What’s the Difference Between Surface-Mount, Through-Hole, and Mixed Assembly?

What you’ll learn:

The different types of PCB assembly methods like surface mount, through-hole, and mixed assembly.
How these PCB assembly processes differ from each other.

The performance of a printed circuit board (PCB) depends on a high-quality fabrication process with the right choice of components and the method of assembly. Based on the circuit complexity and electronic parts included in the design, you can select the precise assembly type for your PCB production.

PCB assembly methods include through-hole technology (THT), surface-mount technology (SMT), and mixed technology. Some PCB assemblies may require just one technology, while others may need more than one, preferably a combination of these specified assembly technologies. Each assembly technology has its process and is different from the other types.

Selecting the right assembly method depends on various factors like end application, cost budget, size requirements, connection density, product-to-market timelines, etc. SMT assembly is the most commonly used assembly process due to its cost-effective and compact PCB assembly capabilities.

In applications demanding high-reliability and harsh working environments like military and aerospace, the THT method of PCB assembly is recommended. Some applications like communication hardware or industrial control circuits would require a combination of both THT and SMT assembly processes. They fall under a mixed assembly category.

Before going ahead with the assembly process, the bare PCBs should be verified for any defects and the manufacturer must ensure that the PCB design follows DFM guidelines. The two types of electronic components used in the PCB assembly are through-hole and surface-mount parts. Based on the component types, PCBs can be assembled using THT or SMT assembly. The steps involved in each assembly process are explained below.

Through-Hole Assembly

In this process, the through-hole parts are inserted in holes drilled in the PCB. The leads are soldered to the pads using molten solder either by manual soldering or wave-soldering methods. The three steps required in this assembly process include:

Component placement: Technicians place the through-hole parts precisely in the designated positions as per the PCB design files. To achieve high performance, it’s recommended to adhere to the regulatory standards of the through-hole mounting process.
Inspection and correction: Automatic inspection techniques are employed to verify the component placement with a standard reference board. If any errors are identified, then the necessary corrections will be done before sending the board for soldering.
Soldering of through-hole parts: There are two possible options—manual soldering and wave soldering. Manual soldering is feasible for low-volume and simple circuit PCBs, while wave soldering involves an automated process. The PCB with through-hole parts placed and inspected is passed through an oven on a conveyer belt. The molten solder is applied to the PCB components at a high temperature from the bottom surface of the board. This is typically used in single-sided PCBs.

Surface-Mount Assembly

The SMT process is an efficient assembly method as it can be completely automated. The surface-mount components, which are smaller than their through-hole counterparts, are mounted using this technique. SMT assembly requires less time compared to the THT method and is highly cost-effective. The three steps of the SMT assembly process are:

Solder-paste application: A stainless-steel stencil is used for the uniform application of the solder paste on the pads. The stencil is reused for multiple batches of PCB assembly. The solder-paste squeezer can be configured to apply the right amount of the solder paste. Furthermore, the PCB will be inspected for uniform solder-paste application before going for component placement.
Component placement: An automated pick-and-place machine is used to place the surface-mount parts on the PCB. The x-y coordinates of all components are fed to the automated machine. With this available information, the pick-and-place machine places the surface-mount parts accurately on the position where solder pastes are already applied.
Reflow soldering: Once the component placement is completed, the PCB is passed through a long furnace at approximately 260°C. The high temperature melts the applied solder paste and the components are connected to the pads on the PCB. Further PCBs are allowed to cool down; the joints are strengthened creating a permanent connection between the SMT parts and the PCB.

Mixed Assembly

With the latest technologies, PCB complexity has grown tremendously. PCBs have shrunk, and the connection density has multiplied. The latest PCB designs include both through-hole and surface-mount components in their circuits; hence the need for a mixed-assembly method. This is a complex process—to achieve a working PCB assembly, one must carefully consider the applied temperature on the board surfaces.

For assembly guidelines of all these types, refer to the IPC-610A and IPC-J-STD-001 standards. After the PCB is assembled using any of the above methods, it’s inspected and tested elaborately for functional validation. The “tested OK” PCBs are cleaned and packed for shipment. Knowing the merits and demerits of the THT and SMT assembly process can assist in choosing the right method for your PCB assembly.

SMT vs. THT

Though the SMT assembly process is more popular, certain applications still require THT assembly, especially products that demand higher consistency and robustness. Components that are inserted in the PCB holes and soldered can establish a stronger bond compared to surface-mount soldering. Surface-mount packages aren’t available yet for some components. But the large size of the through-hole components consumes more space on the PCB and hence increases the board size.

SMT PCBs are smaller in size and light in weight. They support compact, high-density circuits and thus are cost-effective. The completely automated process speeds up the PCB assembly, providing quicker product-to-market timelines. The disadvantage of SMT assembly is that the surface-mount parts are sensitive to high temperatures and are susceptible to vibrations and environmental stress.

PCBs involving transformers, electrolytic capacitors, through-hole connectors, etc., will require a THT assembly process. Aerospace and military applications that are exposed to harsh environments prefer the THT assembly method. In applications like medical devices, telecom equipment, and commercial gadgets, where lightweight and miniaturization are the essential features, SMT assembly is the best option for PCB production.

For entrepreneurs or small businesses planning for a PCB product, it may not be feasible to invest in huge inventory, qualified review teams, or expensive test setups. There’s a quick and cost-effective assembly process known as Turnkey PCB assembly that offers a ready-made PCB assembly solution. Many contract manufacturers (CMs) provide this one-stop PCB assembly facility with seamless management of the complete assembly process. CMs can assist in circuit design, components procurement, functional testing, product shipping, after-sales support, etc., along with PCB fabrication and assembly services.