Ag Nanoparticle Paste Research Articles

Rapid preparation of sintered Ag nanoparticles pastes for high-power electronic packaging

This study developed a low-temperature pressureless sintered Ag nanoparticles (AgNPs) paste with high thermal conductivity (TC), which can be produced on a large scale through rapid pre-treatment. Commercially available unmodified AgNPs were coated with polyvinylpyrrolidone (PVP), and subsequently, citric acid (CA) was used to remove the native oxide layer from the AgNPs surface and improve sintering activity. Compared to the treatment strategy of CA activation followed by PVP coating, applying PVP coating prior to CA activation could prevent their spontaneous bonding that will significantly promote the formation of large grains and strong sintering necks, reducing the interface scattering and leading to an ultra-high TC (≥200 W/mK), thereby meeting the packaging requirements of high-power electronic applications.

Low-resistance joints for YBCO-coated conductors with Ag nanoparticle paste

Due to the limited available piece length of YBCO-coated conductors (i.e. tapes or wires) and the different requirements for magnetic field, joints are inevitable for manufacturing high-temperature superconducting magnets. In this study, a sintering nano-silver (Ag) process was developed and used to connect YBCO tapes stabilized by anAg layer with low-temperature and short-time sintering of Ag nanoparticle (NP) paste. The thermodynamic characteristics of Ag NP paste were explored using a TG/DSC setup. The effects of sintering temperature, mechanical pressure and lapped length on microstructures and electrical properties of joints were comprehensively investigated. It is found that the pre-volatilization of low-boiling-point solvent in the paste is beneficial to improve the densification of sintered structure, thus contributing to increasing the critical current I c of the joint. With increasing sintering temperature, the I c of the joint will be close to that of the virgin tape, and the joint resistance experiences small fluctuations, but joint connectivity is enhanced. As the temperature reaches 205 °C, I c decreases to 84% of the virgin tape, and joint resistance increases obviously. In addition, the axial tension strength at room temperature is improved with the increase in mechanical pressure, while the resistance does not demonstrate distinct variation. Considering the electromechanical properties, the optimal joining process is determined as sintering at 180 °C and 30 MPa for 10 min. The joint with this technology possesses a closely connected interface and a well-sintered nano-Ag microstructure with pores. By further extending the lapped length, a YBCO joint resistivity as low as ∼10.56 nΩ cm2 is obtained, which is around a quarter of that of the soldering joint, and the process is much easier than that of the Ag diffusion joint.

Fabrication and Sintering Behavior of Nano Cu–Ag Composite Paste for High-Power Device

Cu and Ag nanoparticle (NP) paste has emerged as a promising choice for high-power devices because of its excellent thermal and electrical properties. However, easy oxidation, high sintering temperature of Cu NPs, high cost, and high porosity of Ag NPs restrict the practical application. Although Cu–Ag composite paste can improve bonding reliability, there have been no reports of fabricating stable and reliable Cu–Ag composite paste joints. In this work, we reported a novel method for fabricating a nano Cu–Ag composite paste with excellent dispersibility and relatively uniform particle size distribution via polyol one-step reduction. These composite NPs exhibit outstanding antioxidant capacity. The sintering behavior of the composite NPs at different temperatures was analyzed. The electrical and mechanical reliability of nano Cu–Ag composite paste joints with different ratios were investigated. With the increase in Ag content, the porosity and resistivity of the joint decrease, and the shear strength increases. The bonding joint presents a low porosity of 1.92% and a high shear strength of 32.6 MPa under a 1:3 ratio of Cu to Ag. The molecular dynamics (MDs) simulation results imply that Ag-rich composite system can achieve low-temperature sintering and maintain the stability of the sintering process.

Fabrication of superhydrophobic coatings by low-temperature sintering of Ag nanoparticle paste

Ag nanoparticle paste was used to fabricate mechanically durable superhydrophobic surfaces via sintering. A facile method was first adopted to synthesize the Ag nanoparticle paste. To obtain multi-level hierarchical superhydrophobic surfaces, a stainless steel mesh was introduced to the sintering process of the Ag nanoparticle paste. The morphology of the steel mesh was replicated onto the Ag surface under sintering pressure, forming a first-level surface roughness. Using low-temperature sintering, a bond was created between the Ag nanoparticles, enhancing the mechanical durability of the as-prepared surfaces. The gap between the Ag nanoparticles was retained under low pressure, leading to a high surface roughness, which is beneficial for preparing superhydrophobic surfaces. After modification with stearic acid, the as-fabricated Ag coatings exhibited good superhydrophobicity, long-term durability, and mechanical stability. The superhydrophobic Ag coatings can also be fabricated on different substrates successfully. Thus, this type of superhydrophobic coating has promising prospects in practical applications.

High Strength Die-Attach Joint Formation by Pressureless Sintering of Organic Amine Modified Ag Nanoparticle Paste.

Sintered silver (Ag) die-attach has attracted much attention in power systems with high power density and high operating temperature. In this paper, we proposed a novel surface modification method for Ag nanoparticles with organic amines as a coating agent for enhancing the pressureless sintering performance. This work systematically introduced the Ag nanoparticle modification process, Ag paste preparation, and sintering process and compared the changes in the sintering performance of Ag nanoparticles after modification with four different alkyl chain lengths of amines. The study showed that the sintered films of Ag nanoparticle pastes modified with n-octylamine (NOA) can achieve the lowest resistivity of the sintered film and the highest shear strength of the bonded joints. The resistivity of the sintered Ag film is affected by the grain size and microscopic morphology, and the strength of the bonded joints is also related to the sintering density and the amount of organic residues. The thermal behavior of the Ag particles coated with different amines is measured by thermal analysis. Finally, the mechanism of NOA-modified Ag nanoparticles to improve the sintering performance is proposed. This study can provide effective data and theoretical support for the further promotion and application of nano-Ag pressureless sintering.

A Review of Sintering-Bonding Technology Using Ag Nanoparticles for Electronic Packaging.

Metal nanoparticles (NPs) have attracted growing attention in recent years for electronic packaging applications. Ag NPs have emerged as a promising low-temperature bonding material owing to their unique characteristics. In this study, we mainly review our research progress on the interconnection of using polyol-based Ag NPs for electronic packaging. The synthesis, sintering-bonding process, bonding mechanism, and high-temperature joint properties of Ag NP pastes are investigated. The paste containing a high concentration of Ag NPs was prepared based on the polyol method and concentration. A nanoscale layer of organic components coated on the NPs prevents the coalescence of Ag NPs. The effects of organic components on the bondability of the Ag NP paste were studied. Compared to the aqueous-based Ag NP paste, the polyol-based Ag NP with the reduction of organic component can improve the bondability, and the coffee ring effect was successfully depressed due to the increased Marangoni flow. The sintering behaviors of Ag NPs during the bonding process were investigated using the classical sphere-to-sphere approach. The mechanical property of joints using this Ag paste was better than that using Pb95Sn5 solders after storage at high temperatures. The sintering–bonding technology using polyol-based Ag NPs was helpful to the low-temperature interconnection for electronic packaging applications.

Electrochemical migration behavior of low-temperature-sintered Ag nanoparticle paste using water-drop method

The electrochemical migration (ECM) behavior of low-temperature-sintered Ag nanoparticle paste electrode was investigated using water-drop method. The effects of applied direct current bias voltage and NaCl concentration on ECM behavior were discussed. Results show that: at low NaCl concentration levels, mean time to failure caused by ECM decreased with increasing NaCl concentration due to the increase of solution conductivity; at medium NaCl concentration levels, dendrite growth was retarded due to the formation of AgOH or other insoluble products, while at high NaCl concentration levels, due to the turbulent hydrogen evolution, the second spreading of water drop resulted in the circuit broken and thus inhibited ECM behavior. Moreover, mean time to failure decreased with the rising applied bias voltage. More importantly, hexadecyltrimethylammonium chloride (CTAC) was proved to act as an effective inhibitor to migrate dendrite growth by selected adsorption. And the ECM inhibition efficiency increased with the concentration of CTAC. Relevant mechanisms of ECM behavior and ECM inhibition effect of CTAC for low-temperature-sintered Ag nanoparticle paste were proposed.

Large-Area Die-Attachment Sintered by Organic-Free Ag Sintering Material at Low Temperature

Bonding a large-area die by Ag nanoparticle paste sintering faces a lot of challenges, and the voids/cracks caused by residual organics of the paste are some of the obstacles. The organic burnt out during the sintering process and degassing of the residue organics during service cause significant concerns. In this study, organic-free Ag nanoparticle material has been developed by pulsed laser deposition technology. Oweing to being organic-free, no void, crack and delamination were observed in the sintered Ag layer of die-attachment, and the large-area die-attachment could be carried out by a simple sintering profile at low temperature. The die shear test indicated that the bonding strength was superior to the standard. Porosity and densification of the sintered Ag layer could be controlled by sintering profiles, including the processing pressure, temperature and time.

Effect of epoxy content in Ag nanoparticle paste on the bonding strength of MLCC packages

Ag particles have been attractive candidates for highly reliable interconnections in electronic packages due to an absence of IMCs, high melting temperature, and high electrical and thermal conductivity. We fabricated various Ag-nanoparticle (NP) pastes containing different epoxy/binder percent weight (epoxy: 4.5, 9.0, 13.5, and 18 wt%) to investigate the effect on the bonding strength and electrical performance of multilayer ceramic capacitor (MLCC) components. MLCC joints were obtained by stencil printing Ag NP pastes on an Al2O3 substrate with Cu electrodes followed by pressureless bonding at 250 °C for 15 min. When using Ag NP pastes containing higher epoxy compositions, more residues were identified while the other organic additives, such as the binder and dispersant, were removed by debinding during the bonding process. The Ag NP-18 wt% epoxy paste showed bonding strength approximately 8.6 times that of the paste containing 4.5 wt% epoxy. However, the electrical resistivity of MLCC components of 18 wt% epoxy paste showed an electrical resistance of 3.5 Ω, which was about 1.8 times that with the 4.5 wt% epoxy paste. We designed Ag NP pastes with varying epoxy ratio, and studied their chemical, bonding, and electrical characteristics for the development of advanced interconnection.

Comparative study of how additives affect sintered silver microstructure in die-attach applications

Abstract Die attach materials based on silver nanoparticles which sinter at temperatures in the range 200–300 °C are a comparatively new technology. The properties of the sintered structure can be affected by a wide range of additives which can alter the physical and chemical characteristics of the joints. In this study, a commercially available Ag nanoparticle paste has been used as the base, and a range of additives have been added principally to determine the effect of each additive on the sintered microstructure immediately after sintering, and after long term thermal ageing. The additives trialled include Au, Sn, Cu, and Zn. In each case the additive powder was mixed with the original paste and the microstructure after sintering was compared to the microstructure after ageing at 250 °C for 24 h. Another method of introducing an additive into the system is adding it as a mesh, interposed between die and substrate and immersed in the silver paste. Au was added in both this form, and in the form of a powder additive. The mesh results in a thermodynamically stable microstructure up to at least 500 °C. The design takes advantage of solid–solid interdiffusion bonding which results in a die attach assembly with a continuous, non-porous gold-silver interdiffusion layer running all the way from the die to the substrate.

Pressureless sintering multi-scale Ag paste by a commercial vacuum reflowing furnace for massive production of power modules

Air is critical to Ag sintering. The Cu surface of the non-metallized direct-bonding-copper (DBC) substrate can be oxidized seriously for bonding power devices using Ag paste in air. In this paper, we presented a pressureless sintering approach for a multi-scale Ag paste in the formic acid vapor by a commercial vacuum reflowing furnace. The study found that the activation temperature of the formic acid vapor had an obvious effect on the sintering behavior of the multi-scale Ag paste. The multi-scale Ag paste could obtain much denser sintered Ag when the formic acid vapor was rapidly injected at 180 °C by the vacuum reflowing furnace. The sintered Ag necks larger than 0.4 μm and the porosity lower than 11.3% were achieved in formic acid vapor, which had better sintering behavior than Ag nanoparticles (NPs) paste. The thermal resistance of 1200 V/50 A half-bridge IGBT modules by sintering the multi-scale Ag paste in formic acid vapor was 0.41 °C/W, which was ~ 12% lower than those of the commercial IGBT modules using Pb92.5Sn5Ag2.5. This work could overcome the contradiction between the Ag sintering and the Cu oxidation of the non-metallized DBC substrate by the formic acid vapor in a commercial vacuum reflowing furnace. The method is extremely useful for the traditional manufacturer, who do not have to invest any new facilities for the sintering.

Rapid formation of Cu–Cu joints with high shear strength using multiple-flocculated Ag nanoparticle paste

A new type of Ag nanoparticle (NP) paste for rapid sintering was prepared by controlling the thickness of the NP capping agent. The Ag NP paste was combined with a rapid thermo-compression (RTC) method to form a reliable Cu–Cu joint. The shear strengths of the Cu/Ag NP/Cu joints were 46.8 MPa and 90.7 MPa after sintering at 300 °C for only 5 s and 20 s, respectively. The sintering sequence phenomenon was observed, and the differences in microstructure between the pressure-assisted and pressureless joints were investigated. Robust bonding at the lattice level between the Ag NPs and Cu substrate was observed by high-resolution transmission electron microscopy, and this bonding contributed to the high shear strength obtained under rapid sintering conditions. A commercially acceptable alternative solution to achieving reliable Cu–Cu joint formation was obtained, especially for thermo-sensitive devices.

Preparation of Ag Nanoparticles Coated with Silver Stearate for Low-Temperature Sinter-Bonding

Silver nanoparticles (NPs) coated with silver stearate [CH3(CH2)16COOAg] have been prepared using a thermal decomposition method. Near-spherical Ag NPs with average size of 4.4 nm were obtained after thermal decomposition at 523 K. The effects of the organic acid and thermal decomposition temperature on the characteristics of the Ag NPs were investigated. For stearic acid and lauric acid with straight-chain structure, a longer chain of the organic acid contributes to the achievement of Ag NPs of smaller size, while ascorbic acid with cyclic structure results in relatively large-sized (approximately 11.7 nm) Ag NPs. Small-sized Ag NPs with uniform size distribution were obtained after thermal decomposition at temperature of 473 K or 523 K. However, after thermal decomposition at temperature of 573 K, the particle size of the Ag NPs increased and obvious aggregation occurred, which may be caused by the absence of the silver stearate coating. Low-temperature sinter-bonding of bare Cu was realized using Ag NP paste at 623 K under pressure of 5 MPa in air atmosphere. The microstructure of the joint revealed formation of a dense sintered Ag filler layer and its favorable interfacial bonding with the Cu substrates.

Effect of SiC reinforcement on the reliability of Ag nanoparticle paste for high-temperature applications

Ag nanoparticle (NP) paste is considered to be a next-generation thermal interface material. However, its sintered microstructure is at risk of volume shrinkage and mechanical property degradation under harsh environments. In this work, silicon carbide (SiC) NPs were mixed with Ag NP paste to improve the stability and reliability of sintered Ag joints. Robust joints were prepared by a rapid thermal compression method at 275 °C for 20 s. Although the addition of SiC NPs degraded the initial shear strengths of these joints, the SiC-containing joints exhibited excellent mechanical properties and microstructural stability during thermal shock tests from − 50 to 150 °C for up to 1000 cycles. When the added amount of SiC NPs was 2%, the joints achieved the highest shear strength of 87.1 MPa (after 1000 cycles). In addition, the shear strengths of the joints were investigated at 250 °C. The addition of SiC NPs is believed to enhance the reliability of joints at high temperature.

Preparation of Oxidation-Resistant Ag-Cu Alloy Nanoparticles by Polyol Method for Electronic Packaging

Ag or Cu metal nanoparticle paste can be used as a bonding material for electronic packaging applications. However, Ag nanoparticle paste has some drawbacks including high cost and being prone to ion migration in high-humidity conditions. The main obstacle to using Cu nanoparticle paste is rapid oxidation in air during heating. In this work, we describe a method to prepare Ag-Cu alloy nanoparticle paste by a polyol chemical reduction method combined with subsequent concentration. Characterization with ultraviolet–visible spectroscopy, transmission electron microscopy, x-ray photoelectron spectroscopy and energy dispersive spectrometry confirm the formation of the Ag-Cu alloy structure. During the synthesis of Ag-Cu alloy nanoparticles, an Ag core forms initially, followed by codeposition of Ag and Cu. Most of the Ag-Cu alloy nanoparticles have a truncated octahedral shape with twin structures located at the edges. This Ag-Cu alloy nanoparticle paste has a good oxidation resistance up to 350°C in air atmosphere. Using the Ag-Cu alloy nanoparticle paste, joints were formed at a low sinter-bonding temperature of 160°C. Shearing tests confirm the formation of robust joints, with an average shear strength of 50 MPa.

Low temperature bonding with high shear strength using micro-sized Ag particle paste for power electronic packaging

Bonding technology using a Ag nanoparticle paste has been a promising substitute for high-Pb-containing solder joining. However, it has some drawbacks, such as high cost and low compatibility, due to its high sintering temperature. Recently, the bonding process using the micro-sized Ag particle paste has been studied for cost-effectiveness, but it is limited in terms of high applied pressure or relatively low joint strength. In this study, a low pressure of 0.4 MPa was applied during bonding by micro-sized Ag particle paste for lower bonding temperature and higher joint strength. The micro-sized Ag paste was composed of chestnut-burr-like and spherical particles. Under low applied pressure, the joint with a high shear strength of 54.6 MPa was achieved when the bonding temperature and time were 260 °C and 10 min, respectively. It was also possible to reduce the bonding temperature and time for the joints having similar strength to that of the pressureless process. The microstructure showed a linear relationship to the shear strength, regardless of the bonding conditions.

Sintering Bonding Process with Ag Nanoparticle Paste and Joint Properties in High Temperature Environment

Ag nanoparticle paste is prepared based on the polyol method and subsequent concentration by centrifuging. The sintering bonding process using Ag nanoparticle paste at different bonding pressures is studied. The joint strengths are increased as the bonding pressure increases from 0 MPa to 7.5 MPa. This is due to the fact that the higher assistant bonding pressure is beneficial to the growth of neck size between the adjacent particles and forms denser sintered Ag layers. The joint strength bonded under 10 MPa is lower than that bonded under 7.5 MPa, which may be due to the residue of organic component in the sintered Ag layer. The joint properties bonded with Ag nanoparticle paste in high temperature environment are evaluated by heat treatments at temperatures ranges of 200–350°C for 50 hours. The results show that the mechanical properties of joint with Ag nanoparticle paste are better than the joint with Pb95Sn5 solder after storage at high temperatures.

Pressureless Bonding by Micro-Sized Silver Particle Paste for High-Temperature Electronic Packaging

Lead-free bonding in high-temperature electronic components is desirable for realizing eco-friendly technology. A pressureless process is more appropriate for electronic packaging because it enables a more automated manufacturing process and avoids any potential damage caused by application of pressure. Recently, Ag nanoparticles were used without pressure to join materials for high-temperature electronic applications. In this study, a Ag paste of micro-sized particles was proposed for electroless nickel immersion gold (ENIG) finished Cu pressureless bonding owing to its advantages of both cost effectiveness and easy manufacturing process compared to Ag nanoparticle paste. The micro-sized Ag paste was composed of both chestnut-burr-like (CBL) and spherical particles. The weight ratios of CBL to spherical particles were 10:0, 7:3, and 5:5. The bonding process was carried out at 573 K for 60 min in a nitrogen atmosphere. The experimental results showed that all of the sintered layers had an open porous structure. ENIG-finished Cu joint using the Ag paste of the 5:5 weight ratio exhibited the shear strength of 18.6 MPa, which is comparable to that of a conventional Pb-5Sn joint.

Die-attaching silver paste based on a novel solvent for high-power semiconductor devices

A new solvent composed of 4-(tert-butyl) cyclohexyl acetate as a dilute agent and HPMDA as a thickener was developed to make micron-sized Ag paste and to bond power semiconductors. The new Ag paste achieved sentinel printing to form sharp Ag patterns with uniform thickness and defined boundaries, which is a key to printing fine pitches in the electronics industry. The electrical resistivity of Ag patterns of 3 μΩ cm was obtained at 280 °C for 30 min. Two Ag-plating copper substrates were successfully bonded with the new Ag paste and the shear strength gradually increased with the bonding temperature. A high strength of 80 MPa was achieved at 280 °C under a small sintering pressure of 0.4 MPa. This value is far higher than the value obtained by using an expensive Ag nanoparticle paste. The detailed mechanism is discussed in this work by comparison with common ethylene glycol solvent.

The critical role of Ag nanowires in the improvement of conductivity and flexibility of circuits fabricated with hybrid Ag nanopaste

Research into printable ink or paste materials has led to continued improvements in the electrical and mechanical properties of electrodes and circuits fabricated by solution-based processes on flexible polymer substrates. We introduce a facile approach to the production of highly-conductive flexible circuits via a hybrid silver (Ag) nanopaste, employing Ag nanowires (NWs) within a Ag nanoparticle paste. The hybrid Ag nanopaste allows for a significantly lower electrical resistivity of 14.2 μΩ cm as compared to that of conventional Ag nanopaste after annealing for 30 min at 150 °C. The addition of Ag NWs also improves the fatigue stability of Ag nanoparticle-based circuits; the electrical resistance of the hybrid Ag nanopaste annealed at 150 °C was sustained at the smallest rate of increase, 12 %, during 106 cycles of repetitive sliding deformation. Microstructural characterization reveals that Ag NWs in the mixture behave as conductive bridges to reduce contact resistance between Ag nanoparticles, and at the same time, as obstacles to suppress the propagation of cracks under cyclic mechanical deformation.