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You are here: Home » Blog » Measurement

Oscilloscope Probing for DDR Memory

February 4, 2015 by Rolf Ostergaard

How do you do probing for DDR memory? Placing an oscilloscope probe to measure fast signals is not an easy task. If you want to measure data signals for a memory chip, there is however a solution that you should know. The interposer.

Interposer: Better probing for DDR memory

A tiny board that can be soldered in between the board and the chip. The interposer has the series termination resistors build right in, so it is in effect the tip of a high-speed oscilloscope probe. In a schematic overview, the cross-section looks like this:

Interposer cross section - this is how to improve probing for DDR memory

And if we zoom in on the physical appearance of the interposer, it looks like this from top and bottom:

Chip facing side - probing DDR memory  PCB facing side - probing DDR memory

Why not measure at the via?

What is the problem with just placing the oscilloscope probe right on a via on the backside of the board (if that is accessible at all)?

Well, you guessed it, the distance from the die pad to the via. Look at the measurement we are trying to do here:

The measurement setup whit interposer probing for DDR memory

The scope measurement shows this data eye measured at the via:

Csope_Measurement_at_via

The question you must ask yourself at this point is: So is this as expected or not?

If you do the simulations, you will (hopefully) find that this matches with the simulated eye:

 

Cscope_Simulated_at_via

But the “real” signal at the die pad, which is what the receiving memory chip “see”, is going to look much nicer (again per the simulation):

Cscope_Simulated_at_die

So it’s clear that you would want to take the measurements as close as possible to the die pad for the most accurate results. This is where the interposer comes in.

Even more useful

In two situations, the interposer is even more useful:

  • If you have memory chips on both sides of the board, it gets even more useful
  • If you use HDI techniques with buried vias (don’t unless you have to, but that’s another story)
  • If you use memory devices where the dies are stacked inside the package because the distance from the ball to the die pad in the package increases

If you want to do the official memory compliance test, be aware that the requirements are defined at the ball of the memory package. Getting as close as possible to measuring at the ball is important. This is very true for DDR3 and DDR4 speed grades.

I am not saying that you will always need an interposer to verify a memory system, but as speed and board density goes up it will be more and more convenient. You make the call. At the end of the day, you will still have to rely on simulation to “de-embed” the interposer or “shift” the measurement point to the die pads.

Notice that the interposer will only help you verify signals the terminates at the memory device (writes etc.). If you want an interposer for the controller also (to verify reads), it may be possible, but obviously more expensive as these are larger devices and not with a standardized footprint.

The interposer shown is developed and marketed by Herman Ruckerbauer of EyeKnowHow.de – get in touch with Hermann if you want some. They cost €299 each (6-pack: €1500).

I hope this helps you in oscilloscope probing for DDR memory. Let me know below what techniques you are using?

Filed Under: Measurement Tagged With: DDR2, DDR3, DDR4, Interposer, Probing

About Rolf Ostergaard

Rolf V. Ostergaard, M.Sc.EE. has worked with signal integrity in many different projects since working for 3Com in 1998 as a colleague to Lee Ritchey in Silicon Valley. While building a consulting business focused on advanced electronics and embedded software in Denmark, Rolf has been helping numerous companies with signal integrity and power integrity both as design, simulations, coaching, measurements, and troubleshooting. He started conducting training in SI in 2004 and has trained hundreds of engineers, which lead to founding EE-Training to further expand this.
You can hire Rolf to do signal integrity training and consulting worldwide and remote.

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