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• Copper diffusion into epoxy was blocked by using a self-assembled layer. • Blocking copper diffusion into epoxy greatly reduced epoxy decomposition. • The self-assembled layer survived the heating at 200 °C for 1000 h. • Isolation from atmospheric oxygen made the self-assembled layer survive heating. The reliability of copper/epoxy bonding is important for power semiconductors. It has been found that copper can diffuse into epoxy and accelerate epoxy decomposition at high temperatures, deteriorating reliability. We proposed a way to inhibit copper diffusion into epoxy by using self-assembled layers. Copper substrates with self-assembled layers on their surface were encapsulated with epoxy and placed at 200 °C for 1000 h. The diffusion of copper into epoxy was evaluated by XPS depth profiling, and the decomposition of epoxy was evaluated by ATR-FTIR. The results showed that self-assembled layers can effectively hinder copper diffusion into epoxy. The possible reason is that the existence of the self-assembled layer greatly reduces the formation of copper ions. Also, we found that the self-assembled layer survived the heating, implying its potential for practical power module applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Effects of high temperature and high humidity on copper/epoxy bond were studied in nanoscale. • Experiments and auxiliary molecular dynamics simulations were performed. • The water absorbed in the epoxy caused complex physical and chemical changes at the copper/epoxy interface. • The crystal orientation of cuprous oxide significantly influenced its bonding with epoxy. • The higher bonding strength is not equal to the higher reliability. Epoxy encapsulation is a promising technique to reduce the size of the power module. Its reliability is a key aspect. Many previous studies developed new epoxy materials or new surface treatment methods to improve reliability. However, reliability mechanisms are still not well understood. This study focused on the reliability of copper/epoxy bonds under high temperature and high humidity conditions because previous studies did not reach a unified conclusion on this issue. The copper/epoxy bonds are fabricated by compression molding. Then, these copper/epoxy bonds are placed in the 85 ℃/85 %RH reliability test for 1000 h. The bonding strength change was measured. Surprisingly, the bonding strength between copper and epoxy increased after the 85 ℃/85 %RH reliability test. The possible reason was investigated by observing the fracture surface and bonding interface in a nanoscale. In addition, molecular dynamics simulations were performed to better understand the experimental results. The results showed that the water absorbed in the epoxy caused complex physical and chemical changes at the copper/epoxy interface. The formation of a new cuprous oxidation layer with aligned crystal orientation is supposed to be the reason for the bonding strength increase. Our findings will provide a better understanding of copper/epoxy bonds. [ABSTRACT FROM AUTHOR]

• The reason for copper diffusion into epoxy under high temperatures was revealed. • Copper diffusion into epoxy was directly observed with TEM. • Impurity elements of sulfur and chlorine were found to promote copper diffusion into epoxy. • The crosslink density of epoxy has little influence on copper diffusion. • Using cycloaliphatic-type epoxy seems to be a solution to avoid chlorine impurities. High reliability is critical for semiconductors, especially power semiconductors, which usually work in high temperature conditions. One important reliability issue for power semiconductors is the copper diffusion into polymers. Copper diffusion accelerated polymer decomposition. Also, severe copper diffusion even causes short circuits. Although copper diffusion has been identified, the reason for the initiation of copper diffusion is still not well understood. Here, we encapsulated copper substrate with five different epoxies to investigate the reason for the initiation of copper diffusion. The influence of epoxy impurity elements and epoxy cross-link density was investigated. The results show that the initiation of copper diffusion into epoxy is highly related to the impurity elements, sulfur (S) and chlorine (Cl). In contrast, the low cross-link density of the epoxy cannot initiate copper diffusion. It implies that using S-free and Cl-free epoxy is necessary for semiconductors requiring high reliability. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract Solid-state porous Ag was utilized to achieve interface bonding under low temperature and low-pressure for large area, high-reliability designs for high performance die-attach power modules in high temperature applications. In this process, solid-state porous Ag was first fabricated by sintering Ag particles onto both a Cu substrate and an Si die with different bonding areas. After polishing the surface of the solid-state porous Ag, the Ag-Ag interface was bonded together under low pressure (0.4 MPa) and low temperature (250 °C and 300 °C). The bonding strength was greater than 30 MPa for a 15 mm x 15 mm bonding area at a bonding temperature of 300 °C. The electrical resistivity of the solid-state porous Ag was about 7 µΩ cm, or half that of Pb-free alloy solder materials. The bonding mechanism of the Ag-Ag interface was analyzed by a transmission electron microscope. It was observed that Ag hillocks and nanoparticles had been generated and bridged the Ag-Ag interface. This resulted in a large interface connection ratio and a high bonding strength. In addition, it was observed that the shear strength of the solid-state porous Ag with a thick bonding layer decreased thermomechanical stress in the die-attached power module and thus improved structural reliability. This bonding process featuring solid-state porous Ag presents an attractive technology for the fabrication of large area bonding and high-reliability die-attached module structures for high temperature applications. [ABSTRACT FROM AUTHOR]

Sintered silver (Ag) die-attach has attracted much attention in assembling power modules for high power density and high temperature operation. We have investigated a method to improve the thermal stability of a sintered Ag die-attach structure at high temperature storage (HTS). One of the issues of die-attach with sintered Ag is the growth of a microporous structure after HTS, since the microporous growth of the sintered Ag layer may have severe influence on bonding strength and reliability. It was verified that the addition of transition metal compound particles such as WO3, Nb2O5, MoO3, and RuO2 to sintered Ag paste successfully prevents microporous structure growth beyond 200 °C. From the results of cross-sectional scanning electron microscope observation, the inhibiting effect of microporous growth has a correlation with the difference between the total-circumference-length measured before and after HTS, among which WO3 and Nb2O5 additives were more effective. We propose this amazing die-attach material to power modules for high temperature operation providing excellent thermal stability. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The fast development of electric vehicles promoted the development of next-generation power modules. Along with this trend, the encapsulation techniques are also transforming from previous gel encapsulation to epoxy encapsulation because epoxy encapsulation reduces the module size significantly. However, the dissimilar bonding between the epoxy and the metal substrate is a weak part of the entire module. Unlike previous studies, which focused on epoxy properties and thermal stress, we investigated the failure mechanisms between the encapsulation epoxy and the copper substrate under high temperatures by considering the interfacial interaction. A high-temperature storage test (HST) was performed at 200 °C until 1000 h for encapsulated packages. We then measured the bonding strength and identified the fracture path at the nanoscale by SEM, XPS, and ToF-SIMS depth profiling. In addition, the changes in the epoxy were characterized by ATR-FTIR, nanoindentation, and XPS depth profiling. The bonding interface was analyzed with AFM-IR, SEM, EDS, and STEM. We found that the fracture happened inside the epoxy rather than the copper/epoxy interface. More importantly, we found that copper atoms diffused into the epoxy reaching approximately 100 nm. The diffused copper atoms and the long-time high-temperature heating promoted the epoxy pyrolysis, forming a 100 nm thick weak layer at the epoxy side, which is the key reason for the high-temperature failure. Our study provided a fresh understanding of the failure mechanisms of the bonding between encapsulation epoxy and the copper substrate under HST, which will contribute significantly to future power module design and material development. • The failure of the epoxy encapsulation under the condition required by next-generation power modules was investigated. • Failure mechanisms of the interface between epoxy and metal substrate exposed to high temperature were revealed. • The copper diffused into epoxy under high-temperature aging, reaching approximately 100 nm on the epoxy side. • Under a long-time high-temperature condition, the diffused copper promoted the pyrolysis of the epoxy. • The weak layer caused by pyrolysis is the key reason for the failure of the epoxy encapsulation under high temperature. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fatigue characteristics of high-purity Cu films from RT to 200 °C were investigated. • TMF characteristics of high-purity Cu were systematically and numerically revealed. • The change in the grain of Cu at high temperatures leads to a reduced fatigue life. • Ʃ3 fraction decreases and the LAGB fraction increases as the temperature increases. • Fatigue life is significantly changed even below the recrystallization temperature. 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. [ABSTRACT FROM AUTHOR]