Testing DDR memory with interposers

PCB layout designers might have already encountered this complain from one or more test engineers, "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 device under test (DUT). It's nearly always a necessary evil, though, because of PCB cost and/or mechanical constraints.

So, say you need to get probes onto a dual data-rate (DDR) memory device and some (or all) of the pins are inaccessible. Here is where chip interposers come in. Chip interposers are designed for probing extremely close to memory components and are essential for just these situations (figure 1).

Figure 1: The anatomy of a chip interposer.

Interposers are installed by soldering the bottom side of the device to the DDR BGA footprint on the target board where the memory component would be soldered. The memory component is then soldered to the top side of the interposer (figure 2). Interposers can be most useful in embedded applications and in those where there are chips on both sides of a dual in-line memory module (DIMM).

Figure 2: This series of photos illustrates how the interposer brings out the DDR BGA's I/Os to make them accessible to test probes.

As helpful as chip interposers can be in testing DDR BGAs mounted on a board, be careful with them when it comes to DIMM applications. As figure 3 illustrates, installation of interposers on the far right- and left-hand chips on a DIMM could interfere with latching of the side clips that secure the DIMM in its board-mounted connector. Thus, it's recommended that interposers be installed on the six central chip locations and not in the end locations.

Figure 3: Be careful when using interposers with DDR DIMMs. Avoid using them on the chips in the end locations, where they may interfere with latches.

In addition to the aforementioned strategy regarding DIMMs, another "best practice" for testing with interposers is to use tweezers to manipulate probe tips when soldering. Tweezers are more nimble than most people's fingers, providing better placement and accuracy during the soldering process.

One final tip: Solder the probe tips to the interposer in advance. It's easier than when the interposer is in place, and the soldered-in tips can be switched in and out as necessary for connection to a probe amplifier.

About the author
David Maliniak is with Teledyne-LeCroy.

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