Optimisation d'un procédé de soudage par difusion à l'état solide pour réaliser des circuits imprimés : Application au système Au-Cu-Sn

A solid-state diffusion bonding technology, called thermodiffusion, has been developed to miniaturizePCB. A promising configuration where thin layers of gold and tin are coated on the two copper layers to be joined has been chosen. The thermodiffusion leads to the formation of intermetallic compounds forming the joint. The microstructure of the joint depends on the respective thicknesses of the filler metals and the thermal loading (time - temperature). The latter controls the mechanical strength of the joint. This work aims to understand the evolution of the joint's microstructure and its influence on the mechanical reliability of interconnections.The microstructure evolution was characterized along thermal cycles at 200 and 240°C from a so-called optimized structure of 1.5 µm gold and 3 µm tin. The different intermetallic compounds present in the joints were identified, as well as the different diffusion steps and their kinetics. During annealing time, binary compounds are rapidly formed, then progressively replaced by a ternary compound called phase B ; in a last step, the B phase starts to be consumed by copper-rich binary compounds (Cu3Sn, AuCu3 or AuCu). An in situ TEM experiment showed that Kirkendall voids are re-capped when the binary compounds consume the B phase.Nanoindentation tests were performed to measure the mechanical properties of the different intermetallic compounds, including the phase B (with a Young's modulus of about 130 GPa and a hardness of 8 GPa). Shear and peel tests with fracture surface analysis and in situ tensile testing under SEM allowed the identification of the fracture mechanisms. The B phase and copper interface can be weakened by cavities and nanometric intermetallics, making it a critical zone.