HDI
High Density Interconnect
HDI (High Density Interconnect) describes PCBs with significantly higher routing density than conventional boards, achieved through microvias, thinner traces, and sequential lamination. HDI is essential for high pin-count BGAs, compact electronics, and applications in smartphones, medical devices, automotive ECUs, and aerospace systems.
What it is
HDI is defined by IPC-2226 as a PCB with higher wiring density per unit area than conventional boards. The key enabling technology is the microvia — a laser-drilled blind via with a finished diameter of 150 µm or less. By using microvias instead of mechanically drilled through-hole vias, HDI designs route between adjacent layers without consuming routing space on intermediate layers.
IPC-2226 defines three main HDI build-up types. Type I has a single microvia layer on one or both sides of the core. Type II adds buried vias inside the core. Type III has two or more microvia layers on at least one side of the core. Designs are often described using "1+N+1" notation — a 1+N+1 board has one microvia layer on each side of an N-layer core; a 2+N+2 has two microvia layers per side.
Microvias can be stacked (placed directly on top of each other across layers) or staggered (offset between layers). Staggered is generally preferred for reliability under thermal cycling, while stacked enables tighter routing at higher cost. The trade-off between density and cost drives most HDI design decisions.
When it matters
HDI is required when conventional through-hole routing cannot accommodate the design. The clearest indicator is high pin-count BGAs at 0.5 mm pitch or below — fan-out routing for these packages typically demands microvias and HDI build-up. Other drivers include miniaturisation (wearables, hearables, medical implants), high-speed signal integrity (where shorter via stubs matter), and any application where board area is constrained.
HDI fabrication adds cost compared to conventional multilayer boards because of sequential lamination, laser drilling, and tighter process control. The cost premium varies with complexity, but 2+N+2 designs typically cost 30-60% more than equivalent conventional multilayer at low volume. For high-volume consumer electronics, HDI is often cheaper overall because smaller boards reduce material cost.
At Nordic PCB
HDI is available across our certified suppliers, including Type I, Type II, and Type III configurations per IPC-2226. For HDI projects, we recommend including stack-up and impedance requirements in the initial RFQ — these affect material selection and laser drilling specifications, and getting them right at quote stage avoids re-tooling later. Our DFM review covers microvia aspect ratio, stacked vs staggered structure choices, and fan-out feasibility for fine-pitch BGAs.
Related terms
- Microvia
A microvia is a small laser-drilled blind via, typically 150 µm or less in diameter (per IPC-T-50M definition), used to connect adjacent layers in HDI boards. Microvias enable routing for high pin-count BGAs and densely packed components that conventional mechanical vias cannot support.
- Stack-up
A PCB stack-up is the cross-sectional arrangement of copper layers, dielectric materials, and bonding films that make up a multilayer board. It defines layer thickness, copper weight, dielectric properties, and is critical for impedance control, signal integrity, and manufacturability.
- Via
A via is a plated hole that connects two or more copper layers in a PCB. Common types include through-hole (passes through the entire board), blind (connects an outer layer to an inner layer), buried (connects only inner layers), and microvia (small laser-drilled via used in HDI designs).
- Multilayer PCB
A multilayer PCB has more than two copper layers separated by dielectric material and bonded together under heat and pressure. Common configurations are 4, 6, 8, 10, and 12 layers. Multilayer designs enable higher routing density, dedicated power and ground planes, and better signal integrity for complex circuits.
- Impedance control
Impedance control is the practice of specifying and manufacturing PCB traces so their characteristic impedance meets a precise target — typically 50 Ω single-ended or 100 Ω differential. Required for high-speed digital, RF, and signal integrity-critical designs, it depends on trace width, dielectric thickness, copper weight, and material properties.