The growth in demand for small form-factor and more capable electronic devices is driving a corresponding need for for advances in high-density interconnect technology to support high-speed, power efficient data transfer between chiplets or between systems-on-chip and memory. This paper presents, for the first time, electrical modeling and characterization results for 2-µ m line and space (L/S) and 1.5-µ m L/S signal wiring in high-density wiring layers in a panel-level organic interposer to address this need. The first part of this paper shows various high-density routing configurations (< 5µ m) formed in organic thin films, explicitly focusing on the effect on signal phase delays, crosstalk, insertion loss, and the range of characteristic impedance that fine line structures can provide; in this discussion, impedance matching to mitigate reflections is considered. To verify high-density signaling models, the second part of the paper focuses on electromagnetic simulations and hardware measurements taken at frequencies up to 20 GHz for 5.9-mm and 1.3-mm single-ended transmission lines. In addition to the agreement between measurement and simulation results for both 2-µ m and 1.5-µ m L/S configurations up to 20 GHz, this paper quantifies measured per-unit-length insertion loss of the high density wires at multiple frequencies to serve as a standard of comparison.