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Study on High Temperature Resistant Die Bonding Formed by Al/Ni Nano-particles Composite Paste
Keywords: SiC Power Devices, Al/Ni nano-particles composite paste, High temperature resistant die bonding
In power modules using SiC devices, high temperature operation at 250 degree Celsius or more is expected. Therefore, a bonding technique having high temperature resistance of 250 degree Celsius or more is required.In recent years, research on low temperature sintering bonding by Ag nano-particels, Cu nano-particles and sub-micron particles has been conducted as a new bonding technology corresponding to SiC power devices. Nanoparticles are sintered at a temperature much lower than the normal melting point because the surface energy of the particles relative to the volume is very large. In this laboratory, we focus on Ni having high melting point (1453 degree Celsius) and excellent corrosion resistance as a new bonding material and are conducting research on high temperature resistant bonding technology using Ni nano-particles.It has been found that bonding is possible at a bonding temperature of 400 degree Celsius or less. In addition, we fabricated a small size SiC module and confirmed high temperature operation above 250 degree Celsius. However, in the case of bonding by metallic nano-particles, the problem is that it is hard to release the thermal stress due to the difference in coefficient of thermal expansion (CTE) as compared with the conventional solder bonding. A porous gap exists in the bonding layer of the metal nano-particles after sintering. A structure in which resin is poured into the gap and buried to relieve stress, and the like have been proposed. The same problem is also raised in joining by Ni nano-particle paste. In this paper, we propose a new stress relaxation bonding material using composite paste in which Ni nano-particles and Al particles are mixed. Since Al is relatively soft among metals, stress relaxation of Al particles in the bonding layer can be expected. Using this Al / Ni nano-particles composite paste, the bonding strength was measured for each chip size of 2.7 mm x 2.7 mm and 5.0 mm x 5.0 mm, and the reduction in the bonding strength was compared. As a result, the bonding strength of the Ni nano-particles alone for the chip size of 5.0 mm x 5.0 mm was reduced by about 44% compared with that of 2.7 mm x 2.7 mm. On the other hand, the paste to which Al particles were added had a reduction in bonding strength of approximately 16%. According to the simulation by the finite element method (FEM), the portion having highest thermal stress caused by the CTE difference was found to be the joint interface between the chip and the bonding layer. In the joining with the Ni nano-particle paste to which Al particles were added, the fracture modes after the shear test were not the bonding interface between the chip and the bonding layer, but it was confirmed that fracture mostly occurred in the bonding inner layer. From these result, it is considered that Al particles act as relaxing layer of thermal stress in the bonding layer by adding Al particles.
Yasunori TANAKA,
The graduate school of Information, Production and Systems, Waseda University
Kitakyusyu, Fukuoka

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