What Is HDI PCB Design?
High Density Interconnect (HDI) PCBs use microvias, fine lines, and advanced lamination techniques to achieve significantly higher wiring density than conventional boards. HDI technology enables smaller form factors, improved signal integrity, and better power distribution—critical for modern electronics from smartphones to medical implants.
Microvia Technology
Microvias are laser-drilled vias typically 150 µm (6 mils) or smaller in diameter. Unlike mechanically drilled through-hole vias, microvias connect only adjacent layers, freeing routing space on inner layers. Common configurations include blind vias (surface to inner layer), buried vias (inner layer to inner layer), and stacked or staggered microvias for connecting multiple layers.
Sequential Lamination Process
HDI boards are built through sequential lamination cycles. A core subassembly is fabricated first, then additional layers are laminated one or two at a time. Each cycle adds copper foil and prepreg, followed by laser drilling and plating. Designs requiring 1+N+1, 2+N+2, or even 3+N+3 build-ups are common in advanced applications.
Critical Design Rules for HDI
- Microvia aspect ratio: Keep at or below 0.75:1 (depth to diameter) for reliable plating
- Annular ring: Minimum 75 µm for laser-drilled microvias
- Trace width/spacing: 75/75 µm (3/3 mil) is common; 50/50 µm requires advanced fabrication
- Stacked microvias: Filled and planarized before stacking; maximum 3 levels recommended
- Pad-in-via: Use via-in-pad with filled and capped vias for BGA fan-out
Applications and Cost Considerations
HDI adds 20–60% to fabrication cost compared to conventional PTH boards, but the total system cost often decreases because of smaller board size, fewer layers in the final construction, and improved electrical performance. Our PCB design engineers evaluate whether HDI is cost-effective for each project. Get in touch to explore HDI for your next design.