Through-hole technology is a basic method for mounting parts on a printed circuit board by passing their leads through drilled holes and soldering them to pads. This article explains plated and non-plated holes, padstack parts, hole size and fit, spacing, heat flow, assembly methods, components, SMT comparison, reliability points, and defects with fixes, all in clear, detailed steps below.
Through-Hole Basics in PCB Design
Through-hole is a method of mounting parts on a printed circuit board (PCB) by passing their metal leads through drilled holes in the board. The leads are soldered to copper pads, which creates both a strong mechanical hold and a clear electrical connection. Because the lead goes through the full thickness of the PCB, the solder joint is held inside the board, not just on the surface. When the hole walls are plated with copper, the hole can also connect copper layers inside the board.
Common terms:
• THT (Through-Hole Technology) - using drilled holes in the PCB to mount and connect parts.
• THM (Through-Hole Mounting) - another name for the same mounting method.
Plated vs Non-Plated Through-Holes
| Hole Type | Full Name | Copper Plating in Barrel | Main Function |
|---|---|---|---|
| PTH | Plated Through Hole | Yes | Provides electrical connection and supports components |
| NPTH | Non-Plated Through Hole | No | Provides mechanical mounting or clearance, no conduction |
Parts of a Through-Hole Padstack
• Drill hole - the opening in the PCB made by a drill or router, where the lead passes through.
• Barrel - the copper on the wall of the hole in plated holes, which lets current flow between layers.
• Outer pads (top and bottom) - copper areas on the outer surfaces of the PCB where the solder bonds to the lead.
• Inner layer pads - copper areas on inner layers that connect to the same electrical path as the hole.
• Annular ring - the ring of copper around the drill hole that keeps the pad connected and helps prevent it from breaking away.
Through-Hole Size and Lead Fit
Through-Hole Size and Lead Fit
The hole size in a through-hole pad needs to match the metal lead, but it should not be the same. The hole must also allow room for copper plating and normal drill variation. A small extra clearance is added above the lead diameter so the lead can slide in smoothly, and solder can flow around it. This helps the joint stay solid and easier to assemble.
If the Hole Is Too Tight
When the hole is too tight, the lead is hard to push through. It can scrape the copper, bend the pad, or put high stress on the barrel. Over time, this stress can cause cracks in the copper or make pads lift from the board, which can damage the connection.
If the Hole Is Too Loose
When the hole is too loose, the gap between the lead and the barrel becomes large. Solder may not fill this space, so that the fillet may be thin or weak. The lead can lean to one side, affecting testing and making the board look uneven. In this case, most of the strength comes from the solder alone, rather than from a snug fit between the lead and the hole.
Padstack Planning for Through-Hole Pads
Outer pads
Outer pads are the copper areas on the top and bottom of the board around the hole. They provide space for solder to bond to the lead, making the joint easy to see and check.
Inner layer connections
The inner-layer pads determine which copper layers on the board connect to the plated barrel. They guide how power and signals travel through the board and help keep the path clear and controlled.
Anti-pads
Anti-pads are precise openings with no copper around the barrel, in copper plane layers on a different net. They keep the barrel from shorting to nearby copper and help control signal behaviour and unwanted noise.
Layer rules
Layer rules set pad sizes, clearances, and thermal relief patterns on each layer. These rules maintain consistent spacing and help the pads heat and cool in a controlled manner during soldering.
Library consistency
Library consistency means using standard padstacks for common lead sizes and keeping names clear and organised. This makes it easier to match footprints, padstacks, and drill charts without mix-ups.
Through-Hole Pad Spacing and Placement
Hole-to-hole and pad-to-pad spacing
• Leave enough space so solder fillets do not touch and do not form bridges between pads.
• A common starting point is edge-to-edge spacing around 1.27 mm, but the exact value depends on the PCB manufacturer’s limits.
Distance to board edges
• Keep through-hole pads and holes away from the outer edge of the board and from break-away tabs.
• Extra distance lowers the chance that pads will crack or break off when the board is cut from the panel.
Nearby signals
• Avoid placing many through-hole pads too close to fast digital traces or sensitive analogue traces.
• Currents in barrels and copper planes can couple into nearby signal lines and affect signal quality.
Thermal Relief and Heat Flow Around Through-Hole Pads
Heat flow and hard-to-solder pads
When a pad is tied directly to a large copper area, the copper pulls heat away during soldering. The pad may not get hot enough, and the solder may not wet the joint properly.
Using thermal reliefs
Thermal reliefs use thin copper spokes between the pad and the plane. This keeps a good electrical path while slowing heat loss, so the pad warms up faster and soldering is easier.
Balancing copper around the joint
Keeping similar copper areas on both sides of the lead helps both sides heat at a similar rate. This supports smoother solder flow and a more even joint.
Planning for power-carrying parts
For pads that carry more current, combine thermal reliefs with copper pours and thermal vias. This spreads heat while keeping the pad solderable and stable.
Assembly Methods for Through-Hole Components
Hand soldering
• Used for prototypes, small batches, and repair work.
• Allows careful control of each joint but is slower than machine methods.
Wave soldering
• The PCB moves over a flowing “wave” of molten solder on the bottom side.
• Solder many joints at the same time and works well when most parts are through-hole.
Selective soldering
• Uses a small solder nozzle to apply solder only to chosen pads and pins.
• Fits mixed boards where one side has SMT parts, and the other side has through-hole parts, reducing masking and limiting heat on nearby parts.
Common Through-Hole Component Types
Connectors
Through-hole connectors are used where plugs, wires, or cables need a firm anchor. Their leads pass through the board and help distribute the pull and push forces between the solder joints, the PCB, and the enclosure, keeping the connection stable over time.
Power parts
Power parts often have greater mass and generate more heat than small-signal parts. Through-hole mounting provides strong mechanical support across the board, and additional hardware, such as screws or clips, can be used with the leads to keep these parts in place.
Radial electrolytic capacitors
Radial electrolytic capacitors provide high capacitance in a relatively small footprint, with two leads that pass through the board. The through-hole leads help keep the body steady during operation and soldering, thereby supporting long-term reliability in the power and filtering paths.
Axial resistors and diodes
Axial resistors and diodes use leads at both ends, allowing them to span a wider distance on the board. Through-hole mounting works well for layouts that need longer lead spacing or higher-voltage stand-off, and it also fits many repair-friendly or older board styles.
Through-Hole Compared to Surface-Mount Parts
| Design factor | Through-hole | SMT (Surface-Mount Technology) |
|---|---|---|
| Mechanical load | Strong support through the board | Lower load capacity without extra support points |
| PCB density | Lower part density | Higher part density on one or both sides |
| Manual rework | Suited for hand soldering and part swaps | More difficult with very small or fine-pitch parts |
| High-volume assembly | Slower insertion equipment | Fast pick-and-place and reflow processes |
| Thin/compact boards | Less suited to fragile, compact products | Well-suited to slim and highly compact layouts |
Reliability Factors for Through-Hole Solder Joints
Solder fillet quality
A good joint has solder that wraps smoothly around the lead and pad with no gaps or cracks. A solid, even surface helps the joint carry current and handle stress.
Barrel plating
The copper in the barrel must be thick enough and firmly bonded to the pads. Cracks or separation in this copper can break the electrical path even if the outside looks normal.
Thermal profile
Soldering time and temperature must be set so the joint heats enough for good wetting without overheating pads or barrels. Too little heat results in weak joints; too much can lift pads or damage the board.
Mechanical support
Heavy or tall parts should not rely only on their leads and solder joints for support. Extra support that limits movement lowers stress on the joints and helps them last longer.
Common Through-Hole Defects and Fixes
| Symptom | Likely cause | Fixes |
|---|---|---|
| Poor wetting / dull joint | Pad not hot enough; flux weak or old | Add thermal relief where needed, adjust the heat profile, and use fresh flux |
| Pin not centred/tilted | Hole too large; loose part positioning | Use a smaller hole size and improve how parts are held during soldering |
| Solder bridges | Pads too close; too much solder | Increase pad spacing, adjust wave or selective settings, and refine solder mask layout |
| Lifted pad | Too much heat or repeated rework | Lower soldering heat and time, limit rework, and add better strain relief |
Conclusion
Through-hole details in this article cover more than just basic drilling. They link hole type, padstack shape, spacing, and copper balance to how well joints solder and hold up over time. Assembly methods and standard parts show that through-hole still fits beside SMT on modern boards. Reliability checks and defect fixes tie everything together so the same rules can guide stable joints from layout through production and long-term field use.
Frequently Asked Questions
What is a standard minimum through-hole size in PCBs?
A standard minimum drill size is about 0.20–0.30 mm. Smaller holes are possible but need special processing.
How thick is copper plating in a plated through-hole?
The barrel copper is a few tens of micrometres thick, enough to carry current and withstand thermal cycling.
How does lead-free solder affect through-hole soldering?
Lead-free solder melts at a higher temperature, so pads and barrels experience higher temperatures and require a carefully controlled profile.
How are through-hole solder joints checked for quality?
They are checked by visual or automated optical inspection for fillet shape, wetting, and pin position, and sometimes by cutting sample boards for cross-section checks.
What does conformal coating do around through-holes?
It forms a thin protective layer around leads and pads to guard against moisture and dirt, leaving masked areas open for later contact or soldering.
How does vibration affect through-hole parts?
Vibration makes leads and solder joints move with the board, which can fatigue joints if the motion is large or constant. Added support and stiffer boards help reduce stress.
