Consumers’ escalating demand for more features in their small and mobile electronics products, such as PDAs and cell phones, is driving a need for smaller feature sizes, process geometries, and pc boards. For engineers dealing with these desires, the need for HDI (high-density-interconnect) technology has become a reality. HemeixinPCB describes HDI technology as a process that lets you produce a pc-board with through-hole, blind, or buried vias of less than 0.006 in. in diameter without using conventional drilling technology. Users of HDI technology must be able not only to assess and implement next-generation technology, but also to understand its boundaries in terms of layer stack up, via and microvia formation, feature size, and the primary differences between it and traditional pc-board technologies.
Several other construction types are available. Type IV construction comprises a “passive” core that can function as a nonelectrical shield or a thermal buffer. “Coreless” construction, which comprises a pair substrates laminated together, is Type V construction. Type VI construction, or colamination, occurs when you simultaneously form the interconnect and mechanical structure.
The multitude of layer stack up that engineers can derive by combining HDI-construction types and a varying number of layers has driven the need for a simple designation scheme to identify them. The identification method is straightforward. For example, a designation of “2 (C4) 2” indicates a layer-stack construction comprising a four-layer pc-board core (C4) with two HDI (buildup) layers on the top and two on the bottom. A designation of “2 (P) 2” indicates a Type IV construction with a passive core, two HDI layers on the top, and two HDI layers on the bottom.
Currently, the size of microvias limits their current-carrying capability. Designers typically overcome this limitation by nesting several microvias in one large area called a plural via. Microvias that directly connect nonadjacent HDI-buildup layers are called skip vias. A variable-depth microvia is a microvia formed in one operation that penetrates two or more HDI-dielectric layers and terminates at one or more layers. Laser vias, conformal vias, filled vias, photo vias, and stud vias are microvias that derive their names from the processes used to form them.
Each HDI construction type allows the use of different combinations of “standard” vias and microvia structures. Type I construction lets you use blind, one-layer-deep microvias and a standard through-hole via. The standard via spans all layers in the stack including the HDI-buildup layers. Type II construction is similar to Type I but adds a buried via that spans all the layers of the pc-board core. Type III adds yet more complexity to the via structures, allowing the use of buried, stacked, staggered, and variable-depth microvias. These many via structures can add a significant level of complexity to the layout of HDI-buildup designs.