when should i use interposer and spacer in sip？
thanks in advance!
There is both a text document solution, as well as a video walking you through spacer and interposer creation in the self-help area. I don't have exact links to them, but they should be reasonably easy to find with a keyword search.
Or, if you just want a basic, rectangular spacer, you can create it dynamically in SiP 16.6 from the Add -> Spacer form by specifying the material, thickness, size, etc.