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Through-hole structures are fundamental elements in printed circuit board (PCB) design, yet many engineering teams underestimate the importance of correctly defining plated-throughholes (PTH) and non-plated through holes (NPTH). Miscommunication in this area often leads to production delays, unnecessary CAM revisions, or—worst—functional failures in assembled hardware. This article explains the technical differences between PTH and NPTH, how they are manufactured, and the design considerations engineers must keep in mind to ensure accurate fabrication.
1. Definition and Functional Purpose
Plated-Through Holes (PTH)
A PTH is a drilled hole whose barrel is intentionally coated with a conductive layer of copper.
The plating electrically connects the pad(s) on the top layer to pad(s) on the bottom layer and to any internal copper layers.
PTHs serve these purposes:
Electrical interconnections between layers
Component mounting (THT components, connectors, header pins)
Thermal vias for heat dissipationVia-in-pad for HDI assemblies (if filled/plugged)
Non-Plated Through Holes (NPTH)
NPTHs are mechanical holes without copper plating in the barrel. No electrical connection exists between layers.
Common uses:
Mechanical mounting holes
Tooling holes for SMT assembly
Alignment and mechanical fixture
Clearance holes (e.g., for screws or standoffs)
2. Manufacturing Process Differences
2.1 Drilling & Tooling
Both PTH and NPTH begin with mechanical drilling or laser drilling. The critical difference is how the hole is treated after drilling.
2.2 Hole Wall Preparation
PTH requires desmear and electroless copper deposition to activate the hole walls, allowing copper plating to bond to the resin and glass fiber.
NPTH skips the desmear and metallization process entirely.
2.3 Copper Plating
This is the most significant divergence.
PTH Workflow:
Desmear
Electroless copper
Electrolytic copper plating
Final plating to meet thickness specs (typically 20–25 μm barrel Cu)
NPTH Workflow:
Drill
Directly apply solder mask or surface finish
Route/finish the board
No plating step is performed
2.4 Fabrication Panelization Differences
Since NPTH does not receive plating, these holes are often drilled:
Before copper deposition, or
After final surface finish
This depends on whether the NPTH is part of the outline, whether it needs tight mechanical tolerance, and the factory's panel workflow.
3. Tolerance and Mechanical Performance
PTH Tolerance
Copper plating increases the hole-wall thickness, so PTH tolerances are generally based on the finished hole size, requiring:
Tighter control
Compensation for copper build-up
Typical tolerance: ±0.075 mm (varies by factory)
PTH barrels also provide:
Higher mechanical robustness
Stronger anchoring for THT components
Better reliability under temperature cycling
NPTH Tolerance
NPTH is controlled as a mechanical hole, where the drill diameter equals the finished diameter, because no plating is added.
Typical NPTH tolerance: ±0.05 mm
Some customers require ±0.025 mm for tight-fit mechanical features.
NPTH is more suitable for:
Precision alignment
Screw mounting
Non-electrical apertures
4. Pad, Annular Ring, and Layer Stack Considerations
4.1 PTH Design Requirements
A standard PTH requires:
Pad on top and bottom layer
Sufficient annular ring (≥ 6 mil typical)
Clearance on internal layers if the via should NOT connect
Thermal reliefs if connecting to copper pours
A poorly defined PTH can cause:
Barrel cracks
Open circuits
Excessive voids during plating
Reliability failures in thermal cycling (common in automotive and aerospace)
4.2 NPTH Design Requirements
NPTH holes must be:
Clearly defined with 0 plating in drill table
Isolated from copper with sufficient clearance (usually ≥ 0.25 mm)
NO copper pads should connect to an NPTH unless specifically requested (e.g., countersunk hardware pads). If copper pads are mistakenly placed around NPTH, CAM engineers will typically remove them to avoid burrs or copper pull-away.
5. CAM/CAD File Requirements
To avoid production errors, engineers must specify hole type clearly:
PTH Indications:
Drill layer: “Plated”
Gerber: Pads visible
Drill table: Mark as PTH or plated
NPTH Indications:
Drill layer: “Non-plated”
No pads or copper connection
Mechanical layer outline sometimes used for board cutouts or slots
Mixed holes (slots with partial plating, castellated holes, or edge plating) must be explicitly labeled, as they require special processes.
6. Impact on Cost and Lead Time
PTH Cost Drivers
Plating time and copper build-up
Higher drill precision
Via filling (if required, e.g., via-in-pad)
Additional reliability tests
NPTH Cost Drivers
Minimal—mostly mechanical drilling
Tight-tolerance mechanical holes may increase cost
NPC (non-plated slots) are slightly more expensive than circular holes
Typical Trend:
PTH > NPTH
Due to plating, inspection, and more complex processes.
7. Design Errors Commonly Found by PCB Manufacturers
Here are the most frequent mistakes encountered during CAM review:
1. NPTH holes placed over copper planes
→ Risk of shorts and solder mask misalignment. CAM will remove copper around NPTH but this may break return paths.
2. PTH defined but missing annular ring on one side
→ Leads to production hold, redesign, and risk of barrel break.
3. Mechanical slots incorrectly defined as plated
→ Causes unexpected plating bridges and mechanical stress.
4. Complex blind vias defined as NPTH
→ Incorrect layer stack connectivity; will require redesign.
5. Mounting holes without tolerance definition
→ Improper fit during assembly.
8. When to Choose PTH vs NPTH
Application Recommended Hole Type Technical Reason
Electrical via / interlayer connection PTH Conductive barrel required
THT component pins PTH Mechanical & electrical connection
Thermal dissipation via array PTH (filled/plugged if needed) Heat conduction
Mechanical screw/mounting NPTH No plating needed; stronger mechanical fit
Alignment holes NPTH Higher dimensional precision
Clearance for metal parts NPTH Avoid shorting
Edge connectors / castellations PTH (partial plating) Conductive outer edges
9. Conclusion
Understanding the distinctions between PTH and NPTH is essential for efficient PCB fabrication. Properly defining hole types impacts cost, manufacturability, routing strategy, electrical performance, and the long-term reliability of the final assembled product. By ensuring clear documentation, correct drill tables, adequate copper clearances, and design rules aligned with fabrication capabilities, engineers can significantly reduce CAM revisions and accelerate time to product