Silver Joints Research Articles

Recurrent neural network (RNN) and long short-term memory neural network (LSTM) based data-driven methods for identifying cohesive zone law parameters of nickel-modified carbon nanotube reinforced sintered nano-silver adhesives

In this paper, we presented three neural network models including deep neural network (DNN), recurrent neural network (RNN), and long short-term memory neural network (LSTM), which are proposed to predict the cohesive zone parameter of sintered silver DCB joint with different contents of nickel modified carbon nanotube, thus reducing the complexity of CZM parameter acquisition for nanoparticle reinforced adhesive. The bilinear CZM model is used as the prediction target model for sintered silver joints with different contents of nickel-modified carbon nanotube filler, and data sets suitable for different networks are established through experimental and numerical simulation results. Three kinds of networks are trained based on the optimized hyperparameters obtained from the Bayesian hyperparameters tuning process. The results show that DNN, RNN, and LSTM frameworks can all predict CZM parameters of nanoparticle-reinforced sintered silver adhesive through load-displacement curves. Based on loss analysis and statistical indicator comparison after K-fold cross-validation, the RNN and LSTM models have better prediction accuracy and performance than the DNN model, and the accuracy of the LSTM model is further improved compared with the DNN model. RNN and LSTM models have high prediction accuracy and stronger recognition ability for the time series data, they can be used as suitable alternative models for inverse recognition of CZM parameters of nanoparticle-reinforced adhesives, and have broad application prospects.

High-Pressure-Assisted Large-Area (>2400 mm2) Sintered-Silver Substrate Bonding for SiC Power Module Packaging.

The emergence of new semiconductor devices puts forward higher requirements for packaging technology. Sintered silver technology has gradually developed into critical packaging technology in silicon carbide power module packaging due to its good heat dissipation performance and reliability. However, high sintering drive requirements, low sintering densification, and high thermal-mechanical stresses limit the application of sintered silver technology for large-area bonding. In this study, the high-pressure-assisted (≥10 MPa) large-area sintered-silver interconnection process between a substrate and baseplate was discussed. C-scan acoustic microscopy, warpage testing, and microanalysis were used to analyze the effects of drying methods, sintering pressure, and holding time on the sintered joints, and thermal fatigue reliability tests were conducted on large-area sintered silver joints. The results demonstrated that the quality of large-area sintered joints obtained via open-face convective drying is higher than that via close-face convective drying. Combining the performance of sintered joints and productivity, the recommended process condition is determined as follows: open-face convective drying, sintering temperature of 250 °C, sintering pressure of 15 MPa, and holding time of 5 min. Large-area sintered joints have outstanding reliability, with slight delamination of the sintered layer at the corners and no cracking after 1000 cycles of temperature cycling.

Infrared Radiation-Assisted Non-Pressure Sintering of Micron-Sized Silver for Power Electronic Packaging

In recent years, silver sintering has gained increasing attention in high-power density electronic packaging due to its characteristics such as a high melting point and excellent thermal and electrical conductivity. Micron-sized silver sintering offers a lower cost, but requires a longer processing time and additional pressure, which constrains its application. This paper presents a low-cost sintering process using infrared radiation (IR) as a heat source. By leveraging the unique properties of IR, the process achieves selective heating. The thermal energy can be mainly absorbed by the specific areas requiring sintering. This innovative approach eliminates the need for external pressure during the sintering process. This feature not only simplifies the overall process but also reduces the processing time required for sintering. The silver joints obtained from IR sintering process for 45 min achieved an average chip shear strength of 38 MPa at a temperature of 225 °C, which is higher than the strength of silver joints obtained from a traditional sintering process for 2 h. Additionally, the IR-sintered silver joints have a resistivity of 9.83 × 10−5 Ω·cm and scanning electron microscope (SEM) images of the joints reveal that the sintered joints obtained through the IR sintering process exhibit less porosity compared to joints obtained through a traditional sintering process. The porosity of the IR silver joints at 225 °C is 6.4% and does not change even after 3000 cycles of thermal shock testing, showing outstanding reliability. A GaN power device using IR silver joint also performed better in thermal and electrical performance testing, showing promising potential for the application of micro-silver paste in power electronic devices.

An Experimental Investigation of a Flexible Sintered Silver Joint for Micro-Joining Based on a Design of Experiments

The increasing electrification of vehicles requires the use of ever higher voltages and electrical power. The thermal stresses to which the assembly joints of electronic chips are subjected are becoming increasingly severe. As a result, thermal stresses acting on the electronic assembly, and particularly on the electrical interconnections, are becoming increasingly severe. The objective of this study is to create, characterize and optimize the structure and properties of a porous silver sintered joint using the design of experiments (DoE) method. Joints consisting of a porous network of silver particles are obtained, showing remarkable flexibility allowing for thermal stresses accommodation and having sufficient tensile strength to ensure a correct bonding of the components. Depending on the selected levels for the sintering process parameters, the microscopy analysis shows a transition of the mode of rupture from the silver layer to the intermetallic compounds forming at the interface with the substrate.

Microstructural and mechanical evolution of sintered nano-silver joints on bare copper substrates during high-temperature storage

PurposeThe purpose of this paper is to study the reliability of sintered nano-silver joints on bare copper substrates during high-temperature storage (HTS).Design/methodology/approachIn this study, HTS at 250 °C was carried out to investigate the reliability of nano-silver sintered joints. Combining the evolution of the microstructure and shear strength of the joints, the degradation mechanisms of joints performance were characterized.FindingsThe results indicated that the degradation of the shear properties of sintered nano-silver joints on copper substrates was attributed to copper oxidation at the silver/copper interface and interdiffusion of interfacial elements. The joints decreased by approximately 57.4% compared to the original joints after aging for 500 h. In addition, severe coarsening of the silver structure was also an important cause for joints failure during HTS.Originality/valueThis paper provides a comparison of quantitative and mechanistic evaluation of sintered silver joints on bare copper substrates during HTS, which is of great importance in promoting the development of sintered silver technology.

Achieving high-quality silver sintered joint for highly-reliable schottky barrier diodes via pressureless method

The fabrication of silver joints was done using the pressureless sintering technology to suit the demand of high-reliability schottky barrier diodes (SBD). Porosity of 10.6% and shear strength of 39.6 MPa were reached under the optimized parameters of 290°C sintering temperature and 40 min residence time. The sintered joint demonstrated good mechanical/thermal/electrical performance in the ultimate reliability assessment testing, including the temperature cycling test, second sintering test, steady-state lifetime test, and intermittent lifetime test. This study demonstrated the viability of pressureless sintering of silver joints with good high-temperature reliability, which has significant application potential for aeronautical high-reliability power electronics.

Influence of Surface Metal and Current Direction on Degradation Behavior of Sintered Silver Joint Under High-Density Current

Influence of Surface Metal and Current Direction on Degradation Behavior of Sintered Silver Joint Under High-Density Current

Inverse identification of cohesive zone parameters for sintered nano-silver joints based on dynamic convolution neural network

The inverse identification of cohesive zone parameters is an important topic in the field of fracture mechanics. In this paper, three data-driven models, including multilayer perceptron (MLP), convolutional neural network (CNN), and dynamic convolutional neural network (DCNN), were presented to predict the cohesive zone parameters of sintered nano-silver end notched flexure (ENF) joints. Based on the construction of experimental and numerical datasets of load versus displacement curves, bilinear cohesive zone model (CZM) parameters of sintered silver joints are adopted as the prediction target. The investigation shows that MLP, CNN, and DCNN are all valid for predicting CZM parameters through load versus displacement curves with reasonable accuracy. However, DCNN has better prediction accuracy and performance than those of CNN and MLP models based on loss analysis, statistical indicator comparison, and K-fold cross-validation. DCNN can be adopted as the suitable surrogate model for CZM parameters inverse identification with high prediction accuracy. Otherwise, DCNN is also not sensitive to the load versus displacement curves data length. However, the computation efficiency of DCNN during the training process is not as high as that of MLP. Those three methods presented in this paper are very hopeful to be adopted for other inverse identifications of CZM parameters for various kinds of adhesive joints.

Effect of epoxy resin addition on properties and corrosion behavior of sintered joints in power modules serviced offshore

Power modules applied in offshore applications are facing risks of corrosion failures on die-attach materials due to high humidity and H2S exposure. To investigate such corrosion behavior for sintered die-attach materials, we conducted a study with four groups of samples fabricated using copper and silver metal particles under different solvent systems. Such samples were firstly subjected to high-humidity-H2S conditions for 168 h to simulate the harsh offshore environment. After undergoing corrosion, the primary compounds formed were CuO/Cu2O and Ag2S through SEM, XRD, and XPS analysis. Notably, the incorporation of epoxy resin into sintered copper joints resulted in a remarkable reduction in corrosion and a substantial improvement in electrical conductivity after the reaction. In contrast, while the addition of epoxy did not evidently reduce corrosion in silver joints, it did lead to a significant increase in shear strength. Furthermore, to gain further insights into the effect of epoxy resin on corrosion behavior, electrochemical analysis, and molecular dynamics simulations were conducted. Finally, the mechanical reliability of the corroded copper and silver joints was evaluated through thermal shock tests. In summary, sintered copper joints exhibited better anti-corrosion behaviors than sintered silver under high humidity and H2S exposure, especially with the addition of epoxy resin. However, the corrosion products of sintered copper suffered from a sharp decrease in shear strength after thermal shock tests than sintered silver, which is probably due to the coefficient of thermal expansion mismatch.

A Micromechanical Analysis to the Viscoplastic Behavior of Sintered Silver Joints under Shear Loading.

Ag paste has been recognized as a promising substitute for Sn/Pb solder in SiC or GaN power electronic devices, owing to its ability to withstand high temperatures and facilitate low-temperature packing. The reliability of these high-power circuits is greatly influenced by the mechanical properties of sintered Ag paste. However, there exist substantial voids inside the sintered silver layer after sintering, and the conventional macroscopic constitutive models have certain limitation to describe the shear stress-strain relationship of sintered silver materials. To analyze the void evolution and microstructure of sintered silver, Ag composite pastes composed of micron flake silver and nano-silver particles were prepared. The mechanical behaviors were studied at different temperatures (0-125 °C) and strain rates (1 × 10-4-1 × 10-2) for Ag composite pastes. The crystal plastic finite element method (CPFEM) was developed to describe the microstructure evolution and shear behaviors of sintered silver at varied strain rates and ambient temperatures. The model parameters were obtained by fitting experimental shear test data to a representative volume element (RVE) model built on representative volume elements, also known as Voronoi tessellations. The numerical predictions were compared with the experimental data, which showed that the introduced crystal plasticity constitutive model can describe the shear constitutive behavior of a sintered silver specimen with reasonable accuracy.

Shear fracture resistance enhancement through micropatterning on copper substrate for sintered nano silver joints

Enhancement of the fracture resistance for sintered silver joints has been one of the critical issues that considered frequently in engineering practice. In this paper, different micropatterned sintered silver joints were fabricated. Through end notch flexural test, the load displacement curves for various kinds of micropatterned joints are presented. The shearing fracture toughness and shearing fracture characteristics for different micropatterned joints are also given. The maximum shearing fracture toughness is obtained under the grid groove samples with the highest shearing fracture toughness obtained at 356.95 ± 27.45 J/m2, which is about 7.8 times of that without micropatterned joints. It is also found that the shearing fracture toughness is strongly dependent on the groove depth of the micropatterns. The cracking morphology shows that the crack extension was mainly in a cohesive mode but was also associated with locally interfacial debonding failure that occurred at the groove locus. Although the micropatterns lead to the increase of shearing fracture toughness of adhesively bonded joints and even some of the tested P-δ curves shows the ductile fracture manner, the macro fracture morphology of the crack in sintered silver shows that the cracking in micropatterned joints still presents brittle fracture manner.

Effect of Oxygen Content on Bonding Performance of Sintered Silver Joint on Bare Copper Substrate

Optimizing oxygen level in sintering atmosphere seems to be a more economical method to realize die-attachment on bare copper substrate with silver paste. In this paper, effects of oxygen content on mechanical and thermal performance of sintered Ag joint on bare copper substrate were investigated. The 5×103 ppm was demonstrated as the most appropriate oxygen content for the optimum bare copper joint, which removed most organics in the paste layer while preventing copper substrate from oxidation. Besides, bonding mechanism of joints sintered under different oxygen levels was clarified. For joint sintered at 1×103 ppm, excess residual organics significantly facilitated the formation of hydrogen bonds, resulting in the adhesive-dominated failure mode and worst overall performance. The increase of oxygen content to 5×103 ppm reinforced the bonding strength due to the Ag-Cu metallic bond formation at the interface and ameliorative densification of sintered-Ag layer. As the amount of oxygen further increased, copper substrate was oxidized, and bonding strength experienced a changing course of decline, rise, and flat, which can be attributed to Ag adhesion on copper surface and the formation of voids at the interface between oxide layer and copper substrate.

An explanation for the effect of Au surface finish on the quality of sintered Ag-Au joints

As an environmentally friendly interconnection material in power electronics, silver paste is widely used in gold surface finish. To explain the delamination at the Ag-Au interface and clarify the mechanism of Ag-Au interconnection, the bonding properties of sintered silver joints on different gold-plated substrates are studied. It was concluded that the effect of grain size on Ag-Au diffusion is greater than that of grain boundary type. The gold-plating layer with larger grains provides fewer grain boundaries, which inhibits the rapid diffusion of Ag atoms due to high angle grain boundaries. In addition, to investigate the effects of Au grain boundary misorientation on Ag-Au interdiffusion, molecular dynamics simulation is carried out on the sintering process. It is not found that high angle grain boundary of Au layer promotes the diffusion of Ag atoms during pre-drying. However, in the sintering stage, the MSD (mean square displacement) of Ag atoms in the substitute-model with high angle grain boundaries is about 4 times that of with low angle grain boundaries. It is proved that owing to lots of high angle grain boundaries in the gold-plated substrate, the diffusion of Ag atoms is faster in the sintering stage.

Thermo-Mechanical Stability of Resin-Added Sintered Silver Joints Between Si Die and Ni-/Au-Electroplated Substrate

Resin-added sintered silver is widely used in high-power electronic devices as a chip attaching material with low elastic modulus and superior thermal performance. The thermo-mechanical stability of joints, which join in resin-added sintered silver on nickel-/gold-electroplated substrate at 200 °C and 250 °C aging, is studied. At 200 °C aging, coarsening silver squeezes the resin into bonding interfaces, leading to a smooth fracture of the joint. The mechanical strength of the joint decreases with the increase in smooth fracture area proportion (SFP). The junction case thermal resistance of the sample increases from 0.97 to 1.12 K/W after 200 °C 500-h aging. At 250 °C aging, excessive silver–gold diffusion depletes the sintered silver and forms silver–gold solid solutions at the bonding interfaces. The mechanical strength of the joint sharply decreases with the increase in SFP. Fick’s second law is used to explain the formation of the solid solution. The diffusion coefficient and dimensionless parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> of gold at 24, 72, and 150 h under 250 °C aging are calculated.

Mode II cohesive zone law of porous sintered silver joints with nickel coated multiwall carbon nanotube additive under ENF test

Various contents of nickel coated carbon nanotube added sintered silver joints are fabricated through mechanical alloying. An analytical method is presented to compute the fracture energy of ENF tests incorporating shearing effect and thickness effect, which shows better accuracy compared with traditional method, which are closer to the solutions of compliance based beam method. Mode II fracture energy, mode II fracture toughness and mode II cohesive zone model for different contents of nickel coated carbon nanotube added sintered silver joints are presented based on the data extraction of the ENF tests. With the increase of adhesive layer thickness, the fracture energy and the mode II fracture toughness improve. However, thickness effect is less significant but not negligible comparing with the strengthening effect on the improvement of fracture toughness, which is caused by adding nickel coated multiwall carbon nanotubes. The relation between mode II fracture toughness and porosity is also discussed. For most of the conditions, the mode II fracture toughness is higher than mode I fracture toughness regardless of additive content due to the crack deflection, crack bridging and pull-out mechanisms during the crack propagation except for those high content of additive whose fracture energy is reduced because of aggregation effect.

Development of Ag-In Alloy Pastes by Mechanical Alloying for Die Attachment of High-Power Semiconductor Devices.

Sintered silver paste is widely used as the die-attachment material for power semiconductors. However, sintered silver joints encounter problems, such as severe coarsening of sintered pores and oxidation issues, in harsh high-temperature environments. These lead to the deterioration of the die-attachment joints. In this paper, a novel method of sintering silver joints is demonstrated, where silver–indium alloy paste is used to improve the reliability of sintered Ag joints. The silver–indium (Ag–In) alloy paste was fabricated through mechanical alloying using the ball-milling technique. The well-bonded sintered Ag–In alloy joints inhibited pore coarsening better than pure sintered Ag joints and significantly enhanced the mechanical properties at high operating temperatures. Lastly, an oxidation mechanism for the sintered joint was proposed, and strategies to prevent such high-temperature oxidation were discussed.

Low Temperature Sintered Silver Nanoflake Paste for Power Device Packaging and Its Anisotropic Sintering Mechanism

Sintered silver pastes have emerged as a kind of packaging material for wide-bandgap (WBG) power devices because of their superior electrical and thermal conductivity. Herein, silver pastes containing monodisperse triangular silver nanoflakes with a size distribution of 40–260 nm were prepared. The sintered silver joints exhibited a significant high shear strength of 49.8 MPa, larger than that of traditional silver paste, and with a relatively low porosity of 5.1%. These remarkable properties were attributed to the special anisotropic sintering process and bridging effect of the silver nanoflakes. Anisotropic sintering of the silver nanoflakes refers to the successive deformation of the corners, edges, and surfaces during sintering. Especially, the deformation of surfaces provides large diffusion paths for the silver atoms, resulting in a denser sintered silver joint. The bridging effect results from two separate silver nanoflakes that are linked by a single silver nanoflake, which increases the connection between the silver nanoflakes and thus improves the mechanical property of the sintered silver joints. This anisotropic sintering mechanism and bridging effect should inspire other researchers to investigate the sintering mechanism of other materials and optimize the performance of the corresponding pastes of these materials.

Low-pressure micro-silver sintering with the addition of indium for high-temperature power chips attachment

Sintered silver paste is a popular material for die attachment technology in power electronics. However, using traditional nano-silver paste when fabricating sintered joints has intrinsic material problems that cannot be overcome, such as the high cost of nano-silver particles and their potential health hazards. In this study, a novel micro-silver paste was utilized for sintering and bonding to overcome the limitations of nano-silver paste. Although the micro-silver paste is an excellent candidate to replace nano-silver paste for sintering processes to reduce the potential harm to humans, the oxidation at copper substrates on both sides of the pure silver joint at 300 °C causes mechanical reliability issues in long-term high temperature applications. Therefore, a micro-silver paste was sintered with the addition of indium to address the Cu oxidation problem in this study. It was found that Ag–In intermetallic compounds (IMCs) formed through a transient liquid phase reaction. The results demonstrated that sintered joints comprising Ag–In IMCs and Ag–In solid solution phases could effectively resolve the oxidation issues and significantly enhance the high-temperature reliability. Furthermore, the Ag–In IMCs exhibited excellent mechanical properties, withstanding the thermal stress induced during the thermal cycling test.

Laser-driven shocks to explore the effects of aging on the adhesion of silver sintered on copper substrate

The adhesion of thin silver joints sintered on metallic substrates is a major issue for their potential use in power electronics modules. Here, we explore how laser shock-induced spallation can provide a way to study this adhesion. Due to wave interactions inside the shock-loaded assembly, dynamic tensile stresses are induced near the copper-silver interface, resulting in partial or full removal of the silver layer. In order to investigate the effects of aging on the interface strength, experiments are performed on both as-sintered and aged samples. Time-resolved measurements of the free surface velocity provide information on wave dynamics and interface failure, but finite-element simulations show that inferring a quantitative value of the bonding strength from these data is still challenging. Nevertheless, post-recovery observations provide original insight into the debonding process, including some complex effects of aging, consistent with trends reported in the literature.

Large-Area Substrate Bonding With Single-Printing Silver Paste Sintering for Power Modules

Sintered silver with high thermal conductivity and reliability has been considered as a promising substrate bonding solution in power modules. The traditional double printing process is complex, and the second layer of silver paste is too thin to ensure complete wetting. We greatly simplified the sample preparation process by single printing and optimized the sintering process by means of open-face contact drying technology and staged heating method. In this article, the effect of predrying steps and heating rate on the quality of sintered silver joint on large area silver-plated DBC substrate and its mechanism are discussed. The microstructure shows that the samples sintered by the optimized sintering process have no obvious defects, and the sintered silver layer is dense and uniform. In order to evaluate the bonding quality of the whole joint, the sample was cut into 16 small pieces by metallographic cutter, the average shear strength was 55.65 MPa, and the strength distribution between the center and the edge of the sample was uniform. The samples sintered by an optimized sintering process show cohesive failure mode and typical ductile fracture morphology. The energy dispersive spectrum analysis shows that the composition distribution of the sintered silver layer in the center and edge of the sample was relatively uniform, which is consistent with the distribution of shear strength. The scanning electron microscopy image of cross section demonstrated that the porosity of the adhesive layer is 3.08%, which is far lower than that of the traditional double printing process, with good interface bonding and no delamination.