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An Ag–Au joint developed by Ag paste joining on electroless nickel/electroless palladium/immersion gold (ENEPIG)-plated direct-bonded aluminum (DBA) substrate was employed in SiC (Silicon carbide) power modules. The ENEPIG plating was modified to possess high mechanical strength by thickening the Ni layer to 20 μm. The reliability of SiC/DBA die-attached module by Ag–Au joining was evaluated during a long-term thermal shock test (TST) from − 50 to 250 °C up to 2000 cycles. The shear strength of as-sintered Ag–Au joint was evaluated to be 37.6 MPa, but it showed a significant decrease after 1000 thermal cycles and maintained stably from 1000 cycles to 2000 cycles. Based on the microstructural evolution via EBSD observation, it was confirmed, by modifying ENEPIG, the Ag–Au interfacial deformation derived from Al plastic deformation was successfully suppressed even after 2000 cycles. In this case, the mechanism for shear strength degradation was found to be the interfacial delamination between Ag paste layer and ENEPIG substrate. This can be ascribed to the large thermal stress caused by different coefficients of thermal expansion (CTE) of power module components. This study not only realized the deformation-free structure for Ag–Au joint during long-term thermal cycling but also provided fundamental insights for fracture behaviors of Ag–Au sinter joint.

Ag particle paste sintering has been developed and shown a good performance in areas such as high temperature reliability and high electronic conductivity. Recently, it was found that the microstructure of a sintered Ag layer exhibits a coarse-dense inhomogeneous distribution after sintering, which depends on the loading speed of a die during the chip mounting process due to the flow characteristics of the Ag particle paste. In this work, the effect of the die mounting speed and the push-in amount (push depth) in the die mounting process on the microstructure of the sintered Ag layer, shear strength and thermal cycling reliability of Ag sinter joint structures were investigated. The results show that the shear strength of the sintered Ag joints was largely influenced by the speed and push depth during mounting process. The fast push speed exhibited a better shear strength, and the push depth can be optimized to obtain a better shear strength. In addition, the reliability during the thermal cycling of the Ag sinter joint structures with different the die mounting speed and the push depth were evaluated during a thermal cycling test from − 40 to 175 °C for 500 h. Cracks propagated after the thermal cycle test at the high porosity area near the die edge when the die was mounted with a slow speed, leading to a shear strength quickly decreasing after the thermal cycle test. The die attached structure showed better thermal cycling reliability in the case where the die was mounted with a fast speed.

This study was carried out to develop a DBA (direct bonded aluminum) substrate with Ni/Ti/Ag metallization to achieve highly functional thermal shock stability of Ag sinter joining in GaN (Gallium Nitride) power modules. GaN /DBA die-attached module structures by Ag sinter joining was performed during harsh thermal shock cycling tests within a temperature range of − 50/250 °C. In the case of DBA without a Ni metallization layer (Ti/Ag), severe degradation occurred at the interface between the sintered Ag and Al due to significant plastic deformation of the Al layer. The shear strength decreased from an initial value of 33.1 MPa to 22.3 MPa after 500 cycles. With EBSD investigation, it was determined that the Al layer underwent sub-grain rotation recrystallization during thermal shock cycles. This led to a non-uniform grain orientation distribution at center and corner locations. On the other hand, Ni/Ti/Ag metallization showed that it can prevent severe Al deformation due to the superior rigidity achieved by Ni metallization. The die-shear strength maintained almost the same value as its initial value, even after 500 cycles. In addition, a numerical simulation analysis determined that the Ag sinter joining structure on the DBA substrate with Ni/Ti/Ag metallization had high functionality in stress relaxation. This study provided a novel approach to design thermal shock stability Ag sinter joining for next-generation power modules in high-temperature applications.

Adoption of wide-bandgap semiconductors in power electronic systems enables these devices to operate at high switching speeds and at high temperatures (above 200 °C). Sinter Ag joining as an attractive die attach material received more and more attention. This study introduced a robust bonding of sinter Ag joining direct on an electroless Ni(P) surface-finished DBC (direct bonded copper) substrate with a low-cost micron-sized Ag flake particle. Die shear strength of sinter Ag joining increased from sintering temperature of 180 °C to 300 °C and achieved 43.6 MPa at 300 °C in air and without pressure during sintering. The shear strength decreased at the sintering temperature of 350 °C due to mass Ni oxide and dense voids generation. The bonding mechanism of sinter Ag joining direct on the Ni-plated structure was discussed via SEM, EDX, XRD and TEM analysis. There is not clear inter-diffusion between Ni and Ag element at the bonding interface. TEM observation reveals that many Ag nanoparticles with the size

Ag sinter joining provides superior mechanical and thermal/electrical properties and is considered to become a leading next-generation wide band-gap (WBG) die-attach material. However, the microstructural evolution and mechanical characteristics of Ag sinter joining when subjected to high temperature have never been directly compared to those same characteristics of solder materials. In this study, we have evaluated the high-temperature and long-term reliability of a GaN/DBA die-attached module by pressure-less Ag sinter joining and Pb–5Sn solder in a harsh thermal aging test. Both the Ag sinter joining and Pb–5Sn solder were subjected to a thermal aging test of up to 1000 h at 250 °C. Initial shear strength of the Ag sinter joint exceeded 42 MPa, and increased stably up to 1000 h without any defects such as interface oxidation, diffusion, or mechanical deformation. The increase in shear strength of the Ag sinter joints was the result of necking growth of the sintered Ag during thermal aging. On the other hand, the shear strength of the Pb–5Sn joints exhibited substantially decreased shear strength (by 60%) after aging 250 h. NixSnx intermetallic compounds (IMC) were also formed and serious interface degradation occurred during the aging process. These microstructure changes and mechanical characteristics have an important influence on mechanical reliability and, with that in mind, the tendency of fracture mechanism was investigated in detail by SEM–EDX. This study systematically examines the fracture mechanism on the microstructure of a DBA substrate and on high-temperature packaging during thermal aging tests for WBG semiconductor device applications.

Sintering Ag paste on Ni/Au surface finish (Ag–Au joint) is a hot topic because Ni/Au surface finish is one of the most commonly used metallizations in the electronics industry. However, it is still difficult to achieve a reliable Ag–Au joint after high-temperature thermal aging due to severe Ag–Au interdiffusion. In this work, we investigated thermal aging reliability of Ag–Au joint via high-temperature storage at 250 °C up to 1000 h. The Ag–Au joints with 0.1 and 0.8 μm Au layer coated substrates were compared to study the Au layer thickness effect on thermal aging reliability of joints, and a substrate preheating treatment was proposed to improve thermal aging reliability of Ag–Au joint. The result clearly shows that preheating treatment can improvement of bonding performance including thermal aging resistance and initial shear strength for both the thickness of Au layer. The shear strength of 1000 h thermally aged Ag–Au joint with the 0.1 and 0.8 Au substrate has been further improved to 11.7 and 16.5 MPa via the substrate preheating treatment, respectively. The substrate preheating treatment can lead to further growth of Au grains by eliminating fine grains, which results in the improvement of thermal aging reliability of joints. In addition, the thicker Au layer showed superior thermal aging reliability. This is attributed to the thicker Au layer less susceptive to massive Ag–Au interdiffusion and can prohibit severe oxidation of Ni because it hosts larger Au grains and fewer grain boundaries. The study would provide a convenient way to improve the thermal aging reliability of Ag–Au joints.

Sintering of silver is a popular method for forming interconnections in power electronics. Owing to their large size and spherical shape, micron- and submicron-sized Ag particles synthesized by a polyol method (denoted as polyol Ag particles) are not expected to undergo low-temperature, pressureless sintering. However, previous studies have shown sound bonding with shear strength of more than 40 MPa at 200 °C with micron and submicron polyol Ag particles. In this work, to understand the bonding mechanism of polyol Ag particles, the sintering behaviors of two Ag pastes, one with polyol Ag particles and another based on hybrid Ag particles consisting of micron-sized Ag flakes and submicron-sized Ag particles, were investigated without any applied pressure at 175 °C via transmission electron microscopy. During the sintering process, Ag nanoparticles formed in situ can significantly accelerate the sintering of the Ag paste, resulting in low electrical resistivity of the sintered Ag paste (9.8 × 10−6 Ω·cm) after only 5 min of sintering at 175 °C. The Ag nanoparticles were likely generated from the reduction of residual Ag ions or the Ag complex in the paste. The results were also verified by washing the Ag particles or adding Ag ions into the paste.

The use of a Co-W-P metallization substrate in SiC power modules is expected to improve high temperature reliability because Co-W-P metallization has been found to induce strong bonding strength to both sintered Ag joints and encapsulation resins. To progress in the development of this technology, the successful applicability of Co-W-P metallization into the module fabrication process is of critical importance. In this paper, the effects of annealing a Co-W-P metallization substrate in the die attach process onto its resin adhesion was studied with representative annealing conditions for Ag sintering. Initially, mild annealing at 200 °C for 1 h was applied and showed strong resin adhesion greater than 15 MPa at 225 °C and an ideal cohesion fracture mode of resin, the same as that found in that of the fresh Co-W-P case. However, more severe annealing resulted in lower resin adhesion. For example, annealing at 280 °C for 1 h resulted in a poor resin adhesion below 15 MPa, as well as a delamination fracture mode between the resin and the Co-W-P metallization. This mechanism was investigated with the use of SEM–EDS and XPS analysis. It was observed that annealing at 200 °C induced a slight oxidization of Co, but Co(OH)2 for the chemical reaction to resin, still remained on the Co-W-P surface. On the other hand, annealing at 280 °C formed an alternative main component (CoO). The shortage of Co(OH)2 on the top surface created by severe oxidization was found to induce poor resin adhesion. The results from this research are significant to future designs and applications of a module fabrication process using Co-W-P metallization substrates, as well as to the fundamental understanding of adhesion behavior on Co-W-P metallization.

One of the applications of wide band gap semiconductors is high temperature operation. That application requires high temperature compatible (i) joining materials such as sinter Ag, (ii) encapsulation resins such as imide type primers or molding compounds, and (iii) metallization for those materials. Ag metallization, the best candidate metallization for sinter Ag materials, has difficulty in bonding to encapsulation resins. Conversely, Ni/Au-flash metallization enables strong resin adhesion but also demonstrates poor reliability for sintered Ag joints. There is no single metallization compatible to both sintered Ag and encapsulation resin for high temperature application. This paper reports on a single metallization, electroless plated CoW metallization, which has demonstrated the capability to achieve both (i) high-temperature reliability (250 °C for 500 h) for sintered Ag joints and (ii) high-temperature adhesion (at 225 °C) for encapsulation resins. Such results have not been achieved with either Ag or Au metallization. The shear strength of sintered Ag joints on CoW metallization exceeded 40 MPa. TEM observation revealed excellent bonding between the sintered Ag and the metal Co of the CoW metallization. Furthermore, CoW metallization also showed strong resin adhesion (about 21 MPa) at 225 °C. XPS analysis identified metal Co for bonding to sinter Ag and, Co(OH)2 and WOx for bonding to resin on the top surface of CoW metallization layer. The foregoing results indicate that CoW may well represent a new metallization process for the fabrication of high reliability and high-temperature compatible SiC power modules.

In this study, the influence of annealing at 250 °C for 500 h on Ohmic contact of backside electrode of Silicon Carbide Schottky-Barrier-Diode (SiC-SBD) was investigated. Considering the prevalence status of the SiC-SBD as power device applications, the experiment was implemented on a commercial based SiC-SBD device. The annealing has brought remarkable transition in the structure and the impurity profile at the surface and the Ag/Ni interface of the device as a result of oxidation. For these evaluations, dual-beam Time-of-flight Secondary Ion Mass Spectrometry is regarded as an available analytical technique on the high sensitivity and imaging ability. In the deeper region of the backside electrode, an amorphous Ni–Ti alloy layer was found between the Ni/Ti interface of the annealed sample, which is also considered the result of diffusion due to the annealing. On the other hands, some nuggets of glassy-carbon and Ti cubic structures were observed in the Ni(Si)–C layer of both the non-annealed and the annealed samples. These structures are deduced to be formed during the fabrication process of the backside electrode and to be stable against the annealing. For these evaluations, Transmission Electron Microscopy (TEM) and Scanning TEM (STEM)—Energy Dispersion X-ray analysis were optimized with the high lateral resolution.

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.

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.

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.

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.

This paper explores the possibility of using Ag paste containing silicon carbide particles (SiC-p) as a novel high-temperature die-attachment solution for the design of power devices. The bonding structure used in this research was composed of silicon dies and a direct bonded copper (DBC) substrate. A SiC-p/microporous Ag composite structure was prepared by sintering a Ag microflake paste containing 2 wt% sub-micron SiC-p under mild conditions (250 °C and 0.4 MPa for 30 min). In addition to the Ag paste, the surface metallization of the DBC substrate was also evaluated in this research. Ag metallization layers deposited by electroplating and sputtering were compared, along with samples also containing a titanium (Ti) diffusion barrier layer between Cu and Ag. The results indicated that the SiC-p-containing Ag sinter paste showed better stability in storage tests than the paste without SiC-p at the temperatures such as 150, 250 and 350 °C. Additionally, the Ti diffusion barrier layer played an active role in preventing the oxidation of Cu and inter-diffusion between Cu and Ag during use at high temperatures exceeding 250 °C. The joint bonded by SiC-p-containing Ag paste on DBC substrate with Ti barrier layer exhibited excellent stability up to 1000 h at 150 and 250 °C.

A cold-rolled Cu/Al clad ribbon was bonded on an electroless nickel immersion gold (ENIG)-finished Cu substrate by ultrasonic bonding. The bonding samples were subjected to harsh conditions such as thermal exposure at 200 °C and thermal shock at −40/250 °C. The microstructural evolution and textural transitions in the Cu/Al clad ribbons were analyzed using electron backscatter diffraction to understand the heel crack propagation mechanism. The heel cracks were initiated at the edge of the Al layer between the bonded and non-bonded zones and were propagated along the coarsened grain boundaries deep into the cladded Cu layer. The continuous dynamic recrystallization (CDRX) transition phenomena around the heel cracks were scrutinized by electron backscatter pattern analysis to reveal the different microstructural and textural evolutions as a function of the ultrasonic bonding process at the bonded part and of lift-up at the non-bonded part. The heel cracks were occurred by grain coarsening due to CDRX within the Al layer during the environmental tests. The region of coarsened grains by CDRX was extended with increasing test periods. Especially in the thermal shock tests at −40/250 °C, severe grain coarsening was found involving significant transformation of sub-grains into grains by rapid CDRX development. Thus, heel crack propagation accelerated more during thermal shock cycles than during high-temperature storage.

A simple and large-scale two-steps polyol process was used to simultaneously synthesize nano and micro Ag particle pastes, which were used to joint Ag-plated Cu sheets. It is found the optimized size distribution of the Ag particles and less organics remainder highly improved the shear strength and conductivity of the sintered Ag paste. After sintering at 200 °C, a shear strength of over 40 MPa and a resistivity of 5.1 μΩ cm were achieved by using a paste composed by nano-Ag particles (AgNPs) with the diameter below 800 nm and micro-Ag particles (AgMPs) with the diameter of 1.5–4.2 μm under a small sintering pressure of 0.4 MPa. In contrast, the Ag paste including particles with unsatisfied size distribution resulted in low shear strengths due to inhomogeneous sintering and the presence of large voids. These results are superior to those obtained with only AgNP pastes and suggested that the Ag paste composed of both the AgNPs and AgMPs with suitable ratio and size distribution will be an excellent die-attachment material in microelectronic packaging.

In recent years, the requirement for environmentally friendly power electronics such as light-emitting diodes and integrated power modules has led to substantial efforts in electronic packaging. The high operating temperatures of advanced power devices particularly require alternative bonding materials to replace the high-temperature solders that contain much Pb. Without limiting the conventional soldering processes that use melting alloys, several candidate materials have been developed, along with the development of devices. In this study, we demonstrate Ag stress migration bonding with various substrate materials to address the intrinsic mechanisms underlying the bonding process. Varying the substrate material changes the thermal expansion mismatch between the sputtered Ag films and the substrate, resulting in different bonding properties, in particular the die shear strength. It has been revealed that a low bonding temperature process can be achieved with a high thermal stress, suggesting that stress migration and resulting stress relaxation at the bonding temperature play key roles in controlling the Ag stress migration bonding. This Ag stress migration bonding opens up a novel opportunity to access wide band-gap devices by realizing low-temperature processes without any application of pressure.

In this paper, we studied the reliability of Cu joint with a Sn–Ag–Bi–In (SABI) lead-free solder under a high electric current stress with or without trace amounts of Co and/or Ni as grain refiners. Electromigration (EM) caused critical failure of the soldered structure at current densities exceeding 10 kA/cm2, over 100 °C. Because of the anisotropic diffusion coefficients along the a/b and c axis of β-Sn tetragonal crystal, the EM resistance of the joints depended on the grain size and their orientation. The observation of microstructure with electron backscatter diffraction revealed that the grain with the addition Co and/or Ni becomes much finer than that without any additive resulting in, an improved EM resistance. The best alloy in the present our study is SABI containing 0.1 wt% Co, exhibiting a lifetime twice larger than that with SABI. The improvements of EM reliability by these additives are significantly different depending on diffusivity of the refiner elements.

Since the thickness of layers of intermetallic compounds (IMCs) at the joining interface has an apparent effect on the joint reliability, understanding the growth mechanisms of IMCs is important for die-attachment applications. In the present study, the interfacial reaction between elements (Ca, Mn, Cr, and Ti) added to Zn alloys and Cu substrate were investigated in detail, focusing on IMCs grown by isothermal aging at 150 °C. The reaction layers included two types of Cu–Zn IMCs i.e., γ-Cu5Zn8 and e-CuZn5 phases. The joining interface with trace elements containing Zn solder reduced the growth rate of the IMCs. In particular, the addition of 0.1 wt% Cr to pure Zn solder decreased the growth rate of the IMCs by approximately 50 %. The mechanism we propose assumes that a phase containing the small Cr atoms exists between the solder and e-CuZn5, which suppresses the diffusion of metal atoms. To summarize, the Cr additive showed beneficial effects in terms of suppressing the growth of IMCs during the solid-state isothermal aging.

Whisker growths from matte tin electroplating have been observed during thermal cycling up to 1,000 cycles either in air or in vacuum. The density, length, and width of thermal stress whiskers depend on the plating thickness of 2 and 5 μm in the present study. Whiskers grown on the 2 μm plating are longer and thinner than those on 5 μm plating. In both cases, whiskers grow thinner and faster in vacuum than in air. These apparent variations come from the grain sizes and the thermal stress distributions in the electroplating, intrinsically different in 2 and 5 μm thick films. The grain structure of whisker root, particularly grain boundary cracks oxidized in air, determines the stress concentration to drive the whisker growth. Cracking caused by oxidation was rarely observed in vacuum hence causes thin and straight whiskers even from thick plating. Our results indicate that the stress concentration at whisker root grain is essential for controlling whisker growth morphology, and has a critical impact on various electronic applications.

Emerging SiC power semiconductor devices are expected to work under the high temperature condition of 250–300 °C while the operation of Si devices is limited up to 180 °C. The die-bonding materials for emerging SiC power devices hence need to have sufficient capability in such extreme operating environments. In this study, we investigated the thermomechanical reliability of the die-attach technology using Ag flake paste, which can be processed by low-temperature and low-pressure sintering. The Ag flakes start to sinter immediately after the organic dispersant layer is removed from the flake surface at 160 °C, and die-bonding consequently becomes possible. The tested Si die-attachments joining with the paste maintained high strength (23 MPa) up to 1,000 thermal cycles from −40 to 180 °C. The stable microstructures without crack and no interfacial debonding assure the reliability of the Ag flake paste die-attach of Si. SiC die-attachments also maintained their high strength (24 MPa) up to 1,000 cycles of −40 and 250 °C, though a slight degradation appeared after 1,000 cycles. The debondings at the sintered Ag flake paste layer/SiC wafer interface were affected to the joining strength with the Ag flake paste. The obtained results indicate that our Ag flake paste die-attach can be applied to both Si and SiC power devices capable of high temperature operations.