High-Density Interconnect (HDI) Printed Circuit Board

November 6, 2025

1. Definition and Core Concept

High-Density Interconnect (HDI) PCB is an advanced class of printed circuit boards characterized by a significantly higher wiring density per unit area compared to conventional PCBs.The core motivation for HDI technology is the relentless drive in modern electronics for miniaturization and enhanced performance. By using highly sophisticated fabrication processes, HDI boards pack more components and routing traces into a smaller physical space, making devices lighter, smaller, and more compact.

2. Key Structural Features

What defines an HDI PCB is the use of specialized structures that enable this high density:

Microvias: These are the single most defining feature of an HDI PCB. A microvia is a tiny conductive hole with a diameter typically $le 150 mu m$ ($le 0.006$ inches). They are usually created using laser drilling (not mechanical drilling) and are crucial for connecting layers.
- Blind Vias: Connect an outer layer to one or more internal layers, but do not pass all the way through the board.
- Buried Vias: Connect two or more internal layers and are completely sealed from the outside.
- Via-in-Pad (VIP): A technique where the microvia is placed directly inside a component's solder pad, allowing for denser placement of fine-pitch components (like Ball Grid Arrays or BGAs). The via is then filled and plated over.
Finer Lines and Spaces: The copper traces (lines) and the gaps (spaces) between them are much narrower, often $le 100 mu m$ (0.10 mm) wide, allowing for a far greater number of traces to be routed on each layer.
Built-Up Layers (Sequential Lamination): HDI boards often utilize a sequential lamination process where layers are added and processed in steps, with vias drilled and filled at each stage. This "build-up" structure results in types like (1+N+1) or (2+N+2), where 'N' is the core layers and the numbers indicate the high-density build-up layers on either side.

3. Advantages of HDI Technology

Advantage	Description
Miniaturization	Allows for a major reduction in overall PCB size and weight, accommodating the need for smaller consumer products.
Enhanced Signal Integrity	Shorter signal paths (due to microvias) reduce inductance, capacitance, and signal reflection (stubs). This improves the board's high-speed electrical performance and reduces signal loss.
Increased Density/Functionality	More components (especially large pin-count components like BGAs) can be placed on the board, often leading to a reduction in total layer count compared to a conventional design.
Improved Reliability	Microvias are typically copper-filled and plated, offering a more robust connection than traditional mechanically drilled through-holes.

4. Common Applications

HDI PCBs are the backbone of most contemporary high-tech electronic devices, including:

Smartphones and Tablets
Wearable Technology (Smartwatches, Fitness Trackers)
Medical Equipment (Diagnostic Imaging, Monitoring Devices)
Automotive Electronics (ADAS modules, GPS systems)
Aerospace and Defense Systems