Polyimide fatigue induced chip surface damage in DRAM's Lead-On-Chip (LOC) packages

The effect of mechanical fatigue on reliability failure was studied, based on plastic fracture mechanics including stress intensity factor, stress singularity, micro-plastic deformation behavior, and stress-strain characteristics. The fatigue has made the mechanical stability of novel lead-on-chip (LOC) packaging technologies a grave concern. The dominant issue is device failure related to fracture in the passivation layers and Al-Si-Cu metal due to polyimide cracking caused by fatigue in temperature cycles. To investigate the effect of polyimide fatigue on chip surface damage, devices were fabricated with different types of polyimides. A fatigue model obtained from the experiment and the simulated results of plastic deformation behavior were discussed. The validity of the proposed fatigue model was verified by experiments on different types of epoxy molding compounds. The results of these characterizations and an explanation of the primary factors affecting polyimide fatigue are presented in this paper.