Analysis of the formation mechanism of coarse-dense structure of silver paste in die bonding

Abstract The flow behavior of silver paste during die bonding was analyzed by numerical simulation to clarify the formation mechanism of coarse-dense inhomogeneous distribution of silver particles in a die-attach layer. Depending on the loading speed of a die, silver particles in the paste flow and form a coarse-dense distribution under a die. The microstructure of the paste before sintering was observed by X-ray computer tomography (X-ray CT) and Scanning Electron Microscope (SEM). At a slow loading speed of 1.0 μm/s, the paste is densely packed with silver particles along the diagonal of the die. A coarse structure was observed in the other area of the paste. On the other hand, at a high loading speed of 60.0 μm/s, there was no clear segregation and the paste layer has an almost uniform structure. The SPH method was used to calculate the flow behavior of particles in the paste, and the flow velocity and shear stress of the paste were calculated. The paste velocity and shear stress in the region along the diagonal of the die is smaller than those in other area of the die. A comparison of the shear stress applied to the paste at different loading speeds showed that the smaller the loading speed, the larger the value of summation of shear stress. The difference between the maximum and minimum shear stress in a lateral direction through the center of the die increases with decreasing of loading speed. Thus, a large difference in local shear stress acting on the paste causes the formation of coarse-dense structure of the silver paste. It was found that it is necessary to eliminate local differences in the shear stress acting on the paste in order to achieve uniform joint the silver paste to the substrate without forming a coarse-dense structure.