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Through-Hole Mounting (THM)
Through-hole mounting is the process by which component leads are placed into drilled holes on a bare PCB. The process was standard practice until the rise of surface mount technology (SMT) in the 1980s, at which time it was expected to completely phase out through-hole. Yet, despite a severe drop in popularity over the years, through-hole technology has proven resilient in the age of SMT, offering a number of advantages and niche applications: namely, reliability.
Through-hole components are best used for high-reliability products that require stronger connections between layers. Whereas SMT components are secured only by solder on the surface of the board, through-hole component leads run through the board, allowing the components to withstand more environmental stress. This is why through-hole technology is commonly used in military and aerospace products that may experience extreme accelerations, collisions, or high temperatures. Through-hole technology is also useful in test and prototyping applications that sometimes require manual adjustments and replacements.
Overall, through-hole’s complete disappearance from PCB assembly is a wide misconception. Barring the above uses for through-hole technology, one should always keep in mind the factors of availability and cost. Not all components are available as SMD packages, and some through-hole components are less expensive.
However, that doesn’t negate that fact that, in a modern assembly facility, through-hole is considered a secondary operation.
Through-hole technology (also spelled "thru-hole"), refers to the 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 (PCB) and soldered to pads on the opposite side either by manual assembly (hand placement) or by the use of automated insertion mount machines.
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 late 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.
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 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 for mass production designs. This is because of difficulties in use with automated component placement machinery, and poorer reliability because of reduced vibration 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.
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 semiconductors 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 and capacitance in wire leads, which would impair circuit function. Ultra-compact designs may also dictate SMT construction, even in the prototype phase of design.