The Importance of Interposers for Design Verification
- Kathleen Love
- 1 day ago
- 3 min read
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 where a probe can access the signal. In practice, with designs becoming increasingly complex and compact, even getting to the ball of the DRAM can be a challenge. Or sometimes, the closest accessible measuring point is centimeters away, which will result in an inaccurate reading of the signal at the ball. And in some soldered-down designs, measurement in the near vicinity may not be possible at all. So, what do you do when you can’t measure at the DRAM ball?
That’s where interposers come 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 target to probe while retaining the integrity of the original signal, 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
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 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 equipment we're using for this measurement is:
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 that you are capturing the correct eye shape, you will need to de-embed the interposer from the measurement result with S-parameter models. De-embedding can be a challenging process, as it requires a careful setup and additional resources. As a general practice, it is critical to make sure that the interposer pads (see the "solder pad for scope probes" element in Figure 2) can be contacted directly when measuring with a vector network analyzer (VNA).
To perform 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 its own ways to implement 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 can provide S-parameter models to help you de-embed the interposer during your measurement. Additionally, we can support you with developing a measurement test plan since we work with test equipment from many different vendors. Reach out to us at info@signaledgesolutions.com if you would like to discuss your project.
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