Introduction: As electronics become smaller and more integrated, HDI (High Density Interconnect) PCBs have become the standard for high-end hardware. This guide explains how HDI orders are defined (1-N, 2-N, any-layer), how HDI PCBs differ from standard PCBs in routing density, via technology, and layer count, and walks through the HDI manufacturing process.
Description: Based on AnyPCBA's small-to-medium volume PCB manufacturing experience, this guide explains HDI order definitions, differences between HDI and standard PCBs, the manufacturing process, and how DFM review ensures HDI manufacturability.
As electronic products become thinner, smaller, and more highly integrated, HDI (High Density Interconnect) boards have become standard in high-end hardware. However, many engineers still have only a partial understanding of "HDI orders" and how HDI boards actually differ from standard PCBs.
This article explains HDI order definitions, the core differences from standard PCBs, and the manufacturing process, from both an engineering and manufacturing perspective.
The order of an HDI board is closely related to its structural design and manufacturing process. Typically, the order (1-N, 2-N, 3-N, any-layer) depends on layer count, routing density, and the number of laser drilling passes.
Structure: The most basic HDI structure. Typically used for simpler circuit designs with fewer layers (e.g., 4 to 6 layers).
Process characteristics: Only one layer of microvias (blind vias) connecting outer and inner layers. Usually requires one laser drilling pass (e.g., in a 6-layer board, blind vias at 1-2 and 5-6).
Applications: Early smartphones and standard consumer electronics.
Structure: Adds more layers and outer copper on top of 1-N. Offers higher routing density and signal transmission speed. Typically 6 to 8 layers.
Process characteristics: Includes outer copper layers, inner copper layers, inner vias, and isolation layers. Requires two laser drilling passes (e.g., in a 6-layer board, blind vias at 1-2, 2-3, 4-5, and 5-6).
Applications: High-end smartphone motherboards and wearables.
Structure: More layers (10+), with every layer connected via microvias. Extremely high routing density.
Process characteristics: Requires multiple laser drilling passes and multiple lamination cycles. Any-layer HDI has the highest manufacturing difficulty and cost.
Applications: Flagship smartphones, miniature medical devices, aerospace.
Summary: Higher orders mean higher routing density and faster signal transmission, but manufacturing complexity and cost rise exponentially.
HDI boards differ from standard PCBs not just in layer count but fundamentally in physical structure and manufacturing process.
HDI boards: Use blind vias, buried vias, and microvias (laser-drilled, typically smaller than 0.1mm) to route more signals in a smaller footprint.
Standard PCBs: Typically use only through-holes (mechanically drilled), consuming significant board space.
HDI boards: Typically 4+ layers, using sequential lamination to build inner core and outer layers step by step.
Standard PCBs: Single-sided, double-sided, or low-layer-count multilayer boards, laminated in a single cycle.
Thanks to high-density routing and microvia technology, HDI boards can achieve the same functionality in a much smaller size than standard PCBs, meeting the demand for compact packaging.
HDI boards: Require higher precision process control (laser drilling, high-precision alignment, via filling, mSAP, etc.). Extremely demanding on equipment and factory capability.
Standard PCBs: Relatively simpler processes, relying mainly on mechanical drilling and conventional etching.
HDI PCB manufacturing is a complex systems engineering process. Taking the most common 1-N HDI board as an example, the core process is:
Design stage: Define dimensions, layer count, routing rules, and impedance requirements.
Inner layer fabrication: Copper foil treatment (acid cleaning, roughening), film application, exposure, development, and etching to form inner layer circuitry.
Lamination and drilling: Laminate inner cores with prepreg, then drill laser vias (for microvias) or mechanical vias (for through-holes).
Via metallization: Form conductive layers on via walls through electroless copper and electroplating.
Outer layer fabrication: Laminate outer copper foil, expose and etch outer circuitry, and form pads.
Surface finish: Apply ENIG, OSP, or HASL to the pads.
Assembly and testing: SMT placement, AOI, X-Ray inspection, electrical and functional testing.
Packaging: Package for shipment to prevent transport damage.
As a factory focused on small-to-medium volume PCB and PCBA manufacturing, AnyPCBA has accumulated years of experience in HDI manufacturing.
Our manufacturing capabilities cover 2 to 64 layers, including 1-N, 2-N, 3-N, and any-layer HDI, with support for HDI, rigid-flex, and high-frequency hybrid. We hold ISO 13485 (medical) and IATF 16949 (automotive) certifications.
During DFM review and production control, we focus on the following for HDI boards:
Whether the order matches the stack-up design: avoiding cost waste from over-design, or routing failures from under-design.
Laser drilling alignment accuracy: microvias are extremely small, and misalignment causes open circuits or via failure.
Via filling plating quality: whether blind vias are fully filled and whether it affects subsequent SMT soldering or impedance.
Blind/buried via stacking method: staggered vs. stacked—huge differences in manufacturing difficulty and cost.
Our engineers help you confirm these critical details at the design stage, so your HDI boards are not only drawable, but stable and reliable to produce.
If you're working on an HDI project, or want to learn more about our HDI manufacturing capabilities, contact us through our website.
