High density interconnects (HDI) are defined as substrates or boards with a higher wiring density per unit area than conventional printed circuit boards (PCB). They have finer lines and spaces (<75 ¦Ìm), smaller vias (<150 ¦Ìm) and capture pads (<400 ¦Ìm), and higher connection pad density (>20 pads/cm2) than employed in conventional PCB technology. HDI is used to reduce size and weight, as well as to enhance electrical performance. In addition to HDI, terms such as "build-up board" in Japan and "sequential build-up (SBU)" or "microvia technology" in the U.S. can be used interchangeably.
Normally, HDI is associated with PCB materials and processes. However, in terms of dimensions, thin film substrates are by definition outstanding for HDI purposes, though they are more known as MCM-D substrates (Multi-Chip-Module, D stands for thin film deposition of dielectric material and metal leads). MCM-D often uses Si wafers as base material, hence "Si-on-Si" is also an expression sometimes used. MCM-L is based on laminates and MCM-C on ceramics. There are also combinations of these technologies; MCM-L/D & MCM-D/C. Another related term is High Density Packaging (HDP).