Pressure dependence in peripheral bonding in gold-gold thermocompression

A study is performed to evaluate the peripheral bonding exhibited during bond formation for both gold studs and plated gold bump joints. Temperature and pressure are two key parameters determining the quality of thermocompression metal joints. While it has been reported that temperature plays a significant role in inducing the onset of bonding, bonding pressure is required to provide sufficient amount of interfacial deformation to reduce asperities and mechanically remove barrier films. This paper focuses on analyzing the role of bonding pressure in peripheral bond formation found in gold-gold joints despite differences in bump geometries. Using a finite element model of a single metal-metal joint representing gold stud and plated bump respectively, applying higher pressure to the model showed higher shear stress at the bonding interface and this stress remains highest at the periphery. It represents the inwardly radial progression of bonding from the perimeter, consistent with experimental observations for both gold studs and plated bumps. These results can be combined into a compression bonding model where the normal pressure applied during bonding induces an interfacial shear stress which translates into a corresponding lateral strain (equivalent to sliding or displacement) between the two contacting surfaces that are allowed to slide, producing bonds. The highest amount of interfacial shear stress occurs at the outermost region of the bump and when this exceeds a critical value, bonding will initiate from these regions and extend inwards. These findings could facilitate the optimization of thermocompression parameters, especially crucial for fragile components and fine geometries.