A through hole is an opening that passes completely from one side of a printed circuit board (PCB) to the other. It may be plated with conductive copper to create an electrical connection, or left unplated for mechanical use. Through holes are commonly used for component leads, connectors, vias, mounting hardware, alignment pins, and slots.
The term can be confusing because it may describe the physical hole, a plated electrical feature, or the component-mounting process known as through-hole technology (THT). Understanding the difference helps designers create accurate footprints, avoid manufacturing errors, and choose the right assembly method.
Key Takeaways
- A through hole extends through the full thickness of a PCB.
- A plated through hole (PTH) has a conductive barrel; a non-plated through hole (NPTH) does not.
- Through-hole components have leads inserted into PCB holes and then soldered or press-fitted.
- A through via also passes through the board, but its main purpose is to connect copper layers rather than hold a component lead.
- Hole size, annular ring, aspect ratio, tolerance, and plating requirements should be confirmed with the PCB manufacturer.
What Does Through Hole Mean in PCB Design?
In PCB design, a through hole is a drilled or routed opening that crosses the entire board thickness. Its construction and purpose determine whether it carries a signal, accepts a component pin, supports hardware, or simply provides clearance.
A typical plated through-hole structure includes:
- A finished hole through the PCB laminate
- Copper plating on the hole wall, called the barrel
- Copper pads, or lands, around the hole on selected layers
- An annular ring, which is the visible width of copper between the hole edge and pad edge
- Clearances, often called antipads, where the hole must pass through a copper plane without connecting to it
- Solder mask openings on one or both outer surfaces when soldering is required
The distinction between electrical and mechanical holes is fundamental. Current KiCad PCB Editor documentation classifies a through-hole pad as electrically connected and plated, while an NPTH mechanical pad has no electrical connection.
Plated Through Hole vs. Non-Plated Through Hole
The two main PCB through-hole types are PTH and NPTH.
| Feature | Plated Through Hole (PTH) | Non-Plated Through Hole (NPTH) |
|---|---|---|
| Hole-wall finish | Conductive copper plating | No conductive plating |
| Electrical function | Can connect a lead or multiple copper layers | Normally none |
| Common uses | Component leads, connectors, press-fit pins, through vias | Screws, tooling pins, locating features, mechanical clearance |
| Copper pad | Usually present on required layers | Usually absent, with a copper keepout if isolation is needed |
| Manufacturing data | Identified as plated | Identified as non-plated |
| Key risk | Barrel defects, insufficient annular ring, poor solder fill | Accidental copper exposure or incorrect fabrication classification |
What Is a Plated Through Hole?
A plated through hole has copper deposited and plated onto its internal wall. This copper barrel can connect pads and traces on the outer layers, internal layers, or both. When used for a through-hole component, the lead is inserted into the hole and joined to the barrel and pad by solder. Some PTHs are designed for solderless press-fit contacts instead.
PTHs are used when the hole must provide electrical continuity or become part of an electrical and mechanical joint. Common examples include pin headers, terminal blocks, transformers, relays, switches, large electrolytic capacitors, and power semiconductors.
What Is a Non-Plated Through Hole?
A non-plated through hole passes through the board without conductive material on its wall. Designers use NPTHs for mechanical purposes such as mounting screws, plastic posts, guide pins, and manufacturing tooling.
Do not assume every mounting hole should be unplated. A chassis-ground mounting point may intentionally use plating and copper pads. In contrast, a screw hole that must remain electrically isolated generally needs an NPTH plus suitable copper and component clearances. The correct choice depends on the electrical, safety, and mechanical requirements.
Through-Hole Components and Through-Hole Technology
Through-hole technology, or THT, is an assembly method in which component leads pass through holes in the PCB. The leads are then soldered, press-fitted, or otherwise terminated to create the required connection.
Common Types of Through-Hole Components
Through-hole components usually have one of these lead arrangements:
- Axial leads: One lead exits each end of the component. Resistors and some diodes are familiar examples.
- Radial leads: Multiple leads exit from the same side of the body. Examples include many capacitors, LEDs, and inductors.
- Multi-pin packages: DIPs, pin headers, sockets, relays, and certain connectors use organized rows or arrays of leads.
- Power packages: Devices such as some TO-style transistors and regulators may use through-hole leads together with a heatsink or mechanical fastener.
How Through-Hole Components Are Assembled
A simplified THT assembly process is:
- Form the leads if the component specification and process require it.
- Insert the leads into their assigned plated holes.
- Hold the component at the required height and orientation.
- Solder the joints by hand, wave soldering, selective soldering, or an approved pin-in-paste process.
- Trim leads when required by the assembly process.
- Inspect the joint for wetting, fill, bridges, damage, and other acceptance criteria.
The exact process depends on component construction, production volume, thermal mass, reliability class, and whether the board uses mixed SMT and THT assembly.
Is a Through Hole the Same as a Via?
Not exactly. A through via is one type of plated hole, but it normally exists to route an electrical connection between PCB layers. A component through hole is designed to accept a component lead, connector pin, or terminal.
| Feature | Component Through Hole | Through Via |
|---|---|---|
| Main purpose | Mount or terminate a lead or pin | Connect copper between layers |
| Component inserted | Usually | No |
| Typical relative size | Often larger | Often smaller |
| Solder interaction | Commonly forms a component joint | May be tented, open, plugged, or filled |
| Footprint relationship | Part of a component footprint | Normally part of routing |
Both features can pass through every PCB layer and use a plated barrel. Their design rules, documentation, and manufacturing purpose are different, so they should not be treated as interchangeable.
Through vias also differ from blind vias, which connect an outer layer to one or more internal layers without passing through the full board, and buried vias, which connect only internal layers.
How Is a Plated Through Hole Manufactured?
The exact PCB fabrication sequence varies, but the basic process is straightforward:
- Laminate the board: Copper foils, cores, and prepregs are bonded into the required stackup.
- Drill the holes: CNC drilling creates round holes; routing may be used for slots or larger shapes.
- Clean and prepare the walls: Debris and resin smear are removed so copper can bond reliably.
- Make the hole conductive: A thin conductive layer is deposited on the nonconductive hole wall.
- Build the copper barrel: Electroplating increases the copper thickness and connects the required layer pads.
- Complete outer-layer processing: The fabricator images, plates, and etches the external circuitry according to its process.
- Apply solder mask and surface finish: Exposed pads receive the specified finish, and the board is inspected and electrically tested.
This sequence shows why the finished hole size is not necessarily the same as the drill-tool diameter. Plating reduces the open diameter of a PTH, so the fabricator normally selects a larger production drill to achieve the finished size on the drawing.
Through-Hole vs. Surface-Mount Technology
SMT places component terminations on pads at the PCB surface, while THT inserts leads through the board. Neither method is automatically better; many products combine both.
| Design Factor | Through-Hole Technology | Surface-Mount Technology |
|---|---|---|
| Mechanical load path | Lead passes through the board | Termination is attached at the surface |
| Component density | Lower in most layouts | Higher in most layouts |
| Routing space | Holes affect multiple layers | Usually affects fewer layers |
| Automated assembly | Possible, but may require a separate process | Well suited to high-speed placement and reflow |
| Prototyping and hand work | Often convenient for larger parts | Efficient with suitable tools and experience |
| Typical applications | Connectors, transformers, large or stressed parts, selected power devices | ICs, small passives, dense digital and RF assemblies |
Advantages of Through-Hole Technology
- Leads passing through the PCB can provide a useful mechanical load path.
- Larger component bodies and pitches can simplify manual prototyping and repair.
- THT supports many high-power, high-voltage, electromechanical, and legacy component packages.
- It remains practical for connectors and controls exposed to insertion, withdrawal, or handling forces.
- Press-fit through-hole connectors can provide solderless termination where the design and process support it.
Limitations of Through-Hole Technology
- Drilling and plating add fabrication steps and cost.
- Holes consume routing area on multiple layers.
- THT usually supports lower component density than SMT.
- Mixed-technology boards may need wave or selective soldering in addition to reflow.
- Long leads and barrels can add parasitic inductance and capacitance, which matter in high-speed or high-frequency circuits.
- Rework can damage pads or barrels if heat and mechanical force are not controlled.
How to Design a Through-Hole PCB Footprint
A reliable footprint begins with the component drawing and the PCB manufacturer’s capabilities. Avoid selecting a hole size from a generic chart without checking tolerances.
1. Start With the Maximum Lead Size
Use the component manufacturer’s maximum lead width or diameter, not only the nominal value. For rectangular or irregular leads, calculate the diagonal or use the supplier’s recommended PCB hole dimensions. Then add enough clearance for insertion, positional tolerance, plating variation, and the assembly process.
A hole that is too small may reject components after plating or force operators to damage the lead. A hole that is too large can reduce solder control and mechanical support. Ask the assembler whether automated insertion, hand placement, or pin-in-paste creates additional constraints.
2. Distinguish Finished Hole Size From Drill Size
State clearly whether a drawing dimension refers to the finished hole or the production tool. For PTH features, specifying the required finished diameter is usually the clearest approach because the PCB manufacturer controls plating compensation and drill selection.
3. Provide an Adequate Annular Ring
The annular ring must remain after drilling, plating, and layer-to-layer registration tolerances are considered. Too little copper can create breakout, weaken the land-to-barrel connection, or violate the applicable acceptance criteria.
IPC-2221 is a widely used design reference covering through-hole attachment, lands, annular rings, plated holes, hole spacing, and aspect ratio. Its public table of contents illustrates why these parameters must be considered together rather than as isolated dimensions. Apply the revision and performance requirements named in your contract, and confirm the fabricator’s capabilities.
4. Check the Hole Aspect Ratio
For a mechanically drilled through hole, aspect ratio is commonly expressed as:
Aspect ratio = PCB thickness ÷ finished hole diameter
A thicker board or smaller hole increases the aspect ratio and makes cleaning and uniform copper plating more difficult. There is no single maximum that applies to every supplier, material, reliability class, and plating process. Use the manufacturer’s published limit and leave margin for production variation.
5. Manage Plane Connections and Thermal Relief
A PTH connected directly to a large copper plane can draw heat away from the solder joint. Thermal-relief spokes may improve solderability, especially during hand or wave soldering. However, a solid connection may be preferred for high current, heat transfer, or mechanical reasons. The final choice should balance electrical performance, thermal behavior, and assembly capability.
6. Define Mechanical Holes and Slots Clearly
Identify every hole or slot as plated or non-plated. For hardware, include the finished size, tolerance, copper keepout, edge clearance, and any grounding requirement. Plated and unplated slots should also be separated in the fabrication notes or data.
7. Generate Unambiguous Manufacturing Data
Provide complete Gerber and NC drill data or an accepted intelligent format such as IPC-2581 or ODB++. Include a fabrication drawing or drill table that identifies PTH and NPTH features, finished sizes, tolerances, and special requirements.
EDA tools often separate plated and non-plated drill outputs. For example, KiCad’s drill-file guidance states that PTH and NPTH features are placed in different Excellon files by default and should be merged only when the manufacturer requests it.
Common Through-Hole PCB Problems
Incorrect Finished Hole Size
Confusing nominal lead size, finished hole size, and drill-tool size can cause poor fit or assembly failures. Check the component tolerance and communicate the required finished dimension.
Insufficient Annular Ring
A small pad, inaccurate drilling, or layer misregistration can leave too little copper around the hole. This may reduce connection reliability and create an acceptance defect.
Barrel Voids or Cracks
Poor wall preparation or plating can create voids. Thermal cycling and Z-axis expansion can stress the barrel, particularly in thick boards and high-aspect-ratio holes. Select appropriate materials, geometry, plating requirements, and qualification testing for the application.
Incomplete Solder Fill or Poor Wetting
Heavy copper planes, oxidized surfaces, unsuitable flux, insufficient heat, or a poor lead-to-hole relationship can prevent solder from filling and wetting the joint correctly. Adjust thermal relief, preheat, flux, temperature profile, and dwell time through controlled process development.
Accidental PTH/NPTH Conversion
If fabrication data does not clearly distinguish plated and non-plated holes, a mounting hole may be plated or a component hole may be left unplated. Separate drill data and a clear drill chart reduce this risk.
Damage During Rework
Excessive heat or force can lift a land, damage the barrel, or separate an internal connection. Use temperature-controlled tools, adequate preheating for thermally heavy joints, and a qualified removal process.
When Should You Use a Through Hole?
Choose a through-hole feature when the design needs one or more of the following:
- A leaded component that is not available or suitable in an SMT package
- A connector, switch, or terminal that will experience repeated mechanical force
- A transformer, relay, inductor, capacitor, or power device with large leads or significant mass
- A press-fit connector or compliant pin
- A mounting, locating, tooling, or clearance feature
- A simple, accessible prototype or educational assembly
Do not select THT solely because it appears stronger. A connector may also need retention tabs, screws, staking, or enclosure support. Likewise, an SMT component can be highly reliable when its footprint, materials, assembly process, and operating environment are properly engineered.
Frequently Asked Questions About Through Holes
Is Every PCB Through Hole Plated?
No. Plated through holes provide an electrical function, while non-plated through holes are generally mechanical. The fabrication data must identify which type is required.
What Is the Difference Between PTH and THT?
PTH describes a plated through hole in the PCB. THT describes through-hole technology, the method of mounting or terminating a component through a board. A THT component commonly uses PTH pads, but the terms do not mean the same thing.
Are Through-Hole Components Obsolete?
No. SMT dominates many compact, high-volume assemblies, but THT remains important for connectors, power components, transformers, controls, press-fit systems, and mechanically demanding applications.
Can Through-Hole Components Be Reflow Soldered?
Some can. Pin-in-paste, also called intrusive reflow, deposits solder paste at a through-hole pad before component insertion and reflow. The component, board finish, paste volume, aperture design, and thermal profile must all be compatible with the process.
What Size Should a Through Hole Be?
The correct finished size depends on the component’s maximum lead dimensions, insertion method, plating, tolerances, soldering process, board thickness, and fabricator capability. Use the component drawing and obtain a DFM review instead of relying on one universal clearance value.
Can a Plated Through Hole Carry High Current?
It can carry current, but capacity depends on finished diameter, copper thickness, barrel quality, temperature rise, connections on each layer, and the applicable reliability requirements. High-current paths may need multiple PTHs, larger holes, heavier copper, or an alternative interconnect validated by analysis and testing.
Conclusion
A through hole is more than a drilled opening. Its plating, pads, clearances, dimensions, and manufacturing data determine whether it becomes a reliable electrical interconnect, a component joint, or a mechanical feature.
Use PTHs when electrical continuity or component termination is required, and use NPTHs when the hole should remain mechanical and isolated. For a manufacturable through-hole PCB, define the finished dimensions clearly, preserve an adequate annular ring, check aspect ratio and thermal behavior, and confirm every critical value with both the component supplier and PCB manufacturer.
