2.5 D interposers are an important part of the 3D IC evolution path. While TSMC prefers to keep interposers and assembly in-house, other foundries are looking to outsourced assembly and test groups, allowing mixing and matching of die, to handle their packaging. Why is this important? In the assembly of 3D IC stacks where interposers may be used, Trojan circuitry may be placed onto the interposer itself or inserted into the stack during assembly. Depending on the process model, it will be important to have uniform in-process inspection for trusted ICs to ensure no malicious circuitry has been inserted into the stack.