Microstructure and Bonding Strength of Low-Temperature Sintered Ag/Nano-Ag Films/Ag Joints

Abstract

Nano-Ag paste is one of the most widely used die-attachment materials in modern electronic devices, which are gaining continuously increasing application in transportation industries. The nano-Ag film in a pre-formed dimension and free from the use of chemical dispersing agents has been proposed to be a promising alternative to nano-Ag paste for the die-attachment application. Although the bonding mechanisms of Nano-Ag paste have been extensively studied, little is known about the relationship between the microstructure and mechanical properties of low-temperature-sintered Ag/nano-Ag film/Ag joints. In this work, the influences of temperature, pressure, and dwell time at peak temperature on the microstructure and the shear strength of low-temperature-sintered Ag/nano-Ag film/Ag joints were systematically investigated. Mechanical properties tests indicate that both temperature and pressure have pronounced effects on the bonding strength of sintered Ag/nano-Ag film/Ag joints. TEM and hot nanoindentation characterizations further reveal that the sintering temperature plays the most determinant role in the coarsening of nano-Ag film and, hence, the bonding and fracture behaviors of Ag/nano-Ag film/Ag joints sintered at 210–290 °C. The diffusion-induced coarsening of nano-Ag particles can be activated, but remains sluggish at 250 °C, and the mechanical integrity of sintered joints is circumscribed by the interfacial bonding between nano-Ag film and Ag substrate after sintering at 290 °C.

1. Introduction

There has been a global increase in the electric vehicle market as a result of both the consumer demands and the government regulations for more environmentally friendly transportation solutions [1,2]. Power electronic devices, which enable the transmission and conversion of electrical power, play a critical role in the propulsion system of electric vehicles [3,4]. However, the constant increase of working voltage and current in a continuously minimizing physical dimension make the Joule heating and associated thermo- and electro-migration become a reliability issue for the integrity of power electronic devices or modules in electronic vehicles [5,6]. In particular, the die-attachment materials in power electronic devices must provide sufficient thermal, electrical, and mechanical integrity at a working temperature typically above 250 °C [7,8], which is above the soldering temperatures of most Sn- or Pb-based solder alloys [9]. Thus, low-temperature nano-Ag paste has emerged as one of the most promising die-attachment materials for power devices [10,11].

Nano-Ag paste commonly refers to a mixture of nano- or micron-sized Ag particles and organic dispersing agents. In the sintering process, the high surface energy of nanoparticles enables the occurrence of surface diffusion of Ag powders at relatively low temperatures [12], resulting in the formation of a reliable bonding that can endure thermal, electrical, and mechanical loads at operation temperatures even higher than sintering temperatures [13,14,15]. The sintering temperature, pressure, and dwell time at peak temperature are key factors that comprehensively affect the microstructure and mechanical properties of sintered nano-Ag paste [16,17,18]. Previous research indicates that the sintering temperature has a most pronounced influence on the shear strength of nano-Ag paste joints: an increase of the sintering temperature from 225 to 300 °C can result in a 30 MPa improvement in the shear strength of nano-Ag paste joints [19,20,21]; meanwhile, increasing the sintering pressure from 1 to 10 MPa only results in an increase of shear strength from 26 to 35 MPa [12], and an elongation of the dwell time from 5 min to 15 min at a sintering temperature of 250 °C has a negligible influence on the shear strength of sintered nano-Ag paste joints [22].

Recently, nano-Ag film, composed of nano-Ag particles in a pre-formed geometry and free from the use of organic agents, has been proposed as an alternative material of nano-Ag paste for die-attachment applications. The pre-formed thickness of nano-Ag film simplifies the printing, screening, and drying processes associated with conventional nano-Ag paste, allowing tailoring of the thickness of the bonding layer in die-attachment arrangement. Despite these promising advantages of nano-Ag film, little is known about the microstructure and mechanical properties of Ag/nano-Ag film/Ag joints fabricated in the low-temperature sintering process. Influences of sintering conditions on the coarsening and microstructure evolution of nano-Ag film, as well as the bonding and fracture behaviors of sintered Ag/nano-Ag film/Ag joints’ strength, are yet to be revealed. This knowledge is of fundamental importance, not only for the advancement of die-attachment packaging technology and materials, but also for a better understanding of the service reliability of power electronic devices in electrical vehicles.

In this work, the microstructure and bonding strength of Ag/nano-Ag film/Ag joints were systematically investigated at sintering temperatures from 210 to 290 °C under an applied pressure ranging from 1 to 10 MPa. The influences of sintering pressure, temperature, and dwell time at the peak temperature on the microstructure evolution and bonding strength were revealed through shear tests and microstructure observation. The sintering mechanism was studied using TEM and hot nanoindentation techniques, in order to expatiate the role of the sintering temperature and pressure in the fracture behavior of Ag/nano-Ag film/Ag joints. Experimental results were discussed to understand the pros and cons of the newly developed nano-Ag film for die-attachment applications.

2. Materials and Methods

As illustrated in Figure 1a, commercially available Ag ingots were cut into cubes in dimensions of 5 × 5 × 3 mm and 8 × 8 × 3 mm, mechanically polished with 4000-mesh sandpaper, and cleaned in an ultrasonic alcohol bath. Then, nano-Ag film in a size of 8 × 8 mm was placed on the Ag substrate in a dimension of 8 × 8 × 3 mm and heated at 120 °C under a pressure of 5 MPa for 6 min. After heating, the polyethylene terephthalate (PET) film was manually removed from the nano-Ag film. As illustrated in Figure 1b, sample assembly was completed by loading an Ag cube in a dimension of 5 × 5 × 3 mm on the prepared nano-Ag film, and sintering was conducted in atmosphere under the conditions listed in

Table 1. Sintering parameters in this work.

Sample No.	Temperature (°C)	Pressure (MPa)	Time (min)
1	210	5	15
2	250	5	15
3	290	5	15
4	250	1	15
5	250	3	15
6	250	10	15
7	250	5	5
8	250	5	30

. The shear strengths of the sintered Ag/nano-Ag film/Ag joints were measured on a Shimadzu EZ-LX universal tester (Shimadzu, Kyoto, Japan) using a customized sample holder. The shear speed was kept at 5 mm/min, and each shear strength value was averaged from at least 5 measurements. Microstructural characterization of the sintered joints before and after the shear tests was performed on a Hitachi SU-1510 scanning electron microscope (SEM). A FEI TECNAI F20 field-emission transmission electron microscope (TEM) was used for phase analysis of the selected sintering sample. TEM samples were prepared using a FEI Helios Nanolab-600i dual-beam focused iron beam (FIB) milling machine. A G200 nanoindenter installed with a hot stage was used to simulate the coarsening behavior of the nano-Ag film in the sintering process at corresponding temperatures.

3. Results

3.1. Microstructures

Figure 2 shows the SEM image of the original nano-Ag film and statistics of the size distribution of the Ag particles. As shown in Figure 2a, nano-sized Ag particles were well-packed. The statistics analysis of the Ag particles’ size distribution was obtained by processing SEM images using Image J software (Version of 1.54g). The average diameter of most nano-Ag particles was around 30 nm. Hence, no agglomeration of the assembly particles occurred in the nano-Ag film before sintering. Figure 2b shows the microscopic morphology of the nano-Ag film after sintering, which is completely different from the morphology of the original nano-Ag film, as the nano-Ag particles were obviously coarsened.

Figure 3 shows the cross-sectional microstructure of Ag/nano-Ag film/Ag joints after isothermal sintering at temperatures of 210, 250, and 290 °C for 15 min, under an applied pressure of 5 MPa. As revealed in Figure 3, the sintering temperature showed an obvious influence on the sintered microstructure of the nano-Ag film, as well as the interface microstructure between the nano-Ag film and Ag substrates. As shown in Figure 3a, after sintering at 210 °C, uniform pores and superfluous grain boundaries could be observed in the nano-Ag film, and a micro-crack was visible at the interface between the nano-Ag film and the Ag substrate. As shown in Figure 3b, an indistinct segregation layer existed at the interface between the nano-Ag film and Ag substrates. As shown in Figure 3c,d, when the sintering temperature was increased to 250 °C, the Ag grains in the nano-Ag film were clearly coarsened, and the grain boundaries merged to form isolated pores. Moreover, the segregation layer disappeared after the sintering temperature was increased to 250 °C. As displayed in Figure 3e,f, a further increase of the sintering temperature to 290 °C resulted in a corresponding coarsening of the nano-Ag grain size, as well as the formation of a well-bonded interface between the nano-Ag film and Ag substrate. It is, therefore, clear that the increase of the sintering temperature promoted the coarsening of nano-Ag grains and the formation of interfacial bonding between the nano-Ag film and Ag substrates. This is similar to a previous study on the nano-Ag paste, where the remaining organic solvents may have impeded the adhesion of Ag particles at temperatures below 260 °C [23].

Figure 4 displays the cross-sectional microstructure of Ag/nano-Ag/Ag joints after isothermal sintering at 250 °C for 15 min, under applied pressures of 1, 3, and 10 MPa. As revealed in Figure 4a,c, after sintering under 1 and 3 MPa, the formation of grain boundaries and pores was much more obvious than that in samples sintered under 5 MPa (see Figure 3c). Once the sintering pressure increased to 5 MPa, the sintered nano-Ag demonstrated a much denser microstructure. In addition, as shown in Figure 4b,d, after sintering under applied pressures of 1 and 3 MPa, the population of interfacial cracks between the nano-Ag film and Ag substrates was clearly higher than that at the interface of samples sintered under 5 MPa (see Figure 3d). When the applied pressure reached 10 MPa, the nano-Ag film was densified, and the bonding between the nano-Ag film and Ag substrates was largely enhanced, as the interfacial cracks were suppressed. At 250 °C, the applied pressure also demonstrated a conspicuous promotion effect on the coarsening of the nano-Ag film and interfacial bonding in Ag/nano-Ag film/Ag joints. The observation in Figure 4 agrees with the previous study [22], as the porosity of sintered nano-Ag paste was decreased from 27% to 17% and 10% after the applied pressure was increased from 0 to 3 and 5 MPa, respectively. Thus, the increase of applied pressure can result in the densification of the nano-Ag film in the sintering process.

Figure 5 displays the cross-sectional microstructure of Ag/nano-Ag film/Ag joints after isothermal sintering at 250 °C for 5 and 30 min, under an applied pressure of 5 MPa. According to Figure 5a,c, after sintering under 5 MPa, the dwell time at a sintering temperature of 250 °C had a negligible influence on the sintering and bonding microstructures of the nano-Ag film on the Ag substrate: after elongating the dwell time from 5 to 30 min, the porosity of the nano-Ag film remained almost unchanged. According to the magnified SEM image in Figure 5b,d, the size and number of pores in the vicinity of the interface between the nano-Ag film and the Ag substrate was almost identical after sintering for 5 and 30 min.

3.2. Mechanical Properties

3.2.1. Shear Strength

Figure 6 depicts the average shear strength and porosity of Ag/nano-Ag film/Ag joints sintered under variable conditions. The porosity values were obtained by processing cross-sectional SEM images of Ag/nano-Ag film/Ag joints using Image J software. As shown in Figure 6a, raising the sintering temperature from 210 to 250 °C resulted in an increase of the average shear strength of the Ag/nano-Ag film/Ag joints, from 2.45 to 14.31 MPa. When the sintering temperature was increased to 290 °C, the average shear strength reached the peak value of 17.89 MPa. Correspondingly, the porosity in sintered Ag/nano-Ag film/Ag joints was decreased with the increase of the sintering temperature: increasing sintering temperature from 210 to 290 °C led to a decrease in the porosity of the nano-Ag film from 8.23% to 4.97%. As shown in Figure 6b, when increasing the applied pressure from 1 to 10 MPa, the average shear strength of Ag/nano-Ag film/Ag joints sintered at 250 °C for 15 min was increased from 4.02 to 22.66 MPa; meanwhile, the porosity of the sintered nano-Ag film was significantly decreased from 9.42% to 5.95%. This result is in a comparable range to a previous study, where the Ag/nano-Ag paste/Ag joints were sintered at 225 °C under 10 MPa (22.2 MPa) [19], but lower than the value obtained using nano-Ag paste samples sintered at 250 °C under 10 MPa (35.0 MPa) [22]. In Figure 6c, the average shear strength of the sintered Ag/nano-Ag film/Ag joints was found to slightly increase, from 11.67 to 14.96 MPa, with the elongation of the dwell time from 5 to 30 min; moreover, the porosity of the nano-Ag film was not obviously suppressed. According to the experimental results in Figure 6, it can be found that both the sintering temperature and the applied pressure facilitated the coarsening of the nano-Ag film and resulted in an increase of the average shear strength of the sintered Ag/nano-Ag film/Ag joints. However, the elongation of the dwell time at 250 °C had a negligible influence on the sintered microstructure of nano-Ag and the shear strength of the Ag/nano-Ag film/Ag joints.

3.2.2. Fracture Morphologies

Figure 7 shows the fracture morphologies of Ag/nano-Ag film/Ag joints sintered under variable conditions, and the residual nano-Ag film is marked by a red dotted line. As shown in Figure 7a, a small amount of residual nano-Ag film was observed on the Ag substrates. This means that after sintering at 210 °C, the fracture was propagated mainly along the interface between the nano-Ag film and the Ag substrate, and hence a poor interfacial bonding was achieved. As revealed in Figure 7b, after the sintering temperature was increased to 250 °C, the area ratio of the residual nano-Ag film was obviously increased, indicating that the fracture along the interface between the nano-Ag film and the Ag substrate was suppressed. As shown in Figure 7c, when the sintering temperature reached 290 °C, the surface of the Ag substrate was almost completely covered by the nano-Ag film, evidencing that the fracture was propagated in the nano-Ag film and a relatively reliable interfacial bonding was attained at 290 °C. As shown in Figure 7d, after sintering at 1 MPa for 15 min at 250 °C, the area ratio of the residual nano-Ag film was also limited, suggesting that the Ag/nano-film/Ag joint was fractured along the interface between the Ag substrate and the nano-Ag film. As revealed in Figure 7b,e, after increasing the applied pressure from 1 to 3 and 5 MPa, the area ratio of the residual nano-Ag film was congruently increased, and the propagation of the fracture along the interface between the nano-Ag film and the Ag substrate was inhibited. Clearly, the increase of both the sintering temperature (Figure 7a–c) and the applied pressure (Figure 7b,d,e) resulted in an increase of the area ratio of the residual nano-Ag film, which corresponds to how the propagation of the fracture was improved at the interface between the nano-Ag film and the Ag substrate. However, it should be noted that, after increasing the applied pressure to 10 MPa and elongating the dwell time at 250 °C to 30 min, the fracture morphology was similar to that of the sample sintered at 290 °C, i.e., the volume of the residual nano-Ag film was increased, but the fracture at the interface between the nano-Ag film and the Ag substrate could still be observed, as seen in Figure 7f.

4. Discussion

4.1. Sintering Mechanisms

The temperature, pressure, and dwell time at peak temperature comprehensively influence the microstructure and bonding strength of Ag/nano-Ag film/Ag joints in the sintering process. In previous studies, the sintering temperature has been found to play a key role in the sintering mechanisms of nano-Ag paste: at 160–250 °C, ripening is controlled by the surface diffusion of Ag nanoparticles, and at 300–350 °C, the volume diffusion of Ag nanoparticles is dominant [23]. In this work, it has been revealed that both the sintering temperature and pressure have a more pronounced influence than the dwell time at the peak sintering temperature. According to the experimental results in Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7 the temperature showed an effect similar to that of applied pressure on the microstructure of the nano-Ag film and bonding/fracture behaviors of the sintered Ag/nano-Ag film/Ag joints sintered at 210, 250, and 290 °C. The increase of both the sintering temperature and applied pressure enhanced the coarsening and ripening of the nano-Ag film and improved the bonding strength of Ag/nano-Ag film/Ag joints. However, it must be noted that, increasing the sintering temperature from 250 to 290 °C under an applied pressure of 5 MPa, or increasing the applied pressure from 5 to 10 MPa at a sintering temperature of 250 °C, yielded an average shear strength of approximately the same value. That is, the underlying mechanisms governing the microstructure evolution, and hence the mechanical integrity, of Ag/nano-Ag film/Ag joints sintered at temperatures ranging from 210 to 290 °C cannot be clarified merely based on the shear test and SEM observation.

To obtain a better understanding of the influence of the sintering temperature and applied pressure on the coarsening behavior of the nano-Ag film, hot-nanoindentation and TEM techniques were used to reveal the coupled effects of the sintering temperature and applied pressure on the sintering mechanism of the nano-Ag film, as well as the microstructure and bonding strength of Ag/nano-Ag film/Ag joints in the early stage of the sintering process at relatively low temperatures. Figure 8 displays TEM images of the nano-Ag film after sintering at 250 °C for 15 min under 5 MPa. As shown in the low-resolution TEM micrograph in Figure 8a, after the sintering reaction at 250 °C, the Ag nanoparticles collided and merged. According to the findings of Yan et al. [24], the volume diffusion between Ag nanoparticles in nano-Ag pastes can be activated at 250 °C. This can be evidenced by the volume diffusion between the (111) and (020) crystal planes of Ag particles, which led to the necking and recrystallization of Ag particles, as shown in Figure 8a. However, as shown in Figure 8b, the Ag particle with a grain size around 30 nm was found in the sintered nano-Ag film. Therefore, the TEM results evidenced that both the volume and surface diffusion could also be activated at 250 °C, but the coarsening and ripening rate was sluggish and the nano-Ag-film could not be fully densified in 15 min under an applied pressure of 5 MPa.

In order to further investigate the effect of applied pressure on the coarsening of the nano-Ag film in the low-temperature sintering process, hot nanoindentation was carried out at 250 °C, and Figure 9 displays the surface morphology of the nano-Ag film after indentation at peak loads of 50, 100, 200, and 300 mN for 10 s and 5 min. As shown in Figure 9a–c, at 250 °C, the use of peak loads of 50, 100, and 200 mN can lead to the deformation of the nano-Ag film. Only after the peak load was increased to 300 mN did the coarsening of Ag particles in the nano-Ag film become obvious. Note that, presuming an even distribution of applied force on the conical tips in the nanoindentation process, the use of a peak load of 50 mN corresponds to an applied pressure of 3.19 GPa, which is three orders of magnitude higher than the maximum applied pressure in this work (10 MPa). Moreover, after extending the dwell time from 10 s to 5 min, it was found that the microstructure of the indented nano-Ag film was similar to that generated under the same peak load and temperature (Figure 9e–h), indicating that the use of an applied pressure of more than 3 GPa cannot guarantee a rapid coarsening of the nano-Ag film. This is also in agreement with phase diagram information, in that the change in applied pressure from 1 to 10 MPa should have a negligible influence on the sintering behavior of the Ag film and the microstructure and bonding strength of Ag/nano-Ag film/Ag joints. However, this inference contrasts with the shear test results in Figure 4 and Figure 6 and in previous studies. Firstly, the increase of applied pressure resulted in an improvement in the bonding strength, and secondly, the measured bonding strength values of Ag/nano-Ag film/Ag joints were considerably lower than their counterparts sintered by nano-paste [19,22].

4.2. Fracture Behavior

Based on the above TEM and hot-nanoindentation results, the influence of the sintering temperature and applied pressure on the microstructure evolution and the bonding and fracture behaviors of Ag/nano-Ag/Ag joints sintered at 210–290 °C can be elucidated. Firstly, it should be noted that, prior to sintering, the nano-Ag film was in a pre-formed geometric dimension and free from dispersing chemical agents. As shown in Figure 1a, the nano-Ag particles in the nano-Ag film were packed together through van der Waals force/electrostatic forces, resultant from the surface energy of nano-Ag particles. This resulted in the effective radius of the packed nano-Ag nanoparticles being much larger than their nominal radius (Figure 10a). When the effective radius reaches the micrometer level, the surface energy of nano-Ag will be decreased, which will obviously impede the coarsening/ripening/densifying of the nano-Ag film, and hence impair the bonding strength of the sintered joints [23]. As revealed by the TEM and hot-nanoindentation experiment results, the diffusion-induced coarsening of nano-Ag particles can be sluggishly activated at 250 °C, and the increase in the applied pressure should have a negligible influence on the coarsening process at 210–290 °C. Therefore, the increase in the bonding strength of the sintered Ag/nano-Ag film/Ag joint can be explained as illustrated in Figure 10b. In the sintering process, the increase in the applied pressure led to enhanced deformation of the nano-Ag film, which consequently dispersed the packed nano-Ag particles, reduced the effective radius of the agglomerate, and increased the contact area between Ag particles [12]. In doing so, the increased applied pressure facilitated the coarsening of nano-Ag particles (Figure 4) and improved the overall bonding strength of the sintered Ag/nano-Ag film/Ag joints (Figure 6).

Finally, the fracture analysis of Ag/nano-Ag film/Ag joints sintered under variable conditions also cast light on the understanding of the failure mode of these nano-Ag-based die-attachment materials, and two fracture models dependent on sintering temperatures are proposed in Figure 11. As illustrated as Model I in Figure 11a, the fracture of Ag/nano-Ag film/Ag joints sintered at a relatively low temperature was mainly initiated at the interface between the nano-Ag film and the Ag substrate, and then propagated from the interface to the sintered nano-Ag film matrix. This indicates that at the lower sintering temperature, the diffusion between the nano-Ag particles and the Ag substrate is too sluggish to form a reliable interfacial bonding. As illustrated by Model II in Figure 11b, when the sintering temperature was increased, i.e., to 290 °C in this work, the diffusion between the nano-Ag particles and the Ag substrate was enhanced and, as a result, a reliable interfacial bonding was prone to form. This inhibited the crack formation at the nano-Ag film and Ag substrate interface, making the crack propagation remain inside the sintered nano-Ag film. Clearly, in order to ensure the integrity of sintered joints fabricated using nano-Ag film, it is of vital importance to achieve a reliable interfacial bonding.

5. Conclusions

In this work, the microstructure evaluation and bonding/fracture behaviors of low-temperature-sintered Ag/nano-Ag film/Ag joints were systematically studied. It was found that, when the sintering temperature was increased from 210 to 290 °C, the bonding strength was increased from 2.45 MPa to 17.89 MPa, and the porosity was decreased from 8.23% to 4.97%. When the applied pressure was increased from 1 MPa to 10 MPa, the joint bonding strength was increased from 4.02 MPa to 22.66 MPa, and the porosity was decreased from 9.42% to 5.95%. TEM and hot-nanoindentation results further revealed that the diffusion-induced coarsening and bonding of the nano-Ag film were mainly controlled by the sintering temperature. The influence of applied pressure on the microstructure and bonding strength was discussed. It was clarified that the formation of a reliable interfacial bonding between the nano-Ag film and the Ag substrate is of vital importance for the mechanical integrity of sintered Ag/nano-Ag/Ag joints.