Formula Used
Finished hole from drill: Finished hole = Drill diameter − 2 × plating thickness per side.
Drill from finished hole: Drill diameter = Finished hole + 2 × plating thickness per side.
Minimum pad diameter: Pad = Drill diameter + 2 × (annular ring + drill position tolerance + image registration tolerance).
Solder mask opening: Mask opening = Pad diameter + 2 × mask expansion.
Copper keepout: Keepout diameter = Pad diameter + 2 × copper clearance.
Aspect ratio: Aspect ratio = Board thickness ÷ finished hole diameter.
The formula estimates a practical land size. Your board supplier rules should always be checked before release.
How to Use This Calculator
Select the working unit first. Choose whether your main input is the finished plated hole or the mechanical drill. Enter plating thickness, annular ring, drill tolerance, and registration tolerance. Add solder mask expansion and copper clearance. Choose a round or oval pad. Press the calculate button. The result appears above the form and below the header.
Use the CSV button for spreadsheet records. Use the PDF button for a compact report. Review the aspect ratio before final release. A high ratio may need a larger drill, a thinner board, or special fabrication approval.
Via Pad Planning Guide
Why Pad Size Matters
A via looks small on a layout, yet its pad controls yield, cost, and service life. The drilled hole must stay inside the copper land after drilling, plating, imaging, and etching errors. A small pad saves space, but it reduces annular ring. A large pad improves tolerance, but it can crowd traces and planes.
What the Calculator Reviews
This calculator starts with a finished hole or a drill size. It then adjusts for plating thickness when needed. It adds annular ring, drill position tolerance, and image registration tolerance to each side. The result is a practical minimum pad diameter. The tool also estimates solder mask opening and copper keepout. These values help you review spacing before sending files to fabrication.
Choosing the Right Input Mode
Use the finished hole mode when the hole size in the drawing is the plated final opening. Use drill size mode when your shop already gave a mechanical drill value. Enter the smallest ring your design can accept. Add realistic tolerance values from your board supplier. Dense boards often need smaller rings and tighter controls. Power vias may need larger pads for heat and current sharing.
Important Design Checks
A good via pad is not only a formula result. It must match the process class, material thickness, aspect ratio, and assembly method. Tall boards with tiny holes have high aspect ratios. They can be hard to plate evenly. Pads near fine pitch parts also need careful solder mask relief. Mask expansion should prevent slivers, but it should not expose nearby copper.
Exporting and Reviewing Results
The output gives the selected unit and a millimeter reference. This makes reviews easier across mixed design teams. The CSV option stores the numbers for records. The PDF option creates a simple report for meetings or approvals. Always compare the result with your fabricator limits. It also supports cleaner handoffs between layout, purchasing, assembly, and quality teams. Their rules should override any calculator when production data is available.
Best Practice
For best results, start with conservative tolerances. Then reduce sizes only after the supplier confirms capability. Check via arrays, thermal vias, blind vias, and buried vias separately. Each structure may have different limits. This page is a planning aid for land sizing and documentation. It does not replace controlled impedance review, design rule checks, or formal manufacturing notes.
FAQs
What is a via pad?
A via pad is the copper land around a drilled via hole. It gives the drill and plating process enough area to form a reliable electrical connection.
What is annular ring?
Annular ring is the copper width between the hole edge and pad edge. More ring usually improves manufacturing margin and reduces breakout risk.
Should I enter finished hole or drill size?
Use finished hole when your drawing specifies the plated final opening. Use drill size when your manufacturer already supplied the mechanical drill value.
Why does plating thickness matter?
Plating reduces the final opening. A drill must often be larger than the required finished hole to allow for copper plated on the hole wall.
What is drill position tolerance?
It is the possible drill location error. Adding it to the pad formula helps keep the hole inside the copper land after fabrication movement.
What is solder mask expansion?
Solder mask expansion is extra mask opening around the pad. It helps prevent mask overlap, but too much expansion can expose nearby copper.
What aspect ratio is acceptable?
Aspect ratio limits vary by fabricator. Lower ratios are usually easier to plate. Always confirm the allowed value before final board release.
Can this replace fabricator rules?
No. This calculator is a planning aid. Use your supplier design rules, stackup notes, and manufacturing review as the final authority.