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Der Artikel untersucht die entscheidende Rolle der Rauheit und Topographie von Oberflächen bei der Bestimmung der Bindungsfestigkeit von silbergesinterten Verbindungen auf Kupfersubstraten, ein entscheidender Aspekt bei der Weiterentwicklung der Leistungselektronik. Es untersucht, wie Unterschiede in den Oberflächeneigenschaften, die durch unterschiedliche Herstellungsprozesse hervorgerufen werden, die Haftung und die Bindungsfestigkeit von Sinterverbindungen signifikant beeinflussen. Die Studie zeigt, dass eine höhere Oberflächenrauhigkeit zwar die mechanische Verzahnung und Grenzflächenverklebung verbessern kann, aber auch zu lokalisiertem Debonding und verringerter Scherfestigkeit führen kann, insbesondere wenn die Rauheit die Größe der Silberpartikel übersteigt. Die Forschungsarbeiten beleuchten das anisotrope Verhalten der Bindungsstärke auf uniaxial geschliffenen Oberflächen und zeigen, dass die Scherkraft richtungsabhängig ist. Darüber hinaus wird die Wechselwirkung zwischen der Größe von Silberpartikeln und der Oberflächenrauheit untersucht, was zeigt, dass eine optimale Bindungsfestigkeit erreicht wird, wenn die Oberflächenrauheit im Bereich der Silberpartikelgröße liegt. Die Ergebnisse unterstreichen die Notwendigkeit eines ganzheitlichen Ansatzes zum Verständnis des komplexen Zusammenspiels zwischen Oberflächeneigenschaften und Bindungsstärke und stellen die Vorstellung in Frage, dass Rauheitsparameter allein die Bindungsstärke vorhersagen können. Der Artikel bietet wertvolle Einblicke in die mikrostrukturellen und mechanischen Aspekte von Sinterverbindungen und bietet ein tieferes Verständnis der Faktoren, die ihre Leistung und Zuverlässigkeit beeinflussen.
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Abstract
The increasing focus on renewable energy and electromobility is driving advancements in power electronics devices, with the industry increasingly utilizing wide-bandgap semiconductors and alternative interconnect technologies such as silver sintering to meet high-performance requirements. This study investigates the interfacial effects of surface roughness and topography on the bonding strength of silver-sintered joints on copper substrates, considering variations in silver particle size. Experimental analysis includes surface modifications, characterization using microscopy techniques, shear testing for bond strength evaluation, and assessment of joint porosity. The study demonstrates that surface roughness and topography significantly impact the bond strength, with surface blasting reducing bond strength and anisotropy effects being observed for ground surfaces. It is suggested that there are limitations on achievable bond strength due to increased surface roughness from mechanical treatment. Additionally, this work emphasizes the significance of surface topography in addition to surface roughness values such as Ra and examines the interaction between silver particle size and roughness at both the micron and submicron levels in terms of bond strength and porosity. Maximum bond strength is achieved at lower roughness levels (Ra < 1 µm) for submicron and micron particles in silver sintered joints. However, contrary to recent research, maximum bond strength of the micron-particle sintered material is achieved at a ratio 0.1 of Ra to particle diameter d, one order of magnitude lower than that of the submicron-particle sintered material. Therefore, achieving strong adhesion and bond strength in silver-sintered joints requires careful consideration of surface properties and particle size.
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Introduction
The rapid evolution of power electronics is driven by global industrialization and the urgent need to achieve future climate goals, requiring increased focus on renewable energy generation and electromobility.1,2 This results in increasing demands of power electronics devices and their need to meet high-performance requirements.3,4 Higher power densities, lower losses, and miniaturization can be addressed by utilizing wide-bandgap semiconductors, leading to higher operating temperatures, which in turn necessitates the development of alternative interconnect technologies between semiconductor devices and metal-ceramic substrates.5,6 Silver sintering technology has gained significant attention as a bonding technique for power electronics devices due to the excellent properties of the sintered joint, including high performance and reliability. This technique involves die attachment using a paste containing silver particles.7,8 The use of highly mechanically stable bare copper ceramic substrates such as active-metal-brazed (AMB) Si3N4 substrates with excellent heat dissipation properties enables a path to fully utilizing the potential of maximum performance and reliability of power modules.9
The adhesion at the joint-to-substrate interface in particular is crucial for ensuring mechanical attachment of the semiconductor device to the substrate, as well as achieving good chemical and physical interconnection for enhanced thermal properties and thermomechanical robustness.10 Silver sintering is well established on noble surfaces such as silver and gold, but the precious metal plating required on bare copper substrates leads to high manufacturing costs, which is why bare copper substrates would preferably be used in industry. The Ag–Cu bond formation is reliant on different mechanisms such as chemical bonding, atomic interdiffusion, or mechanical interlocking discussed by Wang et al.11 Variations in metal-ceramic substrate manufacturing processes and copper types can result in different surface properties, including a wide range of surface roughness or topographies.12 Surface roughness is an important parameter in characterizing surfaces, since it potentially affects parameters such as surface energy, adhesion, or bonding of the respective material. Various studies have reported that surface roughness and topography play a crucial role in determining the adhesion and bond strength of sintered joints.11‐18 Zheng et al. observed that bond strength is reduced by a rougher chip surface, which may result in localized debonding during the densification process. However, this hypothesis has yet to be empirically validated.13 Moreover, Buttay et al. observed a reduction in bond strength, including a high degree of variation, when bonding to rough copper surfaces. This was attributed to the fact that the silver layer thickness was as thin as the surface roughness, resulting in significant variations in joint thickness after sintering, which in turn led to the formation of voids.14,15 At sufficient joint thickness, Wereszczak et al. reported higher bond strength with rougher surfaces resulting from mechanical interlocking effects induced by different surface modifications. This study also postulated bond strength anisotropy effects but did not provide any results so far.16 Meanwhile, Du et al. studied electroplated, annealed copper surfaces with different grain sizes, resulting in varying surface roughness without mechanical treatment of the surfaces. They found that higher surface roughness can facilitate interdiffusion of Ag and Cu, enabling stronger interfacial bonding through higher mechanical interlocking.17 Therefore, increasing the connection ratio between silver and copper at the interface can be seen as a key factor for improving shear strength, as investigated by Chen et al.18 However, Wang et al. noted enhanced bond strength with higher surface roughness of sandpaper-polished surfaces due to mechanical interlocking only up to roughness values comparable to the silver particle size. An increase in the roughness well beyond the silver particle size led to localized debonding and weakened interfacial adhesion.12
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The influence of surface roughness on bond strength remains an under-explored topic in the current literature, with no clear consensus. It is not sufficient to attribute variations in bond strength solely to roughness parameters, as there are overlapping effects of several other factors. These include roughness anisotropy, surface topography, and silver particle size and shape, each of which has a significant effect on adhesion properties. In addition, the comparability of different studies is difficult due to the inherent problems associated with roughness values. The lack of a bijective relationship between surface profiles and roughness values further complicates the accurate assessment and comparison of research in this area. This complex interplay highlights the need for a more holistic approach to the study of the relationship between surface roughness and adhesion. Therefore, this study aims to expand the understanding of interfacial effects induced by substrate properties, such as surface roughness and topography, by examining the bond strength with respect to various copper surfaces—mechanically treated and untreated—also considering directional dependency or anisotropy of bond strength. In addition, submicron- and micron-sized silver sintering pastes are used to investigate the influence of the ratio between roughness and silver particle size on bond strength, joint porosity, and bonding mechanisms.
Experimental
Materials
Bare copper AMB Si3N4 substrates with dimensions of 27 × 38 mm and a copper foil thickness of 0.3 mm were used as the base material. Two commercial AMB substrates with different grain sizes of approximately 250 µm (fine) and 960 µm (coarse), generated through different manufacturing processes, were used. Silver particles, which were mixed with organic solvents to form a workable paste, acted as bonding material after the sintering process. Two silver sinter pastes with different average silver particle sizes were used to investigate the impact of surface roughness and topography as well as the silver particle size on the bond strength of the sintered joints. The first paste contained particles ranging from 1 µm to 11 µm, with an average particle size of 6.3 µm, thus named micron paste. The second paste contained particles ranging from 0.1 µm to 1.3 µm, with an average particle size of 0.7 µm, thus named submicron paste. Figure 1 shows the scanning electron microscopy (SEM) images of the silver particles used in the micron paste (a) and submicron paste (b). Mechanical silicon dies measuring 4 × 4 mm with 700 nm Ag backside metallization and 250 µm thickness were used for die attach.
Fig. 1
SEM images of the silver particles used in the sinter pastes: (a) micron particles and (b) submicron particles.
To generate different surface roughnesses and topographies on AMB substrates, uniaxial grinding with different grinding tools as well as glass bead blasting was performed. For grinding, diamond (D) and cubic boron nitrite (CBN) grinding tools with various grit sizes ranging from 46 µm to 126 µm were used. Glass beads with a diameter of 40–70 µm were utilized for blasting. Figure 2 compares the different surface topographies in the initial states (a, b), after grinding with 46 µm grit size diamond (c), and after glass bead blasting (d) by light microscopy and SEM (e–h). After surface processing, all substrates were treated by a 10% solution of citric acid in water to ensure clean and oxide-free surface conditions before sintering.
Fig. 2
Light microscopy images and SEM images of different surfaces in the initial states: (a, e) coarse-grained and (b, f) fine-grained, (c, g) after grinding with 46 µm diamond grit, and (d, h) after glass bead blasting.
Confocal laser scanning microscopy (CLSM) with NanoFocus µsurf custom equipment was used for measuring the roughness and topography of the different surfaces as two-dimensional and three-dimensional reconstructions. Since uniaxial grinding leads to anisotropy effects such as different roughness parameters depending on the measuring direction, line profiles with a length of 5.6 mm were taken parallel (x) and perpendicular (y) to the grinding direction to analyze Ra, Rz, and Rmax. In summary, there are six different surface conditions, with three categories of surface topography: initial with different grain sizes (two), ground (three), and blasted (one). The three-dimensional topography images for these three categories and the surface roughness values Ra, Rz and Rmax, in two different orientations for each sample, are shown in Fig. 3 and Table I. For initial and blasted surfaces, roughness results are similar in both measurement orientations. For ground surfaces, Ra, Rz, and Rmax are significantly higher perpendicular to the grinding direction than parallel. This plays an important role with respect to the die shear test, as anisotropy effects can be expected. Ra, Rz, and Rmax are highest for the blasted specimen, while grinding results in a slight increase in the roughness parameters perpendicular to the grinding direction and a smoother surface parallel to the grinding direction compared to the initial surfaces. Summarizing these results, surface processing by grinding and blasting leads to different roughness parameters and topographies, also depending on the orientation of measurement, as anisotropy is observable for uniaxial ground surfaces.
Fig. 3
Three-dimensional topography images for three categories of surface states: (a) initial, (b) ground, and (c) blasted.
Surface roughness values Ra, Rz, and Rmax in two different orientations for all samples
Sample
Orientation
Ra, µm
Rz, µm
Rmax, µm
Coarse-grained
y
0.37
3.11
7.16
x
0.41
3.04
5.69
Fine-grained
y
0.75
5.24
8.52
x
0.77
5.63
8.58
Ground (D46)
Perpendicular
0.89
6.81
10.40
Parallel
0.42
2.59
4.82
Ground (CBN)
Perpendicular
0.86
5.90
7.56
Parallel
0.28
2.14
5.24
Ground (D126)
Perpendicular
1.47
11.20
14.90
Parallel
0.37
2.56
5.79
Blasted
y
2.43
24.90
60.10
x
2.35
21.30
42.00
Silver Sintering Process
The steps for preparing the silver sintered samples are illustrated in Fig. 4, following the process described by Stegmann et al.19 The application of the micron paste onto the substrates was carried out using a 150 µm stencil, creating eight pads for die attachment. The submicron paste was utilized following the same procedure for sintering on initial and blasted surfaces. Following the application of the paste, the samples were dried in a convection oven under nitrogen atmosphere, with drying parameters set at 140°C for 20 min to facilitate solvent evaporation. Subsequently, the silicon dies were attached with a force of 20 N and an attachment time of 2 s. To ensure adherence of the semiconductor devices to the paste prior to progressing to the pressure sintering process, the placement was conducted at an elevated temperature of 75°C. The pressure sintering followed the die placement, utilizing a temperature of 230°C, pressure of 10 MPa, and sintering time of 180 s in a PINK sintering system. Two substrates with eight chips were prepared for each substrate state.
The bond strength of the sintered samples was evaluated using a die shear test (Nordson Dage 4000Plus). The fracture mode was inspected with light microscopy to determine where failure had occurred. Subsequently, a detailed view of the microstructure was taken using SEM of ion-etched cross-sections. The porosity of each specimen was determined by calculating the visible pore areas from the cross-sectional view using ImageJ image processing software. The SEM image of the cross-section of the sintered sample (Fig. 5a) was used to determine the porosity for each sample creating binary images (Fig. 5b). By creating a binary image, pores can be distinguished from sintered material based on the gray level where pores are represented by the black area. Porosity is determined by calculating the percentage of black area to the total area (Fig. 5c).
Fig. 5
Porosity determination process from (a) SEM image to (b) binary image with subsequent porosity calculation (c).
A conventional die shear test was used as a destructive method to evaluate the bonding performance of the sintered micro silver joint on different copper AMB substrate topographies. Figure 6 shows the shear strength in MPa of the sintered samples with respect to the different surface modifications and their Ra, Rz and Rmax values in the y-direction, which is perpendicular to the grinding direction for the ground surfaces. In general, the die shear strength is highest for the initial surface with fine grain size (44.3 MPa). Sintering on the glass bead-blasted surface results in a significant decrease in the die shear strength to 30.7 MPa. Furthermore, against expectations, the shear strength could not be increased by grinding compared to the natural AMB surfaces, even if the Ra value was higher than in the initial state. However, the trend of higher shear strength with higher roughness was observed on the natural surfaces (fine-grained, coarse-grained) without changed morphology, as shear strength could be increased from 40.7 MPa to 44.3 MPa, supporting the work of Du et al.17
Fig. 6
Shear strength in MPa of the sintered micron paste samples with respect to the different surface modifications and their Ra, Rz, and Rmax values in the y-direction.
Figure 7 shows the bond line thickness for three representative surface treatments. It is apparent that the bond line thickness is reduced in the case of the blasted surface (Fig. 7c). Possible reasons for this could be the residual stresses generated by surface blasting resulting in convex bending of the substrate. Consequently, the stencil printing process is slightly modified, ultimately leading to a reduction in wet paste and joint thickness. Nevertheless, the layer thickness is homogeneous across the entire area and is also sufficiently thick in comparison to the roughness, thereby preventing any undesirable effects, such as direct contact of substrate and chip, as observed in a study by Buttay et al.14 The strong decrease in shear strength after surface blasting is unlikely an effect of the reduced bond thickness. In contrast, localized debonding occurs in the depths of the rough substrate and is seen to be the dominant factor for weaker adhesion. Additionally, at the copper surface, deformation caused by grinding and blasting may result in the change in diffusion paths, such as grain boundaries at the substrate surface, where diffusion preferentially occurs.20 These results show the importance of surface topography in addition to surface roughness values such as Ra.
Fig. 7
SEM images with bond line thickness of sintered micron paste samples on (a) initial, (b) ground, and (c) blasted surface.
The die shear test was additionally performed in two directions for the ground surfaces D126, CBN, and D46, since their roughness is strongly dependent on the measurement orientation due to uniaxial grinding. Figure 8 shows that the die shear strength is almost halved for shearing parallel to the grinding direction relative to shearing perpendicular to the grinding direction. The pronounced anisotropy can be attributed to the variation in the interfacial shear stress along the shearing direction. When testing parallel to the grinding direction, the shear stress is relatively homogeneous across the joint-to-substrate interface. In contrast, the local shear stress varies substantially when testing perpendicular to the grinding direction due to the constant changes in surface inclination along the shearing direction and the shielding effect caused by the roughness peaks. In this scenario, a crack that nucleates and propagates along the interface will encounter local minima in shear stress along its path that can potentially stop it. A crack propagating parallel to the grinding direction that does not encounter such stress variations can more easily propagate up to a critical length causing catastrophic failure. A crack perpendicular to the grinding direction can also (partially) pass through the sintered layer or substrate instead of just following the surface, which means that the shear strength is no longer limited by the adhesion alone. The better adhesion in this case compared to the parallel shearing is therefore the result of the mechanical interlocking effect, which has also been observed by other researchers.12,16 The anisotropic behavior of the bond strength represents a noteworthy observation, as it suggests that the damage may also occur in an anisotropic manner during operation in the application. For such surface topographies, it is suggested that the bond strength should be assessed in the two principal directions. Looking at the fracture mode of sample D126 in Fig. 9, the mechanical interlocking effect becomes obvious when shearing perpendicular to the grinding direction, where Cu can be found on the backside of the chips (embedded in the Ag joint). The fracture mode is similar for all samples, showing adhesive failure at the joint-to-substrate interface.
Fig. 8
Shear strength in MPa of the sintered micron paste on ground samples dependent on the shear direction (parallel versus perpendicular) with respect to the grinding direction.
Fracture mode of sintered sample on ground surface D126 with enlarged section of the chip backside demonstrating mechanical interlocking effect when the shear direction is perpendicular to the grinding direction.
As mentioned previously, several researchers have attempted to determine the effects of roughness on the bond strength of sintered joints. In Fig. 10, measurement values in this study are compared to those from the latest studies,12,17 with samples sheared parallel to the grinding direction labeled separately.
Fig. 10
Recent literature data for shear strength in MPa (Refs. 12, 17) plotted logarithmically over Ra in microns compared with measurement values of this study.
The study findings align with prior research when neglecting the impact of shear direction, indicating that bond strength is highest at approximately 0.7 µm Ra and decreases with increasing surface roughness, following the trend observed by Wang et al.12 However, three data points deviate significantly from recent research, suggesting strong deviations attributed to anisotropy effects. When shearing perpendicular to the grinding direction of the ground surfaces, mechanical interlocking stabilizes the shear strength, whereas shearing parallel to the grinding direction weakens the shear strength. These results highlight the importance of considering not only the Ra value but also the actual surface topography. Furthermore, it is possible that surface deformation resulting from surface treatment contributes to weaker bond formation. This may be due to the fact that higher roughness values are only achieved through mechanical treatment. It is believed that achieving higher surface roughness without mechanical surface treatment could further enhance shear strength, aligning with the trend observed by Du et al. 17. It is important to note that the different studies utilized varied sintering process parameters, semiconductor materials and sizes, and silver particle sizes. The influence of silver particle size is elaborated in the following.
Particle Size and Surface Roughness Interaction
The interaction between silver particle size (micron versus submicron) and surface roughness through die shear test including porosity analysis is discussed in the following. The micron- and submicron-particle-containing pastes were sintered on three different surfaces: coarse-grained (Ra = 0.37 µm), fine-grained (Ra = 0.76 µm), and blasted (Ra = 2.43 µm). The microstructure and porosity of the silver sintered joints were analyzed via SEM using ion-etched cross-sections. Figure 11 compares the Ag layer microstructure to the interfaces of both sintered pastes with respect to the different surface states. The microstructure varies with silver particle size, as demonstrated by the different porosity values of approximately 19% for the sintered micron paste and approximately 15% for the sintered submicron paste in the coarse-grained surface state. Additionally, increased porosity is observed at the joint-to-substrate interface compared to the chip-to-joint interface for all samples, highlighting the joint's weakness at the substrate interface, which is also evident in the fracture modes after the die shear test.
Fig. 11
SEM images of the chip-to-joint and the joint-to-substrate interface and calculated porosity of sintered micron paste (a) and submicron paste (b) samples on coarse-grained, fine-grained, and blasted surface states.
The microstructure of the sintered joints is affected by the roughness of the copper surfaces in terms of the joint-to-substrate porosity, which increases for the micron paste to 21.9% and for the submicron paste to 28.5%, respectively, if the surface roughness increases. Additionally, the die shear strength of both sintered materials is influenced by the surface topography. Figure 12 shows the correlation between the die shear strength and the joint-to-substrate porosity. Both materials show decreasing bond strength from 45.6 MPa to 32.8 MPa (submicron) and from 44.3 MPa to 30.7 MPa (micron) with increasing porosity.
Fig. 12
Shear strength in MPa of the sintered micron and submicron paste samples over joint-to-substrate porosity in %.
Following Wang et al.,12 optimal results are obtained when the surface roughness falls within the range of the silver particle size. Figure 13 shows the ratio of Ra and particle size d in relation to the die shear strength. In this study, the submicron-particle-based paste with an average particle size of 0.7 µm showed the highest die shear strength at Ra of 0.72 µm, supporting this assumption (Ra/d = 1). Higher roughness resulted in increased interface porosity and weaker bond strength. However, the micron paste has an average particle size of 6.3 µm and still exhibits decreased die shear strength at Ra of 2.43 µm. For micron paste, maximum shear strength was achieved at Ra/d = 0.1, indicating a potential limitation on the maximum achievable bond strength by surface roughness increase for the silver pressure sintering process of micron-particle-containing pastes. Furthermore, as previously stated, high roughness was achieved through mechanical surface processing, resulting in highly deformed surfaces. This can introduce several influencing factors, such as lower bond line thickness, changes in pressure distribution, localized debonding, and porosity variations, affecting the bond strength of the sintered joints. Therefore, it is not sufficient to consider only roughness parameters such as Ra. Surface topography and type of surface treatment also have a significant effect on the interface porosity and consequently on the interfacial bond strength of sintered silver joints. Future research is needed to shed some light on the influence of processing parameters on the interface porosity as well as on the resulting shear strength.
Fig. 13
Shear strength in MPa of the sintered micron and submicron paste samples plotted logarithmically over the ratio of Ra to particle size d.
Surface roughness and topography: Six surface conditions were generated, which were categorized into three types: initial, ground, and blasted. Uniaxial ground surfaces exhibited strong anisotropy effects, while blasting resulted in significantly increased roughness. This demonstrates how grinding and blasting impact surface roughness and topography, leading to highly deformed surfaces.
Bond strength on various surface topographies: The die shear test revealed that the substrate topography significantly affects the bond strength. Sintering on a glass bead-blasted surface with high roughness led to reduced shear strength due to localized debonding. Anisotropy effects were observed for ground surfaces, with significantly higher shear strength perpendicular to the grinding direction compared to parallel. Achieving higher surface roughness without mechanical treatment may further enhance shear strength. The results align with previous findings regarding bond strength and surface roughness trends. However, significant deviations are attributed to anisotropy effects caused by the surface treatments.
Particle size and surface roughness interaction: Microstructure, porosity, and bond strength of the sintered joints were influenced by the interaction between the silver particle size and the surface roughness. Consistent with prior research, optimal results were achieved when the surface roughness was within the range of the silver particle size for submicron powder. However, the effects were different for micron-sized silver particles, which resulted in decreased bond strength even when the roughness was much lower than the average particle size.
In conclusion, the study demonstrates that surface roughness and topography significantly affect the bond strength of sintered silver joints with different particle sizes. The findings suggest that there may be limitations on the maximum achievable bond strength due to an increase in surface roughness caused by mechanical surface treatment. Achieving higher surface roughness without mechanical surface treatment may enhance shear strength, as high roughness from mechanical surface processing introduces several factors that affect bond strength. This study highlights the importance of considering surface topography and silver particle size in addition to roughness values, as these parameters determine secondary properties of the joint, such as porosity and adhesion at the substrate interface, which affect the global bond strength of the joint.
Conflict of interest
The authors declare that they have no conflicts of interest to report regarding the present study. All authors confirm that there are no relevant financial or non-financial relationships or affiliations that could have influenced the work reported in this manuscript.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
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