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What is an SMT production line? SMT assembly line and manufacturing process
Assembled PCBs enter the cleaning stage of the SMT production line to remove solder residue, such as flux and any other substances harmful to humans. This process is completed using a cleaning agent. However, this process is not required if a no-clean soldering technology is used.
With SMT production lines becoming increasingly popular in the tech world, many involved in the high-tech electronics industry must have heard of it. Its improved quality and increased manufacturing automation have largely replaced through-hole technology (THT) and grown to become the dominant PCB assembly line.
In the past, manufacturers had to connect electronic components to printed circuit boards via holes using wires. This was complex and time-consuming. As components became smaller, electronics needed a significant advancement to automate assembly lines and improve precision. The SMT assembly line was born. It completely eliminated the need for wires and allowed smaller components to be mounted onto PCBs.
In this article, we will guide you through understanding exactly what an SMT production line is, the key processes and equipment in an SMT production line, the types of SMT production line layouts, and the advantages of SMT production lines over traditional PCB assembly lines.
What is an SMT Production Line?
SMT is an abbreviation for Surface Mount Technology, referring to the process of mounting electronic components such as resistors, capacitors, transistors, and integrated circuits onto the surface of an electronic product's printed circuit board (PCB). This technology was initially invented by IBM in 1960 and subsequently widely used in high-tech electronic PCB assembly. The electronic components mounted on the SMT assembly line are known as Surface Mount Devices (SMDs).
Surface mount components come in several forms. One is the single-sided form, meaning only single-sided PCBs need to be assembled. However, if we need to assemble SMCs or surface mount devices on both sides of the PCB, then a dual-sided SMT production line should be used. Sometimes PCB component mounting exists in many places, in which case an additional insertion assembly line may be needed. The insertion assembly line will be based on the original PCB SMT assembly line.
SMT vs. THT Production Lines
Although the SMT method makes the production of circuit boards more precise and faster, not all components can be surface-mounted to the top of a PCB. Components suitable for SMT production lines are generally smaller. Therefore, the older through-hole technology (THT) is still needed.
THT components are larger, with axial or radial leads, requiring manual assembly. The assembly process is done by inserting the leads into drilled holes on the circuit board and soldering them in place. Although SMT lines are more popular, THT components work just as well, and sometimes even better, than SMD components in many situations.
Additionally, there are situations where both THT and SMT assembly lines are needed, known as mixed assembly. Plug-in and surface mount components are combined to form a hybrid production line.
SMT Assembly Line Process
What processes are included in an SMT production line? Understanding the SMT production line process is essential before choosing a good SMT service provider. The general process of a single SMT production line includes the following steps:
1. Material Preparation and Inspection (conducted by the QC department)
Material preparation and inspection is a step in our PCB assembly factory's SMT production line. QC inspectors should carefully check the PCBs and SMT components for any defects and pick out unqualified materials. At this stage, these boards are flat with pads.
2. Printing (using a solder paste printer)
This step involves printing solder paste or adhesive onto the SMT stencil in preparation for soldering the SMT components.
3. Dispensing (using a dispensing machine)
After entering this stage, a dispensing machine is used to dispense glue to the fixed positions on the SMT stencil. Its main function is to fix the surface mount components to the PCB during wave soldering. Therefore, solder paste inspection (SPI) should be performed after dispensing.
4. SMC Placement/Mounting (via pick-and-place machine)
The next step in the SMT production line is to use a pick-and-place machine to mount the surface mount components onto the solder-printed PCB. The pick-and-place equipment picks up the electronic components and places them on the board. This process is fully automated to avoid human error.
5. SMD Adhesive Curing (using a curing oven)
After the component mounting process is complete, the surface mount board is moved to the adhesive curing station, a crucial process in the SMT production line. The operator places the SMD adhesive into a curing oven to cure it, ensuring that the surface mount components and the SMT stencil are firmly bonded together.
6. Reflow Soldering (using a reflow oven)
In this stage, the solder paste melts to enhance the adhesion between the components and the circuit board. The complete reflow soldering process in the SMT production line includes four stages, namely four temperature zones.
7. Cleaning
Next, the assembled PCB enters the cleaning stage of the SMT production line to remove solder residue, such as flux and any other substances harmful to humans. This process is done using a cleaning agent. However, if no-clean soldering technology is used, this process is not necessary.
8. Inspection
The final process in a normal SMT production line is inspection, using various tools to check the actual quality (i.e., soldering and assembly quality) of the finished printed circuit board. The specific inspection items depend on customer requirements. Common ones include automated optical inspection (AOI) with a microscope, X-ray inspection, appearance inspection, and performance testing of in-line testers and first-article inspection machines, etc.
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