Though surface mount technology has become the main mounting method in PCB manufacturing, there are still some components that are not suitable in SMT assembly. Then SMT assembly and THT assembly need to be applied on the same board. This combination of assembly technologies is called a mixed assembly, which does not use solder paste during the manufacturing process.

Most of the components are welded on the board in surface mount configuration, but for some certain components that are not available in the SMT process, then mixed PCB assembly is necessary. The comparison between SMT assembly and THT assembly First, SMT components are welded on the board through the reflow machine, and the process is fully automated. While THT assembly needs pre-drill holes on the board and using leads to connect the components and circuits, and both wave-soldered and hand soldered are conventional methods in the THT assembly process.

Secondly, the PCBs with surface mount devices (SMDs) can be reflow or wave soldered, but the PCBs with through-hole components only can be wave-soldered. Therefore, there are more steps in the assembly process if both SMT components and THT components are used on the board. Usually, the SMT process is performed first and then THT assembly.

Thirdly, Advanced and precision machines allow SMT assembly is achieved with high accuracy and fast speed. Those small-sized and thin components can be placed on the board precisely, making SMT is more suitable in high-density and small size PCB applications. And THT is preferred by those components with large size and high-reliability requirements because it provides more robust connections than SMT components.

All in all, SMT assemblies feature with high productivity, high precision, lightweight, and less cost. SMT is more economical and faster for mass production. THT assemblies usually are highly reliable, high-stress tolerance, heavier, and more expensive. THT is the preferred assembly method for small quantity PCBs and prototyping fabrication.

The applications of mixed assembly Mixed assembly is usually used in applications that require a combination of through-hole components and SMT components. You can find mixed technology in the following applications:

CPUs
Smartphone accessories
Communication hardware
Server boards
LED lighting products
Video processing
Industrial controller assemblies