Everything Gadgets

Question for Electrical/Computer Engineers…

I was playing around with my camera last night and I was taking photos of random objects and one of the things I grabbed was a 2GB SODIMM module.  I’ve never really looked at one closely before but I noticed that on the center of the board, on both the front and the back, were these serpentine traces.  What purpose do these serve?

Any thoughts or ideas?

Update  via Twitter from Dave Jones (eevblog.com)

“It matches the trace length to the one next to it. For timing. RT @brianstuckey: what is the point of curvy tracks? bit.ly/u8zUX5”

3 Comments

  1. I’ve seen these serpentine traces on other computer printed circuit boards as well. They appear to be small planar inductors. A planar inductor in a memory component might be isolating (decoupling) a DC power supply circuit from a computer 2GHz+ CPU clock or a clock-driven circuit.

    The US FCC Part 15 rules require low RF-noise design, as mentioned in many instruction manuals. High-level RF AC (like clocked bit pulses) must be mostly prevented from conducting to the power supply or other circuits. This minimizes stray RF radiation interference to gigahertz frequency range communications (like 2.4 GHz wireless LAN).

    An inductor in series connection to another circuit greatly reduces conduction of AC current (like noise), with little reduction in DC current (like power). The higher the AC frequency in a circuit, the smaller the inductor needed for a given reduction (like decoupling) of AC current.

    Larger electronic inductors are classically coils of wire, some with inductance values in millihenries or microhenries. These PCB serpentine examples may have miniscule inductance values, possibly in the nanohenry or picohenry range. According to the reference below, their values aren’t easy to determine.

    Each back and forth cycle of the PCB serpentine is geometrically equivalent to one coil turn. But the exact dimensions of a small planar inductor are so significant that some shaped traces reportedly can have less inductance than a straight trace. Ref: Printed Inductor (PLANAR INDUCTOR) – Edaboard.com thread

  2. These are not inductors, these are simply length matching the traces (command inputs, for the case of this unbuffered DDR3 DIMM). This is a requirement for DDR signals, otherwise, skews in different routes of the signals will skew the results, reducing margin and speed on this DIMM.

    The meanders seen here increase the distance the signal travels for shorter routes, such that the shorter rightside signals are matched. If the length is the same, then the delays are similar (other physical things occur, like dispersion and crosstalk, which are normally ignored and outside of scope of this question).

  3. Let me clarify here: the speed of signal propagation is finite (roughly 60% the speed of light, or 140 trillioniths of the second per inch), just like the speed in your car is finite. It is important for the signals to arrive at the exact same time to your memory IC’s, so if the road is the same length, then the speed is maximized. That is all this is showing 🙂

Leave a Reply