Abstract-Wire bonding is the most popular method to connect signals between dies in System-in-Package (SiP) design nowadays. Pad assignment, which assigns inter-die signals to die pads so as to facilitate wire bonding, is an important physical design problem for SiP design because the quality of a pad assignment solution affects both the cost and performance of a SiP design. In this paper, we study a pad assignment problem, which prohibits the generation of illegal crossings and aims to minimize the total signal wirelength, for die-stacking SiP design. We first consider the two-die cases and die-stacks with a bridging die, and present a minimum-cost flow based approach to optimally solve them in polynomial time. We then describe an approach, which uses a modified left edge algorithm and an integer linear programming technique, for pyramid die-stacks with no bridging die. Finally, we discuss extensions of the two approaches to handle additional design constraints. Encouraging experimental results are shown to support our approaches.Index Terms-System-in-Package, pad assignment, wire bonding, die-stack.