DDR Memory Testing Part II: Using Interposers

Figure 1: The anatomy of a chip interposer

If you're a PCB layout designer, you've probably heard one or more test engineers complain: "Why can't you lay out the board so that it can be tested?" All too often, components that need to be accessible to oscilloscope probes are physically inaccessible, whether it's because of close proximity of adjacent components or ball grid array (BGA) mounting of the DUT. It's nearly always a necessary evil, though, because of PCB cost and/or mechanical constraints.