Base loop characterization to alleviate short tail issue in stack die package

NAND flash drive is a form of non-volatile memory storage technology that does not require power to store large memory capacity. With solid-state drives (SSDs), multiple NAND dies can be stacked together. The higher the number of dies, the higher the capacity of memory stored. Multi-stack NAND dies require die-to-die bonding using Au wire to form a closed electrical circuit. Die-to-die bonding, also called cascade bonding, forms cascade wires by landing the second bond to the top surface of the ball bond from the bottom die stack. This unstable surface imposes a challenge to bond the second bond due to its unevenness. In multi-stack devices, short tail in Au wire bonding challenges the bonders' efficiency and productivity. Short tail occurs when wire prematurely tears before reaching the desired tail length after second bond formation, resulting in too short or no tail for the next FAB formation. Hence, this paper analyses the factors and mechanism of short tails and highlights the innovative solution to develop a more robust looping formation for cascade wire bonding. Parameters namely the first and second kink, loop base and desirable second bond offset were optimized to eliminate short tails, creating a more stable process.