Nanosilver Paste Research Articles

Densification mechanism and surface mechanical failure evolution of complex silver nanoparticle interconnection interface via pressure-assisted forced bonding

Nano-silver paste is considered the most promising die-attach material due to the small size effect and low-temperature sintering characteristics. Pressure-assisted manufacturing process can accelerate the sintering of silver (Ag) nanoparticles, forming high-quality chip interconnect interface. In this paper, molecular dynamics (MD) simulation was performed for the first time to reveal the densification mechanism and surface mechanical failure evolution behavior of complex Ag nanoparticle interconnect interface in pressure-assisted manufacturing processes. The roles of sintering pressure, sintering parameters, and nanoparticle diameter in the densification process of Ag nanoparticles were evaluated via atomic strain, mean square displacement, sintering density, and atomic layer fault analysis. External pressure can promote the formation of sintering necks through a forced mechanical action. Changing the sintering temperature can regulate the crystal evolution mode of the interface. To describe the mechanism of densification process, we propose a competitive model that correlates sintering pressure, sintering temperature, optimal steady-state sintering time, and initial system stability. Shortening the optimal sintering steady-state time of the system can make a positive contribution to sintering performance. From the perspective of improving mechanical performance, we should try to avoid the occurrence of nanoparticle morphology characteristics inside the sintered interconnect interface, which may cause sintering neck failure. The expansion and failure of random defects are beneficial for increasing mechanical reliability. This study can provide comprehensive and effective theoretical guidance for manufacturing process of high-performance chip interconnect interface.

Fractional damage model of cyclic behaviors for nano-silver paste

Sintered nano-silver paste, extensively utilized in electronic packaging technology, is crucial for evaluating the lifetime of electronic devices. In this paper, a fractional damage model is proposed to characterize ratcheting strain of nano-silver paste under cyclic loading. It can be achieved by incorporating a damage coefficient into the classical fractional model, which directly correlates the ratcheting strain with cyclic number. The power law damage coefficient describes well both the non-damage behavior within a limited number of cycles and the rapid increase of ratcheting strain at higher cyclic numbers. Furthermore, the effects of temperature or stress rate on ratcheting strain of sintered silver have been investigated to establish a quantitative relationship between parameters and loading conditions. It is also numerically validated by various experimental datasets. Ultimately, the damage coefficient is physically interpreted based on evolution of porosity.

Effect of sintering time on the characteristics of YBCO coated-conductor joints using nano-silver paste

Jointing YBa2Cu3O7-z (YBCO) superconducting coated-conductor (CC) tapes is becoming increasingly significant for magnet applications because of the limited length of single tapes. In this work, YBCO CC tapes stabilized by silver were connected by low-temperature sintering of nano-silver paste. The effect of sintering time and lapped length on the electrical properties of YBCO joints were systematically investigated. And the correlation between microstructure and bonding force of the joints with extending the sintering time was established. It is found that joints sintered within 1∼5 min exhibit relatively lower resistance, while the maximum axial tensile strength at room temperature (RT) was improved with increasing sintering time. Considering the electromechanical properties, ten min is selected as the optimal sintering time for nano-silver paste. The joint by this efficient technology possesses closely connected interface, similar critical current and axial tensile strength (RT) to the single CC tape. The joint resistivity is as low as ∼12.5 nΩ·cm2, which is much less than the traditional soldering joint.

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.

Study on preparation and sintering properties of nano-silver-coated tin slurry

A nano-sized silver-coated tin (Sn@Ag) slurry was prepared by heterogeneous flocculation method by adjusting the pH value of solution and selecting different dispersants. The slurry improved the oxidation resistance of tin and its dispersibility in silver matrix. The sintering strength of nanometre Sn@Ag slurry increases with the increase of Sn content. When the Sn content reaches 5%, the shear strength of the joint reaches the highest 50 MPa, which is more than 10 MPa higher than that of the pure nanometre silver slurry sintered joint. The increase of shear strength is due to the fact that the equilibrium phase formed after sintering is Ag–Sn replacement solid solution and intermetallic compound Ag 3 Sn, which have the effect of replacement solution strengthening and dispersion strengthening, respectively. It is proved by experiments and analysis that the application of nano-silver paste in chip interconnection is feasible. The research of this subject provides experimental reference and theoretical basis for the application of new generation interconnect materials in power devices and promotes the development of microelectronics packaging technology.

Microscale hybrid 3D printed ultrahigh aspect ratio embedded silver mesh for flexible transparent electrodes

Increasing the aspect ratio (AR) of embedded metal meshes (EMMs) is an effective approach to solve the “trade-off” effect of flexible transparent electrodes (FTEs). However, achieving low-cost and green manufacturing of ultrahigh AR EMM still faces great challenges. Here, we report a novel solution for the ultrahigh AR EMMs by microscale hybrid printing nano silver paste. An improved printed nozzle is introduced to facilitate the stable electric-filed-driven 3D printing, then the imprinting process is used to achieve an EMM with AR of up to 20.1, which is the highest AR ever reported. The fabricated FTE exhibits unprecedented photoelectric performance with a figure of merit of up to 88,194 and excellent optoelectronic properties after rigorous mechanical and environmental stability tests. Its practical application was also demonstrated to be feasible through flexible transparent heater and electromagnetic shielding. The proposed method provides a promising strategy to fabricate FTEs with ultrahigh AR EMM.

A Fluorinated Polyimide Based Nano Silver Paste with High Thermal Resistance and Outstanding Thixotropic Performance.

Because of high conductivity, acceptable cost and good screen-printing process performance, silver pastes have been extensively used for making flexible electronics. However, there are few reported articles focusing on high heat resistance solidified silver pastes and their rheological properties. In this paper, a fluorinated polyamic acids (FPAA) is synthesized by polymerization of the 4,4'-(hexafluoroisopropylidene) diphthalic anhydride and 3,4'-diaminodiphenylether as monomers in the diethylene glycol monobutyl. The nano silver pastes are prepared by mixing the obtained FPAA resin with nano silver powder. The agglomerated particles caused by nano silver powder are divided and the dispersion of nano silver pastes are improved by three-roll grinding process with low roll gaps. The obtained nano silver pastes possess excellent thermal resistance with 5% weight loss temperature higher than 500 °C. The volume resistivity of cured nano silver paste achieves 4.52 × 10-7 Ω·m, when the silver content is 83% and the curing temperature is 300 °C. Additionally, the nano silver pastes have high thixotropic performance, which contributes to fabricate the fine pattern with high resolution. Finally, the conductive pattern with high resolution is prepared by printing silver nano pastes onto PI (Kapton-H) film. The excellent comprehensive properties, including good electrical conductivity, outstanding heat resistance and high thixotropy, make it a potential application in flexible electronics manufacturing, especially in high-temperature fields.

High-performance printed electrode with rapid fabrication based on UV and IPL light processes without thermal treatment

We developed flexible and ultraviolet (UV)-curable electrodes with improved electrical conductivity using two light processes. Existing microparticle electrodes have poor durability. In this study, we improved electrode durability by facilitating UV light transmission using nanosilver particles and increased electrical conductivity using the photonic sintering process as a posttreatment process. The formulation of the UV-curable nanosilver paste developed this way had no problem in the cross-cut tape test and showed excellent pencil hardness (>3H). Furthermore, the electrical resistivity was 2.76 × 10−5 Ω·cm, and the resistance change rate was <1 % even after 50,000 times of repetitive tests with a 3-mm radius of curvature. When twelve electrode patterns with LED installation and bent were manufactured, we confirmed that there was no change in brightness. Finally, as polyethylene terephtalate, a low-temperature substrate, was not damaged even after the process was completed, the paste and process showed sufficient performance even in the low-temperature process.

Porosity Dependence of Thermal and Electrical Properties in Nano-Silver Paste

Nano-silver paste is considered one of the most promising die-attach materials for high-temperature applications contributing to its excellent thermal and electrical properties and low bonding temperature. However, it is difficult to measure the thermal conductivity and electrical resistivity of nano-silver in practical applications. In this work, we proposed a finite element modeling method to predict the thermal conductivity and electrical resistivity of nano-silver. Nano-silver flake and film with different porosity were prepared by adjusting the sintering temperature, and the thermal and electrical properties were measured. The simulated results are in good agreement with the experimental results under all porosity. The error values between the experimental and simulated temperatures are less than 10%. With the increase of porosity, the thermal conductivity of nano-silver decreases and the electrical resistivity increases. Pores will cause heat flow and current concentration, and increase the possibility of failure of nano-silver in practical applications. The results indicate that the porosity plays a more important role in the thermal and electrical properties of the nano-silver compared with pores location.

Reliability Inspection for Bilayer Nanosilver Paste Ensemble Pressureless Sintered Embedded Cooling SiC Power Module

Various hollowing substrates have been invented to equipped in the embedded cooling power modules. Through a lower stress residue, multilayer nanosilver paste ensemble pressureless sintering to obtain the qualified thermal interface material (TIM) joints becomes an essential process for those fragile stacking packages. As the Anand viscoelastic constitutive model prediction, the maximum residual stress inside the TIM joints is only 63.18 MPa, far below the encapsulated materials’ tensile strengths. From all the scanning electron microscope (SEM), X-ray, and scanning acoustic microscope (SAM) image investigations, the upper and beneath nanosilver paste ensemble pressureless sintered TIM joints (50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm ~10~\mu \text{m}$ </tex-math></inline-formula> thickness) contained few cracks and delamination (only 1.6% ± 0.1% and 8.5% ± 0.1% void ratios, respectively). The measured upper and beneath sintered TIM joints’ bond strengths were up to 38 ± 2 and 63.8 ± 2 MPa, higher than the industry requirement (30 MPa), respectively. After 21 000 power cycles, the upper and beneath sintered TIM joints’ thermal resistance increased only by 10.3% and 5.3%, far underneath the failure standard (20%), respectively. This presented multilayer nanosilver paste ensemble pressureless sintering process is a promising technology for the embedded cooling power module stacking package.

Study on the preparation process and sintering performance of doped nano-silver paste

AbstractAfter tin–lead solders are banned, the widely used electronic packaging interconnect materials are tin–silver, tin–copper, and other alloy solders. With the application of high-power devices, traditional solders can no longer meet the new requirements. Nano-silver (Ag) paste, as a new solder substitute, exhibits excellent properties, such as excellent thermal and electrical conductivity, sintering at lower temperatures, and service at high temperatures. However, many organic devices still cannot withstand this temperature, and are often not suitable for the connection of nano-Ag paste packaging materials, therefore, it is very urgent to further reduce the sintering temperature of nano-silver paste. Based on the transient liquid phase sintering technology, by doping the nano-Ag paste with the nano-tin paste with a lower melting point to make the two uniformly mix, pressureless sintering at low temperature can be realized, and a sintered joint with a connection strength greater than 20 MPa can be formed. Considering that tin is easy to be oxidized, and the core–shell material can prevent the oxidation of tin, and at the same time ensure the uniform distribution of tin in silver, the doping scheme of the core–shell structure is determined. Heterogeneous flocculation method refers to the particles with different properties of charges which attract each other and agglomerate. It is a continuous reduction method for preparing core–shell materials. This method has the advantages of mild reaction conditions and less equipment investment, so the heterogeneous flocculation method is selected to prepare Sn@Ag core–shell nano paste. And research its sintering performance and strengthening mechanism.

Conductivity Evaluation Before and After Photo-sintering of Non-solvent Type UV-curable Nano Silver Paste

Conductivity Evaluation Before and After Photo-sintering of Non-solvent Type UV-curable Nano Silver Paste

High-Throughput Manufacturing of Flexible Thermoelectric Generators for Low- to Medium-Temperature Applications Based on Nano-Silver Bonding

The melting point of the commercial soldering materials is a limiting factor for the functionality of the thermoelectric generators (TEGs) in medium-temperature ranges higher than 200 °C. Hence, using a proper bonding method which could be formed at lower temperatures and withstand higher temperatures is inevitable to unlock the full potential of the commercial thermoelectric materials. Nano-silver bonding is known as the “low-temperature joining technique” due to its lower processing temperature compared with the other conventional bonding approaches. This study is focused on presenting a fabrication process of flexible TEGs using nano-silver (Ag) paste bonding. The pellets of thermoelectric materials were prepared by wire cutting of bismuth telluride ingots in desired sizes. The electrical interconnects between the thermoelectric legs are nano-silver paste traces printed by a digitally control dispenser printer. Reliability of the thermoelectric module was evaluated from room temperature up to 300 °C as well as conducting the cyclic bending test. The proposed fabrication technique is similar to the flexible hybrid electronics (FHE) concept, which is scalable for high-throughput manufacturing, such as roll-to-roll or sheet-to-sheet printing.

Reliability Behavior of A Resin-Free Nanosilver Paste at Ultra-Low Temperature of 180°C

• Large-area (20 × 20 mm 2 ) bonding successfully achieved by a trimodal particle system • The resin-free trimodal paste could sintered under ultra-low temperature of 180°C • Excellent thermo-mechanical reliability was achieved by proposed resin-free paste In this paper, excellent thermo-mechanical reliability of resin-free silver sintering for large-area (20 × 20 mm 2 ) bonding was successfully achieved by using a trimodal particle system composed of nano-, submicron-, and micron-sized Ag particles. After 1000 cycles of the thermal shock (TS) test, the transient thermal impedance ( Z th ) increase of the proposed resin-free silver paste sintered at 180°C under 2 MPa and 5 MPa is only 5.8% and 6.1%, respectively. Due to the avoidance of resin degradation, the obtained resin-free silver paste as-sintered under 5 MPa exhibt outstanding shear strength (>12.5 MPa) even after 1000 cycles TS test. The obtained cross-sectional microstructure can confirms the excellent thermo-mechanical reliability of the sintered resin-free silver paste, which exhibits a denser and more homogeneous bonding layer with a porosity as low as 12.5%. Furthermore, distinct indications of plastic flow can be observed on the fracture surfaces of corresponding joints before and after aging, which further confirm the superiority of proposed resin-free sintering method. The development of novel resin-free silver paste successfully promotes the thermo-mechanical reliability of silicon carbide (SiC) power devices at a low processing temperature of 180°C, and greatly benefits the practical application of SiC power devices.

Optimization and simulation of nano-silver paste sintered copper interconnection process

Optimization and simulation of nano-silver paste sintered copper interconnection process

A Reliable Way to Improve Electrochemical Migration (ECM) Resistance of Nanosilver Paste as a Bonding Material

Electrochemical migration (ECM) of sintered nano-Ag could be a serious reliability concern for power devices with high-density packaging. An anti-ECM nano-Ag-SiOx paste was proposed by doping 0.1wt% SiOx nanoparticles rather than previously used expensive noble metals, e.g., palladium. The ECM lifetime of the sintered nano-Ag-SiOx was 1.5 to 3 times longer than that of the sintered nano-Ag, due to the fact that the SiOx could protect the Ag from oxidation. The thermo-mechanical reliability of the sintered nano-Ag-SiOx was also improved by sintering under 5 MPa assisted pressure. The lesser porosity and smaller grain boundaries of the sintered nano-Ag-SiOx could also be beneficial to retard the silver ECM. In the end, a double-sided semiconductor device was demonstrated to validate the better resistance to the ECM using the sintered nano-Ag-SiOx.

Large-Area Bonding by Sintering of a Resin-Free Nanosilver Paste at Ultralow Temperature of 180 °C

In order to achieve superior interfacial heat dissipation, large-area (&#x2265;400 mm²) sintering of a novel nanosilver paste at ultralow temperature (180 &#x00B0;C) was successfully realized using extra terpineol solvents at interfaces by promoting the diffusivity of Ag nanoparticles, i.e., Ag NPs, into device metallization as well as extra diffusion channel. Under auxiliary pressure of &#x2264; 5 MPa, strong joint can be obtained with a shear strength of &#x003E; 500 kgf and a steady-state thermal resistance of 0.273 &#x00B0;C/W, close to that of traditional nanosilver joints sintered at &#x003E;250 &#x00B0;C. Thanks to the increase of diffusion mobility due to the terpineol solvent as well as more fast diffusion channel, much more Ag NPs can be diffused to the chip metallization, improving the adhesive bonds at interfaces greatly for achieving large-area bonding at such low sintering temperature.

Sintering Nano-Silver Paste by Resistive Joule Heating Process for 2G HTS Tape Joints.

Developing a joining technology for 2G HTS tapes without significantly reducing their superconducting property is crucial for numerous applications (MRI, motor/generator, power transmission, etc.). In this study, low sintering temperature (~230 °C) nano-silver paste was used as solder to join two 2G HTS tapes. In addition, two heating methods, i.e., furnace heating (heat flux outside-in) and resistive Joule heating (heat flux inside-out), were studied. This study indicates that the heat flux from internal by resistive Joule heating method shows less deteriorating impact to the 2G RE-Ba-Cu-O tape (RE: rare earth element) during the sintering process with the best specific resistance of 0.074 μΩ∙cm2 and Ic retention percentage of 99% (i.e., Ic reduced from 100 A before joining to 99 A after joining). This study indicates that nano-silver paste together with resistive Joule heating can possibly be used as soldering materials to join 2G HTS tapes.

Joint Analysis and Reliability Test of Epoxy-Based Nano Silver Paste Under Different Pressure-Less Sintering Processes

Abstract Recent years, the sintered silver paste was introduced and further developed for power electronics packaging due to low processing temperature and high working temperature. The pressure-less sintering technology reduces the stress damage caused by the pressure to the chip, improves reliability, and is widely applied in manufacturing. Currently, most existed studies are focused on alcohol-based sintered silver pastes while resins have been demonstrated to improve the bonding properties of solder joints. Hence, the performance and sintering mechanisms with epoxy-based silver paste need to be further explored. In this work, a methodology for multifactor investigation is settled on the epoxy-based silver paste to reveal the relationship between the strength and the different influence factors. We first analyzed the characteristics of commercialized epoxy-based silver paste samples, including silver content, silver particle size, organic composition, sample viscosity, and thermal conductivity. Samples were then prepared for shear tests and microstructure analysis under different pressure-less sintering temperatures, holding time, substrate surface, and chip size. Full factor analysis results were further discussed in detail for correlation. The influence factors were ranked from strong to weak as follows: sintering temperature, substrate surface, chip size, and holding time. Finally, a thermal cycling test was carried out for reliability analysis. Epoxy residues are one of the possible reasons, which result in shear strength decreasing exponentially.

The Sintering Process and Vibration Characteristics for Leadless Package Structure of Pressure Sensors

A key factor restricting the development of high-temperature pressure sensors is the packaging structure. The traditional lead package with silicone oil as the medium is difficult to meet the performance requirements in high-temperature environments. In this article, the anodic bonding method is used to bond the SOI pressure-sensitive chip and the borosilicate glass to form a vacuum chamber, which avoids the use of silicone oil. The glass hole is filled with high-temperature nanosilver paste, and the glass paste is printed on the surface. After being butted with the pins of the Kovar base, the sensor is sintered under a vacuum to realize the package of the sensor. Taking the shear strength of joint and porosity of silver paste as evaluation indexes, the optimal process parameters of silver paste and glass paste integrated sintering are studied by the Taguchi orthogonal experiment. Finite element simulation was used to analyze the vibration characteristics of the sensor, and its short-term vibration reliability was verified through vibration tests. The static output of the sensor shows that this packaging process can make it maintain good nonlinearity at high and low temperatures.