Understanding the SMT Process and Its Importance for OEMs

Original Equipment Manufacturers (OEMs) are organizations that manufacture or sell products to another company. This company then uses these components as part of a wider process to create products for their customers under their own branding. A significant part of creating products to sell to other companies involves the SMT process.

What is SMT?

Surface mount technology (SMT) is a process where components are mounted onto the surface of a printed circuit board. The components are specifically designed to be directly mounted, rather than hardwired, onto the circuit board, which is the standard for most electronics.

Surface mount technology has been widely used since the 1980s. This widespread adoption is due to the SMT process being automated, which saves time and costs for both the manufacturer and the customer. Previously, methods involved hand wiring through holes between the boards to connect components. This manual labor introduced risks of human error. The SMT process, however, is more adaptable to future technological advances than the through-hole wire method.

Other advantages of surface mount technology include higher component density and smaller components that offer better performance under pressure. Additionally, SMT allows components to be mounted on either side of the board.

What is the typical SMT process?

The SMT process consists of three main steps: solder paste printing, component placing, and reflow soldering.

Solder Paste Printing

The solder paste printing process is performed by a machine to ensure accuracy and speed.

In this phase, a printer applies solder paste using a pre-made stencil of the PCB and squeegees. This solder paste, usually a mixture of flux and tin, is used to connect the SMC and solder pads on the PCB.

Each pad must be covered with the correct amount of paste to establish a connection when the solder melts in the reflow oven. Controlling the quality of the solder paste printing process is crucial. Undetected printing defects at this stage can lead to other defects further down the line. Therefore, the stencil's design is key, and the assembly team must ensure the process is repeatable and stable. Fortunately, most solder paste printers offer automatic inspection options to smooth the process.

However, external machines are sometimes used to assess the printing quality. These solder printer inspection machines use 3D technology for a more thorough inspection, checking for solder paste volume per pad, not just the print area.

Component Placement

Once the PCB passes inspection, it moves to the component placement phase of the SMT assembly process.

During this phase, each component to be mounted on the PCB is removed from its packaging using a vacuum or a gripper nozzle. A machine then places the component in its programmed location. These machines are highly accurate and incredibly quick, with some advanced models placing 80,000 individual components every hour.

After placing all individual components on the PCB, an inspection ensures correct placement. This step is crucial because undetected placement errors, if soldered, can lead to high volumes of rework, which is both costly and time-consuming.

Reflow Soldering

After passing inspection, the placed components move to the reflow soldering phase. During this part of the SMT process, the PCB is placed into a reflow soldering machine (commonly referred to as reflow ovens).

In this machine, all electrical solder connections between the components and the PCB are formed using heat. The previously applied solder paste is converted into solder. Accuracy is vital at this stage because overheating can damage the parts or assembly, while underheating may prevent a connection from being established.

To ensure the best results, all PCBs within the soldering machine are placed on a conveyor belt. They are gradually heated in a series of zones before being passed through a cooling zone.

To avoid joint defects, the PCBs must remain in each zone for the correct period and be completely cooled before handling or moving. If not, they may warp.

After passing through the reflow solder machine, the PCBs undergo a final inspection. This inspection is usually performed by a 3D automated optical inspection machine (AOI) to ensure the solder joint quality meets expectations and no mistakes were made during the SMT process. Machines are preferred for this task as they are quicker and more accurate than humans.