through-hole technology
Mounting scheme used for electronic components that involves the use of leads on the components that are inserted into holes drilled in printed circuit boards and soldered to pads on the opposite side.
Through-hole technology: wires through boards, soldered underneath
Through-hole technology is an assembly method where component leads pass entirely through drilled holes in a printed circuit board (PCB) and are soldered to conductive pads on the reverse side. The process creates a mechanical and electrical connection that holds the component firmly in place while establishing continuity with the board's trace network. This contrasts with surface-mount technology (SMT), where components sit flat on the board surface.
Component leads typically range from 0.5 mm to 1.0 mm in diameter, depending on the part type and current rating. Holes are drilled at precise spacing, commonly in 0.1 inch (2.54 mm) grid increments, matching standard component lead pitches. Common through-hole components include dual inline packages (DIPs), radial and axial capacitors, resistors, electrolytic capacitors, and connectors. Larger components like transformers and power modules often rely entirely on through-hole mounting because their weight and heat dissipation requirements demand mechanical anchoring.
The assembly process involves inserting component leads into holes before soldering. Traditional wave soldering passes the entire board over a wave of molten solder, wetting all through-holes simultaneously and creating solder fillets on the underside. In modern hybrid boards, selective wave soldering or hand soldering addresses through-holes after SMT components have been reflow soldered to the opposite side. Lead bending and clinching can improve mechanical hold before soldering, reducing movement during the process.
Why through-hole persists
Despite SMT dominance in high-volume consumer electronics, through-hole remains essential in power supplies, industrial control boards, and any application demanding high current or thermal robustness. Through-hole joints withstand thermal cycling and mechanical stress better than surface-mount solder joints, and the lead acts as a mechanical anchor that prevents component pull-off. Repairability also favors through-hole, since components can be desoldered and replaced more easily without specialized heating equipment.
The chief drawback is assembly cost and board space. Through-holes require drilling operations, use more PCB material, and limit component density compared to SMT. Lead waste is generated during trimming after soldering. Many modern boards combine both technologies: SMT on one side for signal processing and logic, through-hole on the reverse for power connectors and test points. This hybrid approach balances density, performance, and serviceability.