Analysis of Stacked Overhang Die Stress During Wire Bonding

With the increasing demand for high capacity and miniaturization in flash memory packages, stacking multiple thin dies in a package with overhang die wire bonding has become a mainstream technique. However, risk of overhang dies cracking poses significant challenge during wire bonding especially when the overhang die is thin and corner bonding pad is close to the die edge. In order to attain a reliable design of stacked die overhang wire bonding, a nonlinear and transient structural model has been used to understand and consequently minimize overhang die stress in the present paper. It accurately captures the mechanical behavior of overhang die stress and deflection. The simulation results show that the die stress increases significantly with decreasing die thickness and with decreasing pad-to-edge distance, which is consistent with experimental observations. It is also shown that the stress generated in an overhang die can be very sensitive to wire bond capillary velocity and bond force. By reducing bonding force and capillary velocity, overhang die stress can be effectively reduced, which also matches the actual wire bond process DOE results. However, the reductions of bond force and velocity may have a negative effect on productivity and bonding interconnection, the optimization of design is also achieved in this paper.