In recent years, semiconductor packaging has evolved with an increased demand for greater functionality, smaller size, and added utility. A modern PCBA design has two main methods for mounting components onto a PCB: Through-Hole Mounting and Surface Mounting.

Through-Hole Mounting (THM):

Through-hole mounting is the process by which component leads are placed into holes on 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: 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.

THM Advantages:

1) Stronger mechanical bonds than SMT;

2) Good for test and prototyping.

Surface Mount Technology (SMT):

SMT is 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.

SMT Advantages:

1) Smaller PCB size, higher component density

2) Lower cost and faster production time.

3) Fewer holes need to be drilled

4) Better EMC performance

5) Better mechanical performance under shake and vibration conditions

Some relative definitions:

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.