration reliability. This article reviews the general principles of electromigration in SnBi solder joints on Cu substrates with current flow, as well as the phenomena of whisker formation, voids/cracks, phase separation, and resistance increase caused by atomic migration due to electromigration. Furthermore, it explores methods to enhance the reliability of solder joint by additives including Fe, Ni, Ag, Zn, Co, RA (rare earth element), GNSs (graphene nanosheets), FNS (Fullerene) and Al2O3. Additionally, modifying the crystal orientation within the solder joint or introducing stress to the joint can also improve its reliability to some extent without changing the composition conditions. The corresponding mechanisms of reliability enhancement are also compared and discussed among the literature.

Influences of low In alloying and aging on microstructure and plastic deformation behavior of Sn-58Bi solder

Influences of low In alloying and aging on microstructure and plastic deformation behavior of Sn-58Bi solder

Microstructural Optimization of Sn-58Bi Low-Temperature Solder Fabricated by Intense Pulsed Light (IPL) Irradiation

In this study, intense pulsed light (IPL) soldering was employed on Sn-58Bi solder pastes with two distinct particle sizes (T3: 25–45 μm and T9: 1–8 μm) to investigate the correlation between the solder microstructure and mechanical properties as a function of IPL irradiation times. During IPL soldering, a gradual transition from an immature to a refined to a coarsened microstructure was observed in the solder, impacting its mechanical strength (hardness), which initially exhibited a slight increase followed by a subsequent decrease. It is noted that hardness measurements taken during the immature stage may exhibit slight deviations from the Hall–Petch relationship. Experimental findings revealed that as the number of IPL irradiation sessions increased, solder particles progressively coalesced, forming a unified mass after 30 sessions. Subsequently, after 30–40 IPL sessions, notable voids were observed within the T3 solder, while fewer voids were detected at the T9-ENIG interface. Following IPL soldering, a thin layered structure of Ni3Sn4 intermetallic compound (IMC) was observed at the Sn-58Bi/ENIG interface. In contrast, reflow soldering resulted in the abundant formation of rod-shaped Ni3Sn4 IMCs not only at the reaction interface but also within the solder bulk, accompanied by the notable presence of a P-rich layer beneath the IMC.

Investigating the impact of cobalt incorporation on the transformation of Cu6Sn5 layer to (Cu, Co)6Sn5: Microstructural and mechanical insights

In this study, cobalt (Co) particles were incorporated as reinforcing agents into the Sn58Bi solder. The solderability of the solder and the strength of the solder joints were investigated. The results indicated that adding Co particles did not alter the low melting point characteristics of the Sn58Bi solder, yet its undercooling was reduced, and the wettability was enhanced. In the Sn58Bi/Cu solder joint, adding Co led to forming free (Cu, Co)6Sn5 intermetallic compound (IMC) and generating the CoSn2 phase through microalloying. The interfacial IMC was gradually transformed from a fan-shaped Cu6Sn5 to a flatter layer-like (Cu, Co)6Sn5 IMC, eventually exhibiting a coral-like structure with increased Co content. Adding Co particles increased the thickness of the IMC layer, but at low levels, the Co could refine the IMC grains. Simultaneously, implementing grain refinement and Orowan strengthening resulted in heightened microhardness and shear strength. In the shear test, the fracture of solder joints predominantly transpired within the solder matrix. Excessive Co particles were prone to aggregating within the solder, forming irregular block-shaped (Cu, Co)6Sn5. Concurrently, the coral-like IMC at the interface led to a decline in the mechanical performance of the solder joints, with some interfacial IMC being exposed in the fracture region.

Microstructural evaluation of interfacial intermetallic compounds between Sn58Bi and ENEPIG

BackgroundLow carbonization and Restriction of Hazardous Substances (RoHS) regulations have promoted the development of low-temperature lead-free solder. The solder and substrate are critical entities of joint connections. MethodsSn58Bi has been considered a promising low-temperature solder. Electroless nickel/electroless palladium/immersion gold (ENEPIG) is a good surface finish that prevents solder joints from oxidizing and extends the shelf life of the substrates. In this study, the interfacial reactions between the Sn58Bi solder and ENEPIG surface finish were systematically analyzed. Findings(Au,Pd,Ni)(Sn,Bi)4 was observed after reflow at 160 °C. Au and Pd dissolved into solder rapidly in the beginning. NiSn4 and Ni3Sn4 with small amounts of Au and Pd dissolution formed at the interface after aging. The morphology of NiSn4 evolved from block-like to needle-shaped and finally to a layered configuration as the aging temperature and time increase. Ni3Sn4 was observed between NiSn4 and substrate above 100 °C. This study established the growth mechanism of intermetallic compounds (IMCs) in the reaction between Sn58Bi and ENEPIG for low-carbon emitting processes.

Thermodynamic Assessment of Molten Bix-Sn1-x (x = 0.1 to 0.9) Alloys and Microstructural Characterization of Some Bi-Sn Solder Alloys.

Properties such as lower melting temperature, good tensile strength, good reliability, and well creep resistance, together with low production cost, make the system Bi-Sn an ideal candidate for fine soldering in applications such as reballing or reflow. The first objective of the work was to determine the thermodynamic quantities of Bi and Sn using the electromotive force measurement method in an electrolytic cell (Gibbs' enthalpies of the mixture, integral molar entropies, and the integral molar excess entropies were determined) at temperatures of 600 K and 903 K. The second objective addressed is the comprehensive characterization of three alloy compositions that were selected and elaborated, namely Bi25Sn75, Bi50Sn50, and Bi75Sn25, and morphological and structural investigations were carried out on them. Optical microscopy and SEM-EDS characterization revealed significant changes in the structure of the elaborated alloys, with all phases being uniformly distributed in the Bi50Sn50 and Bi75Sn25 alloys. These observations were confirmed by XRD and EDP-XRFS analyses. Diffractometric analysis reveals the prevalence of metallic Bi and traces of Sn, the formation of the Sn0.3Bi0.7, Sn0.95Bi0.05 compounds, and SnO and SnO2 phases.

Preparation and soldering performance of SAC305@Sn-bi Core-shell solder balls based on eutectic co-deposition

SnBi eutectic thin film alloys are widely used in microelectronic devices, sensors, biomedical devices due to their high resistance, low temperature coefficient, chemical stability, and biocompatibility. However, the large standard electrode potential difference (454 mV) between Sn (−0.136 V) and Bi (+0.3118 V) makes them difficult to be co-deposited, especially with eutectic composition. In this study, cathodic polarization curves were used to analyze the negative shift of the cathodic reduction potentials of Sn and Bi induced by gelatin and EDTA-2Na, respectively, which reduced the potential difference between the two to 184 mv (Sn:-0.958 v; Bi:-0.774 v) and enabled the co-deposition of Sn and Bi; cathodic overpotentials were analysed to show that catechol not only improves the degree of cathodic polarization, but also raises the overpotentials, resulting in flatter surface coatings; By adjusting the temperature, current density, and pulse current, a SnBi eutectic thin film coating with good weldability, controllable thickness, flat surface, density, and uniformity was obtained on copper foil using a platinum as the anode in a methanesulfonic acid solution. The results indicate that it is feasible to deposit SnBi solder alloy at eutectic points (Sn41.53Bi58.47, At.%). Under the synergistic effect of gelatin and unidirectional square wave pulse, a typical eutectic network morphology can be obtained, which is the same as the microstructure of industrial SnBi eutectic solder. Based on these, a core-shell composite soldering material for low-temperature soldering was obtained by using SAC305 solder balls as cores and a SnBi low-temperature soldering material coating was roll-plated on their surfaces. This study provides a reliable technical way and theoretical guidance for low-temperature soldering, preparation of low-temperature solderable core-shell composite soldering material, and application of sensors and optoelectronic devices.

Mechanical Response of Cu/Sn58Bi-xNi/Cu Micro Solder Joint with High Temperatures

Sn58Bi solder is considered a promising lead-free solder that meets the performance requirements, with the advantages of good wettability and low cost. However, the low melting point characteristic of Sn58Bi poses a serious threat to the high-temperature reliability of electronic products. In this study, Sn58Bi solder alloy based on nickel (Ni) functionalization was successfully synthesized, and the effect of a small amount of Ni on creep properties and hardness of Cu/Sn58Bi/Cu micro solder joints at different temperatures (25 °C, 50 °C, 75 °C, 100 °C) was investigated using a nanoindentation method. The results indicate that the nanoindentation depth of micro solder joints exhibits a non-monotonic trend with increasing Ni content at different temperatures, and the slope of the indentation stage curve decreases at 100 °C, showing that the micro solder joints undergo high levels of softening. According to the observation of indentation morphology, Ni doping can reduce the indentation area and accumulation around the indentation, especially at 75 °C and 100 °C. In addition, due to the severe creep phenomenon at 100 °C, the indentation hardness rapidly decreases. The indentation hardness values of micro solder joints of Cu/Sn58Bi/Cu, Cu/Sn58Bi-0.1Ni/Cu, and Cu/Sn58Bi-0.2Ni/Cu at 100 °C are 14.67 ± 2.00 MPa, 21.05 ± 2.00 MPa, and 20.13 ± 2.10 MPa, respectively. Nevertheless, under the same temperature test conditions, the addition of Ni elements can improve the high-temperature creep resistance and hardness of Cu/Sn58Bi/Cu micro solder joints.

Improvement of Solder Joint Shear Strength under Formic Acid Atmosphere at A Low Temperature.

With the continuous reduction of chip size, fluxless soldering has brought attention to high-density, three-dimensional packaging. Although fluxless soldering technology with formic acid (FA) atmosphere has been presented, few studies have examined the effect of the Pt catalytic, preheating time, and soldering pad on FA soldering for the Sn-58Bi solder. The results have shown that the Pt catalytic can promote oxidation-reduction and the formation of a large pore in the Sn-58Bi/Cu solder joint, which causes a decrease in shear strength. ENIG (electroless nickel immersion gold) improves soldering strength. The shear strength of Sn-58Bi/ENIG increases under the Pt catalytic FA atmosphere process due to the isolation of the Au layer on ENIG. The Au layer protects metal from corrosion and provides a good contact surface for the Sn-58Bi solder. The shear strength of the Sn-58Bi/ENIG joints under a Pt catalytic atmosphere improved by 44.7% compared to using a Cu pad. These findings reveal the improvement of the shear strength of solder joints bonded at low temperatures under the FA atmosphere.