High-density interconnect PCBs.

HDI (High-Density Interconnect) PCBs are circuit boards that incorporate very dense interconnects between components and the board itself. Achieving extreme pad density requires several design technologies that all interact — microvias, via-in-pad, fine lines, and often sequential lamination. Adding these technologies has real implications for reliability and cost, so they need to be selected against the end-use of the board.

Most modern HDI applications use microvias of 0.006″ (150 μm) and smaller, which require a "pad-in-via" design. Pad-in-via almost always requires the via to be filled — with conductive epoxy, non-conductive epoxy, or copper — and then capped with a solderable surface.

Key characteristics

Microvias

• Laser-drilled vias typically ≤ 150 μm (6 mil) diameter.
• Used instead of mechanically drilled through-holes.
• Provide shorter electrical paths and improved signal integrity.

Blind and buried vias

• Blind vias connect outer layers to inner layers.
• Buried vias connect inner layers only.
• Both enable higher routing density and free up surface area for components.

Fine line and space

• Typical trace/space values of 75/75 μm (3/3 mil) or below.
• Required to break out high-pin-count BGAs and fine-pitch parts.

Sequential lamination

• HDI boards are built using multiple lamination cycles.
• Allows layer-by-layer interconnect buildup.

Thinner dielectrics

• Reduced dielectric thickness improves impedance control and electrical performance.

What is via fill, and why use it?

Via fill is a process where a hole is plated to create the connection between copper layers, then filled with epoxy and the surface planarized. For HDI designs with little real estate to spare and lots of BGAs, micro-BGAs and IC footprints, via fill enables hole-structure reliability, prevents solder wicking, and allows traces to route more openly inside BGA fields.

The three common fill options

• Non-conductive epoxy — most common and most economical. A resin-based ink fully encapsulates the barrel of the hole and a copper cap is then plated on the surface.
• Conductive epoxy — silver- or copper-loaded resin. Better thermal dissipation than non-conductive fill, with a robust top-down hole structure.
• Copper-filled vias — traditionally used on microvias / blind vias. A specialized plating bath fills the hole completely with copper. Possible (carefully) on through-holes too. Highest thermal performance and reliability — but the longest, most expensive process.

HDI build-up structures

Advantages of HDI

• Miniaturization — smaller boards while maintaining or increasing functionality.
• Improved signal integrity — shorter interconnects, lower parasitics, better high-speed performance.
• Higher reliability — microvias reduce thermal stress vs through-holes.
• Routing efficiency — more channels per layer; sometimes fewer layers overall.
• Better electrical performance — improved impedance control, reduced crosstalk and signal loss.

Manufacturing process

• Laser drilling of microvias.
• Desmear and via preparation.
• Electroless and electrolytic copper plating.
• Sequential lamination cycles.
• Fine-line imaging and etching.
• Via filling — conductive, non-conductive or copper.
• Copper cap plating.
• AOI, X-ray inspection and electrical test.

Common materials

• High-performance HDI-grade FR-4.
• Low-loss laminates for high-speed HDI.
• Resin systems optimized for laser drilling.
• Thin copper foils (1/3 oz, 1/2 oz).

HDI vs conventional multilayer

HDI typically reduces overall system cost by enabling smaller form factors, fewer layers and better performance.