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
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:
LED lighting products
Industrial controller assemblies