Surface Mount Technology (SMT) refers to the practice of mounting or soldering components directly on a Printed Circuit Board (PCB). The assemblies created through this process are generally termed as Surface Mounted Devices (SMDs). These boards are different from the traditional Through-Hole (TH) PCBs, where components with long leads and contacts are passed through pre-drilled holes on the PCB, and subsequently soldered. Additionally, SMT allows the use of extremely small components when compared to their TH counterparts. Component sizes of 0.4 × 0.2 mm (0.016 × 0.008 in) have already been standardized and even smaller sizes of 0.25 mm x 0.125 mm already exist.
Though the technology has been around since the 1960s, SMT took off in earnest during the 1980s largely due to the efforts of IBM, and within a short span of time, other companies started investing heavily in SMT too. Today, SMT-based PCBs have all but replaced the conventional TH assemblies.
One of the foremost reasons for the success of SMT circuit boards and SMDs has been the increased circuit density that allows manufacturing of extremely compact devices. The miniaturization of SMDs and similar devices over the years led to higher adoption rates and consequently, the demand skyrocketed. This in turn led to a revolution in the way consumers used electrical and electronic devices and kick-started the process of technology penetration that is so universally present in our lives today. Technologies that emerged later made extensive use of SMT-based circuit boards to generate expeditious improvements in product quality and service. The finest example is perhaps mobile phones which went from being bulky communication devices to sleek, multi-function smartphones we value and love.
From an economic standpoint, the reduction in real estate and weight translates to lower costs in setting up and maintaining production in addition to increasing time efficiency. Moreover, SMT component are able to withstand higher mechanical impacts and shocks and the board itself requires drilling fewer holes (if at all). These factors add up to a highly efficient production line that can be quickly and easily customized to meet current demands and future forecasts.
From technical and design perspectives, the easily automated double sided circuit boards assembly, in addition to the extremely high circuit density, make SMT a clear winner against TH PCBs. Fewer errors in component placement lead to higher quality standards besides reducing the cost associated with rework or re-assembly of entire batches. The contacts of the components also have lower resistance and inductance which minimize unwanted interference and make it easier to calculate, control and if need be, compensate for. The smaller component sizes additionally translate to a marked improvement in electromagnetic compatibility.