PCB blind vias: selecting blind, buried, and filled via structures
Blind vias connect outer and inner layers; buried vias connect internal layers. Specify start/end layers and finished requirements for each type. Laser drilling, via filling, and via-in-pad are separate process conditions requiring individual definition. CAD connectivity does not establish feasible lamination and drilling sequences.
| Structure | Primary Use | Conditions that must be clear |
|---|---|---|
| Mechanical Vias | Through-board connection | Finished/drilled hole diameter, hole-wall copper, and total board thickness |
| Laser Microvias | High-Density Interconnect in Added Layer Areas | Start/end layers, dielectric thickness, hole opening/bottom, and capture pads |
| Core Buried Vias | Internal layer-to-layer connection | Fabrication sequence, post-lamination reliability, and testing |
| Filled Via-in-Pad | Prevent solder wicking and provide flat pads | Via-fill material, copper caps, dimples, and acceptance methods |
| Backdrilled Vias | Reduce signal stubs | Direction, stop layers, residual stubs, and clearance |
Aspect ratio cannot be mixed
The board-thickness/hole-diameter ratio for through holes and dielectric-depth/hole-diameter ratio for laser microvias describe different structures. Hole diameter may also mean drilled or finished diameter and must be defined in drawings. Historical through-hole aspect ratios of 12:1/16:1 and laser blind-via diameters of 0.075/0.05mm do not establish microvia capability for arbitrary materials and dielectric thicknesses.
Difference between via filling and via capping
Solder-mask tenting, plugging, resin filling, and plated filling solve different problems. Via-in-pad must meet flatness needs for subsequent soldering; solder-mask coverage alone does not automatically create a flat structure supporting a pad. Mark filling scope and representative test locations on drawings.
Input and confirm output
Inputs are via layer-pair diagrams, dielectric thicknesses, hole-diameter definitions, pads, filling/copper caps, and reliability requirements. Outputs are manufacturing sequences, representative microsections, thermal-stress verification, and acceptable deviations; complex via structures require representative samples first.
HDI build-up structures, Layer Stackup, Submit drill drawing
Engineering reference | Tenting, plugging, and filling: specifying via-in-pad requirements: methodological reference; third-party design methods or material documentation do not represent KFPCBA manufacturing capabilities.
HDI examples with one, two, or three build-up layers per side
The diagram's blue core buried vias and red blind microvias in the build-up layers are fabricated separately; core and buildup structures jointly determine total layer count. Define mechanical holes, laser microvias, resin filling, and plated filling by group in drawings.
Lamination and hole processing sequence
| phase | Items Requiring Confirmation | Inspection Objects |
|---|---|---|
| Core board | Internal traces, buried-via layer pairs, and metallization | Inner-layer artwork, buried-via hole-wall copper, and representative microsections |
| First Build-Up Lamination | Added dielectrics on both sides, microvia target layers, and capture pads | Buildup registration, microvia structures, and filling requirements |
| Subsequent Build-Up Laminations | Second/third-cycle via layer pairs and staggered/stacked arrangements | Corresponding interface and representative hole group inspection |
| Final through-hole / Back drill | Hole groups, stop layers, layers protected from damage, and residual stubs | Electrical continuity, microsections, and specified residual-stub measurements |
Press-fit holes provide a mechanical fit between connectors and finished plated holes and are not equivalent to blind/buried vias; plated castellations provide board-edge connections and also require separate definition.
Press-Fit Hole PCB → Plated Half-Hole PCB → Compare via tenting, plugging, and filling →
