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HDI PCB Design Guide: Micro-Vias, Any-Layer & High-Density Routing

2026-06-23
HDI PCB Design Guide: Micro-Vias, Any-Layer & High-Density Routing

HDI PCB Design Guide: Micro-Vias, Any-Layer & High-Density Routing

How high-density interconnect technology enables smaller, faster, and more capable circuit board designs.

High-Density Interconnect (HDI) PCB technology uses laser-drilled micro-vias — typically 0.1–0.15 mm in diameter — to connect layers at far higher density than traditional through-hole vias. Originally developed for smartphone miniaturization, HDI is now standard across automotive ADAS, medical implants, aerospace avionics, and AI accelerator boards. Superb Automation provides full HDI design support from 1-N-1 through any-layer (ALIC) architectures, with DFM review covering via structure optimization, material selection, and stack-up verification to ensure manufacturability and field reliability.

What Is HDI Technology?

HDI is defined by IPC-2226 and comes in several build-up levels. A 1-N-1 board has one micro-via layer on each side of a conventional through-hole core. A 2-N-2 board adds a second micro-via layer. Any-layer HDI (ALIC) extends this concept so that every layer can connect to every other layer through stacked micro-vias — eliminating through-holes entirely for maximum routing density.

1-N-1
1 build-up layer
4–8 layers
2-N-2
2 build-up layers
6–12 layers
Any-Layer
ALIC interconnect
8–16+ layers
mSAP
≤15 μm L/S
IC substrate

Micro-Via Types & Structures

Blind Micro-Via

  • Outer layer → inner layer

  • Does not pass through entire board

  • Laser-drilled, 0.1–0.15 mm Ø

  • Copper target pad stops the laser

Buried Micro-Via

  • Inner layer → inner layer only

  • Completely enclosed within board

  • Invisible from board surface

  • Created during sequential lamination

Stacked vs Staggered

  • Stacked: aligned vertically, saves space, higher cost

  • Staggered: offset, more reliable, preferred for auto/medical

Reliability trade-off: Stacked vias save the most board area but demand tighter process control and cost more. Staggered vias offer higher reliability and are preferred for automotive and medical applications where field failure is not an option.

Design Rules for HDI

Micro-via aspect ratio should not exceed 1:1 for best reliability — a 0.1 mm diameter via should connect layers no more than 0.1 mm apart. Laser-drilled vias require a copper target pad on the layer below to stop the laser at the correct depth and prevent damage to underlying layers.

Via-in-pad design — placing micro-vias directly in SMT pads — maximizes routing density but requires the vias to be filled with conductive or non-conductive epoxy and planarized flat. This adds cost but is essential for high-density BGA fanout in compact designs where every millimeter of board space counts.

  • Via aspect ratio ≤ 1:1 for reliable plating

  • Copper target pad required on layer below each laser via

  • Via-in-pad requires filling + planarization

  • Stacked via count: reliability decreases with each additional stack level

  • Material must be laser-drill compatible (laser-friendly resin system)


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