Home » Manufacturing » The Difference Between SMT, SMD, and THT, and their Significance in the PCB Industry

Understanding the assembly processes and different component types is key to building high-quality PCBs.

A printed circuit board assembly (PCBA) is the backbone of all electronic gadgets in the market. It is a finished board with all components mounted using either through hole technology (THT) or surface mount technology (SMT). Understanding the assembly techniques will help you to choose the right process for your PCB design.

Surface mount devices (SMD) have become popular with the growing demand for compact PCBs in the latest tech applications. While SMDs are installed on a PCB using the surface mount assembly method, through-hole parts are soldered to the holes drilled into the circuit board.

In some complex designs, SMDs and through-hole parts may be used together. Such PCBs with mixed components are assembled by using both assembly techniques. THT is the older method and is best suited for harsh environment applications. Boards assembled by the THT method can sustain vibration and pressure effectively as the component leads are inserted in the holes and soldered. SMT process is widely accepted in the PCB industry as it offers multiple advantages like small board size, improved performance, and reduced manufacturing cost.

THT assembly process

The components used in this method can be either axial lead parts or radial lead components. You can find the leads on both sides of the component in an axial lead part, while the leads are on one side only in radial lead parts. Component placement is done manually by experienced staff. The leads should be trimmed and inserted accurately in the holes.

After the component placement, the board is inspected for any mistakes or corrections required. Generally, through-hole parts are wave soldered. Here, the circuit board is exposed to a wave of molten solder followed by coolers to establish robust connections. Rework of through-hole components is easier compared to SMDs.

SMT assembly process

In this method, a design stencil is used as a reference to apply the solder paste on the board. This ensures the correct location and quantity of solder paste for each component to be mounted. A solder paste printer is used to achieve this task.

Components are placed on their respective pads using an automated pick-and-place machine. It also assists in the accurate positioning of the surface mount parts on the PCB. In bulk production, this automated process saves a lot of time.

Further, the PCBs are moved into a reflow oven and the solder paste melts to form the necessary connection. The boards are thoroughly cleaned and inspected for any visual faults before sending for testing.

Surface mount devices

SMDs are electronic components that can be installed directly on a PCB surface. As they do not have leads inserted through the board, they enable component placement on both sides of the PCB. Usually, SMDs are smaller compared to their through-hole counterparts and thus support higher component density. Resistors and capacitors come in tiny SMD packages like 0201, and 01005 which are essential in miniaturized electronics. A popular package type for SMD transistors is Small Outline Transistor (SOT) e.g., SOT-23.

Integrated circuits in SMD packages are available in the following types:

• Small Outline Integrated Circuits (SOIC) – standard IC package

• Small Outline Package (SOP) – in digital logic, analog amplifiers

• Plastic Leaded Chip Carrier (PLCC) – on prototype PCBAs

• Quad Flat Pack (QFP) – in microcontrollers

• Ball Grid Array (BGA) – FPGAs, Processors

• Package On Package (POP) – HDI design, microprocessors

The SMDs are smaller and lighter components compared to through-hole parts. The fine-pitch packages make them the best choice for high-density circuits. Due to the shorter lead lengths, they improve electrical performance. SMDs are suitable for the automated assembly process and hence improve line efficiency.

SMDs in the SMT process

While SMT is the PCB assembly method, SMDs are the component types used in the SMT process.  Various SMD packages can be easily mounted on a PCB using this method. SMDs are cost-effective and available in small form factors. Dense assemblies and mass productions are possible by using SMDs in the PCB design.

The latest design feature like micro vias is compatible with advanced SMD packages. It improves PCB routing by decreasing the hole size and adding flexibility in the placement of the holes. This can save expensive space in HDI boards. SMDs require minimal clearance for soldering. They deliver reliable PCBs at a profitable price.

Benefits of THT and SMT in the PCB Industry

To choose from SMT and THT assembly, you have to consider the end application of the PCB product. The power and performance requirements should be evaluated and accordingly through-hole parts or SMDs should be used in the design.

High-power circuits use large through-hole parts and require THT assembly. PCBs designed for military equipment and avionics are subjected to vibration, pressure, and extreme temperatures. THT assembly can build rugged PCBs for such applications. More often THT assembly is a manual process and hence not suitable for bulk productions. But for prototypes with frequent component changes, THT is more cost-effective than the SMT process. Also, the assembly rework of THT parts can be done manually without the need for expensive equipment.

SMT assembly is more commonly used in the PCB industry due to its reduced PCB manufacturing cost and quick automated assembly. This method supports a compact PCB design suited for medical applications and consumer products. The SMD PCBs offer reliable performance at high operating frequencies and are ideal for communication applications. The ability to assemble high-density circuits makes the SMT technique a perfect choice for miniature electronics.

Conclusion

THT assembly requires the drilling of holes in the PCB which adds to the manufacturing cost. In multilayer PCBs, these holes run through every layer and hence reduce the routing flexibility for your design. PCB substrates used in SMT assembly should withstand high soldering temperatures. Applying a correct amount of solder paste is critical in SMT assembly to avoid any soldering defects. This requires expensive automated apparatus in the assembly line. Thus, THT and SMT methods have both advantages and disadvantages. Understanding the assembly processes and different component types is key to building high-quality PCBs.

About the Author

Ken G works as Sales Engineer at Technotronix. He brings 15 years of experience in PCB sales and technical client services. You can connect with him on Twitter.