Introduction
When you are working with DDR memory, probing at the DRAM pins as required by JEDEC standards can be challenging. You will often need to probe hard-to-access circuit locations on memory boards and DIMMs while maintaining secure, consistent connectivity without damaging the probe tips or the circuit under test. In addition, you will need to take steps to relieve strain on your probe tips. Other considerations relate to deciding whether to use interposers to access signals of interest and whether to remove probe damping resistors to improve measurement accuracy. Proper grounding of your device under test is also necessary for optimum results. A look at some specific examples of DDR probing best practices can help you avoid mistakes and get accurate results quickly.
Relieve Strain on Probe Tips
Hands-free probing is useful in any probing application. Teledyne LeCroy designed its first hands-free probe with weighted feet and configured it so that the weight of the probe itself would apply pressure to keep the tip in contact with the device under test.
For DDR test, however, the preferred approach is to essentially turn the probe upside down, providing a reverse-mounted connection that acts as a counterweight, removing force from the probe tip and providing strain relief during DDR testing.
You can also use an accessory that provides what you might call "gooseneck" strain relief. This probe accessory features a flat geometry and a rubberized flex circuit lead. To probe pins on one chip, use an adhesive to securely mount the accessory's base to another chip. You can also use this accessory to probe discrete components.
A third approach to providing strain relief when probing is to use a chip clip—a device similar to a clip you might use to reclose a bag of snacks. A chip clip can attach to the corner of a printed circuit board or to the edge of a computer chassis to prevent probe-platform cable-assembly movement during DDR testing. When attached to a chassis, a chip clip can allow the neck of a probe to extend between two DIMMs, for example, to reach the device under test.
Secure Connections
When performing DDR testing, keep in mind that fragile solder connections can be easily disrupted if the board under test is disturbed. To protect them, use Kapton tape, which can easily be applied to and removed from your circuit. However, avoid taping directly over your probe tips to avoid disruption of delicate structures within the probe leads. The tape is an insulator, so taping under the probe lead tip can prevent metal parts of your probe from accidently shorting or coupling to conductive nodes underneath the probe. In addition, Kapton tape can keep delicate connections in place if you need to move your board under test to another location within your lab. Finally, you will find it useful to attach labels to your board indicating the signals being probed.
Kapton tape and labels can be particularly useful if you need to ship your board under test to a remote location. The tape helps keep the connections secure, and the labels can help an operator at the remote location correctly reconnect the probe amplifiers that were disconnected for transport.
CAUTION:do not use ESD bag material when testing a powered-up board, because the material's outer surface is conductive.
Remove DIMM from Chassis to Connect Tips
When working with DDR memory, you may need to test a device on a DIMM mounted in a slot in a computer chassis. To test a device such as a DDR4 DIMM, remove the DIMM from the chassis to solder in the probe tips and tape them down. Then, reinsert the DIMM into the chassis before connecting the probe amplifiers to the tips, applying appropriate measures of strain relief as discussed above.
Use, but Don't Overuse, Hot Glue
Another technique is to use—but not overuse—hot glue to assist with maintaining secure connections. For example, you can hot-glue probe tips to the backside of a BGA ball-out on a single-sided DIMM before inserting the DIMM into a DDR slot. Apply the hot glue along the top of the probe tips, from where you can subsequently easily peel it off without damaging the probe. Take care not to submerge the probe in a puddle of glue, which can result in probe damage when you try to remove the glue.
Probe the Backside of a BGA When Accessible
Single-sided DIMMs provide fairly easy access that enables you to probe the back side of the BGA, which minimizes reflections and optimizes signal integrity.
However, many DIMMs have DRAM chips on both sides, hindering such easy access. For double-sided DIMMs and other cases where you can't access the back side of the BGA ball-out, you can use an interposer, which breaks out signals of interest so you can probe them. The interposer resides between the BGA and the target board, with its bottom soldered to the BGA footprint on the target. Some interposers come with standard DDR sockets on top, ready for memory component insertion. Alternatively, the memory component can be soldered to the top of the interposer. You can use tweezers to assist with the soldering process.
Interposers are useful in connecting to otherwise difficult-to-access signals, and they generally provide reasonable signal fidelity. However, they impose additional complexity to install correctly, and their relatively large footprint (compared with the DDR device) may cause problems on crowded boards. The interposer will alter the electrical characteristics of your circuit, so use the model supplied by the interposer manufacturer to de-embed the interposer from your signal path. In general, use an interposer only when you have no other options.
Note that if an interposer is installed on a DIMM's end chips, the interposer may prevent the side clips that secure the DIMM to the main PCB from latching. To avoid this problem, you can choose to install interposers on one or more of the central chip locations, based on factors such as simulation data or path lengths.
Avoid Long Leads and Square Pin Adapters
For high-speed applications, avoid long leads with nonstandard, unmatched lengths. Long leads add inductance, and nothing creates a worse measurement problem than a long lead acting as an antenna coupling noise into your circuit under test. In addition, different lead lengths between the data-plus and data-minus ends of the probe can result in intra-pair skew within the probe.
Except for DDR2 and slower signals, avoid using square-pin adapters, which reduce rated bandwidth to about 3 GHz. For the higher speeds found in DDR4 and DDR5 devices as well as some DDR3 devices operating at 1,600 mega-transfers per second, use a solder-in tip, which can have a bandwidth of 13 GHz, 16 GHz, 25 GHz or more. The bandwidth reduction of the square pins would slow down fast DDR edges, reducing slew rate in a way that could affect timing measurements between data and strobe signals, for example.
Remove Damping Resistors if Gain Accuracy Becomes a Problem
Also consider the probe damping resistors. When making measurements with an interposer, you can remove the damping resistors and achieve slightly better signal fidelity by connecting directly to the signal. AC response will improve by about 3%, and DC gain accuracy will improve by about 4%. The drawbacks include the need to modify the probe. Consequently, the probe will no longer be suitable for general-purpose use by others in your lab who share your probes until the resistors are replaced, and a colleague may attempt a measurement using the probe without realizing that the resistors have been removed.
If you are passing compliance tests by a 20% to 30% margin, you will find no benefit from removing the resistors, and most users do not do so. If your measurements indicate that your circuit is within 4% of passing compliance tests, removing the resistors may be beneficial. If you do remove the resistors, replace each with a 34-gauge wire that extends 3 mm beyond the PCB edge of the probe tip.
Use the Probe Ground Lead when Measuring Floating Signals
Finally, consider grounding issues when working with DDR devices. When you are making measurements, you may observe signal anomalies, such as the clock signal being pinched off for a brief period of time.
Such anomalies can occur when your device under test is floating with respect to earth ground while your oscilloscope and active differential probe are earth grounded. Your probe may be capable of measuring a signal's peak-to-peak signal (5 V peak-to-peak, for example), but a floating DUT can cause common-mode voltages to exceed the probe's limits (4 V, for example). To solve this problem, connect the DUT's ground to earth ground using the probe's ground connection.
Conclusion
DDR test poses many challenges, but you can take steps to get accurate measurements quickly. For example, probe the backside of a BGA if accessible; otherwise, consider using an interposer to connect to the signals of interest. In addition, provide strain relief for your probe connections, use tape or hot glue to secure those connections, avoid long leads and square pin adapters for high-speed applications, and consider removing probe damping resistors if your test margins are tight. Finally, make sure to properly ground your device under test.
See our DDR instructional videos at: https://go.teledynelecroy.com/ddr-series.
