With the increasing trend of miniaturization, small surface mount components are widely adopted to meet this demand. To adapt to this shift, assembly technology has transitioned from traditional THT to SMT. In traditional THT assembly, it is necessary to drill holes on the PCB, insert component pins, and perform soldering. While robust solder joints can be achieved, the components are larger, and the production process is relatively cumbersome. In contrast, SMT assembly has achieved success by introducing surface mount devices. This technology eliminates the need for pins and mounts the component's pads onto the surface of the printed circuit board before soldering. This subtle change allows for miniaturization and compactness of components on the board, enabling designers to create denser and more complex PCBs. In addition to the advantage of improving circuit board space utilization, the SMT process also significantly reduces manufacturing costs through automated processes. This article will focus on the SMT process flow, covering specific process steps and providing necessary precautions to help you implement this process more effectively.

　　 Step 1: Familiarize Yourself with the Equipment

　　Unlike through-hole assembly, SMT typically uses surface mount devices, as shown in the figure below. These components are small, with many but shorter pins, making manual assembly challenging.

　　Therefore, the use of automated equipment is crucial for production. Here are some key pieces of equipment in the FS Technology SMT production line:

　　Board Loader: Located at the beginning of the production line, it stores PCBs that are about to enter the production line.

　　Conveyor Belt: Acts as a bridge connecting various equipment in the production line, used for PCB transfer.

　　Stencil and Squeegee: Used for solder paste printing, ensuring accurate application to designated areas of the PCB.

　　SPI: Detects the quality of solder paste printing, identifying potential problems.

　　Pick and Place Robot: Accurately picks up components and places them on the PCB.

　　Reflow Oven: High-temperature equipment used for soldering.

　　AOI: Performs visual inspection, identifying any issues related to assembly and solder joints.

　　 Step 2: Preparation

　　It's crucial to understand that this is an automated process. How does the equipment work? What components are used? Where are the components placed on the PCB? What is the soldering temperature? This is a meticulous process requiring careful circuit design and proper machine setup before assembly begins. Therefore, some necessary preparations are required before formally starting the assembly process.

　　 Material Preparation

　　After the audit and confirmation of cooperation, the manufacturer begins preparing the materials required for the project, including:

　　Component Procurement: Procurement of components from qualified suppliers according to the BOM document.

　　Stencil Fabrication: Construction of PCB stencils based on your design documents, typically using laser technology to ensure quality.

　　Solder Paste Preparation: Solder paste is transported in a refrigerated manner and stored in a storage box at 0° to 30°C for 48 hours.

　 　Step 3: Component Placement

　　Solder paste is a mixture of tin and flux that, when heated, firmly bonds pins and components. In this stage, the solder paste is evenly applied to the stencil, then passed over the stencil surface with a squeegee. Under pressure, the solder paste drops through the holes in the stencil onto the designated locations on the PCB. The correct use of solder paste is crucial for the entire SMT process, including solder paste type, squeegee parameter settings, etc. In addition to traditional stencil printing, jet printing technology can also be used in the SMT assembly process, which is more advantageous for large-scale PCB assembly.

　 　b. Solder Paste Inspection

　　After solder paste printing, the equipment integrated into the production line performs SPI inspection. This equipment consists of a 3D scanning system and an AOI unit, capable of capturing surface images of the PCB. The light source provides the necessary conditions for detection to ensure correct capture by the camera. Specific details can be observed on the screen. By comparing with the "golden board," quantity deviations, excess, or misalignment can be immediately investigated and a new solder paste printing performed if necessary.

　　 c. Pick and Place

　　First, the manufacturer loads the operating program of the pick-and-place robot, including component coordinates, pick-up sequence, placement location, etc. Typically, the robot is equipped with an advanced vision system capable of accurately identifying and locating these positions.

　　Then, the pick-and-place robot uses a vacuum nozzle on its arm to pick up components from the feeder, and uses suction to adhere them. The robot uses a multi-axis motion system to move in three-dimensional space and places the components in the designated locations.

　 　Step 4: Soldering

　　The SMT PCB manufacturing process uses reflow soldering technology, where the machine heats the entire assembly to a specified temperature to form the crucial electrical solder joints between the surface mount devices and the printed circuit board. First, the assembled SMT board is conveyed to the reflow oven.

　　The oven temperature gradually increases, with a standard heating rate of 1.0 to 2.0 degrees Celsius. Do not allow the temperature to rise too quickly. After about 90 seconds, the temperature inside the oven reaches 140 to 160 degrees Celsius.

　　Subsequently, the reflow soldering process enters the tin-dipping reflow zone, at which point the oven temperature rises to the pyramidal section, between 210-230°C, reaching its peak, the solder paste completely melts, and fully mixes with the component pins.

　　Afterwards, the oven temperature gradually decreases, entering the final cooling stage. The solder paste transitions from a liquid to a solid state, ultimately forming robust solder joints.

　 　Step 5: Finishing Work

　　After soldering, the basic SMT assembly process is essentially complete, but to ensure the quality of the PCBA board, some finishing work is still needed.

　　One of these steps is cleaning. The solder paste and flux used in the soldering process contain compounds that, if left on the board surface for a long time, may cause corrosion when exposed to moisture and current. The preferred cleaner used in this process is isopropyl alcohol.

　　Next is the inspection phase, including AOI inspection and functional testing. In addition, customized PCB testing services may be required depending on the application and specific requirements.

　　 Conclusion

　　The complete SMT PCB assembly process is outlined above, emphasizing the importance of accurate design documents, precise temperature control during reflow soldering, and the integration of inspection systems in the production line. These aspects play a decisive role in achieving accurate SMT processes.