Reuse-Based Optimization for Prebond and Post-Bond Testing of 3-D-Stacked ICs

Three-dimensional (3-D) stacking of integrated circuits (ICs) using through-silicon-vias (TSVs) is a promising integration platform for next-generation ICs. Since TSVs are not fully accessible prior to bonding, it is difficult to test the combinational logic between scan flip-flops and TSVs at a prebond stage. In order to increase testability, it has been advocated that wrapper cells (WC) be added at both ends of a TSV. However, a drawback of WC is that they incur area overhead and lead to higher latency and performance degradation on functional paths. Prior work proposed the reuse of scan cells to achieve high testability, thereby reducing the number of WC that need to be inserted; however, practical timing considerations were overlooked and the number of inserted WC was still high. We show that the general problem of minimizing the WC is equivalent to the graph-theoretic minimum clique-partitioning problem, and is therefore NP-hard. We adopt efficient heuristic methods to solve the problem and describe a timing-guided and layout-aware solution. We evaluate the heuristic methods using an exact solution technique based on integer linear programming. We also present design-for-test optimization technique to leverage the reuse-based method during post-bond testing. Results are presented for 3-D-stack implementations of the ITC'99 and the OpenCore benchmark circuits.