I. What is an HDI Board

HDI stands for High-Density Interconnect board. Its core feature is the adoption of laser-drilled micro blind & buried vias and build-up processes to achieve ultra-high routing density.

HDI boards are manufactured via the build-up method and classified by the number of build-up layers:

1st-order HDI: Basic single build-up process.

2nd-order, 3rd-order and higher-order HDI: Multiple rounds of build-up, supporting complex structures such as stacked vias, plated filled vias and direct laser drilling.

Process Advantages: For circuits with more than 8 layers, HDI solutions deliver better overall cost-effectiveness compared with conventional multi-layer lamination processes. They outperform ordinary PCBs in signal integrity, EMI suppression, heat dissipation and ESD performance.

Application Scenarios: Mobile phone motherboards, notebooks, camera modules, automotive electronics, smart wearables and other miniaturized high-end electronic products.

Key Knowledge Point: A circuit board with blind and buried vias is not automatically an HDI board.

Boards with buried vias alone do not qualify as HDI. To fall under the HDI category, the board must feature a structure combining outer-layer blind vias plus build-up lamination, with blind vias establishing electrical connections between outer layers and inner layers.

Simple Distinction between 1st-order, 2nd-order and 3rd-order HDI

1st-order HDI: Mature technology with low process control difficulty. Laser blind vias on outer layers connect adjacent inner layers; this is the most widely adopted type on the market.

2nd-order HDI: Significantly higher process difficulty, with three common structures:

Two sets of 1st-order blind vias arranged in offset layout, routed through intermediate layers for signal transition.

Vertically stacked blind vias (stacked via structure, requiring plated via filling and stringent reliability requirements).

Laser drilling directly from the outer layer to connect traces on the 3rd layer.

3rd-order and higher-order HDI

Additional build-up layers are stacked on the basis of the 2nd-order process. The production workflow is lengthy, with extremely strict controls for alignment, drilling and electroplating. These are extensively used on mainboards for flagship high-end electronic devices (e.g., early-generation smartphone motherboards adopted 5th-order HDI).

II. What is a Conventional PCB (Traditional Printed Circuit Board)

Conventional PCBs rely mainly on mechanically drilled through vias to form electrical connections across all layers.

Characteristics: Through holes are formed using mechanical drill bits with relatively large diameters; routing layout is relatively loose, and micro blind vias are rarely used. Featuring simple structures, short production cycles, low technical barriers and favorable cost performance, they are widely applied to power boards, controllers, small home appliances, general industrial control equipment and other products without strict size constraints.

III. Core Differences: HDI Boards vs. Conventional PCBs

1. Via Types and Fabrication Methods (The Most Fundamental Distinction)

✅ Conventional PCB: Dominated by mechanical through vias with larger hole diameters, which cannot support dense routing; no build-up structure.

✅ HDI Board: Adopts laser micro blind vias combined with buried vias, stacked vias and filled via processes. The smaller hole size enables pad-in-via construction directly within BGA pads, greatly freeing up routing space.

2. Manufacturing Process: Build-up Process vs. One-Time Lamination

✅ Conventional Multi-layer PCB: All core substrates are laminated and formed in a single procedure, followed only by mechanical drilling.

✅ HDI Board: Implements multiple rounds of build-up lamination. Inner core substrates are laminated first, then outer build-up layers, laser drilling and metallization are added sequentially through repeated cycles. More build-up layers correspond to higher orders, costs and manufacturing challenges.

3. Substrate Selection

Early HDI boards generally employed resin-coated copper foil, as conventional lasers struggled to penetrate fiberglass-reinforced substrates. New-generation high-energy laser drilling equipment supports direct processing on FR-4 fiberglass boards, narrowing the gap in substrate selection between HDI and ordinary PCBs.

4. Routing Density and Product Form Factor

Conventional PCBs feature wide clearances between traces and pads, limiting miniaturization. HDI technology supports fine-line routing and fine-pitch BGA layouts to enable thinner, compact device designs.

5. Electrical Performance

Short trace layouts on HDI boards improve signal integrity, reduce RF interference and EMI, and deliver superior thermal conduction and electrostatic discharge performance, making them ideal for high-speed signal circuits.

IV. Clarification of Common Misunderstandings in Design & Procurement

Misunderstanding 1: Any board with blind and buried vias is HDI

Correction: Boards equipped only with isolated buried vias without outer-layer blind via build-up structures remain traditional multi-layer PCBs and cannot be priced as HDI products.

Misunderstanding 2: Boards with more layers must be HDI

Correction: 8-layer or 10-layer boards using only through-via structures still belong to conventional multi-layer PCBs. The judgment criterion depends on whether build-up lamination and laser blind via structures are adopted, not solely the total number of layers.