Definition of HDI (per IPC-2226)
- PCB trace width and spacing ≤ 100 μm
- Via diameter ≤ 150 μm and pad size < 400 μm
- Higher connection pad density than conventional PCBs (> 20 pads/cm²)
Definition of Microvia (per IPC-2226)
- Pad diameter ≤ 350 μm
- Plated hole diameter ≤ 150 μm, formed by laser or mechanical drilling
- Aspect ratio (depth/diameter) = 1:1 (typical PTH aspect ratio is 10:1)
Common Via Types
- Plated Through Hole (PTH): Through via from outer layer to outer layer
- Blind via: From outer layer to inner layer
- Buried via: Between inner layers only
Via Aspect Ratio (AR)
The ratio of via depth to via diameter.
- PTH: typically 6:1 to 10:1
- 10:1 is the practical limit (lower reliability)
- 6:1 is recommended for best reliability
- Thicker board requires larger via size
Example:
- 1.60 mm board, 0.40 mm via → 4:1
- 1.60 mm board, 0.20 mm via → 8:1
Microvia Features
- Aspect ratio: 1:1 / 1:0.8
- Staggered microvias
- Copper-filled (via-in-pad)
- Stacked microvias
- Skip microvias
- Buried microvias
HDI PCB Manufacturing Flow
General process:
Copper etching → Lamination → Drilling → Plating
- Core etching
- Lamination 1 (hot press: copper + inner board + PP + copper)
- Drill 1
- Buried via filling
- Lamination 2
- Drill 2 + Laser drill 1
HDI Stage Definition
The “stage” in HDI refers to laser drilling stages.
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