MCC has developed a rapid prototyping technology for manufacturing high density interconnections for multichip modules (MCMs). By using laser direct writing to customize a high density pre-manufactured programmable substrate, the time required to make an MCM is reduced from several months to weeks. No photomasks are required, so the non-recurring engineering costs that are typical of full custom designs are significantly reduced or eliminated. The substrates are manufactured by commercial manufacturers using their standard fabrication processes, so the technology offers high performance, reliability, and robustness. The substrates are laser customized with subtractive and additive metallization processes. The subtractive processes are used to divide wiring segments. The additive processes are used to link the segments into the design-specific nets. A rerouting process is used to enable flip chip connections to area array and perimeter pad ICs so that prototype flip chip MCM work can be done immediately with pre-existing IC perimeter pad layouts. The rerouting process uses photoablation of vias in polyimide and laser lithography to define an interconnect metal between substrate wiring and the flip chip landpads. The laser prototyping process was used to make a signal integrity multichip module that has 1296 flip chip connections and 192 I/Os and designed to operate at over 400 Mb/s between ICs.