Editing Printed circuit board (section)

=== Component mounting ===
[[File:Resistors (1).jpg|thumb|Through-hole (leaded) resistors]]
[[File:MOS6581 chtaube061229.jpg|thumb|Through-hole devices mounted on the circuit board of a mid-1980s [[Commodore 64]] home computer]]
[[File:Box of 02in pcb bits.jpg|thumb|A box of [[drill bit]]s used for making holes in printed circuit boards. While tungsten-carbide bits are very hard, they eventually wear out or break. Drilling is a considerable part of the cost of a through-hole printed circuit board.]]
[[File:Surface Mount Components.jpg|thumb|Surface mount components, including resistors, [[transistor]]s and an integrated circuit]]
[[File:Mouse printed circuit board both sides IMG 0959a.JPG|thumb|A PCB in a [[computer mouse]]: the component side (left) and the printed side (right)]]

"Through hole" components are mounted by their wire leads passing through the board and soldered to traces on the other side. "Surface mount" components are attached by their leads to copper traces on the same side of the board.  A board may use both methods for mounting components. PCBs with only through-hole mounted components are now uncommon. Surface mounting is used for [[transistor]]s, [[diode]]s, [[Integrated circuit|IC chip]]s, [[resistor]]s, and capacitors. Through-hole mounting may be used for some large components such as [[electrolytic capacitor]]s and connectors.

The first PCBs used [[through-hole technology]], mounting electronic components by lead inserted through holes on one side of the board and soldered onto copper traces on the other side. Boards may be single-sided, with an unplated component side, or more compact double-sided boards, with components soldered on both sides.  Horizontal installation of through-hole parts with two axial leads (such as resistors, capacitors, and diodes) is done by bending the leads 90 degrees in the same direction, inserting the part in the board (often bending leads located on the back of the board in opposite directions to improve the part's mechanical strength), soldering the leads, and trimming off the ends. Leads may be [[Soldering|soldered]] either manually or by a [[wave soldering]] machine.<ref>{{cite book |first= |last= |chapter=Electronic Packaging:Solder Mounting Technologies |chapter-url= |editor-first=K.H. |editor-last=Buschow |title=Encyclopedia of Materials: Science and Technology |publisher=Elsevier |location= |date=2001 |isbn=0-08-043152-6 |pages=2708–9 |url=https://books.google.com/books?id=TyVVAAAAMAAJ }}</ref>

[[Surface-mount technology]] emerged in the 1960s, gained momentum in the early 1980s, and became widely used by the mid-1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the PCB surface, instead of wire leads to pass through holes. Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies with much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labor costs and greatly increasing production rates compared with through-hole circuit boards. Components can be supplied mounted on carrier tapes. Surface mount components can be about one-quarter to one-tenth of the size and weight of through-hole components, and passive components much cheaper. However, prices of semiconductor [[surface mount device]]s (SMDs) are determined more by the chip itself than the package, with little price advantage over larger packages, and some wire-ended components, such as [[1N4148 signal diode|1N4148]] small-signal switch diodes, are actually significantly cheaper than SMD equivalents.