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When you need to design high-reliability products that require sturdier connections between layers, through-hole components are the best option. SMT components are tightened only by solder on the surface of the board, conversely, leads of through-hole components bound through all layers, allowing connections to endure the environmental strain. In that regard, through-hole technology is commonly utilized in military and aerospace technologies that may experience extreme conditions: accelerations, collisions, or high temperatures. Through-hole technology is also beneficial in test and prototyping applications that sometimes need improvements and replacements by hand.
There are two types of through-hole components by lead connections - axial and radial lead components. Axial leads run through a component in a straight line (axially), with each end of the lead wire attached to the component on either end. On the other hand, radial lead components protrude from the board, as its leads are located on one side of the component.
As discussed, axial leads protrude from each end of a typical 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 PCB (printed circuit board) or even otherwise unsupported connections through an open space in point-to-point wiring. Axial components do not protrude much above the surface of a PCB, producing a low-profile or flat configuration when placed parallel to the board. In that regard, axial-leaded components allow designers to create devices fit to narrow spaces with ease.
Radial leads outline more or less in parallel from the same surface or aspect of a component container, rather than from opposite ends of the package. Originally, radial leads were defined as more-or-less following a radius of a cylindrical segment (such as a ceramic disk capacitor). Over time, this definition was generalized in contrast to axial leads and took on its prevailing form. When placed on a board, radial components stand up perpendicular, occupying a smaller footprint than their counter-components, making them useful in many high-density projects. The parallel leads projecting from a single mounting surface provides radial components flexibility and mobility, promoting their use in high-speed automated component insertion machines in manufacturing.
Surface mount products have a tendency to be smaller than an equivalent through-hole component. However, this does not necessarily mean the cost of a surface mount component is always cheaper simply because fewer raw materials are used in manufacturing these parts. Surface mount components may cost a similar price as an equivalent through-hole component in features. But, once automated assembly costs per component are considered, the total cost per surface mount component tends to be cheaper than a through-hole component with the same component values, power/voltage ratings, and tolerances. This difference arises because placing through-hole components requires drilling holes in your PCB, which incurs tooling and assembling costs. Conversely, drilling is not required with surface mount components, which accounts for the cost difference. However, there are reasons why through-hole components are still in the market and not labeled as "out of production" yet.
First of all, through-hole mounting technology is exceptional for prototyping and testing. Even before designing a PCB, through-hole technology allows designers to test their designs on a breadboard. And, in addition to prototyping and testing, through-hole components have firm physical bonds to the board as they are soldered from both the top and bottom. In that regard, through-hole mounting technology is very durable and efficient. Through-hole components also have high-temperature tolerance in comparison to surface mount components.
In detail, through-hole components have an advantage as compared to SMDs (surface-mount devices) because through-hole components are mechanically sturdier than most SMDs. Their leads run through the board, allowing them to withstand environmental stress better than SMDs. For instance, high-power or high-voltage parts (such as transformers) are best secured via through-hole technology to ensure they can withstand mechanical stress and high heat. Also, through-hole components are perfect for products that may experience extreme accelerations, high temperatures, or collisions, which is why they are commonly used in the automobile, aerospace, and military industries. And, through-hole components are the best option for testing and prototyping because they can be manually adjusted or replaced with relative ease. Nevertheless, they have some disadvantages too in comparison to surface mount components.
The shortcomings of through-hole components are that they need holes to be drilled in the board, which can be high-priced and time-consuming. Since the holes go through all of the layers, they also restrict the available routing area on multilayer boards. And, the soldering procedures and methods used on through-hole components, while manufacturing, are less reliable and repeatable than those used for SMDs.