Transient Thermal Modeling of Die Bond Process in Multiple Die Stacked Flash Memory Package

As the demand on data storage continues to explode, flash memory package moves towards high capacity such as 8 dies and 16 dies stacking. However, a challenge in die bonding process is the significant decrement of die bonding temperature for multiple die sequent bonding, which may lead to poor adhesion and die to die peeling. The degradation of bonding temperature mainly comes from two aspects. One is the increasing thermal resistance with the number of dies and Die Attach Film (DAF). The other is the cumulative cooling effect from bond head, which is more important, as the bond head is at room temperature and larger thermal mass compared to the ultra-thin die. In this paper, the transient die bonding temperature response of 4 and 8 dies stacking including the pick-up and bonding durations are investigated. Owing to that the heating from bond stage is continuous while the cooling from bond head is intermittent, therefore, the bonding temperature during the entire process runs in a dynamic changing manner. It is not practical to measure the transient temperature due to the extremely short time of die bonding and pick up. However, Finite Element Analysis (FEA) modeling can be employed for this problem. The simulation results clearly demonstrate a huge gap between the die bonding temperature and bond stage temperature, especially for the top die. Moreover, 8 die stacking is also worse than 4 die stacking, which is also consistent with the experimental result of the top die peeling rate between 4 die and 8 die. The improved methods to increase bonding temperature are also studied including longer bonding time, higher bond stage temperature, and using hot bond head.