Enhancing Mechanical Reliability of Silver-Sintered Joints With Copper Nanowires in High-Power Electronic Devices

The worldwide advancements in automotive, aviation, space, and nuclear industries have constantly triggered a quest of smaller, faster, and more powerful electronic devices suitable for modern technological era. This insatiable demand has laid a foundation for making progressive advancements in the field of semiconductor technologies, propelling innovation to unprecedented levels [1].

Amidst this technological renaissance, the most crucial improvement is significant from the material development and its improvement in the electronic device fabrication—the process of die attachment. Primarily, die attachment serves as the most important factor in the semiconductor industry, bridging the gap via the packaging interconnection, physical protection, and mechanical support. The performance of die-attach material in general has profound impact on the performance, reliability, and longevity of these micro-electronics technology.

The most used materials are tin-based solders or high lead solders, commonly used as die-attach materials in many applications at operating temperatures below 200 °C. But the higher technological require research and development in novel packaging materials and technologies for higher temperature applications. In this context, silver sintering has emerged as an alternative promise in the field of die attachment technology to meet these stringent requirements. The promising properties of sintered silver such as high thermal conductivity, high melting temperature (960 °C), high electrical conductivity, and a low coefficient of thermal expansion have made it game changer die attachment material in advanced semiconductor technologies [2].

Additionally, heat can be efficiently conducted away from the semiconductor chip, ensuring optimal operating temperatures, and mitigating the risk of overheating, which can lead to performance degradation and even catastrophic failures. Similarly, the low coefficient of thermal expansion of silver with copper substrate bolsters its rigidity from expansion and contraction with changes in temperature. Hence, this material demonstrates good reliability and prevents the creation of stresses and strains within the assembly that could otherwise lead to cracking, delamination, or other forms of mechanical failure. These properties turn out to be an excellent fit for reliable, high-performance electronic devices due to their robust thermal management and mechanical stability [3].

While silver sintering has indeed emerged as a robust die attachment technique, it is not without its own set of challenges. Among the foremost concerns is the susceptibility to brittleness and porosity within the silver-sintered layer and interface [4]. Consequently, the electronics industry has recognized the pressing need for innovative solutions to fortify the mechanical and thermal–electrical properties of die attachment interfaces.

To counter the issue in sinter silver, this research work has focused on the incorporation of copper nanowires (NWs), which is reported as formidable material due to their exceptional mechanical strength and electrical conductivity [5].

One of the standout features of copper nanowires is their innate ability to facilitate the creation of highly conductive and transparent electrodes [5]. However, the transformative potential of copper nanowires extends beyond their prowess in electrical and optical performance. These nanowires also hold the key to bolstering the very foundation of die attachment interfaces. There are studies that show that adding Cu nanowires to the solder joint can improve the wettability of base solder to a certain degree and can effectively enhance the shear strength of the solder. It is also found that adding Cu NWs can refine the microstructure of SAC105-xCu NWs solder. In addition, the IMCs layer thickness of the SAC105-xCu NWs solders is smaller than that of plain Sn1.0Ag0.5Cu solder, which indicates that Cu NWs can effectively suppress IMC formation and growth [6].

By integrating copper nanowires into the die attachment process, it is possible to enhance the adhesion strength and ductility of the silver-sintered interface, leading to improved reliability and performance of electronic devices.

Adhesion strength in the packaging technology is crucial because it determines how effectively the chip remains securely attached during the device's operational life. Traditional silver sintering techniques, while effective in many regards, can be vulnerable to mechanical stresses and thermal fluctuations, potentially leading to detachment or microcracks within the interface.

Among the above issues, ductility is another critical aspect of this material that is affecting its reliability. As per definition, ductile materials are characterized by an ability deform and absorb energy without fracturing [7]. In silver sintering, a ductile interface is essential because it can accommodate the stresses and strains that electronic devices often encounter during their operational life. The incorporation of copper nanowires contributes significantly to the ductility of the silver-sintered interface [8–10].

This research work is not just focused on the immediate effects of these nanowires; it extends to understanding their impact over time and under challenging temperature conditions. It was closely monitored about the behavior of the interfaces during the high-temperature storage tests, looking for signs of structural degradation and adhesion strength alterations.

In this study, polycarbonate track-etched (PCTE) membranes are utilized as a crucial element in the growth and fabrication of copper nanowires within the substrate. PCTE membranes have gained recognition and widespread use in nanowire synthesis due to their distinct and advantageous characteristics [11].

First and foremost, the decision to employ PCTE membranes stems from their outstanding chemical and thermal stability. These membranes can endure a wide range of chemical environments and thermal conditions without deteriorating, making them an ideal choice for the controlled synthesis of nanowires. This stability ensures the reliability and reproducibility of the nanowire growth process, which is paramount in scientific research and technological applications.

One of the key strengths of PCTE membranes lies in their ability to precisely control nanowire dimensions. In our study, we aimed to cultivate copper nanowires with specific characteristics to suit our objectives. These membranes provided us with the capability to tailor the nanowires to our desired specifications, including an average diameter of 1 μm, a pitch of 2.236 μm, and a length extending up to an average of 10 μm. This configuration was strategically chosen based on its capacity to deliver a harmonious balance of attributes, each contributing significantly to the enhancement of die attachment interfaces in advanced semiconductor technologies.

The choice of a 1-μm diameter is particularly notable as it aligns with the requirements of electrical conductivity [8]. Copper nanowires of this size are renowned for their excellent electrical conductivity, a trait that is pivotal in ensuring efficient electrical connections within electronic devices. This property facilitates the seamless flow of electrons, reducing resistance and enabling optimal device performance.

Furthermore, the 2.236-μm pitch, which refers to the spacing between nanowires, contributes to mechanical flexibility. This configuration allows for the creation of an interconnected network of nanowires that not only facilitates flexibility but also maintains structural integrity, even when subjected to mechanical stresses and strains. This mechanical flexibility is essential for ensuring that the die attachment interface can withstand dynamic environments without compromising its adhesion strength or structural stability [9].

Finally, the length of up to 11 μm serves to enhance thermal stability. Longer nanowires possess a greater surface area for heat dissipation, making them well-suited for applications where thermal management is crucial. This thermal stability is pivotal in maintaining the performance and reliability of electronic devices, particularly those that generate significant heat during operation [9] (Table 1).

Nanowire samples are fabricated using the process shown in Fig. 1 based on the method described by Barako et al. [12]. To initiate the synthesis of copper nanowires, the substrates undergo an initial cleaning process involving an acetone rinse. The PCTE membrane filter is then thoroughly cleaned with a combination of acetone and isopropanol to eliminate any impurities or contaminants that may interfere with the nanowire growth. This critical step is crucial to ensure that the resulting nanowires are of high quality and exhibit excellent electrical properties.

Once the membrane is deemed clean, it is securely attached to the substrate using de-ionized water and tape to establish a robust bond. This step is vital in ensuring that the membrane remains in place during the subsequent stages of the synthesis process.

After the membrane is securely attached, the electroforming process is initiated. This process involves depositing a layer of copper ions onto the membrane filter, which are subsequently reduced to copper atoms. These atoms then grow into nanowires within the pores of the membrane filter, resulting in the synthesis of high-quality copper nanowires with excellent electrical properties.

Once the nanowires have grown to the desired length, the electroforming process is stopped, and the copper overplating is peeled off from the surface of the membrane using gentle force. The PCTE membrane filter is then dissolved in a suitable solvent such as dichloromethane, leaving behind a layer of highly uniform and high aspect ratio copper nanowires.

The final step involves sonicating the nanowires in acetone to remove any residual solvent and then drying them in a vacuum chamber to ensure the removal of any moisture. Figure 2 shows a scanning electron microscopy (SEM) image of the copper nanowires after being synthesized.

Silver sintering is a promising alternative to traditional bonding methods, such as soldering, in the electronics industry due to its high temperature stability and thermal conductivity. The particle-based material allows for bonding at lower temperatures, reducing the risk of thermal damage to sensitive electronic components, while maintaining the melting temperature of bulk silver [11].

Sintered silver particles also exhibit similar properties to bulk silver, in terms of a high melting point of approximately 961 °C and a high thermal conductivity of 140 W/m K. These properties ensure that the bond remains strong even under high temperatures and that heat can efficiently dissipate, preventing overheating and subsequent component failure [12]. In principle, the low bonding temperature plus these properties are meant to facilitate thermal cycling resilience, by reducing the effects of coefficient of thermal expansion mismatch during bonding, while maintaining high temperature stability.

The die attachment process is critical to the overall reliability and performance of electronic systems. For this study, silicon dies with a gold coating (100 Å) and dimensions 4.4 × 5.7 mm were utilized, and the process flow depicted in Fig. 3 was implemented to ensure a consistent and robust bond between the die and the substrate.

First, a 50 μm thick stencil was used to apply the silver sinter paste onto the copper substrate. This approach helped to ensure precise placement of the paste and minimize the risk of uneven bonding. The excess paste was then removed to prevent voids and achieve a uniform bond.

To enhance the uniformity of the bond, a graphite fixture was utilized to hold the copper substrate and silver sinter paste in place. A weight of 500 g was used to apply a sintering pressure of 132 MPa, facilitating diffusion and ensuring consistent bond strength across the entire attached area [13].

During the sintering process, a nitrogen environment was utilized to prevent oxidation and ensure that the bonds were not compromised by impurities.

Finally, Fig. 4 provides a clear illustration of the silver-sintered reflow profile used in the study, which helped to ensure that the silver paste was properly sintered, and the bond was strong and uniform.

The primary mechanism governing deformation in sintered silver is plasticity, where particles undergo microscopic deformation and flow to facilitate the material's adaptation to applied stresses [14]. Low-pressure sintering, introduced to subject the material to the sintering process under reduced pressure, plays a pivotal role in altering the material's microstructure. This alteration predominantly results in an increase in porosity [15].

To investigate the initial bond quality of the sample as well as the degradation observed during the thermal aging test at 300 °C for 96 h, the samples underwent encapsulation, cross-sectioning, and polishing using a Buehler Ecomet 3000 variable-speed polisher equipped with an Automet 2000 power head. The sample preparation procedure was adapted from the Buehler SumMet polishing guide [16].

This involved initial cutting of the samples to the desired size, followed by mounting in epoxy resin, grinding, and polishing to achieve a smooth surface, and finally etching the polished surface to reveal the internal structure of the samples. The FEI Nova Nanolab 200 present at the Nano & Bio Materials Characterization Facility of the University of Arkansas was utilized to perform SEM and energy-dispersive X-ray spectroscopy (EDX) analyses. These techniques were used to examine the microstructure and composition of the samples before and after thermal aging, allowing for a detailed analysis of the changes that occurred in the silver sinter layer and the underlying substrate.

Figure 5 displays a 2500× magnification cross-sectional SEM image of the silver-sintered interface without the presence of copper nanowires. The image shows a uniform distribution of silver particles that are fused, forming a robust and uniform bond without significant voids or cracks. The lack of voids and cracks in the silver-sintered interface signifies that the initial quality of the silver sinter bond is excellent.

The SEM analysis of the copper nanowires samples shown in Fig. 6 confirmed that the silver sinter particles, both nano- and microsized, effectively filled the gaps between the nanowires. With a 5000× magnification, the nanowires can be captured as well as the porosity behavior of pressure less sintering.

The incorporation of copper nanowires in the substrate interface created a three-dimensional network that enhanced the adhesion strength and ductility of the silver-sintered interface. The copper nanowires acted as a scaffold for the silver sinter particles, providing a larger contact area and allowing for better penetration of the silver sinter into the gaps between the nanowires. This phenomenon is highly relevant in Figs. 5 and 6.

Apart from the voids, the EDX images also revealed the appearance of an oxidation layer in the copper interface after thermal aging at 300 °C which is absent in Fig. 7 where aging has not been performed.

The EDX analysis was employed to evaluate the degradation of the silver sinter die attachment after 96 h of thermal aging at both 175 °C and 300 °C. At 175 °C, no significant degradation was observed since the number and size of voids in the bond did not increase, indicating good adhesion and stability of the material.

However, as shown in Figs. 8 and 9, thermal aging at 300 °C led to a considerable increase in the number and size of voids within the bond area, indicating a loss of adhesion and structural integrity of the material. In an elevated-temperature setting, sintered silver experiences coalescence, resulting in the emergence of gaps at the interface between the sintered silver and copper [17].

This oxidation layer is formed due to the reaction between the copper and oxygen in the surrounding environment, and its presence can contribute to the degradation of the bond by reducing the contact area between the copper and silver [18].

In Fig. 9, it is shown that the incorporation of copper nanowires in the die attachment copper interface has a positive effect on the thermal stability of the silver sinter bond. The three-dimensional network created by the copper nanowires provides additional stability, reducing the thermal stress on the bond area and thus mitigating the formation of voids. Furthermore, the use of copper nanowires as a scaffold for the silver sinter particles increases the contact area between the two materials, allowing for better interdiffusion and adhesion. The findings depicted in Figs. 8 and 9 provide clear evidence of the significant reduction in void size within the silver sinter attachment layer after subjecting the samples to 96 h of thermal aging storage at 300 °C, achieved by introducing copper nanowires into the substrate interface.

To gain a comprehensive understanding of the mechanical reliability of the die attachment joints, our research employed a rigorous evaluation process that involved subjecting the samples to thermal aging under controlled conditions. This meticulous testing aimed to assess how the shear strength of the joints evolved over time and under varying temperature conditions, offering critical insights into the long-term performance and stability of the interfaces.

The first step in this investigation involved subjecting the samples to thermal aging within a precisely controlled environment. The samples were placed in an air oven and exposed to elevated temperatures of 175 °C, 220 °C, and 300 °C. These temperature regimes were chosen to simulate the harsh operating conditions that electronic devices may encounter during their operational life, especially in demanding applications.

Throughout the thermal aging process, the shear strength of the die attachment joints was monitored at regular intervals. Specifically, measurements were taken every 24 h, extending up to a total duration of 96 h or until failure occurred. This systematic approach allowed us to capture the dynamic changes in shear strength over time as the samples endured thermal stress and aging. Figure 10 shows a representation of the die shear test process.

Figure 11 shows the results of the thermal aging tests conducted at 175 °C, 220 °C, and 300 °C indicating a significant difference in the performance of the silver sinter layer. At 175 °C, the die shear strength is not affected by the aging through the entire test, with no significant degradation observed. This suggests that the silver sinter layer is highly stable at this temperature and is well-suited for use in applications that operate at or below this temperature range.

In the 220 °C temperature storage test, degradation can be observed. At this temperature degradation in the shear strength can be observed and the impact of the addtion of the copper nanowires acting as an enhancement in the silver sinter die attachment in the long term. The reduction in shear strength throughout the test is decreased.

However, at 300 °C, a decrease of approximately 75% from the initial shear strength value was observed, indicating substantial degradation of the silver sinter layer. This is likely due to the thermal stresses and reactions that occur at high temperatures, which cause the silver sinter layer to degrade and lead to the formation of voids or other defects in the bond.

Exposure to thermal aging 300 °C markedly enlarges both the dimensions of silver particles and pores, consequently diminishing shear strength [19].

In the initial samples without aging, the porosity from low-pressure sintering shifts the observed failure mechanism toward intergranular fracture. This fracture type occurs along the boundaries between individual grains or particles within the material, closely associated with the distribution of porosity along these grain boundaries in the examined sintered interconnections. These samples without aging exhibited cohesive failure [20].

During thermal aging, the increased voids depicted in Figs. 8 and 9 weaken the cohesion between grains, making the material more susceptible to failure along these boundaries and resulting in substrate interface failure—an adhesive failure. The incorporation of copper nanowires demonstrated a reduction in void formation, enhancing shear strength and predominantly exhibiting cohesive failure.

To further analyze the impact of the addition of copper nanowires in the silver sinter die attachment, a discrete analysis was carried out in jmp to compare the shear strength data values with and without nanowires and evaluate the difference in location and scale. This study was carried out for each temperature of the high-temperature storage test (Table 2).

A t-test was conducted to assess the difference in location (mean) between two datasets, one with nanowires and one without. At temperatures of 175 °C and 220 °C, the results indicated a Pi value below alpha (0.05), leading to the rejection of the hypothesis of no effect on varying the location parameter. However, at 300 °C, the Pi value exceeded alpha, suggesting that, due to the extreme temperature, the impact of copper nanowires on shear strength is unclear [21] (Table 3).

For the scale parameter analysis, a test on the hypothesis “Tests that the variances are equal” was performed. The results for each temperature revealed a Pi value larger than alpha, indicating that the hypothesis is not rejected. Therefore, it can be concluded that the data for both samples at all temperatures can be represented with the same scale parameter.

Statistical analyses were conducted using jmppro to examine how the three parameters—temperature, time, and addition of copper nanowires—influenced the shear strength of the silver sinter die attachment [22]. When assessing the significance of both single-parameter and three-parameter models for each of these variables, the findings revealed that the temperature parameter had the most substantial impact on shear strength response in the temperature storage test, followed by the thermal aging time. This observation aligns with expectations, as higher-temperature testing results in a more rapid decrease in shear strength, and prolonged exposure of the die attachment material to thermal aging leads to a greater degree of degradation [23].

On the other hand, the addition of the nanowires appears to be in the limit to consider its addition statistically significant. The three-parameter model proved to be statistically insignificant and was consequently rejected (Fig. 12).

In order to analyze which parameters were impactful in each thermal aging test, the results were modeled independently for each temperature.

At 175 °C, as shown in Fig. 11, the shear strength did not decrease significantly. The results indicate that incorporating copper nanowires has a statistically significant impact on the silver sinter joints over the 96-h aging period studied. However, a longer aging time at this temperature could provide a clearer distinction in the degradation between samples with and without nanowires (Fig. 13).

For 220 °C, the effect of nanowires becomes evident as both parameters statistically influence the shear strength value. Over time, there is a noticeable decline in shear strength; however, this decrease is statistically mitigated in the samples with copper nanowires (Fig. 14).

Finally, at 300 °C, as previously mentioned, there is a substantial reduction in shear strength within the first 24 h, observed in both samples, whether with or without nanowires. Despite the appearance of the graph suggesting that nanowires mitigate the reduction in shear strength, it is important to note that statistical analysis indicates that the nanowires do not have a significant impact under these extreme temperature conditions. Conversely, the aging time proves to be statistically significant, resulting in a reduction in shear strength over time [24] (Fig. 15).

In summary, this study conducted a thorough examination of the mechanical reliability of silver-sintered die attachment interfaces in challenging operating conditions and explored the potential advantages of incorporating copper nanowires to strengthen these crucial bonds. The results demonstrated a substantial improvement in interfacial resilience with the introduction of copper nanowires, acting as robust mechanical reinforcements that enhanced structural integrity. This enhancement is pivotal for ensuring the long-term reliability and performance of electronic devices exposed to extreme temperatures and mechanical stresses while minimizing susceptibility to degradation during thermal aging.

The observed reduction in void formation, coupled with improved mechanical properties, adds an extra layer of stability and durability to the bond. High-temperature storage tests, shear tests, and SEM analysis, complemented by statistical software, provided a comprehensive assessment of the impact of Cu nanowires on the Ag sinter die attachment. The findings indicated enhanced mechanical reliability under aging temperatures below 220 °C, notably reducing shear strength degradation, and significant minimization of voids and oxidation at extreme temperatures (300 °C), enhancing overall bond resilience.

These results assure that the benefits of enhanced mechanical properties remain uncompromised in typical device operating conditions. The study underscores the multifaceted benefits of nanowires in the die attachment process, mitigating potential weaknesses and defects, and highlights the significant potential of pioneering materials and methods to enhance the reliability and durability of electronic devices. It emphasizes the essential role of continuous investigation and experimentation in fully unlocking the capabilities offered by materials such as copper nanowires and deepening our comprehension of their influence on die attachment interfaces.

The completion of this work was made possible through funding support from the Office of Naval Research and NSF via Power Optimization of Electro-Thermal Systems (POETS) Engineering Research Center. Morover, the High Density Electronics Center (HiDEC) facilities and staff at University of Arkansas are acknowledged for assistance in preparing samples, for which the authors express their gratitude. The opinions, findings, and conclusions or recommendations expressed in this work are solely those of the author(s) and do not necessarily reflect the views of the Office of Naval Research or NSF.

• National Science Foundation: POETS ERC (Award No. EEC-1449548; Funder ID: 10.13039/100000001).

• Office of Naval Research (ONR) (Funder ID: 10.13039/100000006).

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

EDX =

energy-dispersive X-ray spectroscopy

NW =

nanowire

SEM =

scanning electron microscope