SMT PCB Assembly Process – 8 Basic Steps

With the rapid advancement of the electronics industry, devices are becoming increasingly smaller and more complex. To meet these demands, engineers have widely adopted the SMT (Surface Mount Technology) assembly and manufacturing method.

 

With the rapid advancement of the electronics industry, devices are becoming increasingly smaller and more complex. To meet these demands, engineers have widely adopted the SMT (Surface Mount Technology) assembly and manufacturing method. When people observe printed circuit boards (PCBs) in electronic devices like smartphones, computers, medical equipment, telecommunication systems, and automotive electronics, they may wonder how the components are attached to the boards. The answer lies in the SMT assembly process.  

 

Curious about the SMT PCB assembly process? Let’s explore how it works and why it plays a crucial role in modern electronics.

 

What is the SMT PCB Assembly Process?  

 

The SMT (Surface Mount Technology) PCB assembly process involves directly mounting components onto the surface of a printed circuit board (PCB). This method differs from the traditional PTH (Plated Through-Hole) assembly, where components are inserted into pre-drilled holes on the PCB, and their pins are soldered to secure them. SMT eliminates the need for drilling holes, offering a more efficient, compact, and modern approach to assembling electronic components.

 

 

7 Steps in the SMT PCB Assembly Process  

 

Understanding SMT PCB assembly process is essential, so let’s dive into its detailed process:  

 

1. Incoming Material Inspection  

 

The first step is ensuring the quality of all incoming materials. Each electronic component, including PCBs and SMD parts, must undergo a thorough inspection. Components should match the customer’s BOM (Bill of Materials) in package type, shape, and size. PCBs must be free from defects like oxidation or deformation. This step is critical to ensure the quality and reliability of the SMT assembly.  

 

2. Preparation Before SMT Assembly  

 

Once materials are inspected and approved, preparations for the assembly process begin. This includes:  

- Preparing stencils for solder paste application.  

- Baking PCBs and BGA chips to eliminate moisture.  

- Generating pick-and-place files for component placement.  

- Ensuring feeders and other equipment are ready for operation.  

 

Each preparation step ensures smooth execution during the assembly process.  

 

3. Applying the Solder Paste

 

With all preparations complete, the surface mount PCB assembly process can begin. The first step is applying solder paste to the pads on the PCB. This is done using stainless steel stencils, which are placed over the circuit board. Solder paste is then printed onto the PCB through the holes in the stencil. Since different PCB designs require solder paste to be applied in specific areas, custom-designed stainless steel stencils are necessary for each design.

 

The holes in the stencil are typically precision-cut using lasers. Once the solder paste is applied to the PCB, it undergoes inspection—either by a solder-paste printing inspection machine — to ensure proper placement. If the paste is correctly applied, the PCB is ready to proceed to the SMT production line for component assembly.

 

In this process, a solder paste machine is used to accurately apply the paste to the bare PCB, in conjunction with the solder stencil. This ensures precision and consistency in the assembly process.

 

4. Mounting Components with a Pick-and-Place Machine

 

Once the solder paste is printed on the bare PCBs and passes quality inspection, the next step is to place the SMD components onto the PCB. This is done using a high-precision pick-and-place machine, such as those manufactured by Yamaha, ensuring components are accurately positioned on the solder pads.

 

SMD components are typically supplied in reels or trays, though loose packages are often used for prototype assemblies. These components are loaded into the pick-and-place machine, which operates with the support of specialized software to prevent errors during the loading process.

 

The pick-and-place machine uses pre-programmed X-Y coordinates to accurately position the SMD components on the PCB. Once all components are placed, the PCB will move to a reflow oven for soldering. However, before the PCB enters the reflow process, an in-line production engineer performs a first article inspection (FAI). This step verifies the accuracy of component placement and ensures the assembly meets specifications. Dedicated first-article inspection equipment is essential to this stage of the SMT manufacturing process.

 

5. Soldering Components on the PCB

 

There are two primary methods for soldering components onto the PCB: vapor-phase soldering and reflow soldering, each offering distinct advantages for different applications. Vapor-phase soldering is typically used for prototype assemblies or for soldering sensitive components. In this method, the PCB is heated in a vapor phase until the solder paste melts. Once the paste reaches its melting point and cools down, the components are securely fastened to the PCB.

 

For mass production, reflow soldering is the more commonly used technique. This process requires a nitrogen atmosphere and the PCB is placed into a controlled chamber. The solder paste is then melted using heated air, and the temperature is gradually reduced to allow the components to be permanently fixed onto the PCB. Reflow soldering ensures strong and reliable connections, making it ideal for large-scale SMT PCB manufacturing.

 

6. 100% AOI Inspection

 

Is the job done once components are soldered onto the PCB? Absolutely not. Ensuring the SMT PCB assembly process is flawless requires additional quality control measures, including Automated Optical Inspection (AOI) for mass production.

 

In AOI inspection, multiple cameras capture high-resolution images of the PCB after components have been soldered. These images are compared against a reference image of a correctly assembled PCB. If discrepancies are detected, the system alerts the operator, identifying the specific board for further inspection and correction.

 

AOI is indispensable for maintaining the accuracy and consistency of the SMT assembly process. Every assembled PCB undergoes 100% AOI inspection to catch issues like bad solder joints or incorrect components, ensuring high-quality production.

 

7. 100% Visual Inspection

 

While AOI effectively detects most defects in the SMT assembly process, visual inspections remain essential for identifying cosmetic issues, such as minor scratches on the PCB surface. Despite being time-consuming and costly, we perform 100% visual inspections to ensure that no defective PCBs are shipped. This final layer of quality control guarantees that every product meets our high standards for both functionality and appearance.