SMD, THD, THT, SMT, THM, SOIC, QFN, the electronics nomenclature can be unnecessarily confusing for the budding electronics hobbyist. But the terminology and technology are a lot simpler than it may first seem. In this article, we will briefly introduce these methods and cover when and why they are used.
SM stands for Surface Mount and TH stands for Through-Hole, which refers to two different methods of mounting components onto a printed circuit board. The T, D, M, C or A that follows the SM or TH simply stand for Technology, Device, Mounting, Components or Assembly respectively, and they tend to be used quite loosely. For example, an SMD is made using SMT, since it uses SMC in SMA processes. A board which utilizes both SM and TH components is referred to as a mixed device.
Initially, all electronic components were through-hole mounted. Through-hole components have metal leads, and these leads are fed through-plated holes in the circuit board. The ends of the leads are then soldered onto pads on the opposite side or solder side. The drill holes and their pads that make up plated through holes can take up valuable real estate on the surface of the PCB, more so in multilayer boards, since the drill holes take up space in all layers. The increasing space limitations gave rise to Surface mount technology, and with it, a new era of more compact and portable electronic devices.
Surface mount components may or may not have leads, but most importantly, they are designed to be soldered onto the surface of the boards directly on the same side as the component body. Thus, both sides of the boards can be easily utilized for mounting, and there is no need for plated drill holes. For the purpose of connecting traces in the circuit layers, one may use vias instead, which are structurally the same as plated through-holes but much smaller. They can even be designed to only connect, and take up space, in specific layers. Therefore, the absence of plated through-holes itself can surmount to significant space savings in SMD devices.
In addition, the size and footprint of SM components can be miniaturized compared to their TH counterparts. The leads may be eliminated completely in favor of contact terminals on the ends of the component body. As a result, many components such as resistors, capacitors, inductors and even LEDs can now be found in SMD packages down to the size of a grain of sand (0201/0603 package).
However, the miniaturization impacts the reliability of SMD devices. Due to complications with soldering and the extra precision required in producing such boards, an SMD assembly line will typically have a large number of defects, and these can be difficult to repair. The final boards will also be delicate and must be handled with extra care. A board assembled with TH components, on the other hand, is very rugged as a result of the large solder joints that extend throughout the width of the board itself. This property is desirable in military or industrial applications that may exert the device to strong impacts and vibrations.
Cost is another key consideration to bear in mind. SM components are typically cheaper than TH components, but the SMA process is much more costly than THA. Surface Mount assembly requires some large and highly advanced equipment and materials such as pick and place machines, reflow ovens and of course, a custom stencil. Whereas, through-hole assembly requires some solder, a soldering iron and a pair of sturdy hands. Although hand soldering a few SM components is possible with a few tools, it can be a fidgety process, especially if you are handling many components that are barely visible to the naked eye. Therefore, it makes sense to use SMT in large-scale productions that have a high degree of automation and speed and to reserve THT for small-scale projects and prototypes that may require manual adjustments.
From a hobbyists perspective, one will most definitely have begun with through-hole components when learning the ropes. And the move to SMT may seem daunting or may not even be necessary. The real advantages of SMT lie in making smaller, more compact devices possible and enabling mass productions to be almost entirely automated. But for small scales production and home projects it can be difficult to realize the cost and speed benefits of SMT. So it is best to stick with the through hole boards unless really necessary.
SMT’s primary advantage is the huge component density and volume reduction that can be achieved using SMT components. The strive for smaller, more compact devices has pushed component form factors to their limits, and in the modern age of electronics, THT has been rendered “unfashionable.” But despite early predictions of its demise, through-hole technology and components are still valuable, and in the long term, it seems it is here to stay.