What is the SMT Process and Why Should OEMs Care?

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

 

What is SMT?

 

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

 

Surface mount technology (SMT) has been widely adopted since the 1980s due to its automation, which saves both time and costs for manufacturers and customers. In the past, components were connected using hand wiring through holes between the boards, a process that required manual labor and was prone to human error. In contrast, the SMT process is not only automated but also more adaptable to future technological advancements compared to the traditional through-hole method.

 

SMT offers several other advantages, including higher component density, the ability to use smaller components, and better performance under stress. Additionally, SMT allows for components to be mounted on either side of the board, further optimizing space and design flexibility.

 

 

What is the Typical SMT Process?

 

The surface mount technology (SMT) process consists of three main steps: solder paste printing, component placement, and reflow soldering.

 

Solder Paste Printing

 

Solder paste printing is the first step, performed by a machine to ensure precision and speed. During this stage, a printer applies solder paste using a pre-made stencil of the printed circuit board (PCB) and squeegees. The solder paste, typically a mixture of flux and tin, connects the surface mount components (SMC) to the solder pads on the PCB. It's crucial that each pad receives the correct amount of paste to establish a solid connection when the solder is melted in the reflow oven.

 

Quality control is vital during this phase, as any defects in the solder paste printing can lead to issues later in the process. Therefore, the design of the stencil is critical, and the assembly team must ensure that the process is repeatable and stable. To aid this, most solder paste printers can include automatic inspection, and sometimes external machines with 3D technology are used for a more thorough inspection, checking for solder paste volume per pad rather than just the print area.

 

Component Placement

 

After passing inspection, the PCB proceeds to the component placement phase. During this stage, each component is removed from its packaging using a vacuum or gripper nozzle and placed in its programmed location on the PCB. The machines used in this process are highly accurate and capable of placing up to 80,000 components per hour.

 

Once all components are placed, they are inspected to ensure correct placement. This step is crucial because any undetected placement errors can lead to costly and time-consuming rework if the components are soldered incorrectly.

 

Reflow Soldering

 

The final step is reflow soldering. In this phase, the PCB is placed into a reflow soldering machine, often called a reflow oven. The heat converts the solder paste into solder, forming electrical connections between the components and the PCB. Accurate temperature control is essential; too high a temperature can damage components or the assembly, while too low a temperature may prevent proper connections.

 

To achieve optimal results, the PCBs are placed on a conveyor belt and heated gradually through several zones before passing through a cooling zone. The PCBs must remain in each zone for the appropriate time and be completely cooled before handling to prevent warping.

 

After reflow soldering, a final inspection is conducted, typically using a 3D automated optical inspection machine (AOI). This inspection checks the quality of the solder joints and ensures no mistakes were made during the SMT process. Machines are preferred for this inspection because they are faster and more accurate than manual inspection.