Blind and buried vias are plated holes used to connect different layers in a multilayer PCB without passing through the entire board thickness.
- Blind via: Routes signal from an outer layer (top or bottom) to one or more inner layers, terminating before reaching the bottom layer.
- Buried via: Connects two or more internal layers and remains completely enclosed within the substrate core, with no connection to the outer layers.
These structures are widely used in HDI and ultra-HDI designs to reduce routing congestion, conserve board space, and support finer-pitch components such as BGAs. Unlike through-hole vias, blind and buried vias allow designers to use only the layers needed for each connection.
Fabricating blind and buried vias requires controlled drilling depths, specialized plating processes, and often sequential lamination. Their manufacturing complexity and cost are generally higher than those of conventional through-hole vias.
Guidelines for designing blind and buried vias
- Follow aspect ratio limits: Keep the via depth-to-drill diameter ratio within your fabricator’s capabilities to ensure reliable plating. Confirm the allowable aspect ratio for each via structure.
- Maintain adequate layer clearances: Provide sufficient clearance between via pads, antipads, and copper features on non-connected layers to prevent unintended shorts.
- Avoid unnecessary via structures: Use blind and buried vias only where routing density or component pitch requires them. Prefer through-holes when they meet the design requirements.
- Consider sequential lamination: Plan the via structures and lamination sequence with your fabricator before finalizing the stack-up. Additional lamination cycles can increase manufacturing complexity and cost.
- Check via registration: Account for layer-to-layer registration tolerances, especially for stacked and staggered microvias.
- Define via structures clearly in the fab notes: Specify the start and end layers for each blind and buried via in the layer stack-up.