Mode Purity Beyond Conductor Count: Miniature Sommerfeld–Goubau Interconnects and Their Engineering Envelope in Advanced Packaging

This preprint proposes Mode Purity Beyond Conductor Count, investigating whether miniature Sommerfeld–Goubau (μ-Goubau) or SSPP slow-wave structures can sustain bound guided-wave propagation inside packaging-scale micro-environments. It constructs an eight-dimensional non-collapsible metric vector for modal power ratio, radiation leakage, insertion/return loss, crosstalk, mode conversion, fabrication sensitivity, and BER. The engineering envelope is confined to 5–30 mm channels with controlled ground proximity, and an explicit retreat to shielded differential striplines is codified when radiation thresholds are breached. This record is a conceptual and theoretical preprint. No original experimental dataset is reported. Ungrounded "return-less" claims are rejected. It is Paper 5 of the SBE/DSR Structural Substrates track.