through-hole technology
Through-hole technology is the older, sturdier way of mounting parts on a board: each component has wire legs (leads) that poke through drilled holes in the PCB and are soldered on the opposite side. Picture sewing a button onto cloth — the thread goes through holes and is tied off on the back. A through-hole resistor or capacitor has long wire leads you bend, push through the board's holes, and solder underneath. It is the classic look of older electronics and of hobbyist breadboard-style builds made permanent.
Mechanically this is a very strong way to mount a part. The lead passes all the way through a plated hole and is soldered, so the joint is anchored through the board, not just stuck to its surface. That makes through-hole the natural choice for parts that take physical stress — connectors that get plugged and unplugged, big electrolytic capacitors, switches, anything a human hand tugs on. It is also far friendlier to hand-soldering and to learning, because the parts are large, clearly labelled, and easy to grip and rework with a basic iron.
Why it matters: through-hole's strength and ease are real, but so are its costs. The leads and holes take up a lot of board space, holes block routing on inner layers, and machine-stuffing leaded parts is slower and pricier than placing surface-mount parts. The lead also adds parasitic inductance, which hurts high-speed and high-frequency performance. So modern boards are mostly surface-mount, with through-hole reserved for connectors, power parts, and anything that must physically endure. The two technologies happily coexist on one board.
A power jack and a chunky electrolytic capacitor are mounted through-hole — their leads pass through the board and solder on the back, so they survive being knocked and tugged — while the tiny resistors and the microcontroller beside them are surface-mount.
Strong and easy to hand-solder, but bulky — kept for connectors and parts under stress.
Through-hole is not obsolete, but it is no longer the default. Using leaded parts everywhere wastes space and money on a modern board; reach for through-hole specifically where mechanical strength or hand-soldering matters, and use surface-mount for the rest.