Through-hole technology (THT)
THT (through-hole technology) is a PCB assembly method in which component leads are inserted through plated holes in the board and soldered, usually on the opposite side. It creates strong mechanical joints and is used today mainly for connectors, large capacitors, transformers and other parts that face mechanical stress or carry high current.
Through-hole technology (THT) explained
Through-hole technology was the standard assembly method before surface-mount technology became dominant. Each component has wire leads or pins that are inserted through drilled and plated holes. The plated barrel connects the lead to traces on any layer, and the solder fills the hole and forms fillets on both sides of the board.
There are two main lead styles. Axial components, such as some resistors and diodes, have leads coming out of each end. Radial components, such as electrolytic capacitors, have leads on one side. Connectors, headers, terminal blocks, relays, transformers and many power components also use through-hole pins.
Components are inserted either by hand or with insertion machines. They are then soldered by wave soldering, selective soldering or hand soldering. Some designs use pin-in-paste, also called intrusive reflow, where solder paste is printed into and around the holes and the through-hole parts are soldered in the reflow oven together with SMT parts. This requires components that can survive reflow temperatures.
Press-fit is a related technique for some connectors, where specially shaped pins are pressed into plated holes without solder.
The key advantage of through-hole technology is mechanical strength: the lead through the board holds the component firmly, which suits parts that are plugged and unplugged, are heavy, or are exposed to vibration. The drawbacks are lower component density, holes through all layers that block routing, and more assembly steps compared with SMT.
When THT matters
Through-hole technology matters wherever a component faces mechanical stress, high current, high voltage or heat. Typical examples are connectors that are frequently plugged, large electrolytic capacitors, transformers, power inductors, terminal blocks and heavy components in vibrating environments.
On most modern designs, THT is used selectively alongside SMT. The number of through-hole parts and where they sit affects the assembly route: few parts may be hand soldered or selectively soldered, while many may justify wave soldering. Bottom-side SMT parts limit the options.
Typical issues include insufficient hole fill, solder bridging between closely spaced pins, wrong hole-to-lead clearance that makes insertion difficult or soldering unreliable, and components inserted in the wrong orientation. Hole sizes must suit the lead dimensions with appropriate clearance. Through-hole parts often also add manual steps, so reducing their number or choosing pin-in-paste-compatible parts can simplify assembly.
THT vs SMT
SMT is the default for nearly all components because it is denser and fully automated. THT remains the better choice for parts that need mechanical strength or handle high power. Most designs combine the two.
THT in your quote request
In your quote request, make it clear in the BOM which parts are through-hole, and note whether any must be hand soldered, pressed in or soldered with pin-in-paste. Include an assembly drawing for orientation of polarised parts and the side each part sits on. State the IPC-A-610 class you need, as it sets the hole-fill requirement. Our free DFM check flags hole sizes that do not suit the component leads.
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PCB assembly →FAQ about THT
What is the difference between THT and SMT?
In THT, component leads go through plated holes in the PCB and are soldered on the opposite side, usually by wave or selective soldering. In SMT, components sit on pads on the board surface and are soldered by reflow. THT gives stronger mechanical joints, while SMT allows much smaller components, higher density and fully automated assembly.
When is through-hole technology still used?
Through-hole technology is still used for components that face mechanical stress, carry high current or are large and heavy. Typical examples are connectors, terminal blocks, large electrolytic capacitors, transformers, relays and power components. It is also chosen for products exposed to vibration and for some high-reliability applications where joint strength is important.
Can THT and SMT be combined on one board?
Yes. Most modern boards that need through-hole parts are mixed-technology boards. SMT components are usually assembled first by reflow soldering, and the through-hole parts are then soldered by wave, selective or hand soldering. Some through-hole parts can also be soldered in the reflow oven using pin-in-paste, if they tolerate reflow temperatures.
Related terms
- SMT
SMT (Surface Mount Technology) is the dominant PCB assembly method where components are placed directly onto the surface of the board and soldered using reflow ovens. It enables smaller, denser, double-sided boards and supports nearly all modern component packages including BGAs, QFNs, and 0402 passives.
- Wave soldering
Wave soldering is a bulk soldering process in which an assembled PCB is passed over a standing wave of molten solder, soldering all exposed through-hole joints on the underside in one pass. It is mainly used for through-hole components and can also solder certain glued SMT components on the bottom side of the board.
- Selective soldering
Selective soldering is an automated through-hole soldering process that solders specific joints one at a time using a precision nozzle of molten solder. Unlike wave soldering, it requires no fixtures and only applies flux to targeted joints. Ideal for mixed-technology boards with SMT on both sides and few THT components.
- PCBA
PCBA (Printed Circuit Board Assembly) is the process of populating a bare PCB with electronic components — resistors, capacitors, ICs, connectors, and other parts — to produce a functional electronic assembly ready for testing or integration into a finished product.
- IPC-A-610
IPC-A-610 is the IPC standard that defines visual acceptance criteria for electronic assemblies. It describes what acceptable and defective solder joints, component placement, cleanliness and damage look like on a populated PCB, using photographs and illustrations, across three product classes. It is the most widely used inspection standard in PCB assembly.