Nanosilver Paste Research Articles

Degradation of high power single emitter laser modules using nanosilver paste in continuous pulse conditions

This paper is mainly reported to a pulse reliability investigation of high power single emitter laser modules with nanosilver paste. Comparative experiments in continuous pulse conditions for the laser modules packaged with nanosilver paste, indium and AuSn solders were conducted. The results indicate that the laser modules attached by nanosilver paste have a longer-term lifetime than those with indium and AuSn solders in continuous pulse conditions. Transient thermal behavior and coupled thermo-mechanical behavior in continuous pulse conditions are simulated by finite element method (FEM). A semi-empirical model based on Arrhenius relationship is established to provide relative reliability assessments for laser modules by combining with the simulating results.

Effects of Die-Attach Material and Ambient Temperature on Properties of High-Power COB Blue LED Module

With the increasing demand of high-power light-emitting diode (LED), multichip modules have made great progress. A die-attach layer as the first-level packaging has a most significant impact on the thermal performance of a power module. In this paper, we introduce a novel die-attach material, nano silver paste, which can be used for connecting multichips on the substrate. This nano silver paste has a higher melting temperature and thermal/electrical conductivity than the conventional Sn3Ag0.5Cu solders and silver epoxy films. In addition, Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ceramic substrates were used to form a high-power LED chip-on-board architecture because of their high thermal conductivity and low coefficient of thermal expansion. Recently, because of their ability to emit high brightness, LED packages are being exploited for the automotive, aircraft, space exploration, nuclear industries, and in most cases are exposed to harsh environments. Therefore, the performance and stability of LED packages will be the key to assuring the reliability of LED modules. In this paper, the properties of the LED modules bonded separately with nano silver, Sn3Ag0.5Cu, and silver epoxy and operating under various ambient temperatures from 27°C to 100°C were determined. The test results showed that the nano silver paste is a very promising die-attach material for high-power multichip modules packaging.

Characterization of Nanosilver Dry Films for High-Temperature Applications

Nanosilver paste seems to be one of the most promising lead-free die-attach alternatives. The emergence of this technology is mainly due to the desired characteristics of the joint, such as high electrical and thermal conductivities, low elastic modulus offering a good thermomechanical reliability, low process temperature, and high operating temperature. Compared to traditional paste sintering, dry films are more appropriate for large area, do not need a drying process, and reduce the short-circuit risks for electrodes with narrow spacing. In this paper, the effects of some of the sintering profile variables (applied pressure, sintering temperature, and holding time) as well as the joint dimensions (thickness and area) on the sintered dry films were investigated by mechanical, thermal, acoustic, and microscopic analysis. The applied pressure and the holding time at the desired sintering temperature affect closely the die shear strength. In our tests, at the highest temperature (350 °C) and lowest pressure (3 MPa), the structural and thermal properties of the joint are almost similar to those obtained at 280 °C at four times higher pressure. These results can be very useful for assembling pressure-sensitive devices. Unlike a nanosilver-paste-sintered joint, the joint dimensions do not influence the shear strength values. Otherwise, the reliability of the joint was tested during high-temperature storage at 300 °C up to 2000 h and during thermal cycles between -40 °C and 160 °C. The storage at 300 °C for 1000 h did not lead to any degradation of the die shear strength, while a continuous decrease of the shear strength is observed during thermal cycling tests for the thinnest joint. The thermomechanical reliability can be improved by increasing the joint thickness.

Temperature Cycling Reliability Assessment of Die Attachment on Bare Copper by Pressureless Nanosilver Sintering

A nanosilver-sintering-enabled die-bonding technique has been widely applied for attaching power semiconductor devices onto the surfaces of noble metals such as silver or gold. Recently, high bonding strength (> 35 MPa) of die attachment has been demonstrated by sintering on bare copper surface. In this work, the temperature cycling reliability of joints processed by nanosilver sintering on bare copper surface was evaluated. Joints of silicon dice on copper were fabricated by pressureless sintering of nanosilver paste in forming gas. All the joints survived 1000 temperature cycles in the temperature range of −40 °C to 125 °C and with a cycling period of 60 min. Die-shear test results showed that the average die-shear strength of joints had no significant drop after 1000 cycles and remained over 30 MPa. X-ray tomography and scanning electron microscopy (SEM) were applied to detect the cracks or delamination generated during cycling. X-ray images show no significant change on the delamination percentage of die-attach area during cycling. Cross-sectional SEM images of the joints show no delamination formed at the silver/copper interface after 1000 cycles. We believe that strong metallurgical bonding and low interdiffusion rate at the silver/copper interface contribute to such great temperature cycling reliability.

Parametric Study on Pressureless Sintering of Nanosilver Paste to Bond Large-Area (≥100 mm2) Power Chips at Low Temperatures for Electronic Packaging

We have developed a new kind of nanosilver paste by reducing the stress and strain of the drying process with the help of some organics to bond large-area (≥100 mm2) power chips without additional pressure. This new nanosilver paste contains nanoparticles and microparticles ranging from 0.02 μm to 2 μm. The effects of sintering temperature (T), heating rate (ν), and holding time for sintering (t) on the microstructure and mechanical properties of sintered silver joints were investigated by the Taguchi method. The various factors were assigned to an L16 (43) orthogonal array. The experimental results showed that neck formation was strongly dependent on the increase of the sintering temperature, while prolonging the holding time and decreasing the heating rate were beneficial to neck growth. The pore size distribution ranged from 0.03 μm2 to 1.6 μm2, and the larger pores (0.8 μm2 to 1.6 μm2) were more irregular and clustered along the interstices between particles. The shear strength increased with increased sintering temperature, prolonged holding time, and decreased heating rate due to smaller pore size and a more homogeneous pore distribution. The fatigue and tensile behaviors of pressureless-sintered silver joints were also compared with those of soldered SAC305 joints.

The shear strength of nano-Ag sintered joints and the use of Ag interconnects in the design and manufacture of SiGe-based thermo-electric modules

Thermo-electric modules (TEMs) can be used to convert heat into electricity by utilizing the Seeback effect. It is now possible to buy BiTe thermo-electric modules that can operate up to temperatures of around 300°C. However, many applications, such as the harvesting of excess gas turbine heat, may occur at higher temperatures. Therefore, new materials and manufacturing processes need to be developed to produce packaged TEMs that can operate at a maximum operating temperature of 650°C. Two critical areas in the manufacture of a SiGe TEM are the choice and strength of materials used to both sintered joint the TE material to the rest of the module and the metal used for the interconnects. The interconnection material needs to be sufficiently strong to withstand large temperature fluctuations while maintaining a low contact resistance, as well as being compatible with the nano-Ag sintered joint. Shear force tests of the sintered thermo electrical leg material showed that the joints are brittle when sintered to W metallized AlN substrates are used and ductile fracture behavior when sintered to Cu metallized AlN substrates using the NanoTach K nano silver paste. Almost all of the joints were found to be brittle when using the NachTach X nano silver paste. Shear testing of the sintered joints showed that the X paste joints were variable in strength and stiffness, having a typical Young’s modulus between 10 and 100MPa at room temperature. The K paste joints were stiffer, but had a similar strength as compared to the X paste joints.

Test Results of Sintered Nano-Silver Paste Die Attach for High Temperature Applications

The emergence of wide band gap devices has pushed the boundaries of power converter operations and high power density applications. It is desirable to operate a power inverter at high switching frequencies to reduce passive filter weight and at high temperature to reduce the cooling system requirement. Therefore, materials and components that are reliable at temperatures ranging from −55 to 200 °C, or higher, are needed. Sintered silver is receiving significant attention in the power electronic industry. The porous nature of sintered nano-silver paste with a reduced elastic modulus has the potential to provide strain relief between the die component and substrate while maintaining its relatively high melting point after sintering. The test results presented herein include tensile testing to rupture of sintered silver film to characterize stress strain behavior, as well as die shear and thermal cyclic tests of sintered silver bonded silicon die specimens to copper substrates to determine shear strength and reliability.

Three-Dimensional Visualization of the Crack-Growth Behavior of Nano-Silver Joints During Shear Creep

Evolution of creep damage in nano-silver sintered lap shear joints was investigated at 325°C. Non-destructive x-ray three-dimensional (3D) visualization clearly revealed the crack-growth behavior of the joint; this could be divided into three stages. In the initial stage, little development of cracks occurred. In the second stage, cracks propagated at a consistent rate. In the final stage, rapid extension of the cracks led directly to fracture of the joint. Three-dimensional volume-rendered images and fractographic analysis showed that the growth of macroscopic initial cracks at the interfaces dominated the creep fracture process. Initial failure of nano-silver sintered lap shear joints often occurred at interfacial nano-silver paste layers. Both the size and position of the initial interfacial cracks had significant effects on the final creep failure of the joints, and higher stresses led to greater porosity and earlier failure.

Characterizations of Nanosilver Joints by Rapid Sintering at Low Temperature for Power Electronic Packaging

Nanosilver paste is a promising lead-free die-attach material suitable for power electronic packaging, particularly for high-temperature applications. Compared with traditional hot pressing to sinter nanosilver, rapid sintering by a pulse current is able to sinter nanosilver in less than a second. To investigate the nanosilver sintering process during rapid sintering, we characterize the temperature field of a nanosilver joint by using an infrared camera. The temperature field is analyzed as a function of current magnitude and current-on time. The relationship between the temperature field and the shear strength of joint is discussed to optimize the rapid sintering parameters. Results show that the joint's temperature–time curve varies with the current-on time. The shear strength can be up to 40 MPa, which is comparable to the robust hot-press sintered joint, when the peak temperature reaches above 400 $^{\circ}\hbox{C}$ . The microstructure of these joints is porous with a particle diameter of $\sim$ 400 nm, and this aids in releasing the internal stresses resulting in higher shear strength.

Migration of Sintered Nanosilver on Alumina and Aluminum Nitride Substrates at High Temperatures in Dry Air for Electronic Packaging

Joining semiconductor chips at low temperature (below 523 K) by sintering nanosilver paste is emerging as an alternative lead-free solution for power electronic packaging, particularly in high-temperature applications, because of the high melting temperature of silver (1234 K). However, silver is susceptible to migration. In this paper, we study the effects of dc bias, electrode spacing, and temperature on migration of sintered nanosilver on alumina (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) and aluminum nitride (AlN) substrates. The “lifetime” of silver migration, which is defined as the time at which the leakage current reaches 1 mA, increases with decreasing bias voltage and temperature but with increasing spacing between the nanosilver electrodes. The lifetime of silver migration on the AlN substrate is much longer than that on the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate. A phenomenological model is proposed to predict well the lifetime of migration of sintered nanosilver on both the AlN and Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrates in dry air. The activation energy of silver migration of sintered nanosilver on both Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and AlN is also obtained and discussed.

Review on Joint Shear Strength of Nano-Silver Paste and Its Long-Term High Temperature Reliability

Soldering has been the main die attach technology for several decades. Recently, in order to meet the high temperature electronic requirements (high temperature-operating SiC and GaN devices) as well as the health recommendations (replacing the toxic lead present in common solder alloys with lead-free alternatives), several new attach technologies have been developed. Among others, the sintering of nano-silver particles seems to be one of the most interesting choices, and has been extensively investigated during recent years. The emergence of this technology is mainly due to the desired high electrical and high thermal conductivities of the sintered joint, its low elastic modulus offering a good thermo-mechanical reliability, its low process temperature, and its high operating temperature. In this paper, a review of parameters affecting the initial shear strength of the sintered silver joint will be summarized as well as the high temperature long-term reliability issues. The sintering cycle (bonding pressure, bonding temperature, sintering dwell time, heating rate, and the sintering atmosphere), the joint size, and the attached materials properties (nature, roughness), are found to closely affect the initially measured shear strength of the joint. The long-term reliability of the joint has been shown to suffer initially from three phenomena: the silver electro-migration, the decrease of shear strength under harsh thermo-mechanical stresses, and the swelling of the sintered layer. While the latter phenomenon is observed during the storage at temperatures above 350°C, the electro-migration and thermo-mechanical stresses can influence the package reliability at temperatures as low as 250°C. However, some suggested precautions during the module fabrication can lead to the minimizing of the effects of these phenomena and the achievem a more reliable joint.

Evaluation of Thermal Cycling Reliability of Sintered Nanosilver Versus Soldered Joints by Curvature Measurement

Low-temperature silver sintering technology is emerging as a lead-free die-attach solution to significantly improve the heat dissipation and reliability packaging of power devices and modules die attached by solder alloys. With the recent introduction of nanosilver materials, which dramatically simplify the bonding process by lowering the required pressure down to a few megapascals, the silver sintering die-attach solution is poised for wide use in the manufacture of electronic products. In this paper, the thermomechanical reliability of sintered-silver joints was studied in comparison with soldered joints of two lead-free solders, SN100C and SAC305. Die-attach samples were fabricated by bonding 10 mm × 10 mm silicon mechanical chips to silver-metalized copper blocks and direct-bond copper (DBC) substrates according to the respective heating profiles of a nanosilver paste and the two solders. The die-attach samples were thermally cycled between -40 °C and +125 °C. The bonding reliability was evaluated by measuring the bending curvatures of the cycled samples and examining the cross sections of the samples under an electron microscope. Bending of the bonded structures, which is the result of mismatched coefficients of thermal expansion between silicon and copper or DBC, offered a nondestructive method for monitoring the integrity of the bond line. The bending curvatures of all of the die-attach samples decreased rapidly after they were thermally cycled. Most of the drop in curvature can be attributed to stress relaxation in the bonding materials without bond-line cracking. However, in the samples on copper blocks, after 800 cycles, the curvatures of the soldered samples decreased to near 0 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , but those of the silver-sintered samples still had about 30% of the original curvatures. Scanning electron microscopy images showed that the joints of the soldered samples with near 0 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> curvature had been cracked almost all the way through, but the joints of the sintered samples were still intact. These results demonstrate that the sintered-silver joints are more reliable than the soldered joints.

Pressureless sintering of nanosilver paste at low temperature to join large area (≥100 mm2) power chips for electronic packaging

We achieve robust bonding of large area power chip (≥100mm2) based on pressureless sintering of silver particles at low temperature, i.e., 250°C. The particle size of silver ranges from 0.02 to 2µm and most organics can be evaporated rapidly from 70°C to 180°C. It is not necessary for metallic bond to be formed entirely by silver nanoparticles. Instead, silver microparticles also can rapidly grow up and aggregated together by atom diffusion from the silver nanoparticles. This point of view leads to a new way to bond the large area chips without additional pressure. The sintered silver joint shows high shear strength, low porosity, high thermal conductivity, and low void ratio.

TSP 센서 전극용 Nano Silver Paste 제조 및 물성

TSP 센서 전극용 Nano Silver Paste 제조 및 물성

A diffusion-viscous analysis and experimental verification of defect formation in sintered silver bond-line

Abstract

Characterization and Reliability of Sintered Nanosilver Joints by a Rapid Current-Assisted Method for Power Electronics Packaging

A rapid current-assisted sintering technology (CAST) was used to bond electronic devices with nanosilver paste, which is a promising alternative compared with traditional solders. Instead of using an external heating source, the specimen and bonding material of nanosilver paste were heated by the alternating current whose current direction is not constant. Die-shear testing and hardness measurement were used to evaluate the bonding strength. The bonding strength and hardness increase with increasing the current and/or current-on time. CAST was able to sinter nanosilver in a very short time, i.e., less than 1 s. Scanning electron microscopy was used to characterize the microstructures of sintered silver joints. The reliability of the sintered joints by CAST was also evaluated by cyclic shearing tests. The joint by CAST could survive much longer time than that by the conventional hot pressing way. The CAST was recommended to bond two pieces of bare copper with nanosilver paste in power electronic applications without any protective gas atmosphere, e.g., bus bar connection.

Chip-Bonding on Copper by Pressureless Sintering of Nanosilver Paste Under Controlled Atmosphere

Pressureless sintering of nanosilver paste has been widely demonstrated for bonding power semiconductor devices on silver- or gold-coated substrates. In this paper, we extended the application of nanosilver bonding to bare copper surface by controlling sintering atmosphere in an effort to prevent surface oxidation of copper during sintering. Three different atmospheres were evaluated: 1) pure nitrogen (N2); 2) nitrogen with 1 vol% oxygen (1% O2/N2); and 3) a forming gas consisting of 4 vol% hydrogen and 96 vol% nitrogen (4% H2/N2). The bonding strengths of sintered joints were evaluated by die-shear test. The joints sintered in 1% O2/N2 had the lowest average bonding strength of approximately 15 MPa due to the formation of copper oxide clearly visible on the failure surface of the substrate. The joints sintered in 4% H2/N2 had the highest average bonding strength of over 40 MPa. Microstructure characterization of the sintered joints showed that the bond-line density was strongly affected by the sintering atmosphere. The joints sintered in the forming gas had a denser microstructure than those sintered in pure nitrogen. It is believed that the higher sintered density is a result of catalytic hydrogenation and hydrogenolysis of organic binder in the paste, which occurred at the sintering temperature with nanoparticles of silver as catalyst. This increase in microstructure density and prevention of copper surface oxidation led to the high bonding strength of the sintered joints formed on copper in the forming gas.

SiC Die Attach for High-Temperature Applications

Eutectic solders AuIn19 and AuGe12 and nanosilver paste were investigated for SiC die attach in high-temperature (300°C) applications. The soldering or sintering conditions were optimized through die shear tests performed at room temperature. In particular, application of static pressure (3.5 MPa) during sintering resulted in greatly improved mechanical behavior of the nanosilver-based joint. Microstructural study of the eutectic solders showed formation of Au-rich grains in AuGe die attach and significant diffusion of Au and In through the Ni layer in AuIn19 die attach, which could lead to formation of intermetallic compounds. Die shear tests versus temperature showed that the behaviors of the studied die attaches are different; nevertheless they present suitable shear strengths required for high-temperature applications. The mechanical behavior of joints under various levels of thermal and mechanical stress was also studied. Creep experiments were carried out on the eutectic solders to describe the thermomechanical behavior of the complete module; only one creep mechanism was observed in the working range.

Low-Pressure Joining of Large-Area Devices on Copper Using Nanosilver Paste

Low-temperature joining technology using nanosilver paste has been widely demonstrated for attaching power chips on silver or gold metallized substrates. In this paper, we investigate the processing conditions of nanosilver paste for bonding large-area chips on a plain copper surface. A double-print, low-pressure-assisted sintering process is developed for attaching the chips on copper. An evaluation criterion used in the process development is the bond strength of mechanical chips that are made of alumina and sintered on direct-bond-copper (DBC) substrates. The bond strength, measured by the die-shear test, is found to be in excess of 40 and 77 MPa at sintering pressures of 3 and 12 MPa, respectively, during sintering. Characterization of the bondline microstructure reveals a void-free sintered joint, the density of which increases, with increasing sintering pressure. Sintering in air causes partial oxidation of the copper surface, but the oxide can be easily removed by dipping in 1% hydrochloric acid solution. To evaluate the impact of the bonding and acid-cleaning process on device characteristics, a large-area insulated-gate-bipolar-transistor (IGBT) chip is bonded to DBC substrate and then wire-bonded for electrical testing. Test results shows that the die-attach process does not alter the IGBT performance.

Creep properties of low-temperature sintered nano-silver lap shear joints

As a potential alternative to conventional solders and adhesive films, low-temperature sintered nano-silver paste, as a result of its high thermal and electrical conductivity (240Wm−1K−1 and 4.1×107Sm−1, respectively) and high melting temperature (961°C), can be used for connecting chips that require high temperature operation and high heat dissipation ability. In the present work, a number of creep tests were carried out to study the effect of stress and temperature on the creep behavior of low-temperature sintered nano-silver lap shear joints. The steady-state creep flow behavior of the joints was discussed. In general, the creep shear strain rate increased as the applied stress and ambient temperature increased. When the creep strain rate was described by the Arrhenius power-law model, we found that the stress exponent and activation energy were temperature dependent and stress dependent, respectively. A modified power-law model with a temperature-dependent stress exponent and stress-dependent activation energy was proposed to describe accurately the creep flow of the sintered lap shear joints.