Complete PCB SMT Assembly Process Flow

As the trend towards miniaturization deepens, 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, holes need to be drilled on the PCB, component pins inserted, and soldered. Although sturdy solder joints can be achieved, the components are larger, and the production process is relatively cumbersome. In contrast, SMT assembly achieves success by introducing surface-mounted devices. This technology eliminates the need for pins and mounts the component’s solder pads on the surface of the printed circuit board before soldering. This subtle change allows components on the circuit board to be miniaturized and compact, enabling designers to create more dense and complex PCB. In addition to the advantage of improving board space utilization, the SMT process significantly reduces manufacturing costs through the automation process. This article by FS Technology will focus on the SMT process flow, covering specific process steps and providing necessary considerations to help you implement this process more effectively.

Note: This article involves the use of automated equipment and is therefore more suitable for the commercial sector rather than personal hobbyist projects.

SMT assembly line

Step 1: Familiarizing with the Equipment

Unlike through-hole assembly, SMT typically employs surface-mount device, as shown below. These components are small, with numerous but shorter pins, making manual assembly challenging.

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

  • Board Loader: Located at the beginning of the line, it stores PCB about to enter the production line.
  • Conveyor: Acts as a bridge connecting various devices in the production line for PCB transfer.
  • Stencil and Squeegee: Used for solder paste printing, ensuring precise application on designated areas of the PCB.
  • SPI: Detects the quality of solder paste printing, identifying potential issues.
  • Pick and Place Robot: Precisely picks up components and places them onto the PCB.
  • Reflow Oven: A high-temperature device used for soldering.
  • AOI: Conducts visual inspections, identifying any issues related to assembly and soldering points.

Step 2: Preparation

The key is to realize that this is an automated process. How do the devices work? What components are used? Where on the PCB are the components placed? What is the soldering temperature? It’s a meticulous process, requiring careful circuit design and proper machine setup before the assembly begins. So, some essential preparations are needed before formally starting the assembly flow.

a. Circuit Design

From concept to finished product, it’s not a spontaneous achievement. You need to visualize your ideas using PCB design software, draw circuit patterns, component layouts, decide on the components to be used, and include various details such as temperature limits, names, etc., to complete the final design.

b. Exporting Files

Having only provided the circuit design might complicate the work. After obtaining SMT assembly services, be sure to submit relevant files to the manufacturer. Design software usually integrates this feature, requiring only a one-click conversion. Some necessary files include:

  • Gerber Files: Contain information about the PCB, such as layout, spacing, traces, and layer information.
  • PNP/CPL Files: PNP represents pick and place, while CPL represents the component placement list. These files instruct the machine on the placement of each part on the circuit board in SMT process steps.
  • BOM File: The Bill of Materials includes a list of all components needed for the project.

c. Material Preparation


After reviewing and confirming cooperation, the manufacturer starts preparing the materials needed for the project, including:

  • Component Procurement: Purchasing components from qualified suppliers based on the BOM file.
  • Stencil Manufacturing: Constructing the PCB stencil according to your design files, often using laser technology to ensure quality.
  • Solder Paste Preparation: Transporting solder paste in a refrigerated manner and storing it in a storage box at 0° to 30 °C for 48 hours.

d. PCB Cleaning


If you opt for FS Technology’s turnkey service, we’ll start with PCB manufacturing services. If it’s just an assembly project, please send us the boards.

Before assembly begins, we thoroughly clean the PCB to remove surface impurities, including dust and grease. PCB cleaning is crucial for SMT production processes, as these impurities directly affect soldering quality, and even fingerprints are not an exception.

Step 3: Component Placement

a. Solder Paste Printing

Solder paste, a mixture of tin and flux, firmly binds pins and components when heated. At this stage, solder paste is uniformly applied to the stencil, and then a squeegee passes over the stencil’s surface. Under pressure, solder paste drops through the holes in the stencil onto the specified locations on the PCB. Proper application of solder paste is crucial for the entire SMT process, including the type of solder paste, and squeegee parameter settings. In addition to traditional stencil printing, jet printing technology can also be used in SMT assembly flow, which is more advantageous for large-scale PCB assembly.

b. Solder Paste Inspection

Solder paste detection in SMT assembly process

After solder paste printing, equipment integrated into the production line performs SPI inspection. This equipment consists of a 3D scanning system and AOI unit, capturing surface images of the PCB. The light source provides the necessary conditions for detection to ensure the camera captures correctly. On the screen, specific details can be observed. By comparing with the “golden board,” any deviations in quantity, excessive amounts, or misplacement are immediately investigated, and a new solder paste printing is carried out if needed.

c. Pick and Place

Picking robot in SMT assembly process

Initially, the manufacturer loads the pick-and-place robot’s operating program, including component coordinates, picking sequence, placement locations, etc. Typically, the robot is equipped with an advanced vision system capable of precisely identifying and locating these positions.

The pick-and-place robot then utilizes vacuum nozzles on its arm to obtain components from the feeder belt, adhering them through suction. Using a multi-axis motion system, the robot moves in three-dimensional space and places the components in the specified positions.

Step 4: Soldering

Reflow soldering in smt assembly process

SMT PCB manufacturing processes utilize the reflow soldering technique, where the machine heats the entire assembly to a specified temperature to form crucial electrical solder joints between surface mount device and the printed circuit board. Initially, the assembled SMT board is conveyed into the reflow oven.

The oven temperature gradually rises, with the standard heating rate ranging from 1.0 to 2.0 degrees Celsius. It is crucial to avoid excessively rapid temperature increases. Around 90 seconds later, the temperature inside the oven reaches 140 to 160 degrees Celsius.

Subsequently, the reflow soldering process proceeds to the soak and reflow zone. At this point, the oven temperature elevates to the cone in the range of 210 to 230 degrees Celsius, reaching its peak. The solder paste completely melts and mixes with the component pins.

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

Step 5: Finishing Touches

After the completion of the soldering process, the basic SMT assembly process is essentially concluded. However, to ensure the quality of the PCBA board, some finishing touches are necessary.

One of these steps is cleaning. The solder paste and flux used during soldering contain compounds that, if left on the board’s surface for an extended period, may lead to corrosion when exposed to moisture and electrical current. The optimal cleaning agent used in this process is isopropanol.

Next is the inspection phase, which includes AOI Inspection and functional testing. Additionally, depending on the application and specific requirements, there may be a need for customized PCB testing services.


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

In conclusion, if you are in urgent need of PCB assembly services, consider FS Technology. With our in-house assembly facility and advanced equipment, we offer customized services to ensure quality at a more competitive price point for our customers!

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