Assessment of the feasibility of ‘multiple chips-to-wafer’ thermo-compression bonding using FEM

In this work FEM is used to conduct a parametric study of the chip-to-wafer thermo-compression bonding process. It is found that the main cause of pressure non- uniformity across stacked chips is due to die-thickness-variation (DTV) resulting from wafer thinning process. The use of a compliant pad is necessary to reduce non-uniform pressure across stacked chips. A compliant pad thickness of about 2.5 - 3 mm is observed to be the optimal thickness for a minimal non-uniformity in pressure at the Cu-Cu interface for a one-level die-stack having a maximum stack thickness of 700 mum. Stress concentrations were found to occur at the edges of the chip, but were minimal and are not expected to result in chip fracture/cracking.