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Sea of Leads (SoL) is an ultrahigh density (>104/cm2) compliant chip input/output (I/O) interconnection technology. SoL is fabricated at the wafer level to extend the economic benefits of semiconductor front-end and back-end wafer-level batch fabrication to include chip I/O interconnections, packaging, and wafer-level testing and burn-in. This paper discusses the fabrication, the mechanical and electrical performance, and the benefits of SoL. SoL can lead to enhancements in reliability, electrical performance, manufacturing throughput, and cost. A chip with 12 × 103/cm2 compliant I/O leads is demonstrated. The mechanically compliant I/O leads are designed to enable wafer-level testing and eliminate the need for underfill between chips and printed wiring boards by mitigating thermo-mechanical expansion mismatches between the two. The fabrication of partially nonadherent, or slippery, leads is desirable as it allows the leads to freely undergo strain during thermal cycling. Compared to adherent metal leads, preliminary results show that slippery leads enhance the overall in-plane compliance. Microindentation experiments show that a polymer film with embedded air gaps provides substantially higher compliance than a polymer film without embedded air gaps.