High Temperature Storage Test Research Articles

Al-clad Cu bond wires for power electronics packaging: Microstructure evolution, mechanical performance, and molecular dynamics simulation of diffusion behaviors

With the advancement of power electronics, aluminum-clad copper thick bonding wires have garnered attentions due to superior electrical and thermal properties, making them well-suited for high-temperature and high-current applications. However, the impact remains unveiled of whether the growth of intermetallic compounds (IMCs) at the bonding interface presents critical challenges to the reliability of wedge wire bonds. Therefore, it is necessary to investigate the evolution behavior of Cu/Al IMCs in Al-clad copper wires. In this study, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were firstly employed to characterize the phase composition and growth behavior of Cu/Al intermetallic compounds (IMCs) at two distinct interfaces—the bonding interface and the core-shell interface—under various annealing conditions during high-temperature storage (HTS) tests, revealing a parabolic relationship between aging time and IMCs thickness. Subsequently, shear and pull tests of Al-clad copper bond wires were conducted to evaluate the bonding strength under different aging conditions, clarifying the correlation between various failure modes of the bonds and the evolution of IMCs at the bi-interfaces of this novel composite across different aging stages. Additionally, molecular dynamics (MD) simulations were employed to explore the diffusion behavior of Cu and Al atoms. It revealed that polycrystalline structures enhanced the mutual diffusion at the interface, with copper serving as the predominant element in the interdiffusion process. In conclusion, this study integrates experimental and numerical approaches to elucidate the growth mechanisms of Cu/Al intermetallic compounds and their effects on reliability, providing valuable guidance for optimizing the performance of composite bonding wires in high-temperature power device applications.

A Study on the Effect of Pd Layer Thickness on the Properties of Cu-Ag Intermetallic Compounds at the Bonding Interface.

This paper conducted a high-temperature storage test (HTST) on bonded samples made of Pd100 (Pd-coated Cu wire with a Pd layer thickness of 100 nm) and Pd120, and studied the growth law of Cu-Ag intermetallic compounds and the inhibitory mechanism of Pd thickness on Cu-Ag intermetallic compounds. The results show that the Kirkendall effect at the bonding interface of the Pd100-bonded sample is more obvious after the HTST, the sizes of voids and cracks are larger, and the thickness of intermetallic compounds is uneven. But, the bonding interface of the Pd120-bonded sample has almost no microcracks, the Kirkendall voids are small, and the intermetallic compound size is uniform and relatively thin. The formation sequence of intermetallic compounds is as follows: Cu atoms diffuse into the Ag layer to form Ag-rich compounds such as CuAg4 or CuAg2, and then the CuAg forms with the increase in diffused Cu elements. Pd can significantly reduce the Kirkendall effect and slow down the growth of Cu-Ag intermetallic compounds. The growth rate of intermetallic compounds is too fast when the Cu bonding wire has a thin Pd layer, which results in holes and microcracks in the bonding interface and lead to the peeling of the bonding interface. Voids and cracks will hinder the continuous diffusion of Cu and Ag atoms, resulting in the growth of intermetallic compounds being inhibited.

Influence of Pd-Layer Thickness on Bonding Reliability of Pd-Coated Cu Wire.

In this paper, three Pd-coated Cu (PCC) wires with different Pd-layer thicknesses were used to make bonding samples, and the influence of Pd-layer thickness on the reliability of bonded points before and after a high-temperature storage test was studied. The results show that smaller bonding pressure and ultrasonic power lead to insufficient plastic deformation of the ball-bonded point, which also leads to small contact area with the pad and low bonding strength. Excessive bonding pressure and ultrasonic power will lead to 'scratch' on the surface of the pad and large-scale Ag spatter. The wedge-bonded point has a narrowed width when the bonding pressure and ultrasonic power are too small, and the tail edge will be cocked, resulting in false bonding and low strength. When the bonding pressure or ultrasonic power is too large, it will cause stress concentration, and the pad will appear as an 'internal injury', which will improve the failure probability; a high-temperature environment can make Cu-Ag intermetallic compounds (IMCs) grow and improve the bonding strength. With the extension of high-temperature storage time, the shear force of Pd100 gradually reaches the peak and then decreases, due to Kirkendall pores caused by excessive growth of IMCs, while the shear force of Pd120 continued to increase due to the slow growth rate of IMCs. In the high-temperature storage test, the thicker the Pd layer of the bonding wire, the higher the bonding strength; in the cold/hot cycle test, the sample with the largest Pd-layer thickness has the lowest failure rate. The thicker the Pd layer, the stronger its ability to resist changes in the external environment, and the higher its stability and reliability.

Microstructure evolution and micromechanical behavior of solvent-modified Cu–Ag composite sintered joints for power electronics packaging at high temperatures

With the increased deployment of power modules in demanding conditions, sintering materials, especially composite sintering materials, have raised growing interest due to their cost-effectiveness and suitability. Therefore, this study explores the viability of Cu–Ag composite sintering material, focusing on solvent influence through microstructure and mechanical behavior analysis. Micron-sized particle-based Cu–Ag composite pastes were designed and compared using eight solvents (four epoxy-free and four epoxy-added) based on fluidity and thermal stability. The sintered joints' performance, assessed through shear strength analysis, showed comparable values to pure silver sintering for both epoxy-free and epoxy-added samples. Optimized samples from each solvent system underwent reliability analysis, demonstrating that Cu–Ag joints with epoxy resin exhibited significantly higher shear strength after high-temperature storage and thermal cycling tests. Micromorphology and elemental composition analysis revealed differences in aging mechanisms, primarily attributed to variations in porosity due to oxide formation and pore filling by epoxy resin under different solvent systems. Further nanoindentation characterization of micromechanical properties, including hardness, modulus, and creep properties, during high-temperature aging, established constitutive models for insights into reliability evolution. In conclusion, the optimized epoxy-added Cu–Ag sintered joints proposed in this study demonstrated exceptional reliability and acceptable micromechanical properties, presenting a promising option for high-temperature power packaging.

Influence of Interfacial Interaction Between Various Metal Substrates and Epoxies on Bonding Reliability Under High Temperture

The shift from gel encapsulation to epoxy encapsulation has been driven by the need to reduce the size of power modules. However, the dissimilar bond between the encapsulation epoxy and substrate is seen as a potential weak point. Previous studies have largely focused on the impact of epoxy materials on package reliability. Our prior research has highlighted the crucial role played by interfacial interactions between copper substrates and epoxy. In this study, we investigate the influence of interfacial interactions between three metal substrates (copper, Ag-plated copper, nickel-plated copper) and epoxy on the reliability of epoxy encapsulation under high temperature. Two types of epoxy (bisphenol A and cycloaliphatic) were used to encapsulate metal substrates. Encapsulated packages were subjected to a high-temperature storage test (HST) at 200°C for 1000 hours, and changes in bonding strength were measured. We then analyzed the fracture surface and bonding interface at the nanoscale. Our results indicate that substrate material has a significant impact on the reliability of epoxy encapsulation. Both epoxies exhibited the highest initial bonding strength with copper, followed by Ag-plated copper and nickel-plated copper. After HST, we found that the catalytic effect of metals caused the epoxy in contact with the metal substrate to decompose. The decomposition was most significant in epoxy in contact with Ag, followed by copper, which resulted in a significant drop in bonding strength. In contrast, epoxy in contact with nickel showed little decomposition and only little change in bonding strength. These findings will greatly benefit power module design.

Reliability evaluation of thick Ag wire bonding on Ni pad for power devices

In recent years, silicon carbide (SiC) semiconductors have gained traction in high-performance power devices due to their ability to operate at high temperatures (above 200 °C). However, traditional interconnection materials are not sufficiently reliable at such elevated temperatures. Typically, in power devices, thick Al wires with diameters of 200 μm or larger are commonly used to bond Al electrodes of semiconductor chips to their corresponding Cu leadframes or substrates. Nevertheless, the low melting point of Al can lead to recrystallization at high temperatures, thereby raising concerns about reliability and a decrease in wire strength and fatigue properties. Although thick Cu wires with higher melting points have been explored as potential solutions, their high hardness and susceptibility to work hardening during bonding can damage the chip beneath the bonding area, making them impractical. In response to these challenges, we investigated the use of thick Ag wires. However, direct bonding of thick Ag wires to Al pads can form intermetallic compounds (IMC) in the bonding area, leading to concerns about decreased reliability. To address this issue, we experimented with a bonding method that utilized thick Ag wires on Ni pads. In this study, we evaluated the reliability of thick Ag wire bonding under various stress conditions, including high-temperature storage tests (HTST) and temperature cycling tests (TCT). Our results showed that thick Ag wire bonding on Ni pads exhibited high bond strength and prevented IMC formation after HTST. Nonetheless, bond failure of the Ag wires could occur after TCT due to the mismatch in the coefficient of thermal expansion (CTE) between the Ag wire and SiC chip, particularly under extremely severe TCT conditions. We found this issue could be mitigated by introducing a soft metal layer, such as Al, between the Ni pad and the SiC chip. This reduced internal stress and enabled the device to pass TCT under harsher conditions with reliable performance.

Transient liquid phase bonding using Cu foam and Cu–Sn paste for high-temperature applications

This study presents a new structure for transient liquid phase (TLP) bonding using Cu foam and Cu–Sn paste. Cu foam and Cu–Sn paste with large reaction surface areas were used to reduce the long bonding time, which is a disadvantage of the conventional TLP bonding process. After applying the Cu–Sn paste at an optimized ratio to the top and bottom of the Cu foam, pressure was applied to fill the pores inside the Cu foam with the paste, and a bonding process was performed at 260 °C under a pressure of 3 kgf. Bonded joints composed of only the Cu skeleton and Cu–Sn intermetallic compounds (IMCs) were confirmed within a bonding time of 5 min. In addition, the proportion of the Cu3Sn phase in the joint increased with the bonding time. After a bonding time of 40 min, the joint was mostly composed of a Cu skeleton and Cu3Sn, with a measured shear strength of approximately 39.2 MPa. After the joint analysis of the as-bonded samples, a high-temperature storage test (HTST) was conducted to evaluate their high-temperature long-term reliability. As a result of HTST, because most of the joint was converted to Cu3Sn after the bonding process, the shear strength did not decrease despite heat treatment at a high temperature of 240 °C, and a similar microstructure of the joint was maintained.

Reliability of the hybrid bonding level using submicrometric bonding pads

Hybrid bonding is a major enabler for devices requiring 3D stacking with high interconnection density. An in-depth reliability study is performed for the specific Cu/SiO2 integration to identify the potential failure mechanisms. A high robustness is evidenced down to bonding pitch 0.81 μm for the standard reliability tests such as Thermal Cycling (TC) tests, High Temperature Storage (HTS) test, Stress induced Voiding (SiV), Time Dependent Dielectric Breakdown (TDDB) or electromigration. The hybrid bonding interface is stable under thermal stress with no atomic or ionic Cu diffusion observed through the hybrid bonding interface. The role of the thin self-formed cuprous oxide at the Cu/SiO2 interface in the case of misaligned bonding pads is believed to play a major role in the high robustness to reliability tests.

Influence of interfacial interaction on the reliability of the bond between encapsulation epoxy and copper substrate

The need for power modules has been promoted by the emergence of electric vehicles. The requirements of small volume, high integrity, and high reliability for the next-generation power module lead to the change of encapsulation method from previous gel encapsulation to epoxy encapsulation. Efforts have been made to enhance epoxy materials, and commercialized high-end epoxy has shown excellent properties, but power module failures still exist. The possible reason may lie in the interfacial interaction between encapsulation epoxy and substrate, which is less noticed previously. This paper seeks to unveil the influence of interfacial interaction on the bonding reliability between encapsulation epoxy and copper substrate. Three reliability tests were performed: high-temperature storage test (HST), temperature cycling test (TCT), and pressure cooker test (PCT). The bonding strength after these reliability tests was evaluated, and the bonding interface and fracture surface were analyzed in a nanoscale. Our study discovered that interfacial interaction plays a vital role in encapsulation reliability. Copper can diffuse into the epoxy and catalyze epoxy thermal oxidative degradation, which results in bonding strength reduction after HST and PCT. The surface structure change caused by copper oxidation after HST and PCT also reduced the bonding strength. These findings will greatly benefit future power module design.

Study on the Solder Joint Reliability of New Diamond Chip Resistors for Power Devices

New diamond chip resistors have been used in high-power devices widely due to excellent heat dissipation and high-frequency performance. However, systematic research about their solder joint reliability is rare. In this paper, a related study was conducted by combining methods between numerical analysis and laboratory reliability tests. In detail, the shape simulation and thermal cycling finite element simulation for solder joints with different volumes were carried out. The optimized solder volume was 0.05 mm3, and the maximum thermal cycling stress under the optimized shape was 38.9 MPa. In addition, the thermal cycling tests with current and high temperature storage tests were carried out for the optimized solder joint, which showed good agreement with the simulation results, clarified the growth and evolution law of intermetallic compound at the interconnection interface, and proved the optimized solder joint had great anti-electromigration, temperature cycling and high temperature storage reliability. In this work, an optimized solder joint structure of a diamond chip resistor with high reliability was finally obtained, as well as providing considerable reliability data for the new type of diamond chip resistors, which would boost the development of power devices.

Failure mechanisms of the bonded interface between mold epoxy and metal substrate exposed to high temperature

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.

High-temperature resistant interconnection using Ni nanoparticles and Al microparticles paste sintered in an atmosphere

Next-generation power devices using wide bandgap semiconductors, such as SiC, are expected to operate at higher temperatures than conventional Si power devices, and their operating temperatures are expected to exceed 250 °C. We developed a novel high-temperature resistant interconnection technology for die-bonding of SiC power devices using Ni nanoparticles and Al microparticles composite paste. The bond strength of the Al-metallized Si chip to Ni-plated direct bonded copper substrate was evaluated using shear tests. The initial shear strength of samples from pressureless sintering at 350 °C for 15 min in the air exceeded 30 MPa. Furthermore, no significant degradation was observed in a high-temperature storage test at 250 °C for 1000 h.

Solderability and Reliability of Sintered Nano-Ag Bond Pads of Printed Re-Distribution Layer

Abstract Redistribution layer (RDL) is one key enabling technology for advanced packaging. RDL is usually fabricated at wafer level by a photolithography process. An alternative approach for implementing RDL by additive manufacturing (AM) method is proposed in this study. This allows RDL to be fabricated on a singulated chip. Nanosilver (nano-Ag) ink is printed on the silicon chip to form routing traces and bond pads. However, the Ag pad may be consumed by the solder quickly if the reflow process is not properly controlled. The mechanical integrity of the solder joint was evaluated by both solder ball shear and die shear tests. A high temperature storage test and humidity test were carried out to evaluate the solder joint reliability. Experiment results showed that the Ag pad fabricated by AM is compatible with surface mount technology (SMT). Reliable solder joints on the Ag pad were achieved. There was no significant change in the Ag3Sn intermetallic compound (IMC) layer thickness and mechanical strength after aging at high temperature and high humidity conditions. There was no early solder joint failure observed. The finding of the present study will serve as a very useful reference for the future practice of forming solder joints on sintered nano-Ag pads.

Simulation and reliability testing of leadless package high-temperature pressure sensor

Most of the current SOI high-temperature pressure sensors are packaged in lead bonding, where the gold wire is in direct contact with the bonding bump and filled with silicone oil as a protective medium. Silicone oil expands at high temperatures, limiting the sensor's operating temperature to 150 °C or less. To solve this problem, this paper uses an anodic bonding process to bond borosilicate glass wafers to SOI wafers, which are subsequently filled with conductive silver paste and sintered at a high temperature to avoid direct contact between the leads and the chip pads. The reliability of the leadless package structure under high temperature, high pressure, and random vibration is verified by multi-field coupling stress simulation. High-temperature storage and random vibration tests were conducted according to the application requirements, and the results show that the sensor has good reliability and stability.

Interface-Mechanical and Thermal Characteristics of Ag Sinter Joining on Bare DBA Substrate During Aging, Thermal Shock and 1200 W/cm2 Power Cycling Tests

In this article, an SiC die was directly attached on a bare DBA (Al/AlN/Al) substrate via micron-sized Ag sintering at 250 °C without pressure. The micron-sized structure of the Ag–Al joint revealed robust bonding (33.6 MPa), which was attributed to the excellent sinterability of the Ag paste. A high-temperature storage test was conducted for 1000 h at 250 °C, and the thermal shock test was conducted from −50 to 250 °C for 2000 cycles. The shear strength was >30 MPa even after 1000 h of high-temperature storage. Furthermore, the online thermal resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ) of the direct bonding of SiC–DBA was measured during the power cycling test employing an n-doped 4 H-SiC thermal engineering group (TEG) chip. The SiC–TEG chip exhibited ultra-high power density (1200 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and ensured that the junction temperature reached 200 °C. The total R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> increased from 0.58 to 0.7 K/W after 10 000 power cycles at a swing temperature, ΔTj, of 175 °C, indicating that the increase in the R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> was 20.7% at such a large power density. The results revealed that the SiC–DBA power module structure exhibited very good bonding and thermal-resistance reliability, which can be employed as a new structure for next-generation SiC power modules.

Selection of 8-channel silicon phototransistor arrays for space applications, based on wafer-level radiation and high-temperature storage tests

Selection of 8-channel silicon phototransistor arrays for space applications, based on wafer-level radiation and high-temperature storage tests

The 3-phenyl-1,4,2-dioxazol-5-one (PDO) Electrolyte Additive for Li(Ni0.6Mn0.2Co0.2)O2 and Li(Ni0.8Mn0.1Co0.1)O2 Lithium-Ion Cells

Electrolyte additives, as a small proportion of the electrolyte, greatly affect the performance of lithium-ion cells. This work performs a comparative study to reveal the difference between commercial 99.8% pure 3-phenyl-1,4,2-dioxazol-5-one (PDO) additive and lab-made 95% pure PDO in NMC622/graphite cells. In addition, a set of experiments were conducted to evaluate the performance of 99.8% pure PDO and its binary blends with vinylene carbonate (VC), 1,3,2-dioxathiolane-2,2-dioxide (DTD) or lithium difluorophosphate (LFO) in NMC811/graphite cells. 99.8% Pure PDO and 95% pure PDO show little difference in the NMC622 cells, with the latter presenting relatively better performance in the best-performing blends for long-term cycling and high-temperature storage tests. Considering all the tests including ultra high precision coulometry (UHPC) cycling, long-term cycling, and high-temperature storage, the NMC811 cells with 2%PDO+ 1%LFO outperformed the other PDO-containing cells. The PDO-based blends were confirmed to be more promising in cells with higher nickel content; that is, PDO could be a useful additive in high-nickel content cells.

A fluorinated carbonate-based electrolyte for high-voltage Li(Ni0.8Mn0.1Co0.1)O2 lithium-ion cells

This work performed a comprehensive investigation on a fluorinated electrolyte (1 M LiPF6 in a 1:9 solvent blend, by mass, of fluoroethylene carbonate (FEC) and methyl (2,2,2-trifluoroethyl) carbonate (FEMC)) proposed by Im et al. [1]. The fluorinated electrolyte addicted with 3 wt% lithium difluorophosphate (LFO), named FF19+3LFO is found to demonstrate better cycling performance in high-voltage Li(Ni0.8Mn0.1Co0.1)O2 lithium-ion cells over the other researched electrolytes. The improvement of cells’ performance under high-voltage conditions is revealed to benefit from the robust CEI layers formed by FF19+3LFO, which can significantly enhance the electrode/electrolyte interface and restrain the side reactions between cathode materials and electrolytes. Moreover, the FF19+3LFO cells also illustrate more competitive performance than the conventional non-aqueous electrolyte based cells in the high-temperature storage tests, accelerating rate calorimetry (ARC) tests, nail tests, overcharge tests, and so on. In short, the electrochemical and safety features of high-voltage cells are effectively improved by FF19+3LFO. More concern into the fluorinated electrolyte is warranted for achieving the high-capacity and high-safety lithium-ion cells.

Evaluation of the Long-Term Reliability of Open-Tube Diffused Planar InGaAs/InP Avalanche Photodiodes under a Hybrid of Thermal and Electrical Stresses

The long-term reliability of open-tube diffused planar InGaAs/InP APDs was investigated via accelerated life testing in this study. For the proposed life testing scheme, both thermal and electrical stresses were applied simultaneously to reduce the testing periods while maintaining statistical significance. Additionally, the Eyring model was used to extrapolate the activation energy. To determine the optimum life testing conditions, high-temperature storage tests, preliminary accelerated life tests, and main accelerated life tests were conducted. From the test results, the mean-time-to-failure was utilized to verify the suitability of the Eyring model. The proposed testing scheme, which utilizes a hybrid of accelerated stress factors, allows us to estimate the device reliability within an acceptable testing period, minimizing the time to market.

High-temperature Resistant Interconnections Formed by Ni Nanoparticles/Al Micro-particles Composite Paste for SiC Power Device Application

Sintering bonding using metal nanoparticles is considered a promising die-attach technique for high-temperature operating power devices, such as SiC. However, the thermal stress produced by the difference in coefficient of thermal expansion (CTE) between the chip and substrate, often affects the reliability of this technique. To address this limitation, we analyzed a sintering bonding using Ni nanoparticles and Al microparticles composite paste. The results revealed that the Al microparticles significantly reduced the voids and cracks in the bonding layer formed by Ni nanoparticles paste. Three chips with different sizes were bonded to Cu substrates, and the bonding strength was measured. The result revealed that the bonding strengths in the samples were higher than those of the samples bonded using conventional metal nanoparticles. In addition, the fracture observation after the shear test revealed that the Al particles suffered plastic deformation. Furthermore, the high-temperature storage test at 250°C confirmed the long-term bonding reliability.