Engineering Reference
PCB Via, Drill and Trace Geometry Reference
Lookup PCB drill, finished-hole, plating, annular-ring, barrel, aspect-ratio, trace and thermal-via geometry with fabrication boundaries.
Geometry Terms
| Term | Meaning | Boundary |
|---|---|---|
| Drill diameter | Fabrication drill/tool diameter before final plating | Not necessarily the delivered hole diameter |
| Finished hole diameter | Usable hole after plating and process tolerances | Use the fabricator's finished-hole tolerance |
| Plating thickness | Copper thickness on the hole wall | Minimum finished plating controls barrel area and resistance |
| Annular ring | Copper radial width around the finished hole | Apply fabrication registration and minimum-ring rules |
| Barrel length | Conductive path through the board | Approximately board thickness for a through via; different for blind/buried vias |
| Aspect ratio | Board/drill depth divided by drilled-hole diameter | Fabrication capability and plating reliability are process-specific |
| Via pitch | Center-to-center spacing | Affects routing, plane spreading, soldering and thermal-via density |
| Trace width | Planar conductor width | Finished copper thickness and etch tolerance determine cross-section |
Via Types
| Type | Definition | Manufacturing note |
|---|---|---|
| Through via | Connects outer layers through the full board | Longest barrel path; broadly available |
| Blind via | Connects an outer layer to selected inner layers | Depth, aspect ratio and sequential-lamination limits apply |
| Buried via | Connects inner layers only | Requires multilayer process planning |
| Microvia | Laser-drilled shallow interconnect | Stacked/staggered construction and reliability rules are fabricator-specific |
| Thermal via | Conductive via used in an array for heat spreading | Barrel is only part of the package-to-board thermal path |
Electrical Barrel Example
Four 0.30 mm finished vias with 25 µm plating and a 1.6 mm barrel give 0.0236 mm² barrel area per via and 0.293 mΩ idealized equivalent resistance.
Thermal Barrel Example
The shared barrel-only model for nine vias gives 19.60 °C/W. This excludes spreading and interface resistances.
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FAQ
What is the difference between drill and finished hole size?
Drill size describes the fabrication tool/hole before final plating; finished hole is the delivered opening after plating and tolerance.
How is annular ring measured?
It is the radial copper width from the finished-hole edge to the pad edge, subject to registration tolerance.
What determines via resistance?
Finished-hole circumference, plating thickness, barrel length, copper resistivity and temperature determine the idealized barrel resistance.
Do parallel vias share current equally?
Only approximately. Placement, plane connections, spreading resistance and manufacturing variation affect sharing.
What is via aspect ratio?
It is commonly hole depth divided by drilled diameter; allowable values depend on the fabrication process.
Can thermal-via resistance be calculated from the barrel alone?
The barrel can be estimated, but total thermal behavior also includes package, pads, planes, spreading and interfaces.
Should thermal vias be filled?
It depends on assembly, solder-wicking, planarity, reliability, cost and thermal needs. Follow package and fabricator guidance.
Are PCB geometry rules universal?
No. Use the selected fabricator's current capability and tolerance table.