When designing printed circuit board assemblies (PCBAs), many factors must be considered. Designers must understand and account for size, materials, layout, and connectivity. The quality of a PCB depends largely on the integrity of contact points. Due to the vast number of options regarding connectivity, various forms of soldering have developed and evolved. The newest step in this evolution is Surface Mount Technology (SMT,) which, consequently, created Surface Mount Devices (SMDs.) While the two terms are closely related, there are key differences as described below.
Surface Mount Technology (SMT)
Soldering connections on printed circuit boards has conventionally consisted of through-hole mounting or Insertion Mount Technology (IMT). This form of mounting simply means that connections are created and soldered through holes in the PCB. A new process developed called Surface Mount Technology. In this modality, connections are made not through holes in the PCB, but on devices attached to the board. SMT refers to the concept and process by which a PCB obtains mounting other than the through-hole variety. While SMT has become more common than conventional through-hole technology, it should be noted that not all connection components are suitable for surface mounting and are, therefore, mounted the conventional way.
Surface Mount Device (SMD)
SMT developed, in tandem, surface mount devices. Simply stated, a surface mount device is a component attached to a PCB that allows for a connector to be soldered to the board. If a hand were to be run along the top of the device, the SMDs could be felt to be physically raised from the board. SMDs are most commonly attached to the PCB using a method called SMD reflow soldering. The ideal scenario for achieving this method is by use of an oven, but a soldering iron can be effective for small-scale or at-home projects. The term Surface Mount Device refers to an item or component which can be touched and/or physically handled.
As long as PC boards require connectivity, they will also need a method of attaching the components. As conventional mounting technology became largely replaced by surface mount technology, the popular term SMT developed along with the term SMD. While the two work hand-in-hand, the most important takeaway is that surface mount technology is a concept or practice of mounting in a manner other than the through-hole predecessor; that is, on top of the device using an intermediary device. The device itself is called a surface mounting device and can be moved, seen, and touched.
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