Laser sintering of thick-film PTC thermistor paste deposited by micro-pen direct-write technology

In this paper, laser sintering has been used to process thermal sensitive material deposited by micro-pen direct-write technology. Lines of thick-film PTC thermistor paste were deposited by micro-pen direct-write technology and irradiated by a continuous-wave laser with varied power density and scanning speed. Line widths, sheet resistivities and temperature coefficients of resistivity were measured as a function of laser power density, scanning speed, coating thickness and overlapped value. The mechanism of laser sintering thermistor paste was discussed. Laser sintering enabled finer thermistor lines to overcome the resolution limitation of micro-pen deposition without the use of any high temperature post processing. The thermistors exhibited excellent electrical performance and resistance tolerance equivalent to traditional oven-fired electronic paste.     

Characterization of direct patterned Ag circuits for RF application

We investigated the effects of sintering temperature on microstructural evolution and electrical characteristics of screen printed Ag patterns. A commercial conducting paste containing Ag nanoparticles was screen printed onto a Si substrate passivated with SiO₂ and sintered under a sintering temperature range from 150 °C to 300 °C. Four point probe method was used to measure the DC resistance, while a network analyzer and Cascade’s probe system in the frequency range from 10 MHz to 30 GHz were employed to measure the S-parameters of the sintered Ag conducting patterns. The resistivity under the application of a DC decreased from 398 μΩ cm to 9 μΩ cm with increasing sintering temperature from 150 °C to 300 °C. From the measured S-parameters, the electrical losses in high frequencies also decreased with increasing sintering temperature (about 1.2 dB at 30 GHz) due to the formation of an interparticle neck after heat treatment at high temperatures.     

Electrical properties and electrochemical migration characteristics of directly printed Ag patterns with various sintering conditions

In this study, we investigated the phenomena of electrochemical migration (ECM) on a silicon (Si) substrate with directly printed Ag pattern using the screen-printing method. The microstructures and electrical characteristics of the directly printed Ag patterns under different sintering conditions were estimated. In addition, the ECM characteristics of the directly patterned Ag circuits on Si substrates were evaluated according to the sintering conditions. Clusters were formed by interparticle necking during the sintering process; the cluster size in the patterns increased as the sintering temperature and time increased. Granular Ag films, which were sintered at high temperatures and for long time periods of time, had excellent electrical characteristics as a result of the formation of interparticle necking of sufficient size. Also, the directly printed Ag patterns exposed to higher sintering temperatures and longer sintering times had higher resistances to ECM than those exposed to lower sintering temperatures and shorter sintering times.     

Microstructure,Electrical Properties,and Electrochemical Migration of a Directly Printed Ag Pattern

In this study, we investigated the phenomenon of electrochemical migration (ECM) on a silicon (Si) substrate with a silver pattern directly printed using the screen-printing method. The microstructure and electrical characteristics of the directly printed Ag pattern under different sintering conditions were examined. In addition, the ECM characteristics of a directly patterned Ag circuit on a Si substrate were evaluated and compared with those of an immersion Ag circuit. Clusters were formed by interparticle necking during the sintering process; the cluster size in the patterns increased with increasing sintering temperature. The granular Ag films sintered at high temperatures had excellent electrical characteristics as a result of the formation of interparticle necking with sufficient size, despite the presence of porosity and large voids. The directly printed Ag pattern had lower resistance to ECM than the immersion Ag electrode.     

A Simple and Facile Iodination Method for Improving Sinterability and Electrical Conductivity of Silver Thick Films

Micro-sized silver powders were decorated with nano-scaled Ag/AgI clusters on the surface via a simple reaction with iodine and subsequent exposure to sunshine. Surface morphologies, crystal structures, and thermal properties of the powders were characterized. The powders with different mole ratios of I:Ag (0:100, 2:100, 10:100) were employed in silver pastes to evaluate sinterability and electrical conductivity of thick films. Microstructures and sheet resistance of the films were investigated by scanning electron microscopy and the four-point probe method. The particles coated with and without nano-sized Ag/AgI clusters showed different sintering behaviors. Moreover, clear necks were formed between the Ag particles with the ratio of 2:100 even at 570°C, whereas those untreated remained discrete. However, over-decoration lowered sinterability and electrical conductivity.     

Low-temperature low-pressure die attach with hybrid silver particle paste

New types of die attach pastes comprising micron-sized Ag particles hybridized with submicron-sized Ag particles were considered as lead-free die attach materials for SiC power semiconductors. Micron-sized Ag particles in alcohol solvent were prepared by mixing the die attach paste with submicron-sized Ag particles. The alcohol vaporizes completely during sintering and no residue exists in the bonding layer. The Ag layer has a uniform porous structure. The electrical resistivity of the printed tracks decreases below 1 × 10^?5 Ω cm when sintered above 200 °C. When sintered at 200 °C for 30 min, the average resistivity reaches 5 × 10^?6 Ω cm, which is slightly higher than the value obtained by using Ag nanoparticle paste. A SiC die was successfully bonded to a direct bonded copper substrate and the die-shear strength gradually increases with the increase in bonding temperature up to 300 °C. The Ag die attach bond layer was stable against thermal cycles between ?40 °C and 300 °C.     

A Simple Process for Synthesis of Ag Nanoparticles and Sintering of Conductive Ink for Use in Printed Electronics

Various-sized Ag nanoparticles capped with oleylamine were synthesized by means of a thermal decomposition process for low-temperature electronic devices. The Ag nanoparticles, which had diameter of 5.1 nm to 12.2 nm, were synthesized in incubation and ripening stages related to nucleation and growth. After the Ag nanoparticles were made into ink with a proper solvent, inkjet printing and thermal sintering methods were used to form a metal thin film with thickness of 100 nm. A type of thermal sintering related to percolation transformation and surface sintering was conducted at a temperature much lower than the melting point of bulk Ag. The electrical resistivity was examined with the aid of a four-point probe system and compared with the resistivity of bulk Ag, showing that the Ag film had much higher resistivity than bulk Ag. To improve the electrical stability and properties, we applied hexamethyldisilazane (HMDS) surface treatment to the substrate and dipped the as-deposited films into methanol. Both treatments helped to diminish and stabilize the resistivity of the printed conductive films.     

无铅银浆烧结工艺与导电性能研究

制备了无铅低温玻璃粉,将其与银粉和有机载体混合配制成无铅导电银浆并烧结。通过SEM和EDX观察浆料烧结银膜的形貌并进行成分分析,用四探针测试仪测量烧结银膜的电阻率,讨论了浆料成分配比、烧结时间、烧结温度等方面对银膜导电性能的影响。确定了无铅导电银浆的最佳配比为:质量分数w(银粉)72%,w(玻璃粉)3%和w(有机载体)25%,最佳烧结温度为580℃,最佳保温时间为5min。     

Improvement of Bondability by Depressing the Inhomogeneous Distribution of Nanoparticles in a Sintering Bonding Process with Silver Nanoparticles

Low-temperature bonding by sintering of Ag nanoparticles (NPs) is a promising lead-free bonding technique used in the electronic packaging industry. In this work, we prepare Ag nanoparticle (NP) paste using both an aqueous method and a polyol method. Sintering bonding trials were then conducted using different forms of Ag NPs. The results showed that use of the aqueous-based Ag NP paste led to inhomogeneous distribution of NPs, known as the ??coffee-ring effect.?? This led to low strength of fabricated joints. We investigated the influence of the coffee-ring effect and ways to depress it by changing the bonding material composition. Our results show that, when using polyol-based Ag NP paste as the bonding material, the coffee-ring effect was successfully depressed due to increased Marangoni flow. The corresponding shear strength of joints was increased significantly to 50?MPa at bonding temperature of 250°C.     

活性钎焊电子浆料流变性能及丝印质量的研究

研究了固含量及有机载体对电子浆料流变性能及丝网印刷质量的影响。采用流变仪测量钎焊电子浆料的黏度和触变性,并结合丝印后形貌特征,通过优化电子浆料的配方来改善丝印质量,对后续烧结过程形成性能优良的陶瓷覆铜板三明治结构具有很好的指导意义。获得了优化的浆料配方为:固含量为质量分数89%,有机载体成分质量组成为:丙二醇苯醚溶剂93.53%,乙基纤维素粘结剂5.97%及LD-9108型流平剂0.5%。获得的浆料黏度为46.039 Pa·s,触变指数为2.6136。     

The Effect of High-Pressure Sintering Process on the Microstructure and Thermoelectric Properties of CoSb<Subscript>3</Subscript>

The high-pressure sintering process is studied for the fabrication of the bulk CoSb₃ thermoelectric material. The CoSb₃ powder is prepared by a solid reaction method, and then the samples are sintered under high-pressure conditions. The emphasis of the present study is on the influence of the pressure on the grain size and the electrical properties of the material. For the present study, the pressure is taken to be from 1.5 GPa to 6 GPa, and the sintering temperature is 723 K. The experimental results show that the major phase of skutterudite and traces of metal impurities of Sb and the CoSb₂ phase coexist in some of the samples, and that the grain size of all the samples increase after sintering. In the range of 3.0 GPa to 6.0 GPa, the grain size increases with increasing pressure. In the range of 1.5 GPa to 3.0 GPa, the grain size also increases, but with a diminishing growth rate. All the materials show p-type transport behaviors, and the sample sintered under 2.0 GPa shows higher electrical conductivity than the 5.7 GPa sample, which may be due to the impurities.     

Room-Temperature Sintering Process of Ag Nanoparticle Paste

Recently, the authors developed a novel room-temperature wiring method using Ag nanoparticle paste. In this paper, the sintering mechanism of the Ag nanoparticle paste was clarified through examination of the adsorption stability and the removal of the dispersant from the Ag nanoparticles. The Ag nanoparticles in the paste are protected by dodecylamine as a dispersant. This paste possesses substantially long shelf life and thermo stability at room temperature. When the printed line of the Ag nanoparticle paste is dipped in a methanol bath, methanol effectively dissolves and removes the dispersant from the nanoparticles. Ag nanoparticles are sintered within a short period. The sintering of Ag nanoparticles is not uniform. Some Ag nanoparticles quickly grow and form a network by sintering (necking). The others maintain a nanometer scale. Large Ag particles and an Ag skeleton continue to grow by absorbing very small nanoparticles. In addition, the use of ethanol and isopropanol as the treatment agent is shown for the first time.     

MLCC制作过程中分层开裂原因分析

MLCC是经过多层材料堆叠共烧后制成的,在其制作过程中,产生分层是比较严重的质量缺陷之一,严重影响MLCC在使用过程中的可靠性。对MLCC产生分层的原因进行分析归类,总结出MLCC产生分层的原因为：制作MLCC内浆与瓷粉TMA（热机械分析）曲线匹配性不佳;MLCC在烧结前存在排胶不良;烧结回火温度或氧含量高。     

An aqueous developable photoimageable silver conductor composition for high density electronic packaging

In our preliminary work, we have formulated aqueous developable photoimageable thick film conductor pastes, consisting of ‘inhouse processed’ submicron sized silver powders (functional material), micron sized lead borosilicate glass frits (permanent binder), epoxy acrylate resin with pendant –COOH group (base photoimageable polymer/temporary binder/organic vehicle) and 2,2′-dimethoxy-2-phenylacetophenone (photoinitiator). The conductor pastes thus formulated were manually screen printed on the alumina substrates, dried, exposed to the ultra-violet light through the desired test patterns, developed in a 1% aqueous sodium carbonate solution, and subjected to a standard one-hour thick film firing cycle. Solid content of the polymer appears to influence the paste performance. Prima facie observations indicate that the paste with the organic:inorganic ratio of 28:72 (corresponding to solid content of 89.9% of the polymer system) exhibits better electrical conductivity, relatively smooth surface finish and line/space resolution of 100 μm with ±5 μm accuracy. Investigations related to the effect of glass content on the properties of photoimageable conductor paste are also furnished in this communication.     

Termination of BME–MLCC Using Copper–Nickel Bimetallic Powder as Electrode Material

Microstructure of termination of a base metal electrode multilayer ceramic capacitor (BME-MLCC) using copper-nickel bimetallic powder as electrode material was investigated. Thick film paste containing nonagglomerated monodispersed copper-nickel bimetallic powder was applied in preparing the termination electrode of a BME-MLCC. Influence of an inorganic binder on the microstructure of the termination of BME-MLCC was studied. The distribution of metallic copper on the surface of glass bubbles in termination was investigated by energy dispersive X-ray. A scanning electron microscope and a polarized light microscope were applied to discuss the microstructure of thick film. The results show that suitable binder and firing temperature result in the excellent connection between internal and termination electrode, as well as one between termination electrode and green chip. The BME-MLCCs with a dense surface of end termination, high adhesion, and qualified electrical behavior were obtained. The rough interface from interfacial reaction between glass and chip gives high adhesion     

采用水基黏合剂制作MLCC工艺的研究

采用水基PVA黏合剂为主要原材料,研究了水基PVA黏合剂在瓷浆制备、流延、生坯层压等关键工艺,制作出MLCC,并与溶剂型PVB黏合剂MLCC比较.结果表明:采用水基PVA黏合剂能够制作出性能良好的MLCC.     

Mechanical Deformation of Sintered Porous Ag Die Attach at High Temperature and Its Size Effect for Wide-Bandgap Power Device Design

The mechanical properties of sintered Ag paste with microporous structure have been investigated by tensile and shear tests, focusing on the temperature-dependent plastic deformation at various temperatures from 25°C to 300°C, corresponding to the target operating temperature range of emerging wide-bandgap semiconductor devices. Specimens were prepared by sintering hybrid Ag paste consisting of microflake and submicron spherical Ag particles, simulating a typical bonding process for power semiconductor die attach. Mechanical tests revealed that the unique microstructure caused a brittle-to-ductile transition at temperature of around 160°C, remarkably lower than that of bulk Ag. The obtained Young’s modulus and shear modulus values indicate obvious softening with increasing temperature, together with a remarkable decrease in Poisson’s ratio. These plastic behaviors at elevated temperature can be explained based on Coble creep in the microporous network structure. Fracture surfaces after tensile and shear tests indicated unique features on scanning electron microscopy, reflecting the variation in the ductile behavior with the test temperature. Furthermore, these temperature-dependent mechanical parameters were employed in three-dimensional finite-element analysis of the thermomechanical stress distribution in wide-bandgap semiconductor module structures including Ag paste die attach of different sizes. Detailed thermal stress analysis enabled precise evaluation of the packaging design for wide-bandgap semiconductor modules for use in high-temperature applications.     

Effect of Ag photo-doping on structural,optical and phase change properties of GeTe chalcogenide films

This paper reports the effect of Ag photo-doping on optical, electrical and structural properties of GeTe:Ag thin films. The absence of sharp diffraction peak confirms the amorphous nature of as-deposited and photo-doped films. The decrease in reflectance of GeTe:Ag bilayer films with photo-doping reaction time has been observed. The RAMAN spectrum showed the characteristic Raman bands for GeTe₄ (127 cm⁻¹), long chain interactions of Te–Te chains (145 cm⁻¹) and a broad peak for Ge–Ge vibrations (275 cm⁻¹) without appreciable change in their position/shape with photodoped Ag concentration. The electrical resistivity measurement shows that photo-doping of Ag led to sharp amorphous-crystalline phase transition along with an increase in transition temperature and resistivity value in both amorphous as well as crystalline state. The annealing of photo-doped GeTe:Ag samples showed an enhancement in the crystallinity of GeTe phase without any segregation of Ag phases in annealed samples. The preferential formation of GeTe (200) phase upon crystallization has been observed for GeTe:Ag films. Different optical parameters have been calculated for photo-doped and annealed samples and are discussed in conjunction with the modification of network structure of GeTe with inclusion of photo-doped Ag content.     

Tensile Behaviors and Ratcheting Effects of Partially Sintered Chip-Attachment Films of a Nanoscale Silver Paste

The mechanical behaviors of partially sintered thick films of a nanoscale silver paste used for attaching semiconductor chips are studied. The films, about 150 μm thick, were made by repeatedly stencil-printing the paste on a ceramic substrate and sintering by a recommended heating profile suitable for device attachment. The partially sintered films were lifted off the substrate, and their tensile behaviors, i.e., stress–strain curves, were measured at temperatures between −60°C and 300°C using a dynamic mechanical analyzer (DMA). The elastic modulus and tensile strength of the sintered silver films decreased with increasing temperature. Ratcheting behaviors of the films under cyclic tension at 150°C were also tested by using the DMA by examining the effects of loading rate, mean stress, and stress amplitude. The ratcheting strain grew with increasing mean stress or stress amplitude and with decreasing loading rate.     

Electrically Conductive Thick Film Made from Silver Alkylcarbamates

A homogeneous electrically conductive silver paste without solid or particle phase was developed using silver alkylcarbamates [(C _n H_2n−1NHCOO)₂Ag, n ≤ 4] as the precursor of the functional phase. The silver alkylcarbamates were light insensitive and had a low decomposition temperature (below 200°C). The paste was a non-Newtonian fluid with viscosity significantly depending on the content of the thickening agent ethyl cellulose. Array patterns with a resolution of 20 μm were obtained using this paste by a micropen direct-writing method. After the paste with about 48 wt.% silver methylcarbamate [(CH₃NHCOO)₂Ag] precursor was sintered at 180°C for 15 min, an electrically conductive network consisting of more than 95 wt.% silver was formed, and was found to have a volume electrical resistivity on the order of 10⁻⁵ Ω cm and a sheet electrical resistivity on the order of 10⁻²–10⁻³ Ω/□. The cohesion strength within the sintered paste and the adhesion strength between the sintered paste layer and the alumina ceramic substrate were tested according to test method B of the American Society for Testing and Materials standard D3359-08. None of the sintered paste layer was detached under the test conditions, and the cohesion and adhesion strengths met the highest grade according to the standard.