Almost all modern electronic equipment has at least one printed circuit board or PCB inside. The PCB is a resin or plastic board, containing copper tracks on its surface on which it is possible to anchor various electronic components. Usually, these components are different types of resistors, capacitors, inductors, transistors, integrated circuits, and many other active and passive components. Once the anchoring is over, the combination of PCB and components is known as a printed circuit board assembly or PCBA.
With the tremendous surge of electronic equipment in almost all spheres of life, there have been continuous improvements in functionality as well. This has resulted in the requirement of a reduction in the sizes of PCBAs, and a choice between the two primary methods of mounting electronic components on the PCB—the Through Hole Technology or THT and the Surface Mount Technology or SMT.
Through Hole Technology
In THT, the printed circuit board requires drilled holes through which the leads of electronic components pass. Before the invention of SMT in the 1980s, THT was the main process for assembling PCBAs. Contrary to the popular belief that THT would become obsolete with the introduction of SMT, the industry continues to use THT because of certain advantages THT exhibits over the latter.
Surface Mount Technology
Components for SMT do not require long leads, as there are no holes in the PCB. Rather, each component has stubs or end caps, and flat pins or solder balls for connecting directly to the pads on the PCB. As the pin count can be high, and the pitch can be very small, components for SMT can be tiny, resulting in very high component density on the PCBA.
Advantages of THT
Fig 3: THT PCB Assembly
With leads of components passing through PCB holes before anchoring, their connection to the board is very strong. Therefore, PCBAs with THT components are capable of withstanding very high levels of mechanical or environmental stress compared to PCBA with SMT.
When it is necessary to expose the PCBA to extreme acceleration, collisions, or high temperatures, boards with THT are more durable than those with SMT are. Therefore, many aerospace and military applications use THT specifically. Moreover, being larger in size, THT components are easier to work with, such as to remove and replace. This advantage makes THT ideal for prototyping, testing, and development.
Disadvantages of THT
Drilling holes into PCBs is both an expensive and a time-consuming process. PCBA with THT components cannot achieve high component density, both on account of the larger size of THT components, and the inability of mounting them on both sides of the PCB.
The presence of a large number of through holes limits the area available for routing in multilayered boards, increasing the number of layers for effective routing. The large number of through holes also makes the PCB mechanically weak and less reliable.
As THT components must rely on wave or manual soldering, THT joints are less reliable compared to the solder joints on SMT components.
Advantages of SMT
Fig 4: SMT PCB Assembly
As boards for SMT do not require holes, it is possible to mount the SMT components on both its sides. With size of SMT components being much smaller, the main benefit of SMT over THT is a substantial increase in component density.
The lack of holes in the PCB also results in faster production throughput, and a substantial decrease in waste, translating into significant savings in cost per PCB. Pick and place machines can mount SMT components at least ten times faster than they can mount THT components.
Reflow ovens and reflow technology for soldering SMT components result in far more reliable solder joints. In combination with rapid mounting of components, PCBAs with SMT perform equally well in applications experiencing shaking and vibrations.
Disadvantages of SMT
If it is necessary to expose the board to high mechanical, temperature, or environmental stress conditions, PCBAs with SMT components alone are at a disadvantage. The industry overcomes this problem by using a suitable mix of SMT and THT components for the PCBA, taking advantage of both the technologies in the process.
It is very difficult to repair or replace components on a board with SMT components.
Which to Use—THT or SMT?
Knowing key differences between THT and SMT processes helps in choosing the most appropriate method for the intended application:
PCBA with SMT components can achieve much higher component densities, as SMT components are smaller, and it is possible to use both sides of the board for mounting them.
The non-requirement of holes allows higher routing densities for boards with SMT components, as a result of which, SMT boards can be much smaller, lighter, faster, and more powerful.
SMT components can have much higher pin counts than compared to what a THT component can. This results in much higher functionality available from an SMT component, and hence from a PCBA with all SMT components. As a result, the economies of scale are in favor of SMT PCBAs, lowering their per-unit cost.
However, SMT requires far more advanced levels of design, production, skill, and technology to implement, as compared to the requirements of the simpler THT. Moreover, setting up machinery and production processes for SMT requires substantially more capital outlay than those required for THT.
Conclusion
At present, both SMT and THT are firmly entrenched processes in the electronics industry, as both have their individual advantages and usefulness. Depending on the application, the designer may decide to use either SMT only, THT only, or a judicious mix of both technologies.
