nano-Ag Paste Research Articles

Ag-Pd nanoalloy film with ultra-high thermal and electrochemical stability for power electronic packaging

Nano-Ag paste is a popular material in power electronic packaging, while its microstructure is unstable at high temperatures and suffers from electrochemical migration. In this work, Ag-Pd nanoalloy film has been prepared by the pulsed laser deposition, and been sintered at low temperature for packaging. The thermal stability is significantly improved at 300 ℃ when Ag is alloyed with Pd. The Pd addition significantly increased the diffusion activation energy of Ag atoms and reduced the diffusion coefficient. The Ag-Pd sintered layer exhibited a stable microstructure at 300 °C or even 500 °C, which indicated that Ag-Pd nanoalloy had the potential to serve at high temperatures. Ag-Pd nanoalloy will preferentially generate a PdO passivation film and cover the anode surface, thereby slowing down the dissolution of the Ag and improving its resistance to electrochemical migration. This work provides theoretical studies and insights into the applications of Ag-Pd nanoalloys in high-reliability power electronics.

Improving thermal stability and reliability of power chips by sintering foam structure layer

In order to ensure the high-temperature reliability of device in high temperature service, a new sintered structure layer, nano-Ag paste filling graphene reinforced Ni foam, was designed to realize the chip packaging in this work. This layer had excellent reliability and high-temperature heat dissipation stability, which benefit from the higher proportion of low-angle grain boundaries, finer grains and excellent heat dissipation capacity of graphene. The foam structure had favorable stress release effect, which made the sintered layer had high service reliability. The excellent heat dissipation ability of graphene overcomes the inherent defect of slow heat dissipation of the device under high temperature environment.

Effect of the paste spreadability on ultrasonic transducer fabricated by pressureless sintering of nano-Ag

Paste spreadability is critical to paste spread layer and thus the quality of final product in pressureless sintering of nano-Ag, but the metrics and the underlying mechanics have not yet been well acknowledged. In this work, various ultrasonic transducers are fabricated by pressureless sintering of various nano-Ag pastes. Particle dynamics mechanism during the paste spreading process and the relationship between paste viscosity and spreadability are clarified. The influences of paste spreadability on dynamic performances of transducer are analysed, and the underlying mechanisms are also investigated. The results show that the normalized mass of Ag NPs is basically positively correlated with the flatness of the coupling layer, and the flatness of the coupling layer is also positively correlated with the peak-to-peak voltage and SNR of the transducer. The attenuation in the far-field scattering process mainly depends on the porosity of the coupling layer.

An analytical review on interfacial reactions in high-temperature die-attach: The insights into the effect of surface metallization and filler materials

This review aims for a comprehensive insight into the interfacial reactions and the effects of surface metallization in high temperature die-attach, which is critical in governing the reliability of resultant interconnects of power electronic modules. With the emerging high-temperature filler materials, the interfacial interactions and microstructural evolutions exhibit various distinctive features, which demands detailed examination in order to identify suitable surface finishes. The metallization is not always beneficial to improving the quality and reliability of the interconnects, and consideration should also be given to the cost-effectiveness and manufacturability. The intermetallic compounds (IMCs) formed during interfacial reactions are of prime concerns for an extended discussion, together with the formation of solid solution at interfaces which can also be an important topic to explore. In this work, five commonly-encountered high temperature solder fillers are examined, including high-Pb, Au-based, Bi-Ag, Zn-Al solders and nano Ag paste, the emphasises are put on the interactions with different metallized surfaces in die-attach. The effects of metallization on interfacial reactions and formation of interfaces are discussed. Finally, the recommendations of suitable metallization are outlined to enable cost-effective and reliable interconnects for the related applications.

High-Temperature Reliability of Nano-Ag Sintered Joints on Ag-Plated Cu Substrates

As an emerging lead-free packaging interconnect material, Nano-Ag paste exhibits superior high-temperature mechanical, electrical, and thermal properties. It can meet the stringent high-temperature and high-density packaging requirements of future high-power semiconductor devices and is garnering widespread attention. However, research on the high-temperature reliability of sintered joints remains rather limited. In this study, we fabricated high-strength hot-press sintered nano-Ag joints using a hot-press sintering process. These joints underwent aging at high temperatures of 200°C and 300°C, yielding insights into the variations in mechanical properties at different temperatures. The reasons behind joint failure were analyzed by investigating the evolution of joint microstructures and fracture surfaces. The results indicate that even after extended aging at 200°C, the joint strength can still be maintained at 176 MPa. However, after aging for over 100 hours at 300°C, the joint fails. The growth of Cu oxides was observed within the joint microstructure, which constitutes the primary cause of joint failure at 300°C.

A novel coupling method for ultrasonic transducer based on pressureless sintering of nano-Ag

The acoustic performance of ultrasonic transducer is directly determined by the coupling between the piezoelectric unit and the medium. At present, the coupling methods of the transducer are relatively single. In this work, a novel ultrasonic transducer coupling method based on pressureless sintering of nano-Ag is reported. The effects of sintering temperature on the micro-morphology and macro-porosity of nano-Ag sintered bulk (i.e. demoulded coupling layer) are systematically investigated. The transducers are design and fabricated by one-time pressureless sintering of nano-Ag paste. The influences of sintering temperature on acoustic performance of the transducers are analysed by experiments in detail. The results show that the number of the pores in the pressureless sintered bulk decrease with the sintering temperature, while the pores size gradually increase. From the perspective of acoustic impedance matching, silver materials have great advantages and universality as the coupling layer between most piezoelectric wafers and steel mediums. The coupling effect of sintered Ag could be improved by increasing the temperature. The best coupling effect could be obtained by sintering at 350 °C, at which the primary echo of the transducer has ideal waveform and bandwidth.

Microstructure and Bonding Strength of Low-Temperature Sintered Ag/Nano-Ag Films/Ag Joints

Nano-Ag paste is one of the most widely used die-attachment materials in modern electronic devices, which are gaining continuously increasing application in transportation industries. The nano-Ag film in a pre-formed dimension and free from the use of chemical dispersing agents has been proposed to be a promising alternative to nano-Ag paste for the die-attachment application. Although the bonding mechanisms of Nano-Ag paste have been extensively studied, little is known about the relationship between the microstructure and mechanical properties of low-temperature-sintered Ag/nano-Ag film/Ag joints. In this work, the influences of temperature, pressure, and dwell time at peak temperature on the microstructure and the shear strength of low-temperature-sintered Ag/nano-Ag film/Ag joints were systematically investigated. Mechanical properties tests indicate that both temperature and pressure have pronounced effects on the bonding strength of sintered Ag/nano-Ag film/Ag joints. TEM and hot nanoindentation characterizations further reveal that the sintering temperature plays the most determinant role in the coarsening of nano-Ag film and, hence, the bonding and fracture behaviors of Ag/nano-Ag film/Ag joints sintered at 210–290 °C. The diffusion-induced coarsening of nano-Ag particles can be activated, but remains sluggish at 250 °C, and the mechanical integrity of sintered joints is circumscribed by the interfacial bonding between nano-Ag film and Ag substrate after sintering at 290 °C.

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.

Effects of Bonding Materials on Optical-Thermal Performances and High-Temperature Reliability of High-Power LED.

The die-bonding layer between chips and substrate determinates the heat conduction efficiency of high-power LED. Sn-based solder, AuSn20 eutectic, and nano-Ag paste were widely applied to LED interconnection. In this paper, the optical–thermal performances and high-temperature reliability of LED with these bonding materials have systematically compared and studied. The thermal conductivity, electrical resistivity, and mechanical property of these bonding materials were characterized. The LED module packaged with nano-Ag has a minimum working temperature of 21.5 °C. The total thermal resistance of LED packaged with nano-Ag, Au80Sn20, and SAC305 is 4.82, 7.84, and 8.75 K/W, respectively, which is 4.72, 6.14, and 7.84 K/W higher after aging for 500 h. Meanwhile, the junction temperature change of these LEDs increases from 2.33, 3.76, and 4.25 °C to 4.34, 4.81, and 6.41 °C after aging, respectively. The thermal resistance of the nano-Ag, Au80Sn20 and SAC305 layer after aging is 1.5%, 65.7%, and 151.5% higher than before aging, respectively. After aging, the LED bonded with nano-Ag has the better optical performances in spectral intensity and light output power, which indicates its excellent heat dissipation can improve the light efficiency. These results demonstrate the nano-Ag bonding material could enhance the optical-thermal performances and high-temperature reliability of high-power LED.

Reduction of thermal resistance of Ag-coated GFs/copper structure using nano-Ag paste as interconnection

Reduction of the thermal resistance between graphene films (GFs) and substrate is crucial to the application of GFs in thermal management. GFs/copper structures were prepared using nano-Ag paste as interconnection material. The effect of the thickness of nano-Ag paste on thermal resistance of GFs/copper structure was investigated. A thin layer of Ag was coated on GFs by physical vapor deposition (PVD) to further reduce thermal resistance. The thermal resistance of GFs/copper structure using Ag-coated GFs is 5.84% lower than that using raw GFs. The thermal resistance of GFs/copper structure decreases first and then increases with the increase of coating temperature and thickness of Ag layer. The minimum thermal resistance of 1.64 mm2·K·W−1 was gained for GFs/copper structure using GFs coated Ag at 300 °C for 60 min.

Self-assembled monolayers for electrochemical migration protection of low-temperature sintered nano-Ag paste

Self-assembled monolayers for electrochemical migration protection of low-temperature sintered nano-Ag paste

Nano Ag sintering on Cu substrate assisted by self-assembled monolayers for high-temperature electronics packaging

Sintering of nano Ag paste on bare Cu has attracted more interests recently for high-temperature electronics packaging, which offers the advantages of high reliability, cost-effective and direct bonding process. However, the current bonding methods normally need a protective atmosphere or metallization on Cu substrate to avoid oxidation. In this study, self-assembled monolayers (SAMs) were deposited on Cu substrate to suppress oxidation prior to nano Ag sintering. Thermal-compression bonding process of Cu/nano Ag/Cu joints was conducted and analysed with and without SAMs treatment. The cross-sectional characterization and shear tests were conducted to evaluate the influence of SAMs treatment. When SAMs applied, shear strength of 12.72 MPa has been achieved in the ambient atmosphere, which is much higher than the value without SAMs treatment (3.77 MPa). It has been identified that the shear mode changed from the interfaces of sintered nano Ag/Cu to inside of sintered nano Ag due to the applied SAMs. This technological approach provides a tangible and cost-effective method for high-temperature electronics packaging.

Effect of Cu Foam and Ag-Coated Layer on the Microstructure and Mechanical Behavior of Nano-Ag Sintered Joint

Abstract Cu foam (Cu-F) and Ag-coated Cu-F were added into nano-Ag paste to obtain Cu-F@nano-Ag composite sintered joint. The microstructure, hardness, and shear behavior of the sintered joints were investigated. Experimental results indicated that the addition of Cu-F and Ag-coated Cu-F suppressed the generation and propagation of cracks at the interface of the sintered joint. As the thickness of the Cu-F increased from 0.1 mm to 0.2 mm, the deformation ratio of the Cu-F sheet raised from 12% to 50%. Thereby, the hardness and bonding strength of the sintered joint was improved due to the microstructural densification. The bonding quality between Cu-F and sintered Ag is enhanced by the Ag-coating treatment. Therefore, the Ag-coated composite joints show higher shear strength than the others.

Cu-Cu joining using citrate coated ultra-small nano-silver pastes

In this study, ultra-small nano-Ag particles with an average diameter of 4.76 nm and excellent stability have been synthesized on a large scale to meet low-temperature, low-pressure, and lead-free packaging requirements for modern high-power electronic applications. Citrate coated nano-Ag particles’ agglomerations have been used in the form of a nano-Ag paste to solder by a robust Cu to Cu direct bonding. The bonding strength of the formed Cu-Cu joint reaches 28.2 MPa when sintered at 260 °C for 30 min and 1 MPa. The bonding mechanism of such a direct Cu-Cu bonding by using nano-Ag paste has been facilitated by the assistance of an in-situ sintering of nano-Ag particles. The analysis of the bonding has been performed using a transmission electron microscopy (TEM) and a focused ion beam (FIB). The results show that the bonding mechanism can be interpreted by two parts: (1) self-sintering process of nano-Ag paste, and (2) interfacial bonding between sintered nano-Ag paste and Cu substrate.

Development and Practical Application of Low-Temperature Sintering Nano Ag for Semiconductor IC Bonding

In recent years, lead-free has been promoted by the RoHS Directive which bans the use of lead in electronic devices, and nano Ag die attach pastes has attracted attentions. Nano Ag particles show high electrical and thermal conductivity, but generally require Au or Ag plating on the surface of the substrate. Therefore, demands for Cu substrate without the plating are also increasing to reduce costs. This paper deals with the fundamental study on nano Ag pastes newly developed with a unique approach. The technology provides a low-temperature sintering capability without pressure during cure process. In addition, resin reinforcing technology and lowering modulus technology have been developed to improve the mechanical properties. By adding epoxy resins, the porous area is filled with the resin and the sintered structure is reinforced. Additionally, the reliability can be more improved by thermoplastic resin particles. To meet the demand for the applications without Ag or Au plating, nano Ag pastes for bare Cu substrate have newly been developed. By using several types of Cu substrates without plating, surface analysis and bonding properties of the pastes were investigated in this study.

Microstructural evolution and degradation mechanism of SiC–Cu chip attachment using sintered nano-Ag paste during high-temperature ageing

The high-temperature reliability of nano-Ag pastes on bare Cu substrates is of great significance in power electronics. Although Cu metallization in sintered joints is susceptible to oxidation in air, methods to inhibit the oxidation under high temperatures are rarely studied. In this paper, a SiC–Cu interconnection with an average shear strength of 161.7 MPa was fabricated using nano-Ag paste by a two-step sintering process. The microstructure of the sintered silver was investigated by the transmitted Kikuchi diffraction (TKD) technique, and the formation of robust joints was attributed to the low porosity, fine gain size and high density of twins. Pores in the sintered silver were isolated, which effectively impeded the penetration of oxygen and growth of Cu oxide during high-temperature thermal storage. Two kinds of failure mechanisms were determined: the growth of a Cu2O band and the evolution of a dense sintered layer. Overall, we propose a sintering method for SiC–Cu interconnections that could guarantee long-term service at 200 °C and short-term service at 300 °C in air.

High Electrical and Thermal Conductivity of Nano-Ag Paste for Power Electronic Applications

The nano-Ag paste consisted of Ag nanoparticles and organic solvents. These organics would be removed by evaporation or decomposition during sintering. When the sintering temperature was 300 °C, the resistivity of sintered bulk was 8.35 × 10−6 Ω cm, and its thermal conductivity was 247 W m−1 K−1. The Si/SiC chips and direct bonding copper (DBC) substrates could be bonded by this nano-Ag paste at low temperature. The bonding interface, sintered microstructure and shear strength of Si/SiC chip attachment were investigated by scanning electron microscopy, transmission electron microscopy and shear tests. Results showed that the sintered Ag layer was porous structure and tightly adhered to the electroless nickel immersion gold surface of DBC substrate and formed the continuous Ag–Au interdiffusion layer. The shear strength of Si and SiC chip attachments was higher than 35 MPa when the sintering pressure was 10 MPa. The fracture occurred inside the sintered Ag layer, and the fracture surface had obvious plastic deformation.

Ag-Sn bimetallic nanoparticles paste for high temperature service in power devices

In this paper, high strength Cu−Cu interconnections were achieved by sintering the paste of Ag-Sn bimetallic nanoparticles at low temperature. Compared with nano-Ag paste, the outer Sn coatings of the nano-Ag particles were found to be favorable for the densification of the bondline. The microstructures of Ag-Sn bimetallic nanoparticles and the bondlines under different sintereing conditions were studied in detail by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The Ag-Sn bimetallic nanoparticles bondline exhibit a high shear strength of 35.3 MPa and a low resistivity of 9.5 μΩ cm, when sintered at 260 °C for 20 min under a pressure of 0.5 MPa. The electrochemical migration time of this sintered Ag-Sn bimetallic nanoparticles was prolonged to be ten times of that of sintered nano-Ag. This bonding technology based on Ag-Sn bimetallic nanoparticles was a promising die attach method for high temperature power device packaging.

Doping low-cost SiOx (1.2

As a die-attach material, nano-Ag paste is extensively applied in the packaging of power semiconductor devices. However, silver is highly prone to migration, particularly at high temperatures. In this paper, we studied the inhibition effect of doping different proportions (0.1% and 0.2%) of low-cost SiOx (1.2 < x < 1.6) nanoparticles (NPs) instead of noble metals to nano-Ag paste on the electrochemical migration (ECM) of silver at high temperatures. The ECM tests showed that silver migration was significantly postponed by the doping of SiOx NPs and the lifetime of sintered Ag-0.2%SiOx was ∼2.6 times than that of sintered Ag at 400 °C and 200 V. The great news was that the mechanical, thermal as well as electrical performances were almost not impacted by the doped SiOx NPs. The mechanism of inhibiting silver migration by doping SiOx NPs was proposed as that the oxidation of SiOx hindered the oxidation of silver. Then a lifetime model was determined to predict the failure of both the sintered nano-Ag and the sintered Ag-SiOx at high temperatures to provide theoretical guidance for the reliability design of the high temperature package of power semiconductor devices.

Microstructure and mechanical properties of nano-Ag sintered joint enhanced by Cu foam

Copper foam (Cu-F) was added into nano-Ag paste to obtain nano-Ag@Cu-F composite sintered joint. The microstructure, hardness, and shear behavior of the joints were investigated in comparison with the nano-Ag sintered joints. Experimental results indicated that the addition of Cu foam significantly decreased the void percentage at the sintered interface. The average hardness of the nano-Ag joints sharply increased from 0.64 to 1.46 GPa as the sintering time extended from 5 to 15 min. In contrast, the nano-Ag@Cu-F sintered layers were condensed within 5 min and the hardness reached 1.41 GPa. Further extension of the sintering time led to slow increase of the hardness. The shear strength of the sintered joints was enhanced due to the addition of Cu foam. Compared with the nano-Ag sintered joints, the composite joints showed greater shear distance to failure during the shear test. The shear fracture occurred in the sintered layer instead of the interface in the nano-Ag@Cu-F sintered joints since the bonding interface was enhanced by Cu foam addition. As the sintering time prolonged, the bonding quality of the sintered joints was improved and led to the increasing of shear strength.