An effort to determine the high-frequency electrical characteristics of the HDI (high-density interconnect) process and validate models and analytical tools is described. There were seven different designs fabricated, five with only passive structures and two with both passive and active structures. These coupons were designed with many different line structures in order to determine the electrical characteristics of HDI at high (>500-MHz) frequencies. A few of the structures used on the coupons and their respective electrical performances are discussed. The analytical tools derived from basic transmission line theory appear adequate to explain observed values of insertion loss, reflection loss, and crosstalk. In addition, the measurements confirmed the commonly accepted values for the electrical parameters of the GE HDI process (dielectric constant, loss tangent, and electrical resistivity). The crosstalk measurements demonstrate that sufficient isolation exists for practical system designs up to frequencies of 9 GHz in the stripline configuration, and even in the less-tolerant microstrip configuration good isolation is obtained without use of excessive spacing or guard conductors. The major limitation at the higher frequencies appears to be the insertion loss, which approaches 10 dB in some of the longer-length lines considered.