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thru-hole

A circuit board packaging technique in which the leads (pins) on discrete components and chips are inserted through holes in the printed circuit board and soldered from beneath. Until the late 1980s and prior to surface mount technology, all devices on circuit boards were thru-hole. Although chips are mostly surface mounted these days, some chips are still thru-hole, and most discrete devices such as resistors and capacitors are thru-hole.

Thru-hole devices have a strong bond with the board but require an extra drilling step. They also eliminate the board real estate underneath from being used for other layers. See surface mount.




Through the Board
In this circuit board from an LED table clock made in 2005, not only the discrete components, but the chip, are thru-hole devices.






Thru-Hole Vs. Surface Mount
The thru-hole chip is not only thicker than the surface mount chip, but it eliminates the real estate underneath because the pins stick through. The board designer must route any traces from other layers around that part of the board.






surface mount

A circuit board packaging technique in which the leads (pins) on the chips and components are soldered on top of the board. Boards can be made faster because surface mount technology (SMT) eliminates drilling holes for thru-hole devices, in which the pins go through the board and are soldered underneath.

Surface mount also allows the real estate under the device to be used for other layers by the board designer. In addition, the boards can be smaller because the chips are smaller; for example, SMT packages such as BGA have no leads extending out from the sides (see BGA). Contrast with socket mount and thru-hole.




Surface Mount Vs. Thru-Hole
Most chips are surface mounted to the circuit board, but some are thru-hole, in which the pins go through the board and are soldered underneath. With surface mounted chips and no holes drilled through the board, the real estate under the device can be used by the circuit board designer for other board layers.






Gull-Wing Surface Mounts
These are examples of the common gull-wing style of leads on surface mount chips. The leads extend slightly out from the package, then down and out again.






Discrete and Surface Mounted
Although most discrete resistors are thru-hole devices, these resistors are surface mounted to save space as well as time making the board.






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