The Importance of Interposers for Design Verification
- Kathleen Love
- May 14
- 3 min read
Updated: May 27
Why Use DRAM Interposers for Signal Measurement?
JEDEC standards define the DRAM input parameters at the ball of the DRAM, since it is the last point a probe can access for signal measurement. In practice, with designs becoming increasingly complex and compact, accessing the ball of the DRAM can be a challenge. Or sometimes, the closest measuring point is centimeters away, which will result in an inaccurate reading of the signal at the ball. In some soldered-down designs, measurement near the DRAM ball may not be feasible. So, what happens when direct measurement isn't possible?
That’s where a DRAM interposer comes in. Interposers work as adapters between the DRAM component and the PCB and are soldered between them (Figure 2). They provide you with a much more accessible probe target while preserving signal integrity, and they’re helpful for more than just measuring at the ball. Interposers can be used to capture signals from the entirety of the ball-grid array (BGA), which is especially important when working with DDR5 and DDR6 boards. In these high-speed systems, it is almost impossible to probe directly on the PCB traces without overloading the circuit or damaging the board. Interposers break out the high-speed Command, Address, Control, and Data (DQ/DQS) signals from underneath the BGA package to accessible surface pads, protecting your board while allowing your test equipment to capture a clean signal. Some interposers, like the EyeKnowHow interposer in Figure 2, also contain a wing as part of their architecture, which extends access to every DRAM signal.
Typical (LP)DDRx Measurement Test Equipment List
DRAM interposers are critical when correlating a (LP)DDRx simulation model to a measurement, and they need to be included on the memory module to ensure that the signal measurement is accurate. A depiction of how the EyeKnowHow (EKH) interposer is used in a DDR measurement is shown in Figure 3.
The interposer is the access point to the signal at the ball of the DRAM for the probe tips shown within the red box, and the measured signal is then shown on the oscilloscope.
The test equipment we're using for this measurement includes:
R&S RTP164 16GHz BW Oscilloscope
RT-ZM160 – 16GHz Modular Probe
RT-ZMA14 Probe Tip
Measuring and Validating with (LP)DDRx Interposers
To validate the performance of the device under test and ensure the accurate capture of the correct eye shape, you will need to de-embed the DRAM interposer from the measurement result using S-parameter models. De-embedding can be a challenging process that requires careful setup and additional resources. It is crucial to ensure that the interposer pads (see the "solder pad for scope probes" element in Figure 2) can be contacted directly when conducting measurement with a vector network analyzer (VNA).
To complete interposer de-embedding, you will require 3-port models for single-ended signals and 6-port models for differential signals. It is important to note that each oscilloscope vendor has different methods to implement interposer de-embedding. Some interposers, like the EKH interposer featured in this blog, come with de-embedding files to help this process. At Signal Edge Solutions, we provide S-parameter models to help you de-embed the interposer during your signal measurement. Additionally, we can support you with developing a measurement test plan since we work with test equipment from many different vendors. Contact us at info@signaledgesolutions.com if you would like to discuss your project.
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