High-Temperature Reliability of Nano-Ag Sintered Joints on Ag-Plated Cu Substrates

As an emerging lead-free packaging interconnect material, Nano-Ag paste exhibits superior high-temperature mechanical, electrical, and thermal properties. It can meet the stringent high-temperature and high-density packaging requirements of future high-power semiconductor devices and is garnering widespread attention. However, research on the high-temperature reliability of sintered joints remains rather limited. In this study, we fabricated high-strength hot-press sintered nano-Ag joints using a hot-press sintering process. These joints underwent aging at high temperatures of 200°C and 300°C, yielding insights into the variations in mechanical properties at different temperatures. The reasons behind joint failure were analyzed by investigating the evolution of joint microstructures and fracture surfaces. The results indicate that even after extended aging at 200°C, the joint strength can still be maintained at 176 MPa. However, after aging for over 100 hours at 300°C, the joint fails. The growth of Cu oxides was observed within the joint microstructure, which constitutes the primary cause of joint failure at 300°C.


Introduction
The miniaturization of electronic devices and the use of high-power components have contributed to the increasing operating temperatures of chips, thereby imposing rigorous demands on thermal interface materials' thermal stability and heat dissipation capabilities [1][2][3][4].To solve this issue, innovative materials and processes have been proposed, such as novel solder alloys, instantaneous liquid-phase bonding [5][6][7], and nano-metal sintering [8], to complete the packaging of high-power electronics.
Traditional Sn-based soldering, due to lower melting point and limited thermal conductivity, is hardpressed to satisfy the requirements of high-temperature service [9].Hence, the Au80Sn20 solder alloy with a melting point of 280 ℃ was developed and reported to have superior thermal stability during aging at 150 and 200 ℃ compared to Sn-3.0Ag-0.5Cu[10].However, the high processing temperature at approximately 300 ℃ and high cost make it difficult for wide applications.Meanwhile, the Au80Sn20-based joints exhibited significantly declined shear strength from 67.23 MPa to 17.34 MPa after aging at 200 ℃ for 360h due to the continuous formation of intermetallic compounds, and almost failed at above 250 ℃, which cannot fully meet the requirement of power-electronics [11].Hence, the transient liquid phase (TLP) bonding has been proposed to complete soldering at relatively low temperatures to form full-IMCs joints with higher remelting temperatures [12].Note that the extensive heating duration required to create full-IMC structures resulted in multiple hours of annealing before achieving a thermodynamically stable joint [13][14][15].Additionally, the ductility and the thermal conductivity were weakened by the IMC phases.Even a transient liquid phase sinter (TLPS) bonding process can be used to get a thermally stable joint within 30 s sintering at 300 ℃, comprising a Cu3Sn IMC with ductile Cu particles, massive holes are usually generated in these joints, leading to other reliability challenges for practical applications [16][17][18].
Thus, nano-metal sintering, typically nano-Ag for its excellent thermal conductivity (410 W/m• K), high strength over 150 MPa and high remelting point like bulk Ag, stands out for its exceptional mechanical properties and long-term reliability [19][20].Owing to the size effects inherent in nanomaterials, the sintering temperature of nanoscale Ag particles is significantly lower than that of bulk Ag.This enables the low-temperature sintering of nano-Ag, which facilitates the interconnection at lower temperatures and avoids the reflow issues of conventional lead-free solders, making it the most promising material for power electronics packaging.However, Ag is prone to migration, which causes significant failure in practical interconnections, but the high-temperature reliability of the Ag-sintered joints has not been fully studied, and the failure mechanism needs to be further investigated [21][22].In this study, the reliability of nano-Ag sintered joints in a high-temperature environment was characterized in detail to understand the microstructural evolution and mechanical property changes of the Ag-sintered joints in high-temperature service and aging conditions.

Experiment
This study investigates nano-Ag paste provided by Shenzhen XinYuan Advanced Materials Co., Ltd.The sintered joint possesses a typical "sandwich" structure, comprising a silicon chip, a nanoscale Ag sintering layer, and an Ag-plated Cu substrate.The Si chip was brought from Beijing Century Golden Light Semiconductor Co., Ltd., and it is coated with Ti/Ni/Ag metallization layers with thicknesses of 500nm/500nm/1000nm, respectively.The Ag-plated Cu substrate has a plating thickness of 2 μm.
For shear strength testing, the substrate dimensions of the sintered connection specimens are 5mm×5mm, while the silicon chip dimensions are 2mm×2mm.The Ag-plated Cu substrate was cleaned in ultrasonic ethanol for 5 minutes.During the preparation of the sintered connection specimens, the cleaned substrate was taken out, and after allowing any residual liquid on the surface to evaporate, a certain pattern of nanoscale Ag paste was printed onto the substrate using a scraper.Subsequently, the chip was gently placed onto the dried nano-Ag paste on the substrate, forming the "sandwich" structure of the specimen.An aluminum foil cover was placed over the small chip, and the assembly was subjected to sintering in a hot press machine, specifically the XL-TC200 model.The sintering pressure was controlled at 5MPa, and the assembly was held at 225°C for 20 minutes to ensure complete sintering of the Ag particles.
Shear strength testing was conducted using the MFM1200 push-pull force tester produced by Draytek Technology Co., Ltd.The interfacial microstructure, reaction product and fracture surface were observed using the HITACHI S4700 SEM.Also, EDS was employed for chemical composition analysis, including line scanning and mapping to get the elemental distribution.

Influence of High Temperatures on Mechanical Performance
The shear strength of the as-sintered Ag-plated Cu joints is measured to be approximately 200 MPa.After aging at 200°C, the shear strength of the Ag-sintered joints was changed.The shear strength decreases at first, attaining 131.8MPa after 50 hours of aging, but increases conversely as the aging progresses, reaching 176.2MPa after 200 hours.Then, this strength remains stable if the aging is further prolonged (up to 500 h), far higher than that of joints formed using Au80Sn20 solders (15-18 MPa) and TLP techniques (10-30 MPa) [11,17].Meanwhile, the shear strength of the joints was also examined in 300°C aging.After 50 hours of 300°C aging, the shear strength has significantly declined from around 200 MPa to approximately 65 MPa.Notably, this strength is maintained even after 100 hours of aging, which is approximately 0 MPa and 15 MPa for Au80Sn20-based and TLP/TLPS joints, respectively, but practically diminishes with further extended aging of 200 hours.Given these results, the Ag-sintered joints show significant superiority over the Au80Sn20-based and TLP/TLPS joints, hence more suitable for power electronics manufacturing.
Figure 1 The shear strength of the Ag-plated Cu joints after high-temperature aging

Influence of High Temperatures on Mechanical Performance
During 200℃ aging, as depicted in Figure 1, it becomes evident that the shear strength of Ag-plated Cu joints exhibits an anomalous increase after 50 hours of aging.Given the intimate correlation between joint strength and microstructure, the post-aging microstructure of the Ag-sintered joints was analyzed.In high-magnification images, the growth of Cu oxide stripes is easily observable.It is worth noting that these oxide stripes gradually extend from the exterior of the joint inward.After 50 hours of aging, Cu oxide stripes begin to traverse the non-pressure sintering zone but have not yet penetrated the interior of the joint.At this point, the oxidation of the Cu substrate can be considered to have a minimal impact on the strength of the joint.From 100 hours, Cu oxides continue to grow, entering the pressurized sintering zone, with their thickness steadily increasing.However, their growth towards the center of the joint is not rapid due to the relatively low aging temperature, resulting in a slower diffusion rate of Ag, Cu, and oxygen atoms.Despite the continuous thickening of the oxide stripes, even after 500 hours of aging, the oxide stripes within the joint are only approximately 1 μm thick.Considering the strength variations illustrated in Figure 1, the negative impact on the joint's strength remains minimal in 50 hours of aging.
Furthermore, it can be observed that with increasing aging time, the Ag plating layer on the substrate gradually diffuses into the sintered Ag within the joint.From the microstructure after 50 and 100 hours of aging, traces of the Ag plating layer, distinct from the sintered Ag, are still faintly discernible.However, after 500 hours, it becomes impossible to differentiate between these two structures in the secondary electron images.
From the data depicted in Figure 1, it is evident that the shear strength of the Ag-plated Cu joint experiences a drastic decline from 73.5 MPa to 2.9 MPa when the aging time at 300℃ rose from 100 to 200 hours, indicating the occurrence of failure in this time intervals.Figure 3 presents the SEM images of cross-sections of the joints after aging for 50 hours, 100 hours, 200 hours, and 500 hours at 300℃.Notably, conspicuous oxide bands emerge within the joint, with oxide infiltration already apparent at 50 hours.With increasing aging time, the thickness of the oxide bands increases.This thickness is contingent upon the extent of Cu-Ag interdiffusion within the joint.As aging time extends, the Cu content within the joint also rises, facilitating the formation of Cu oxides through reactions with oxygen diffused into the Ag joint.Simultaneously, this process depletes Cu elements within the sintered Ag, resulting in their enrichment within the oxide bands. .Elemental composition analysis of the oxide bands, as obtained through EDS analysis, is illustrated in Figure 4.The oxide bands exhibit two distinct contrasts, with the deeper contrast region possessing an oxygen content of 37.9 at.%, surpassing the oxygen content in the lighter-contrasted sections of the oxide bands.Figure 5 presents a binary phase diagram of Cu oxides [23].Due to the prolonged temperature stability during high-temperature aging, the Cu, oxygen, and Ag within the joint reach equilibrium.Considering the approximate content of Cu and oxygen elements in the oxide bands obtained from the phase diagram and EDS analysis, it can be deduced that the deeper-contrasted oxide is a mixture of CuO and Cu 2 O, while the oxide bands with lower oxygen content and lighter contrast consist of a mixture of Cu and Cu2O.Additionally, the Kirkendall voids, resulting from the differential diffusion coefficients of Cu and Ag, eventually lead to cracks at the Ag-Cu interface after extended aging.In Figure 4, no conspicuous or continuous Kirkendall voids were observed within the joint at 50 and 100 hours of aging.However, several independent Kirkendall voids were detected after 200 hours of aging, and coalesced to form a continuous microcrack after another 300 hours.Hence, the failure of the Ag-plated Cu joint after 200 hours of aging can be attributed to two potential factors: the growth of Cu oxide bands within the joint; and the microcracks induced by long-term aging through Kirkendall voids.As substantial Cu oxide bands had already formed and penetrated deep into the joint by 50 and 100 hours of aging, with nearly identical shear strength, it is reasonable to conclude that the primary cause of joint failure after 200 hours is microcrack propagation induced by Kirkendall voids.Under the influence of shear forces, these microcracks continue to propagate, resulting in failure.

The Fracture Mechanism of Sintered Nano-Ag Joints
The Ag-sintered joints' strength can reach well above 200 MPa before aging, significantly surpassing the strength requirements for practical industrial applications.Examining the fracture mode of the Agsintered joints before and after aging helps to identify vulnerable areas within the joint.This, in turn, facilitates the identification of methods to enhance high-temperature reliability to provide pertinent guidance for the practical industrial application of nano-Ag paste.
As described in Figure 6, the fracture surface of the Ag-plated Cu joint before aging exhibits distinct characteristics.The joint fractures at the interface between the sintered Ag and the substrate, even revealing the inherent texture of the substrate itself.Despite forming a well-diffused bonding structure at the interface, the junction remains the weakest point within the joint.Under the influence of shearing forces, cracks first initiate and propagate from this location.
Figure 6 Fracture of as-sintered Ag-plated Cu joint before aging Macroscopically, the sintered Ag does not display significant plastic deformation, and the fracture plane within the sintered Ag exhibits a certain angle to the upper and lower substrates, a characteristic feature of brittle fracture.However, the densely distributed ductile dimples could be discerned.This may be attributed to the voids generated within the sintered Ag resulting from particle sintering fusion.Under external shear forces, these voids continuously expand, aggregate with adjacent voids, ultimately forming the ductile dimples at the fracture surface and deforming in the direction of the applied stress to create the necking dimples.Simultaneously, larger cracks become apparent in the sintered Ag on the lower pad.The fracture surface appears remarkably smooth at the Ag and Ni plating layers' interface, with some residual sintered Ag and Ni oxides still present, which indicates that the oxides formed at the defect locations are not distributed continuously or in a sheet-like fashion.In some regions of the upper substrate, residues of Ni oxides persist, suggesting that cracks initiate from the interface between the oxides and the sintered Ag in locations where Ni oxides are present.In summary, the primary fracture  After aging (Fig. 7a), exposed Cu is observed at the fracture surface of the lower substrate, and corresponding areas on the upper substrate reveal the presence of Cu oxides through EDS analysis.In other regions, fractures occur at the interface between the upper substrate and the Ni plating, signifying a noticeable improvement in the bonding strength, with fewer large exposed plating areas visible.With the progress of aging, fractures occur closer to the upper substrate on the sintered Ag, with the exposed Cu and Cu oxides still evident at the fracture surface.The bond strength of the sintered Ag with the upper substrate is further enhanced on this occasion.Dimples are less formed in the fracture, and instead, there are typical traces of plastic deformation, with the undeformed sintered Ag observable beneath the deformed layer.After 500 hours of aging (Fig. 7b), the Ni plating layer on the upper substrate fractures, with most bonding regions fracturing at the interface between the lower substrate with sintered Ag.A significant portion of the sintered Ag layer adheres to the upper substrate, and dense dimples with larger central voids compared to the joints aged for 50h were developed.
During the early stages of aging, the elevated atomic energy at high temperatures enhances atomic diffusion between the interfaces and leads to the coalescence of small grains within the sintered Ag, resulting in the growth of larger grains.Additionally, the mismatch in thermal expansion coefficients between the upper and lower substrates and the sintered Ag causes increased internal stresses within the joint.Hence, the joint undergoes drastic structural changes, leading to the formation of larger cracks and a significant reduction in shear strength.As aging progresses, the diffusion-driven adhesion between the sintered Ag and the upper and lower substrates continuously increases, resulting in a corresponding rise in strength.Prolonged aging and annealing reduce stress concentrations within the joint, eliminating the formation of large cracks.However, the oxidation at the interfaces (Cu-Ag interface on the lower substrate and Ni-Ag interface on the upper substrate) leads to a continuously weakened sintering interface.Furthermore, engulfing small grains by larger grains at high temperatures, also known as resintering, results in grain growth and further sintering shrinkage within the structure, creating larger voids.All these phenomena have negative effects on bonding and finally lead to a continuous increase in shear strength after aging.
Figure 8 shows cross-sectional images of the Ag-plated Cu joint after aging at 300°C.Note that the joints have already completely failed after 200 and 500 hours of aging, where prominent black Cu oxides grow at the Ag-Cu interface of the lower substrate, the SEM morphologies were not collected for these fractures.In 50 hours of aging, the entire joint fractures within the sintered Ag, and little Cu oxides could be observed.There are clear signs of shear-induced plastic deformation with inconspicuous dimples.With the progress of aging, the oxygen flow and diffused continuously within the porous sintering structure, led to the oxidation of Cu substrates, while the sintered Ag grains were re-sintered significantly at this high temperature, as figured in Fig. 8 c-d.Hence, several continuous Cu oxide layers were discovered within the cross-section after 100 hours of aging, and more and larger voids were generated in the joints for the Ag re-sintering, for which the majority of the joints still fracture in the sintered Ag.After 200 hours of aging, the entire Ag-Cu interface has oxidized, resulting in an easy failure from this interface and a shear strength nearing zero.In summary, the cracks preferentially initiate from the oxidized Ag-Cu interface, and lead to exceptionally poor mechanical after long-term aging for the continuous oxidation.

Conclusion
This study examines the reliability of hot-pressed sintered nano-Ag joints after aging.The shear strength for Ag-plated Cu sintering joints subjected to high and low-temperature aging is tested.The microstructural evolution in the sintered nano-Ag joints at different aging temperatures was analyzed, and the failure mechanisms were investigated.The main conclusions of this study are as follows: (1) The Ag-plated Cu sintering joints exhibit superior high-temperature strength over Au80Sn20 and TLP/TLPS joints.At 200°C, the shear strength drops to 131.8 MPa after 50 hours of aging, but steadily rises and stabilizes at around 175 MPa with continued aging.But when subjected to 300°C, the shear strength significantly decreases to 65 MPa in 100h and eventually fails after 200 h aging.

Figure 2
portrays scanning electron microscope (SEM) images of cross-sections of Ag-plated Cu sintering joints after aging for 50 hours, 100 hours, 200 hours, and 500 hours at 200℃.

Figure 2
Figure2The microstructure of nano-Ag sintered Ag-plated Cu joints after aging at 200℃ for a-b)50h, c-d) 100h, e-f) 200h and g-h) 500h.In high-magnification images, the growth of Cu oxide stripes is easily observable.It is worth noting that these oxide stripes gradually extend from the exterior of the joint inward.After 50 hours of aging, Cu oxide stripes begin to traverse the non-pressure sintering zone but have not yet penetrated the interior of the joint.At this point, the oxidation of the Cu substrate can be considered to have a minimal impact on the strength of the joint.From 100 hours, Cu oxides continue to grow, entering the pressurized sintering zone, with their thickness steadily increasing.However, their growth towards the center of the joint is not rapid due to the relatively low aging temperature, resulting in a slower diffusion rate of Ag, Cu, and oxygen atoms.Despite the continuous thickening of the oxide stripes, even after 500 hours of aging, the oxide stripes within the joint are only approximately 1 μm thick.Considering the strength variations illustrated in Figure1, the negative impact on the joint's strength remains minimal in 50 hours of aging.Furthermore, it can be observed that with increasing aging time, the Ag plating layer on the substrate gradually diffuses into the sintered Ag within the joint.From the microstructure after 50 and 100 hours of aging, traces of the Ag plating layer, distinct from the sintered Ag, are still faintly discernible.However, after 500 hours, it becomes impossible to differentiate between these two structures in the secondary electron images.From the data depicted in Figure1, it is evident that the shear strength of the Ag-plated Cu joint experiences a drastic decline from 73.5 MPa to 2.9 MPa when the aging time at 300℃ rose from 100 to 200 hours, indicating the occurrence of failure in this time intervals.Figure3presents the SEM images of cross-sections of the joints after aging for 50 hours, 100 hours, 200 hours, and 500 hours at 300℃.Notably, conspicuous oxide bands emerge within the joint, with oxide infiltration already apparent at 50 hours.With increasing aging time, the thickness of the oxide bands increases.This thickness is contingent upon the extent of Cu-Ag interdiffusion within the joint.As aging time extends, the Cu content within the joint also rises, facilitating the formation of Cu oxides through reactions with oxygen diffused into the Ag joint.Simultaneously, this process depletes Cu elements within the sintered Ag, resulting in their enrichment within the oxide bands.

Figure 3
Figure 3 The microstructure of nano-Ag sintered Ag-plated Cu joints after aging at 300℃ for a) 50h, b) 100h, c) 200h and d) 500h.Elemental composition analysis of the oxide bands, as obtained through EDS analysis, is illustrated in Figure4.The oxide bands exhibit two distinct contrasts, with the deeper contrast region possessing an oxygen content of 37.9 at.%, surpassing the oxygen content in the lighter-contrasted sections of the oxide bands.Figure5presents a binary phase diagram of Cu oxides[23].Due to the prolonged temperature stability during high-temperature aging, the Cu, oxygen, and Ag within the joint reach equilibrium.Considering the approximate content of Cu and oxygen elements in the oxide bands obtained from the phase diagram and EDS analysis, it can be deduced that the deeper-contrasted oxide is a mixture of CuO and Cu 2 O, while the oxide bands with lower oxygen content and lighter contrast consist of a mixture of Cu and Cu2O.Additionally, the Kirkendall voids, resulting from the differential diffusion coefficients of Cu and Ag, eventually lead to cracks at the Ag-Cu interface after extended aging.In Figure4, no conspicuous or continuous Kirkendall voids were observed within the joint at 50 and 100 hours of aging.However, several independent Kirkendall voids were detected after 200 hours of aging, and coalesced to form a continuous microcrack after another 300 hours.Hence, the failure of the Ag-plated Cu joint after 200 hours of aging can be attributed to two potential factors: the growth of Cu oxide bands within the joint; and the microcracks induced by long-term aging through Kirkendall voids.As substantial Cu oxide bands had already formed and penetrated deep into the joint by 50 and 100 hours of aging, with nearly identical shear strength, it is reasonable to conclude that the primary cause of joint failure after 200 hours is microcrack propagation induced by Kirkendall voids.Under the influence of shear forces, these microcracks continue to propagate, resulting in failure.

Figure 4
Figure 4 Oxidation strips and EDS analysis of Cu-plated Ag joints aged at 300°C for 500 hours.

Figure 7
Figure 7 Fractures of the Ag-plated Cu joints after aging at 200℃ for a) 50h and b) 500hAfter aging (Fig.7a), exposed Cu is observed at the fracture surface of the lower substrate, and corresponding areas on the upper substrate reveal the presence of Cu oxides through EDS analysis.In other regions, fractures occur at the interface between the upper substrate and the Ni plating, signifying a noticeable improvement in the bonding strength, with fewer large exposed plating areas visible.With the progress of aging, fractures occur closer to the upper substrate on the sintered Ag, with the exposed Cu and Cu oxides still evident at the fracture surface.The bond strength of the sintered Ag with the upper substrate is further enhanced on this occasion.Dimples are less formed in the fracture, and instead, there are typical traces of plastic deformation, with the undeformed sintered Ag observable beneath the deformed layer.After 500 hours of aging (Fig.7b), the Ni plating layer on the upper substrate fractures, with most bonding regions fracturing at the interface between the lower substrate with sintered Ag.A significant portion of the sintered Ag layer adheres to the upper substrate, and dense dimples with larger central voids compared to the joints aged for 50h were developed.During the early stages of aging, the elevated atomic energy at high temperatures enhances atomic diffusion between the interfaces and leads to the coalescence of small grains within the sintered Ag, resulting in the growth of larger grains.Additionally, the mismatch in thermal expansion coefficients between the upper and lower substrates and the sintered Ag causes increased internal stresses within the joint.Hence, the joint undergoes drastic structural changes, leading to the formation of larger cracks and a significant reduction in shear strength.As aging progresses, the diffusion-driven adhesion between the sintered Ag and the upper and lower substrates continuously increases, resulting in a corresponding rise in strength.Prolonged aging and annealing reduce stress concentrations within the joint, eliminating the formation of large cracks.However, the oxidation at the interfaces (Cu-Ag interface on the lower substrate and Ni-Ag interface on the upper substrate) leads to a continuously weakened sintering interface.Furthermore, engulfing small grains by larger grains at high temperatures, also known as resintering, results in grain growth and further sintering shrinkage within the structure, creating larger voids.All these phenomena have negative effects on bonding and finally lead to a continuous increase in shear strength after aging.

Figure 8
Figure 8 Fractures of the Ag-plated Cu joints after aging at 300℃ for a) 50h and b)100h, and c-d)the correlated cross-sectional morphology of the sintered Ag in the joints, from which more and larger voids could be observed after aging at 300℃ for 100h.