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Copper sintering with nano-structured particles and formate dissociation

Subject Area Synthesis and Properties of Functional Materials
Materials in Sintering Processes and Generative Manufacturing Processes
Mechanical Properties of Metallic Materials and their Microstructural Origins
Term from 2021 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 496149269
 
Final Report Year 2024

Final Report Abstract

Two strategies for improving the joint formation of joining materials for chip mounting in microelectronics were investigated. Joint formation by copper sintering, which is limited to a parameter space of ≤ 300 °C; ≤ 40 MPa and ≤ 15 min in the context of assembly and connection technology in microelectronics, could be greatly accelerated with the help of additives of a complexed copper salt and the necessary process temperature could be reduced from 275 °C to 250 °C. The approach contributes to the improvement of the joining materials, which is reflected, for example, in lower resource consumption (lower process temperatures, shorter process times). The second strategy, pre-treatment of the particles by selective etching to increase the surface energy, also enabled a strong acceleration of the joint formation; however, agglomeration and chloride residues as a side effect meant that this approach is not yet suitable for industrial use, but still needs to be further developed. Previously undescribed structures in the plane of the compound layer (in contrast to the usual cross-sections) were revealed and examined using a new type of test setup, namely removing the chip after sintering. This made it possible for the first time to directly determine the cohesion strength of different layer areas using a scratch test. In addition, the heat distribution in layers with such a structure could be determined using lockin thermography. The thermal resistance of the interface could be determined using transient thermal analysis and the mechanical properties of the interface could be determined using the stress of the components detected by µ-Raman spectroscopy. These specific properties of the copper compound layers are caused by gap pores, which were recognized as characteristic of the microstructure of sintered flake particles. They could be described by improved measurement methods (segmentation of the pores via a circularity threshold, as well as statistical evaluation of mappings of the so-called local thickness), whereby it was found that their extent is determined by the process pressure and does not change significantly even with longer sintering times, whereas small, rounder pores grow and thus reduce surface energy, which corresponds to the typical sintering behavior. The pore clusters formed areas with reduced cohesive strength and hardly any reduction in thermal conductivity, although the material bond was still very good overall, as evidenced by high shear strengths of 50 - 60 MPa. The sinter pastes based on copper flakes with and without the addition of copper salts are therefore suitable in principle for use as a joining material for die attachment.

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