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″
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