Why do PCB designers opt for serpentine tuning and switchback tuning over spiral patterns when addressing length matching in their designs?

Trombones can fill any available space without adding additional vias.  Are you worried about the corners in a trombone path causing impedance discontinuity?

Thanks for your reply!

In Allegro, accordion, trombone, and sawtooth patterns are commonly used for trace detours during PCB design. I'm curious why these specific shapes are preferred and why other shapes, such as spiral patterns like below, are not typically used. Can you provide any insight into this design choice?

Routing space requirements in the majority of pcbs call for a more optimal, direct pattern of flow for tracks. We typically think of usable space in terms of routing channels. For example, if I have 20 tracks to be connected across a horizontal channel of limited vertical width, then the maximum width of that channel is dictated by the largest obstacle in the vertical direction.

Tuning a track in the accordion pattern minimises the physical impediment in the vertical direction which would otherwise reduce the channel width for other tracks. Further additional & subsequent tuning of that track can then be later implemented once again without subtracting more space from the channel width, assuming some additional straight sections of the trace remain available for further tuning. In other words, it is more efficient to add the extra trace length along the direction of travel of that trace. It maximises channel width.

Additionally, there are concerns for higher inductance of the track using the spiral pattern. Spirals are a good pattern for designing inductors into your board: See the video link titled "How to Design Spiral Inductors on a PCB" https://www.youtube.com/watch?v=-M91ig6o0Uw using the Allegro Productivity Toolbox to do so.