THT

Through-hole technology refers to the assembly method in which component leads pass completely through drilled holes in a printed circuit board and are soldered to pads on the opposite side. The leads emerge from the component body, poke down through the board, and anchor the part mechanically and electrically. This differs fundamentally from surface-mount technology, where components sit flat on the board surface.

THT dominated electronics manufacturing from the 1960s through the 1990s and remains in use today, particularly in industrial equipment, power supplies, and amateur radio. Resistors, capacitors, diodes, transistors in TO-220 packages, and large integrated circuits all commonly used THT. The board itself must be drilled to precise tolerances, typically 0.8 mm to 1.2 mm depending on lead diameter. Lead spacing on through-hole components follows standardized pitch values: 0.1 inch (2.54 mm) is the most common for ICs and component headers.

The soldering process for THT involves either wave soldering, where the assembled board passes over a wave of molten solder, or selective hand soldering for repair and prototyping. Wave soldering is faster at volume but requires careful board design to avoid bridging between adjacent pads. Lead bend reliability became a known failure mode in vibration-prone applications; mechanical stress at the solder joint could initiate cracks over time, particularly with stiff leads and rigid mounting.

THT versus surface-mount trade-offs

THT components are larger, use more board space, and require drilling operations that add cost at volume. However, they are easier to repair, reflow, and hand-solder. Component availability remains broad; many high-power devices and passive components are still sold primarily in THT form. Hobbyists and maintenance technicians often prefer THT for its accessibility. Mixed assemblies, combining both THT and SMT on a single board, became common in the 2000s as a compromise.

Today THT is seldom chosen for new consumer products or high-density designs, but it remains standard in industrial control boards, laboratory equipment, and anywhere serviceability outweighs density. The term itself is now somewhat archaic in cutting-edge manufacturing contexts, but it remains in use whenever discussing board-level assembly methods or component availability.