Why are these traces on the PCB zigzag?

Question

I recently found my old Raspberry Pi laying in a drawer and I took a look at it. I noticed that the conductor traces are in a zigzag form.

What's the reason behind this? Especially because there's plenty of space to just make a straight trace.

I am sure there has to be reason for this because I have never noticed this on the PCBs I deal with at work.

Here's someone asking about _exactly the same tracks_, come on. https://electronics.stackexchange.com/questions/172508/why-do-circuit-boards-use-strange-paths — pipe, Jun 12 '18 at 18:00
The word "tracks" is OK but "traces" or "conductors" might be better. In the U.S. we say "zig-zag" instead of "zick-zack", just so you know. — Elliot Alderson, Jun 12 '18 at 18:01
@pipe Oh yeah, I'm sorry. Thanks for informing me about that! — user18596, Jun 12 '18 at 18:04
Lots of reasons: Trace length matching, controlling impedance and adding inductance spring to mind. In this case I would suspect trace length matching to control prorogation delays. — Warren Hill, Jun 12 '18 at 18:09
@pipe, actually, the first link "as duplicate" is wrong. The second one is correct, but again is designated as duplicate wrongly. The first link shows the serpentine on one trace of a differential pair, to equalize the mismatch within the **same** differential transmission line. The second "duplicate" is exact match, and shows the dual-serpentine on diff-lines, just for overall length equalization **between different pairs** of the camera interface. — Ale..chenski, Jun 12 '18 at 18:19
@AliChen Indeed, but it's a little weird when the second question is marked as a duplicate of the first. As you point out, they should probably be separate. — pipe, Jun 12 '18 at 18:22

score 1 · Answer 1 · answered Jun 12 '18 at 18:01

They are used for trace length matching. Differential high-speed signals need to have their traces routed with lengths as close as possible to each other, so by adding curves like this one can add more length to one trace than the other and bring the trace lengths closer to each other.

score 0 · Answer 2 · answered Jun 12 '18 at 18:04

Those are probably differential pairs for a LVDS comms scheme. To make sure the signals arrive at the same time, all the traces are the same length. That means some of them need to be scrunched up.

As an aside, I haven't heard zick-zack before, but I'd recognize zigzag, which means a back-and-forth pattern. Hope that's the word you were looking for.

Spehro Pefhany · Answer 3 · 2018-06-12T18:24:12.273

There are two features there.

One is the pairs of tracks close to each other. That's because they are designed to have a controlled characteristic impedance. The thickness and dielectric constant of the PCB through to the ground plane underneath and the desired impedance determines the width and spacing of the conductors. That keeps the signals relatively free from reflections and clean. Each pair represents a current path to and from the receiver. It takes stripline signals about 1 to 1.1 nanosecond to travel 15cm (6").

The second feature is the switchback configuration of the pairs. That's to equalize the length between chip and connector (for each pair), so that the differential signals arrive at the connector at very close to the same time.

Why are these traces on the PCB zigzag?

3 Answers3