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Novel solder-mesh interconnection design for power module applications
Keywords: Lead-free soldering, Thermal and mechanical reinforcement, Finite element modeling
Research and development on packaging of power modules aims at miniaturization, 3D stacking and ever-increasing power densities with lead-free, low-temperature joining solutions. Since soldering can meet the requirements for large-scale production, various studies have contributed to the improvement of solder joints by optimizing metallization and interfacial intermetallic compound (IMC) layers [1-3], strengthening Sn-based solders with alloying elements [4-7], using high-temperature Zn- or Au-based solders [8-10], adding nano-sized reinforcement materials [11-15] or using Sn-coated metallic particles [16-18]. The present work targets a novel approach and applies composite structures that consist of Sn-based solder and thin metallic meshes in order to increase the thermal conductivity and achieve mechanical reinforcement of the interconnections. In the first step, we intended to prove the feasibility of such mesh-solder joints. Soldering was carried out with Cu substrates that had dimensions of 20 x 20 x 2 mm and 10 x 10 x 2 mm, respectively, and 99.96% purity. Cu and Ni meshes with 70 μm filament diameter as well as 200 μm thickness and mesh width were cut into a size that was slightly larger than the cross sectional area of the smaller Cu substrate. Commercial Sn-3.0Ag-0.5Cu (SAC305) solder paste was applied on the mesh to fill its empty space. Samples assembled with only SAC305 solder were used as reference. Pressureless bonding was conducted in a reflow furnace (UniTemp GmbH) at a holding temperature of 240 °C for 1 min. The process was run under N2, vacuum and a combination of N2 and vacuum, respectively. The microstructure of the Cu-SAC305-Cu, Cu-Ni mesh/SAC305-Cu and Cu-Cu mesh/SAC305-Cu samples was analyzed by optical microscopy (OM, Olympus SZX7) and scanning electron microscopy (SEM, Hitachi S-3000H) after conventional metallographic preparation, i.e. grinding and polishing, and ion beam cross section cutting (JEOL SM-09010), respectively. The joined assemblies were subjected to shear tests with a cross-head speed of 0.01 mm/s. The analysis of the fracture surfaces was performed by OM and SEM, respectively. Scanning acoustic tomography (SAT, Hitachi FineSAT) was used to inspect the volumetric quality of the mesh-solder joints. Microstructural analyses proved a good match between the Cu substrate, the Cu and Ni mesh and the SAC305 solder. Scallop-like Cu6Sn5 IMC with an averaged thickness of 2 μm were found at the upper and lower interface between the Cu substrate and the SAC305 solder as well as between the filaments of the Cu mesh and the SAC305. Needle-like and faceted Ni3Sn4 IMC formed at the interface between the Ni mesh and the SAC305 solder. IMC also evolved at intersection points of two filaments which mechanically connected the initially loose parallel and perpendicular filaments and additionally stiffened the mesh-solder composite. A homogeneous joint thickness of around 250 μm was determined, which is slightly larger than the mesh thickness, because SAC305 solder is located between the mesh and the Cu substrates. A low amount of residues was spread over the joint bonded under a N2 process atmosphere. A systematic parameter study identified process conditions with N2 and vacuum that lead to defect-free joints. The results were doublechecked by SAT. It was shown that SAT provides reliable information on the volumetric quality between the wavy mesh and the Cu substrates. Residues were predominantly found between the mesh and the upper substrate because of the lower density compared the solder. The presence of residues provoked an increase of the joint thickness or a slight tilting of the upper substrate. Both phenomena can be easily measured and used for quality assurance. The shear strength was determined as 32, 33.7 and 35.3 MPa for Cu-SAC305-Cu, Cu-Ni mesh/SAC305-Cu and Cu-Cu mesh/SAC305-Cu samples, respectively, produced under N2 atmosphere. Cohesive fracture within the solder joint with ductile deformation features (dimples) was observed on the SAC305 solder. Interestingly, the mesh locally cracked despite its higher specific strength. Although fracture mainly occurred between the mesh and the upper substrate (as expected), the fracture path could apparently change toward the lower substrate side. Further analysis showed that the crack propagated through the mesh where residues were located. Miniaturized 3D finite element (FE) models of the Cu-SAC305-Cu and Cu-Cu mesh/SAC305-Cu systems were set up using Ansys Workbench 17.2. The geometries and dimensions from the experimental section were implemented considering elasto-plastic properties of the materials. Additionally, circular defects with 600 μm diameter were added in order to mimic residues in the experimental joints. The models were loaded with a shear force that provoked the strain-to-failure within the SAC305 solder. Plastic strain versus external stress plots could be derived from the simulations which proved that the mesh mechanically reinforces solder joints with and without defects as it was suggested by the experimental shear test results. Stress peaks within the mesh appeared when a defect was located in the joint which supports the experimental observations and confirms that fracture of the mesh is caused by residues, i.e. weak spots of the structure, by which the external load is transferred from the solder into the mesh. Thermal simulations were performed on the Cu mesh-solder composite with 250 μm thickness in which a defined and constant heat flux was applied on one surface that generated temperature distributions T1(x,y) and T2(x,y) on the heated surface and the opposite one, respectively. Based on Fourier’s law, a theoretical thermal conductivity of 164 W/mK was obtained for the Cu mesh-solder composite which is around three times higher than the value for SAC305. In other words, the thermal resistivity of a 250 μm thick Cu mesh-SAC305 joint is supposed to be equal to that of a 90 μm thick SAC305 joint. The Cu filaments serve as effective paths for heat transport. A conventional solder joint is a bottle neck for heat dissipation in power module applications. Because of the apparently enhanced thermal conductivity of mesh-solder joints, it can be expected that a certain power density will lead to lower temperatures in the semiconductor chip that can improve the lifetime (or increase the power density) of power modules. Since the present study could prove the feasibility of mesh-solder composites as interconnections, future work will investigate the resistance of Cu/DBC-mesh/SAC305-Si assemblies against elevated temperatures in cycling and storage tests. Moreover, the thermal conductivity of the solder-mesh composites as a key property of the presented approach must be experimentally determined.
Adrian Lis, Assistant Professor
Graduate School of Engineering, Osaka University
Suita, Osaka

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