Redistribution Layer Routing for Wafer-Level Integrated Fan-Out Package-on-Packages

The wafer-level integrated fan-out (InFO) package-onpackage (PoP) is a promising 3D packaging technology, which usually consists of a bottom package with the InFO technique, and a top package stacked on the bottom package. Different from the traditional PoPs, there are frontside and backside redistribution layers (RDLs) in the InFO PoP for signal redistributions. To the best of our knowledge, there is still no previous work specifically tackling the RDL routing for the InFO PoP. Previous works on RDL routing mainly deal with the following three types of routing: the free-assignment, pre-assignment, and unified-assignment routing for single or multiple chips. In this paper, a new RDL routing problem for the InFO PoP is formulated. To remedy the deficiencies of lacking the interactions between frontside and backside RDLs, we present the first work in the literature to handle the unified-assignment multi-layer multi-package RDL routing problem (without RDL vias), considering layer assignment, layer number minimization, and total wirelength minimization. We propose an algorithm based on extracting increasing subsequences (IS), which transforms a routing sequence into two directed acyclic graphs (DAGs), namely, IS-DAG and Constraint-DAG. By minimizing the number of vertices on the longest path on the Constraint-DAG, we implicitly minimize the layer number. Furthermore, we perform backtracking on the IS-DAG to efficiently assign the connections to appropriate layers to avoid long detours. Experimental results show that our router can achieve 100% routablility for all given test cases, while the previous works with extensions fail all test cases even with more frontside RDLs.