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

Question

I am currently researching the length matching problem in the PCB EDA domain and am curious as to why tools such as Allegro and Altium primarily use accordion, trombone, and sawtooth patterns to address these issues. Are there any particular reasons for not considering spiral patterns or other alternative patterns in this context?

John T · Answer

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.

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

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