Through-hole Technology
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electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
, through-hole technology (also spelled "thru-hole") is a manufacturing scheme in which
lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...
s on the components are inserted through holes drilled in
printed circuit board A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a laminated sandwich str ...
s (PCB) and soldered to pads on the opposite side, either by manual assembly (hand placement) or by the use of automated insertion mount machines.


History

Through-hole technology almost completely replaced earlier electronics assembly techniques such as point-to-point construction. From the second generation of computers in the 1950s until surface-mount technology (SMT) became popular in the mid 1980s, every component on a typical PCB was a through-hole component. PCBs initially had tracks printed on one side only, later both sides, then multi-layer boards were in use. Through holes became plated-through holes (PTH) in order for the components to make contact with the required conductive layers. Plated-through holes are no longer required with SMT boards for making the component connections, but are still used for making interconnections between the layers and in this role are more usually called vias.


Leads


Axial and radial leads

Components with wire leads are generally used on through-hole boards. Axial leads protrude from each end of a typically cylindrical or elongated box-shaped component, on the geometrical axis of symmetry. Axial-leaded components resemble wire jumpers in shape, and can be used to span short distances on a board, or even otherwise unsupported through an open space in point-to-point wiring. Axial components do not protrude much above the surface of a board, producing a low-profile or flat configuration when placed "lying down" or parallel to the board. Radial leads project more or less in parallel from the same surface or aspect of a component package, rather than from opposite ends of the package. Originally, radial leads were defined as more-or-less following a
radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
of a cylindrical component (such as a ceramic disk capacitor). Over time, this definition was generalized in contrast to axial leads, and took on its current form. When placed on a board, radial components "stand up" perpendicular, occupying a smaller footprint on sometimes-scarce "board real estate", making them useful in many high-density designs. The parallel leads projecting from a single mounting surface gives radial components an overall "plugin nature", facilitating their use in high-speed automated component insertion ("board-stuffing") machines. When needed, an axial component can be effectively converted into a radial component, by bending one of its leads into a "U" shape so that it ends up close to and parallel with the other lead. Extra insulation with heat-shrink tubing may be used to prevent shorting out on nearby components. Conversely, a radial component can be pressed into service as an axial component by separating its leads as far as possible, and extending them into an overall length-spanning shape. These improvisations are often seen in breadboard or prototype construction, but are
deprecated In several fields, especially computing, deprecation is the discouragement of use of some terminology, feature, design, or practice, typically because it has been superseded or is no longer considered efficient or safe, without completely removing ...
for mass production designs. This is because of difficulties in use with automated component placement machinery, and poorer
reliability Reliability, reliable, or unreliable may refer to: Science, technology, and mathematics Computing * Data reliability (disambiguation), a property of some disk arrays in computer storage * High availability * Reliability (computer networking), ...
because of reduced
vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, suc ...
and mechanical shock resistance in the completed assembly.


Multiple lead devices

For electronic components with two or more leads, for example, diodes, transistors, ICs, or resistor packs, a range of standard-sized semiconductor packages are used, either directly onto the PCB or via a socket.


Characteristics

While through-hole mounting provides strong mechanical bonds when compared to SMT techniques, the additional drilling required makes the boards more expensive to produce. They also limit the available routing area for signal traces on layers immediately below the top layer on multilayer boards since the holes must pass through all layers to the opposite side. To that end, through-hole mounting techniques are now usually reserved for bulkier or heavier components such as electrolytic capacitors or
semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
s in larger packages such as the TO-220 that require the additional mounting strength, or for components such as plug connectors or electromechanical relays that require great strength in support. Design engineers often prefer the larger through-hole rather than surface mount parts when prototyping, because they can be easily used with breadboard sockets. However, high-speed or high-frequency designs may require SMT technology to minimize stray
inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of t ...
and
capacitance Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized a ...
in wire leads, which would impair circuit function. Ultra-compact designs may also dictate SMT construction, even in the prototype phase of design. Through-hole components are ideal for prototyping circuits with
breadboards A breadboard, solderless breadboard, or protoboard is a construction base used to build semi-permanent prototypes of electronic circuits. Unlike a perfboard or stripboard, breadboards do not require soldering or destruction of tracks and are h ...
using microprocessors such as
Arduino Arduino () is an open-source hardware and software company, project, and user community that designs and manufactures single-board microcontrollers and microcontroller kits for building digital devices. Its hardware products are licensed und ...
or PICAXE. These components are large enough to be easy to use and solder by hand.


See also

* Point-to-point construction * Board-to-board connector *
Surface-mount technology Surface-mount technology (SMT), originally called planar mounting, is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referred ...
* Through hole *
Via (electronics) A via (Latin for ''path'' or ''way'') is an electrical connection between copper layers in a printed circuit board. Essentially a via is a small drilled hole that goes through two or more adjacent layers; the hole is plated with copper that fo ...


References


Further reading

* * * *


External links

* {{Wikibooks-inline, Practical Electronics, PCB Layout#Holes, Hole sizes for through-hole parts Chip carriers Printed circuit board manufacturing