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Ag sinter-joining technology has become one of the leading candidates to replace conventional high-Pb soldering technology for wide bandgap (WBG) power modules. In many cases of die-attach applications, Ni/Au plating is an inevitable choice for both die-back faces and substrates. In this study, a preheating treatment of the Ni/Au plating was utilized to increase the bond strength of the Ag sinter-joined structure of Cu-to-Cu to the electroless Ni/Au plating. Our research determined that the microstructure of the Ni/Au plating can be modified by performing an optimized preheating treatment. The thickness of the Ni underlayer was fixed at 5 μm, and the Au was plated with two different thicknesses of 0.1 μm and 0.3 μm. Ni/Au plated Cu substrates and Cu chips were preheated at various temperatures from 150°C to 350°C and then bonded with hybrid Ag particles at a low temperature of 250°C with a pressureless sintering process. Without the preheating treatment, average shear strength was 14.2 MPa (as plated Au), however, after a 1 h preheating treatment at 250°C, the interface microstructure change resulted in a 26.3 MPa shear strength, an increase of about 85%. The same improvement effect was also confirmed for the Au plating layer with a thickness of 0.3 μm. Energy dispersive x-ray spectrometry (EDS) and x-ray diffraction (XRD) analysis indicated that the preheating treatment influences the grain structure of the Au plating layer and removes defects, resulting in an increase of bond strength for the Au plated joint structure.

A novel die-attach material, Cu particle paste with self-reduction and self-protection characteristics, was designed by simply adding ascorbic acid (AA) into Cu paste. The self-reduction characteristic is due to the addition of AA into the paste, as it reduces the Cu oxide layer on the metal Cu, even at room temperature. The self-protection characteristic is due to the decomposition of AA preventing further oxidation during sintering process. These characteristics are beneficial for the sintering of Cu particles and for enhancing the bondability. The increase to bonding strength is the direct result of the concentration of AA in the Cu paste. The bonding strength of Cu joints sintered from Cu paste with 1.3 wt% AA addition was as high as 24.8 MPa, thus making it attractive as a die-attach material. On the other hand, the bonding strength of those sintered from Cu paste without AA addition was only 9.7 MPa. These results strongly suggest that the self-reduction and self-protection characteristics resulting from AA addition are necessary and crucial for sintered joints fabricated from readily oxidized Cu particle paste. This novel Cu paste is an efficient, safe, and environmentally-friendly die attachment material, especially well-suited for applications in high power semiconductor devices. Keywords: Cu particle pastes, Self-reduction, Self-protection, Ascorbic acid, Sintered Cu joints, Power electronics

A simple and large-scale polyol method was utilized to successfully synthesize bimodal Cu particles in the air. The size, size distribution, and morphology of Cu particles were determined by the reaction parameters, such as Cu source and additives. The insoluble Cu source is suitable for the formation of bimodal Cu particles as they provide a slow and steady supply of fresh Cu nuclei, which modify the nucleation and growth of Cu particles. In addition, sodium sulfide (Na2S) additive has the ability to control the size of Cu particles due to its reduction capability. As-synthesized bimodal Cu particles were used to fabricate sintered Cu joints for high-power electronic packaging. The results show that the shear strength of sintered Cu joints was determined by the size and size distribution of Cu particles. Sintered Cu joints with high shear strength above 20 MPa can be readily achieved at 300 °C with a slight applied pressure of 0.4 MPa. This strength is superior to joints utilizing nanoparticle Cu paste. The size-matching effect and the high oxidation resistance of the bimodal Cu particles were attributed to the excellent mechanical performance of the sintered Cu joints. Our results indicate that this affordable paste utilizing bimodal Cu particles of a suitably designed size and with a uniform distribution is highly promising as the die-bonding material for next-generation power devices.

Silver nanowires (AgNWs) show great potential in the fabrication of high-flexible transparent conductive films (TCFs) and reduced graphene oxide (rGO) has been considered as one of the most promising overcoating materials for improving the long-term stability of AgNW TCFs. However, transmittance, conductivity, and flexibility of AgNW TCFs are often compromised during the assembly of rGO film. Herein, a highly efficient process of preparing a high-performance and highly stable AgNW TCF is proposed. Firstly, a water-based, self-assembled coating method is used to achieve GO/AgNW TCFs. Then, the light absorption property of AgNWs is utilized to assist the reduction of the graphene oxide (GO) layer through the use of a microsecond intense pulsed light (IPL) irradiation treatment, resulting in a high-performance rGO/AgNW TCF. The instantaneous high-temperature rising from AgNWs provides additional and abundant thermal energy during the irradiation to reduce the ultra-thin GO film without damage to the nanostructure of the GO layer or to the AgNW TCFs itself. The achieved rGO/AgNW TCFs have a low sheet resistance to 8 Ω/sq and a high transmittance of 86.2% owing to the in-situ reduction of rGO film from thin GO film and welding between the AgNWs. In addition, this technique presents a robust enhancement of AgNW TCFs, not only with regard to their electrical stability in high-temperature and high-humidity atmospheres but also to their overall mechanical stability, which significantly enhances the application prospects of AgNW TCFs for use in flexible optoelectronic devices.

Homogeneous columnar Cu film with fully embedded nanotwins was successfully fabricated on Ti/Cu seed layer on silicon wafer. The nanotwins with thickness of tens of nanometers are generally parallel to the silicon surface, showing a strong (111) preferred orientation. The acid concentration was found to be important in influencing the formation of nanoscale twins. By adjusting the acid concentration, the nanotwins can be induced from the top columnar grain to middle columnar grain and reach the bottom equiaxed grain, and a microstructural transformation model was given. A theory focusing on the cathode overpotential was proposed to reveal the effect of acid concentration on the growth mechanism of nanoscale twins. An appropriate adsorption proportion of hydrogen on cathode (acid concentration 17 ml L−1) could increase the overpotential which supplies adequate nucleation energy for nanoscale twins formation.

Next generation power semiconductors, e.g. SiC and GaN, are emerging for the further minimization and high current/voltage of power devices with high reliability covering wider operating environments than those based on Si. To implement high reliability operation, the key technology is the control of the temperature distribution in the module, and thermal stress caused by the heat generated by power loss. In the present study, we have developed SiC micro-heater chip with temperature probe to evaluate thermal characteristics of an assembled system of Ag sinter die-attach on metalized ceramic substrate (Cu/Si3N4/Cu) during the repetitive power cycling. The test specimens were fixed on a water cooling system, and steady-state heat resistance of the system was measured during the power cycling. For comparison, Pb-Sn, Sn-Cu-Ni-P, Sn-Ag-Sb-Cu solders were used as die-attach material bonded on the same metalized ceramic substrates. The maximum applied power exceeds 200 W with cycles of 2 seconds of heating and 5 seconds of cooling, and the test cycles was over 5000 cycles. The power cycle number dependence on the temperature swing and thermal resistance characteristics would be discussed, in connected with the power cycle testing for real power devices.

Ag sinter-joining is an ideal connection technique for next-generation power electronics packaging due to its excellent high-temperature stability and excellent thermal conductivity. In this work, we applied Ag sinter-joining to die attach of power electronics and focused reliability of Ag sinter-joining under a harsh thermal cycling condition, which ranges from − 50 to 250 ℃. The bonding quality of as-sintered die attach had a shear strength of over 45 MPa and remained over 25 MPa after a 500-cycle test. However, the shear strength drastically degraded to less than 10 MPa due to a failure of metallization layer detachment between dummy chip and sputtering layer after 750 cycles. Meanwhile, thermal resistance of die attach with different bonding materials was also evaluated by a T3ster, which suggests the Ag sinter-joining owns a superior property of thermal conduction than the traditional solder joining. This investigation indicates that the Ag sinter-joining has a long lifetime under a severe operating condition of power electronics.

N. Wakasugi, C. Chen, K. Hirao, S. Nagao and K. Suganuma, "Thermal evaluation of metalized ceramic substrates for use in next-generation power modules toward international standardization," 2020 IEEE 8th Electronics System-Integration Technology Conference (ESTC), Tønsberg, Vestfold, Norway, 2020, pp. 1-4, doi: 10.1109/ESTC48849.2020.9229827., Wide bandgap (WBG) semiconductors such as SiC and GaN have opened their market as the next generation of high power modules. As advanced electronic power modules must deal with increasing power density, power modules with an insulating ceramic substrate are subjected to extremely high temperature due to high current and voltage. Currently, the junction temperature of Si power devices is kept low enough, around 150 , to maintain the module durability. Nevertheless, the temperature will be soon beyond 200 for WBG devices. Thus thermal dissipation performance has one of the key technologies in designing modules. Directly bonded copper (DBC) or aluminum (DBA) substrates have been widely used as two of the typical ceramic insulating substrates for high power modules. If the extensive heat from a junction is kept inside a module, the module will be easily damage due to the increasing temperature resulting in severe thermal stress inside. Designing a proper module structure requires not only each material property but also complex component shape/structure/layout including each interface heat transfer. An accurate measurement method of precise thermal properties for such a complex metallized ceramic substrates is currently still missing. To meet the urgent requests for the next generation of high power modules, a new and simple measurement method of the thermal properties has been proposed for complex metallized ceramic substrates. First we have designed a micro heater SiC chip as an accurate and controllable heat source instead of a SiC active die. The heater chip was die-attached on metallized ceramic substrate mimicking real module packaging structure. The thermal resistance of the metallized ceramic substrate was evaluated. Due to the high power of the chip, i.e. 1 kW/cm2, the obtained thermal resistance has an excellent accuracy within a few percent error, when adequate cooling system is used. The thermal resistance includes those of die-attach material and thermal interface materials (TIM). The developed method thus enables precise comparison in thermal properties of high power modules, which was proposed as an ISO standard.

C. Chen, Z. Zhang and K. Suganuma, "Evaluation of high temperature reliability of SiC die attached structure with sinter micron-size Ag particles paste on Ni-P/Pd/Au plated substrates," 2020 IEEE 8th Electronics System-Integration Technology Conference (ESTC), Tønsberg, Vestfold, Norway, 2020, pp. 1-5, doi: 10.1109/ESTC48849.2020.9229850., Next generation power modules empowered by wide bandgap semiconductors like SiC and GaN can operate for a higher power than Si-based semiconductors at a high temperature over 250 °C. Ag sinter joining as a die attach material with a high melting point and good thermal conductive properties have obtained many attentions for the next generation power modules. This study investigated the bonding properties and the high temperature reliability of Ag sinter joining on ENEPIG (Electroless Ni plating (EN), Electroless palladium plating (EP) and immersion gold plating (IG)) metallized DBA substrates. The initial die shear strength for SiC (3x3 mm2) power modules was 33.9 MPa at 250°C sintering without assisted pressure in air conduction. The high temperature reliability of Ag sinter joint structure was evaluated at 250°C aging for 1000 h. The Ag-Au interface bonding mechanism, fracture behaviors and microstructure evolution of sinter Ag-Au joint were systematically analyzed. In addition, the connected Ag particles change to coarse with pore size increase during the aging process. The Ag-Au inter-diffusion layer also increased with the aging time but not clearly delamination occurrence. The die shear strength was slightly increased from the initial to 36.5 MPa after aging 1000 h, which meaning the Ag-Au joint possessed a good high temperature reliability. This study should add the understanding of the reliability issues of Ag sinter joining on Au finished substrate for its applications at high temperature.

SiC die attach bonding based on a micronsized Ag sinter paste direct on DBA (direct-bonded aluminum) substrate was developed, which achieve a high shear strength about 35 MPa at a low temperature of 200 °C without assisted pressure. Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) observation provided a trustworthy interface bonding mechanism, where self-generated Ag nanoparticles from the Ag paste during the sintering formed a dense layer which touched on the DBA substrate top surface. In addition, a thin Al oxide can be found at the top surface of DBA substrate, which was an amorphous structure with the thickness less than 10 nm. The selfgenerated Ag nanoparticles uniformly adhesion on the Al oxide layer and thus achieve a robust interface bonding. Shear strength still larger than 30 MPa after aging at 250 °C for 1000 h, indicating that this bonding structure have a good high temperature stability. The encapsulation polyimide resin for high temperature application was first time used to bonding with Al substrate. The average shear strength of pudding cup on Al substrate was evaluated as 20 MPa by a shear test.

Sinter Ag particles for connection of power devices is a hot topic due to the superior thermal and electrical properties of Ag to conventional solder paste. However, there are still some dilemmas for the sintering Ag particles such as high cost of nano Ag particles, requirements of pressure and surface metallization. In this work, we realized a robust die attachment on bare direct-bonded aluminum (DBA) substrate with sintering micron Ag flake particles. The bonding strength of die attachment can reach about 35 MPa under pressureless, atmospheric and 200°C sintering condition. By analysis the structure of sintered Ag particles, we found the micron flakes can be sintered into a uniform porous structure as low as 200°C. The sintered Ag structure shows an imitate attachment to the bare DBA substrate according to TEM observations of bonding interface. The high-temperature storage results indicate this die attachment has excellent thermal reliability that no significant degradation occurred even after 1000 h aging at 250°C.

Sinter silver (Ag) paste has been considered as promising joint materials for SiC and GaN power modules due to its excellent thermo-stability and high electrical/thermal conductivity. Here, we investigated the mechanical properties of sintered Ag paste including fatigue and creep from room temperature to high temperature (≥200 °C), which strongly related with reliability and lift time of power modules. At the room temperature, the sample did not break even at 10 to the 6th power when the stress amplitude was less than 4 MPa. From the S-N curve, the fatigue is close to the Morrow equation but not is the Coffin–Manson law. In addition, in the case of high temperature test (200 °C), the fracture behavior is different with room temperature, large deformation appeared at the necking part of the sintered Ag paste for both fatigue and creep test.

Sintering Ag particles for connection of power devices is a hot topic due to the superior thermal and electrical properties of Ag to conventional solder paste. However, there are still some dilemmas for the sintering Ag particles such as high cost of nano Ag particles, requirements of pressure and surface metallization. In this work, we realized a robust die attachment on bare DBA substrate with sintering micron Ag flake particles. The bonding strength of die attachment can reach about 35 MPa under pressureless, atmospheric, and 200 °C sintering condition. By analysis the structure of sintered Ag particles, we found the micron flakes can be sintered into a uniform porous structure as low as 200 °C. The sintered die attachment structure shows a imitate attachment to the bare DBA substrate according to TEM observations of bonding interface. The high-temperature storage results indicate this die attachment has excellent thermal reliability that no significant degradation occurred even after 1000 h aging at 250 °C.

In industrial manufacturing, alloying can contribute to the passivation of active metals and markedly improve their corrosion resistance. This inspires us to solve the current critical problem of Ag nanowires (Ag NWs) that have poor stability against chemical and electrochemical corrosion. These problems have seriously limited the applications of Ag NWs in optoelectronic devices where they are used for transparent conductive electrodes. Here, a kind of transparent conductive electrode based on Ag@Pt alloy-walled hollow nanowires (Ag@Pt AHNWs) is successfully fabricated by introducing 12 mol % Pt into long Ag NWs to form Ag@Pt alloy. The as-synthesized electrodes exhibit better optical transmittance (82% at the wavelength of 550 nm) under high electrical conductivity (28.73 Ω/sq–1), high thermal stability up to 400 °C for 11 h, and remarkable mechanical flexibility (remaining stable after 5000 cycles bending), as well as high resistance against chemical and electrochemical corrosion. The Ag@Pt AHNWs electr...

GaN die-attach/DBC substrate joint structure bonded by Ag sinter paste/W thin film/Ag sinter paste sandwich die-attached layer was designed and its high-temperature reliability was investigated. GaN chips were bonded to DBC substrate using Ag sinter paste with thin W film at 220 °C and at 0.4 MPa for 60 min. The joints structure was subjected to thermal shock testing in temperature range of −50 to 250 °C for duration up to 1000 cycles. The initial average die shear strength of the GaN/DBC joint structure was about 30 MPa, and was maintained up to 1000 cycles. These results reveal that Ag sinter paste/W thin film/Ag sinter paste sandwich die-attached layer can provide a superior thermal shock resistant. The microstructure evolution of GaN/DBC joint structure during the thermal shock testing was observed by FE-SEM and EDX analysis. In addition, FEM numerical simulation has also been implemented in this study, which revealed that the GaN/DBC joint structure with the sandwich die-attached layer shows a stress-shielding effect from the external force, and thus achieves a reliable power module structure for wide-bandgap semiconductors for its application in high-temperatures.

Improving the reliability of solder joints for die attachment in power modules is one of the most important issues in creating environmentally friendly vehicles such as hybrid electric vehicles. Power modules must have highly reliable solder joints that must be thermostable at temperatures over 175 °C in the future. In die attachment, soldering surfaces are often finished with Ni plating, so for Cu/Ni plating/Sn–Cu solder joints it is necessary to suppress both Ni diffusion into the solder as well as growth of the (Ni,Cu)3Sn4 intermetallic compound (IMC). Ni diffusion in Ni plating can be suppressed by the presence of a continuous (Cu,Ni)6Sn5 IMC layer at the Ni plating/solder interface. To form this IMC, we investigated the interfacial reactions and growth behavior of IMC layers in the presence of composite Sn–0.7Cu solder with added Cu balls. Adding 2.5 mass% of Cu balls prompted the formation of a continuous (Cu,Ni)6Sn5 IMC layer at both the electroless Ni–P and the electrolytic Ni plating, and this IMC layer worked well as a Ni diffusion barrier during a high-temperature storage test at 200 °C for 1000 h.

In this study, we used a light-irradiated environmental cell to directly observe the formation of hydrogen nanobubbles on platinized La5Ti2AgS5O7 photocatalyst particles; this approach enabled us to verify whether the site of hydrogen evolution was governed by the one-dimensional conductivity of the photocatalyst material. This work constitutes substantial progress as such visualization of reaction sites while they are operational and helps us to determine the reaction mechanism, which can lead to the improved performance of particulate photocatalysts.

The flexible polyaniline (PANI) thin films with modified nanostructure are prepared on flexible indium tin oxide (ITO)/polyethylene terephthalate (PET) substrates by combination of galvanostatic and cyclic voltammetric electrodeposition techniques. The electrochemical properties of as-prepared PANI nanomaterials are investigated by using the PANI film as an active material of the electrode for primary electrochromic device and electrochemical supercapacitor. This film presents a remarkable mechanical flexibility, high coloration efficiency of 80.9 cm2 C−1 at 630 nm and noticeable multicolor performances with reversible color change of transparent, pale yellow, green, blue and blue-purple. Further, a smart supercapacitor is presented, and the level of energy storage can be monitored by the change of optical performance. The film displays the highest specific capacitance of 473.3 F g−1 at the scan rate of 0.03 V s−1. The high-performance PNAI film exhibits promising features for various emerging flexible and multicolor electronics and optoelectronic devices combining energy storage and electrochromism.

Corrosion mechanism of Zn-30Sn high-temperature, lead-free solder in neutral 0.5 M NaCl solution are investigated by using FE-SEM, XRD, XPS and SIMS. The results confirm that α-Zn phases are corroded preferentially. Moreover, during initial corrosion process, a double-layer structure of ZnO and Zn(OH)2 is formed, providing beneficial barrier effect hindering the penetration of Cl−. However, subsequent morphology transformation weakens barrier effect to Cl− penetration and results in severe corrosion. Mechanism of surface layer breakdown and internal corrosion propagation are further examined, where it is determined that aggressive Cl− plays an important role in the corrosion process of Zn-30Sn solder.

This study reports a novel high-temperature and high-current wire interconnection by printing Ag sinter paste for WBG semiconductor power devices. The Ag sinter paste wire interconnection (APW) was successfully fabricated connecting two Cu electrodes coated by Ag. Both the high-temperature reliability and the high-current reliability of the APW were evaluated by aging test at 250 °C for 1000 h and by electro-migration (EM) test at 2.2 × 104 A/cm2 for 2000 h. In the aging, a resistivity of about 4.1 × 10−6 Ω cm was achieved, which further decreased to 3.4 × 10−6 Ω cm after aging for 1000 h. This reduction can be attributed to further sintering of the sintered Ag network. The initial shear strength was 17 MPa, which, while high enough, was maintained even after an aging test of 1000 h. In the EM test, the resistance of APW was not changed significantly up to 1000 h and then only slightly increased at 2000 h. This only slight change in resistance demonstrates greater stability than that of conventional wire interconnection material such as Al and Cu under high temperature and high current density. The influence of EM on the interface between APW and a Cu substrate was observed by scanning electron microscopy with EDX. The change of resistance of the APW was induced by the formation of Cu oxide at a cathode side, which leads to a crack extension along the interface between APW and a Cu substrate. Thus, the results obtained in the present work show the APW as a promising new alternative to wire bonding technology for WBG power electronic devices.

Low-stress design of bonding technology with a sandwich structure of sintered Ag and tungsten (W) thin film was developed for SiC power die-attached modules. The die-attached bonding layer was designed as sintered Ag/W/sintered Ag structure. Experiment results show that the initial bonding strength was larger than 65 MPa for this die-attached structure and larger than 35 MPa with a thermal shock test from − 50 to 250 °C for 1000 cycles. These results are largely better than that almost all sintering Ag technology reported in previous studies. Furthermore, the sandwich structure also compared with the sintered Ag structure which just using sintered Ag paste as bonding layer. The thickness of Ag paste is set as 100, 200 and 500 µm in the sintered Ag structure. The results show that the initial bonding strength of sintered Ag structure was about 60–70% of the value of W sandwich structure and about one-third of that after 1000 cycles. X-ray and SEM observation revealed that sandwich structure significantly decreased the size of crack extension in the sintered Ag layer during the thermal shock test. Finite element analysis reveal that the shear stress at the pore location of sandwich structure decreased to almost half values of the sintered Ag structure with the thickness of sintered Ag of 500 µm, and decreased almost 20% compared with the thickness of sintered Ag of 100 µm. The bonding technology with the W sandwich structure should be an attractive for low stress design in SiC power die-attached modules, which significantly increased its function for long-term high temperature applications.

Solid porous Ag–Ag interface bonding was introduced with the hillocks growth on a solid porous Ag surface. Porous Ag was fabricated on both Cu substrate and Si die by sintering a hybrid Ag paste. The size, density of grown hillocks was investigated at different heating temperature and time to provide an optimized condition in the interface bonding. The growth mechanism of hillocks on the solid porous Ag structure was discussed and analyzed in detail based on stress-induced migration and finite element stress analysis where solid porous Ag was reduced to a polycrystalline structure. The interface between two porous Ag structures bonded together in the die-attached structure with the bonding strength over 30 MPa, considerably stronger than that of traditional Sn–Pb solders. Furthermore, TEM observation revealed that Ag particles with the size of tens nanometers formed between the solid porous Ag–Ag interface, accumulate together and thus bridged the interface. This study provided a new attractive bonding technology based on the solid porous Ag structure in the die-attached module structures for large area bonding and thicker bonding layer design.

In the present study, we introduce a heater dummy chip equipped with temperature sensor, with extremely high capability of high heat-density using SiC wafer. As often used in high voltage/temperature SiC devices exhibit their advantage of high thermal stability and heat conductivity. Our testing heater chip (thermal test engineering group, TEG) consist of insulated SiC and Pt thin-film heater and temperature sensor realizes extreme heat density of 250 W/mm2 with reliability above 300°C of high temperature. The TEG chip was the n-doped 4H SiC chip, which is used generally for SiC devices, coated with an alumina insulation film and molded the Pt heater and sensor by lithography technique. Using the TEG chip, we demonstrate simple but accurate heat resistant measurements of several types of metal bonded ceramic substrates. The combination of SiC TEG chip and heat resistance measurement system is also applied to power cycle testing, avoiding numerous problems arising from using actual working device chips. Therefore, this study systematically was carried out the thermal measurement technology of SiC with repetitive power cycle by TEG chip as new concept heat system. Through this progress, the technologies developed in the study may have further applications in evaluation and testing of various packaging materials targeted for high temperature/wattage power modules.

Electrochemical techniques were employed to study the electrochemical corrosion behavior of Sn-9Zn-xTi (x = 0, 0.05, 0.1, 0.2 wt.%) lead-free solders in neutral 0.5M NaCl solution, aiming to figure out the effect of Ti content on the corrosion properties of Sn-9Zn, providing information for the composition design of Sn-Zn-based lead-free solders from the perspective of corrosion. EIS results reveal that Ti addition was involved in the corrosion product layer and changed electrochemical interface behavior from charge transfer control process to diffusion control process. The trace amount of Ti addition (0.05 wt.%) can refine the microstructure and improve the corrosion resistance of Sn-9Zn solder, evidenced by much lower corrosion current density (icorr) and much higher total resistance (Rt). Excess Ti addition (over 0.1 wt.%) led to the formation of Ti-containing IMCs, which were confirmed as Sn3Ti2 and Sn5Ti6, deteriorating the corrosion resistance of Sn-9Zn-xTi solders. The main corrosion products were confirmed as Sn3O(OH)2Cl2 mixed with small amount of chlorine/oxide Sn compounds.

Micron Ag paste had a more affordable price, feasible large-scale synthesis, and longer storage life compared to nano Ag paste, thus it attracts much industrial interest for die attachment of high-power devices. However, the previous studies of high-temperature reliability were mainly focused on nano Ag joints, the research about reliability of micron Ag joints, especially low-temperature and pressureless, was very limited. Therefore, we evaluated high-temperature stability of low-temperature and pressureless micron Ag joint, involving in the changes of mechanical behaviors, evolution of microstructure and interfacial reliability. The average joint strength of micron Ag joints was independent of aging time and kept approximately 35 MPa after aging for 1000 h. The fracture of the micron joint was dominated by the ductile deformation of Ag grains during the fracture process. On the other hand, the microstructure of porous structure evolved greatly during aging process. Ag grains were oriented randomly before and after aging process, but the Ag grains increased slightly from 827.2 nm initially to 1178.4 nm after 1000 h aging. Meanwhile, the pores size in porous structure increased, the number decreased significantly, and the porosity also decreased slightly. Moreover, the barrier layers at interfaces of micron Ag joint remained stable and reliable during aging at 250 °C. The results would promote the large-scale application of the commercially available micron Ag paste in high-power devices.

Although the diamond/Al composite is a good heat sink material due to its high thermal conductivity and low cost, its surface cannot be wetted directly for microelectronic packaging. Using modified electroless plating procedures and recipes, a glossy, continuous, and uniform Ni–P film was successfully fabricated on the diamond/Al composite surface for the first time with a bonding strength of 15.1 MPa. This Ni–P coating layer is solderable for both lead-free SAC305 and Sn63Pb37 alloys on Cu dummy chips. A continuous intermetallic compound (IMC) layer was formed at the interface without any crack or void defects, and the shear strength of both solder joints was in the range of 39.9–41.2 MPa. During thermal storage tests, the growth of IMC was slow at 125 and 150 °C, but it elevated sharply at 175 °C. According to growth kinetics analyses, a Ni element can hinder IMC growth. The diffusion activation energy (Q) of (Cu,Ni)6Sn5 was calculated as 62.6 kJ/mol in SAC305 and 97.7 kJ/mol in Sn63Pb37 solder joints. Both microstructures and mechanical properties of the solder joint using the Ni–P coated diamond/Al composite can satisfy the requirement of microelectronic industrial applications.

A bonding technology is introduced by using surface polished porous Ag in die-attachment structure. Bonding strength did not change much as the chip size varied from 3 × 3 mm2 to 15 × 15 mm2. This confirms that the technology was not influenced by the chip size, and thus can be used in large area bonding. Bonding mechanism based on stress-induced migration was discussed with the three dimensional finite element analyses. Transmission electron microscopy (TEM) observation further confirmed that single crystal hillocks and Ag particles formed at the bonding interface, bridging the interface together.

Submicron Cu particles mixed with Cu complex are used successfully to fabricate highly conductive Cu films for printed electronics. This study investigates the effects of Cu particle size on microstructure, conductivity and on the long-term stability of printed Cu films. In particular, the oxidation behaviors of printed Cu films at high temperatures are studied from the evolutions in microstructure and chemical composition. The submicron Cu particles are sintered efficiently due to the help of in-situ formed Cu nanoparticles from the decomposition of the Cu complex. Low resistivity of 5.8 μΩ cm is easily achieved. At temperatures of 140 °C and 180 °C, the printed Cu films prepared from 800 nm Cu particles are more stable than that from those 350 nm particles, which can be attributed to larger Cu particles possessing higher oxidation resistance. At 220 °C, the result becomes opposite because the loose structure with many large voids in the printed Cu films from large particles provides sufficient space for oxygen and accelerate the break of pathways between adjacent particles by the formation of Cu oxides layers. This indicates the long-term stability of printed Cu films is attributed to not only the intrinsic oxidation of Cu to Cu2O but also the degradation of microstructures. At all events, the printed Cu films from submicron Cu particles with Cu complex exhibit excellent oxidation resistance and are superior to those from Cu nanoparticles. This presents significant potential and favorable prospects for the fabrication of highly reliable and cost-effective printed electronics.

Topographical patterns are widely applied in many manufacturing areas due to the unique role in modifying performance related to physical, chemical and biological fundamentals. The patterns are usually realized by buckling or wrinkling, self-assembly or epitaxy, and lithography techniques. However, the combination of satisfactory controllability, ridge robustness, cost and dimensional precision is still difficult to achieve by any of the strategies above. A novel, simple and low-cost nanopatterning technique named "photodegradation copying method" with high technological flexibility has been initially proposed in this study. As a perfect example, a nanoridge-patterned surface has been successfully realized on a polymeric film thanks to the selective photodegradation of polymer and the shielding effect of silver nanowire (AgNW) networks. Roughness, wettability and transmittance of the polymeric film became simply and effectively controllable by adjusting the photodegradation time or the size and distribution of AgNWs. In addition, the ridge-patterned film could also be employed as a substrate in transfer printing for more flexible devices. Various topographical nanopatterns are expected to be simply realized by the photocopying method, just replacing nanowires with other masks like nanodisks, nanocubes, nanotriangles, and so on. This promising photocopying technique is believed to play an important role in the development of topographical nanopatterns, and enable more intriguing applications simply, flexibly and inexpensively.

In the silver sinter joining process, not only the bonding materials but also the metallization layer play important roles in bonding reliability. Compared with Ag, Au is utilized less often due to its higher cost, however Au plating is the inevitable choice for many applications which require a multiple reflow processes. In our research, the influence of the sintering process on the bonding strength of joints using Ag paste, as well as the influence of the electroplated Au layer on the joining property of SiC chips bonded on a printed circuit board (PCB) substrates with various types of Ag pastes was investigated. Electroplated layers of Ni/Pd/Au were prepared as a substrate metallization for this research. The shear test results revealed that higher shear strength was achieved on substrates with 0.8 μm-thick Au layer compared to those with 0.15 μm and 0.3 μm-thick Au layers. Fracturing inside the porous Ag near the substrate side was the main fracture mode for all joints in this research. The cross-section morphologies revealed that a weak bonding layer would form on the substrate side which was caused by the migration of Ag towards Au plating during the sintering process. The migration rate, which is in inverse proportion to the grain size of exterior Au plating, would impact the weak bonding layer, and thereby influence the bonding strength of joints. According to our results, higher bonding strength can be achieved on Au plating with larger grain size.

Ag sinter joining technology has been used in the advanced power applications to replace conventional soldering technology due to its high temperature stability, along with its excellent electrical and thermal conductivity. In this paper, we report the high-temperature reliability (250 °C for 1000 h) for die-attachment structures using Ag sintering technology on Cu substrates with different top metallization layers (Au and Ag), formed via different deposition processes (electroplating and sputtering). The bonding strength over 40 MPa and high-temperature reliability of sintered Ag on the sputtered Ag surface was the best among the systems studied here. Bonding quality and the bonding fracture behavior of sintered Ag on the different metallization substrates were characterized. Ag–Au solid solution was formed due to metallizaion Au atoms diffused into sintered Ag layer, leading to decreased shear strength under high temperature process. The influence of grain structure on the bonding quality at the interface between sintered Ag and the metallization Ag layers were discussed. It revealed that the grain size and orientation of the top metallization Ag layer influenced the bonding quality. The sintered Ag layer formed by Ag hybrid particles may have a selective orientation of metallization layer on the surface (111) of the Ag crystal. These results will be helpful to understand both technological perspectives for design and the applications of sintered Ag from the viewpoint of high-temperature reliability, as well as the fundamental understanding of its bonding quality mechanism with top metallization layers.

The thermal stability of silver (Ag) paste sintering on coated copper (Cu) and aluminum (Al) substrates has been investigated. Instead of conventional zincating or nickel plating, magnetron sputtering was used to achieve coating with titanium (Ti) and Ag. Silicon (Si) chips were bonded to coated Cu and Al substrates using a mixture of submicron Ag flakes and particles under 250°C and 0.4 MPa for 30 min. The joints were then subject to aging testing at 250°C for duration of 200 h, 500 h, and 1000 h. Two types of joints exhibited satisfactory initial shear strength above 45 MPa. However, the shear strength of the joints on Al substrate decreased to 28 MPa after 1000 h of aging, while no shear strength decline was detected for the joints on Cu substrate. Fracture surface analysis revealed that the vulnerable points of the two types of joints were (1) the Ag layer and (2) the interface between the Ti layer and Cu substrate. Based on the results of scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and simulations, cracks in the Ag layer were identified as the cause of the shear strength degradation in the joints on Al substrate. The interface evolution of the joints on Cu substrate was ascribed to Cu migration and discontinuity points that initialized in the Ti layer. This study reveals that Al exhibited superior thermal stability with sintered Ag paste.

We introduce a Ag-Ag direct bonding process with the aid of ultrasonic bonding, which uses silver “Nano-volcanic eruption of Ag” caused by stress migration at 250 °C. Various preheating temperatures before the ultrasonic pulse and various sputtering methods such as direct current (DC) and radio frequency (RF) have also been evaluated. A novel bonding process which combines ultrasonic bonding and stress migration bonding (SMB) method is established. The bonding achieved a die-shear strength of more than 50 MPa and a nearly-perfect voidless bonding interface. High temperature storage (HTS) test at 250 °C was used to evaluate the thermal-resist ability of bonded SMB joints. The results indicate that the bonded joints show no obvious changes in the interfacial morphology and the die-shear stress after HTS test for 1000 h is still higher than 30 MPa. The thermally-stable SMB joints can be applied to advanced devices such as thin-wafer multi-chip integrations and next-generation power devices.

This work introduces the possibility of using Ag sinter-paste as a novel high-temperature and high-current wire bonding solution. We investigated the electromigration (EM) behavior and lifetime of the sintered Ag wiring under high current density and high temperature required for the design of power electronic devices. The sinter Ag wiring fabricated on the two Cu substrates were tested under current densities of 2.7 × 104 A/cm2 at temperature of 250 °C. The microstructure evolution of sintered wiring was characterized after EM test. The resistance of sintered wiring did not change even after EM test for 300 hours, which confirms that the Ag-paste sinter wire bonding is rather stable than aluminum wire bonding under high temperature and high current density. No degradation was observed in microstructure of sintered wiring after EM test. Thus, it is expected that Ag paste sinter wire bonding is one of potential alternative interconnection technology for power electronic devices.

Metal paste sintering die-attach is recently attracting much attention as an alternative to Pb containing high temperature solders, particularly required for power device packaging with post-Si wide band-gap semiconductors. For high voltage and high power devices, which are used in electric vehicles, railway trains, or power grid systems, SiC MSOFET/SBD devices are emerging replacing Si IGBT devices. These SiC devices have two prominent advantages to traditional Si based devices: fast switching and high maximum junction temperature TJ. The excellent characteristics serve for miniaturization of the device module; the former allows to use smaller capacitor and reactors because of the high frequency, and the latter excludes cooling system without affecting the device life time. However, the thermal reliability should be critically tested before used in industrial applications. We have hence conducted comprehensive reliability tests using several types of metal sintering die-attach including Ag and Cu. High temperature storage tests at 250°C certify that the device structure is truly thermostable, and thermal cycling between −50°C and 250°C indicates that the thermomechanical stress caused by device package design is the key for high reliability of power devices. Power cycling demonstrates the usefulness for effective acceleration tests to estimate the device life time. Our results conclude that present metal paste die-attach is ready for use in the product instead of high temperature solders.

This study conducts stress-controlled cyclic tension-tension tests from room temperature (RT) to 200 °C, at 50 °C intervals, on high-purity Cu films to comprehensively understand the temperature-dependent fatigue life and the related microstructural and fracture characteristics. As evidenced by the electron backscattering diffraction analysis, the Ʃ3 boundary fraction decreases and the low-angle grain boundary fraction increases as the temperature increases. The significant change in the grain structure at high temperatures leads to a reduced fatigue life. The fatigue striations are closely spaced at RT and the fracture surface develops into the typical fatigue striation and the void zones.

In this work, we applied a Ag flake formed paste for sinter-joining on bare Si, SiC, and GaN surfaces. The sintered joints possess a high shear strength of over 40 MPa under a pressureless sintering condition at 250°C. The high bonding strength of the joints is achieved by a tight adhesion between Ag and bare surfaces, which attributed to an excellent sinter-joining ability of the Ag flake. The flakes, acquired by mechanical milling, can be rapidly sintered into a homogenous porous structure at a pleasureless and low-temperature sintering condition due to its dislocation-rich nano grain structure. During sintering, a drastic morphology reconstruction of Ag flake, from a flattened flake to a drop-like particle, can introduce robust interfacial connection structures, which is important for the robust bonding. This Ag flake paste can be regarded as a promising sinter-joining material for the connection of bare surfaces in high-temperature applications.

The fracture behaviors and Ag–Au joint interface evolution of sintered micron-sized Ag particles paste joined on an electroless nickel/electroless palladium/immersion gold (ENEPIG)-plated direct-bonded aluminum (DBA) and direct-bonded copper (DBC) substrates are evaluated during an extreme thermal shock test (TST) from −50–250 °C. The die shear strength of the as-sintered Ag joint on the DBA substrate is evaluated to be 33.5 MPa at a sintering temperature of 200 °C without any assisted pressure, which decreased gradually with an increase in the thermal cycling number. The shear strength declined slightly but remained at approximately 20 MPa; subsequently, it decreased considerably to 11.2 MPa after 1000 cycles. Coarsening of the sintered Ag layer is observed as the microstructure inhomogeneity and vertical cracks increased after 1000 cycles. In addition, the Al layer induced a greater undulate deformation, resulting in a sintered Ag layer exhibiting partial compression and tension after a TST. The sintered Ag layer became dense with a significant decrease in porosity at the compression parts and large horizontal cracks appeared at the tension parts. Both of horizontal cracks and vertical cracks led to fracture mode change and shear strength decrease. The Ag–Au joint interdiffusion layer became thicker during thermal shock with the gradual diffusion of the Au atoms into the sintered Ag layer. The die shear strength of the as-sintered Ag joint on the DBC substrates is evaluated as 34.4 MPa at 200 °C sintering but decreased to 0 MPa after 250 thermal shock cycles owing to the stress-induced delamination between the Ag and Au interdiffusion layer and sintered Ag layer. This study provides insights into the interface reaction and evolution of sintered Ag on ENEPIG-finished DBA and DBC substrates for applications at high temperatures.

This paper proposes a method to prepare aluminum nitride (AlN) ceramic–metal joints by nano-grained Ag flake particle sintering under low temperate pressure-free conditions. A robust shear strength larger than 25 MPa was achieved at 280 °C sintering for 3 mm × 3 mm joints. Moreover, large chip (5 mm × 5 mm, 10 mm × 10 mm) bonding confirmed that the technology was not influenced by the size and can be applied to large-scale ceramic–metal joining. Further investigation confirmed that nano-grained Ag micro flake particles were easily necked together, and the AlN/Ag interface could be tightly bound at a low temperature of 180°C. This study provides a new method for low-temperature pressure-less conditions for bonding ceramic-metal structure materials on a large-scale.

Previously, we reported the introduction of the low-temperature, pressure-less Ag direct bonding method, which has a high bond strength. In spite of this, when Ag was deposited directly on the sapphire surface of a GaN-based light-emitting diode die as a reflective layer, there were still some remaining problems with unreliable adhesion. Hence, this paper focuses on improving adhesion of Ag to sapphire by modifying the properties of Ag. Specifically, doping oxide into an Ag layer is investigated. As a result, excellent improvements in adhesion, and particularly in thermal shock resistance, are achieved. A similar effect is observed with various oxides. Additionally, oxide-doped Ag films have a high reflectivity that is almost equal to pure Ag. It is proposed that the adhesion improvement is caused by the formation a pseudotransition layer at the interface between the sapphire substrate and the Ag layer; oxide and Ag coexist in the pseudotransition layer.

Electrochemical behavior of Zn- x Sn ( x = 40 wt%,30 wt%,20 wt%) high-temperature lead-free alloys was investigated in aerated 0.5 M NaCl solution by open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. OCP results show that the corrosion state of the alloys working surface reached a relatively dynamic equilibrium after the alloys were immersed in 0.5 M NaCl solution for 100 h. EIS analysis reveals that the total corrosion resistance R t of alloys depended on the amount of Sn, and Zn-40Sn possessed the highest value 2228.2 Ω cm 2 . No passivation behavior was observed during corrosion process by potentiodynamic polarization measurement. The dominant corrosion products were confirmed as Zn 5 (OH) 8 Cl 2 ·H 2 O, ZnO and Zn(OH) 2 . Moreover, the surface and cross-section morphology of corrosion product suggested that the corrosion layer was more homogeneous and denser for higher Sn-content, Zn-40Sn obtained the best protective layer from corrosion product. The results recommend that Zn- x Sn high-temperature solder alloy with higher Sn content appears to be more attractive in terms of superior corrosion properties. The exact corrosion process was also discussed in detail.

Thermostable die-attach method alternative to soldering with Pb content is urgently required for utilizing wide-bandgap power semiconductors like SiC or GaN with a high maximum junction temperature TJ. Ag nano-or micro particle paste sintering attracts wide attention for this purpose, but Cu paste has also being developed mainly because of the cost pressure. Recently Cu sub-micron particle paste dispersed polyethylene Glycol solvent has demonstrated low temperature sintering at 250°C in N2 gas, resulting in high strength of die-attach. In the present study, the developed Cu paste die-attach has accepted to SiC Schottky barrier diode in TO-247 standard packaging with highly thermostable mold materials. The produced sample devices are subjected to high temperature storage tests at 250°C and thermal cycling tests between −50°C and 250°C. The device including the novel die-attach structure exhibited excellent thermal stability. Furthermore, power cycling tests has been conducted, and the results appear prospective for long life time and high reliability with high TJ exceeding 250°C. The presented study thus conclude that Cu submicron paste sintering is a promising candidate of power device die-attach usable for heavy industrial applications.

Solution method fabrication of copper indium gallium selenide (CIGS) solar cells has attracted researchers due to easy fabrication and cost effective potential. Despite the encouraging results, the process of fabrication of solution processed solar cell faces challenges like use of toxic chemicals, high temperature processing to achieve large grain and dense CIGS absorber film for highly efficient device. Even though many methods have been developed, the voids generated in the CIGS absorber film after high temperature processing adversely affect the performance of fabricated CIGS solar cell to reach its maximum efficiency up to 20% which has already been achieved for vacuum deposited CIGS absorber film. A dense CIGS absorber film can be deposited by tailoring the size of nanoparticles in the ink solution. In this work, two types of nanoparticles, one having 200–500 nm size nanopowder prepared by ball-milling and other with 20–30 nm nanoparticles prepared using chemical solution route are mixed to prepare ink solution and deposited on the substrate to achieve dense and voids free absorber CIGS film . The smaller CIGS nanoparticles were found to occupy voids generated among the bigger CIGS nanopowder under low temperature sintering process. The performance of fabricated solar cells was found to improve when the concentration of smaller nanoparticles increases in the mixture. Solar cell fabricated with CIGS absorber layer deposited using mixture of nanoparticles exhibited 2.1% efficiency whereas only bigger nanopowder absorber film hardly exhibited any photovoltaic response under sintering at 250°C in ambient atmosphere.

Micron silver paste enables a low-temperature and pressureless sintering process by using an ether-type solvent CELTOL-IA (C x H y O z , x > 10, boiling point of approximately 200°C) for the die attachment of high-powered devices. The conductive patterns formed by the silver paste had a low electrical resistivity of 8.45 μΩ cm at 180°C. The paste also achieved a high bonding strength above 30 MPa at 180°C without the assistance of pressures. These superior performance indicators result from the favorable removal of the solvent, its thermal behavior, and its good wetting on the silver layer. The results suggest that the micron silver paste with a suitable␣solvent can promote the further spreading of next-generation power devices owing to its marked cost advantage and excellent performance.

A one-step polyol method was employed to synthesize bimodal Cu particles with average diameters around 200 nm and 1000 nm, respectively. The bimodal Cu particles were mixed with a reductive solvent of polyethylene glycol (PEG) to form a paste. The Cu paste was used as die bonding material to prepare Cu joints under N2 or vacuum sintering atmosphere. The results showed that the strength of the Cu joints in N2 atmosphere was always higher than that in vacuum. The shear strength of a Cu joint processed at 350°C under only 0.4 MPa bonding pressure in N2 was above 40 MPa, which was far higher than that obtained using single-sized nano-Cu particle paste. It is related to the dense packing of the bimodal Cu particles and slow decomposition behavior of the reductive PEG solvent. The reductive PEG solvent in the Cu paste, which effectively removed oxides on the surface of the Cu particles, was necessary for easy-oxidized Cu pastes. These results suggested that Cu pastes with suitable particle sizes, reducing solvent and sintering atmosphere could be a proper candidate for low-temperature and low-pressure bonding process.

Silver sintering joining technology provides lead-free die attachment with excellent thermal conductivity and compatibility with high-temperature (≥300 °C) applications. However, sintering affordable micron-scale silver particles requires high temperature and the assistance of high pressures, which can damage chips and increase the manufacturing cost. In this article, we achieved the low-temperature, pressureless sintering of micron-scale silver particles by employing an active Ag 2 O additive as a bridge among the micron-scale silver particles. The Ag 2 O was reduced in situ to nanoscale silver to activate the surface of the micron-scale silver particles, resulting in the rapid diffusion and densification of silver particles at low temperature. An optimized composition of Ag-10% Ag 2 O exhibited a bonding strength of over 40 MPa at only 180 °C without any additional pressure. The results suggest that affordable micron-scale metal particles can be sintered using a suitable activating additive under an energy-efficient and eco-friendly fabrication process.