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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.

Axial vs. Radial Lead Components

There are two types of through-hole components: axial and radial lead components. Axial leads run through a component in a straight line ("axially"), with each end of the lead wire exiting the component on either end. Both ends are then placed through two separate holes in the board, allowing the component to fit closer, flatter fit. Radial lead components, on the other hand, protrude from the board, as its leads are located on one side of the component.

Both through-hole component types are "twin" lead components, and both have their distinct advantages. While axial lead components are used for their snugness to the board, radial leads occupy less surface area, making them better for high density boards. Generally, axial lead configuration may come in the form of carbon resistors, electrolytic capacitors, fuses, and light-emitting diodes (LEDs). Radial lead components are available as ceramic disk capacitors.  

Advantages: THM provides stronger mechanical bonds than SMT, making through-hole ideal for components that might undergo mechanical stress, such as connectors or transformers. Good for test and prototyping.

Disadvantages: On the bare PCB side, THM requires the drilling holes, which is expensive and time consuming. THM also limits the available routing area on any multilayer boards, because the drilled holes must pass through all the PCB’s layers. On the assembly side, component placement rates for THM are a fraction of surface mount placement rates, making THM prohibitively expensive. Further, THM requires the use of wave, selective, or hand-soldering techniques, which are much less reliable and repeatable than reflow ovens used for surface mount. Most of all, through-hole technology requires soldering on both sides of the board, as opposed to surface-mounts, which only -- for the most part -- require attention to one side of the board.

Surface Mount Technology (SMT)

SMT the process by which components are mounted directly onto the surface of the PCB. Known originally as “planar mounting,” the method was developed in the 1960s and has grown increasingly popular since the 1980s. Nowadays, virtually all electronic hardware is manufactured using SMT. It has become essential to PCB design and manufacturing, having improved the quality and performance of PCBs overall, and has reduced the costs of processing and handling greatly.  

The key differences between SMT and through-hole mounting are (a) SMT does not require holes to be drilled through a PCB, (b) SMT components are much smaller, and (c) SMT components can be mounted on both side of the board. The ability to fit a high number of small components on a PCB has allowed for much denser, higher performing, and smaller PCBs.

Through-hole component leads, which run through the board and connect a board’s layers, have been replaced by "vias" -- small components which allow a conductive connection between the different layers of a PCB, and which essentially act as through-hole leads. Some surface mount components like BGAs are higher performing components with shorter leads and more interconnection pins that allow for higher speeds.

Nomenclature

There are perhaps too many terms that describe different aspects of surface mount technology. Here’s what they mean:

SMA (surface-mount assembly) – a build or module assembled using SMT.

SMC (surface-mount components) – components for SMT.

SMD (surface-mount devices) – active, passive, and electromechanical components.

SME (surface-mount equipment) – machines used for SMT.

SMP (surface mount packages) – SMD case forms.

SMT (surface-technology) – the act and method of assembling and mounting electronic technology.