SMT (surface mount technology) is popular in the electronic manufacturing service industry. The steps of the SMT assembly process flow include: incoming material inspection, solder paste printing, solder paste inspection, components pick and place, reflow soldering, AOI (Automated Optical Inspection), and X-ray testing. This article will introduce them in detail, step by step.
#Step 1. Incoming Material Inspection
This is a key step to ensure PCBA board quality, including the following inspections:
• Component inspection: Check the part numbers, specification, and quantity, and check whether the appearance is damaged or the pin is deformed, etc.
• Raw material inspection: Check if the solder paste, glue, etc. are in the validity time, meet the production requirements, and do performance inspections.
• PCB board inspection: Includes inspection of appearance, pads, solder joints, and electrical specifications. The PCB’s appearance must have no scratches and corrosion; the pads must be flat, and the aperture and hole spacing must meet the requirements; the solder joints must be firm and uniform; the electrical properties must meet the standards of insulation resistance, conductivity, etc.
In addition, the inspection process must meet strict standards and acceptance levels, such as implementing the IPC-A-610D standard and setting different acceptance quality limit values for fatal defects, major defects, and minor defects. Through all these incoming material inspections, the quality of the SMT PCB assembly process can be effectively controlled and improve the PCBA’s reliability.
#Step 2. Solder Paste Printing
Solder paste printing is one of the important steps in the production of printed circuit board assemblies. It includes printing solder paste evenly on the PCB boards’ pads to prepare for the following SMT PCB assembly processes. According to our on-site process analysis over 10+ years, solder paste printing is the most important process of all SMT assembly processes, because 70% of SMT assembly defects are related to this process. Solder paste printing has the following steps:
• Preparation phase: Select the appropriate solder paste and ensure that it has good printing performance during its service life. Then warm the solder paste according to the rules, place the solder paste on the stencil, and ensure the right amount.
• Equipment debugging: Adjust the parameters of the solder paste printer, such as scraper pressure, printing speed, separation speed, etc., to meet the requirements of different PCB boards and solder pastes.
• Printing process: Place the PCB board on the printer and start the printing program. The solder paste printer will evenly apply the solder paste on the pads of the PCB board through the scraper to form a continuous solder paste pattern.
#Step 3. SPI (Solder Paste Inspection)
SPI solder paste inspection is crucial to ensure the quality of the PCB SMT assembly process. It focuses on evaluating the quality of solder paste printing. By measuring the volume, area, height, and other parameters of solder paste on the circuit board, it can detect whether the solder paste is insufficient, excessive, offset, short circuit, foreign matter or poor shape.At PCBbee, we use an advanced SPI machine, which provides strong quality assurance for our PCBA manufacturing process with its high precision, high efficiency, intelligence, and resource-saving features.
#Step 4. Components Placement
Components placement is to use the pick and place machine to accurately mount electronic components on the designated position on the PCB board after the solder paste is printed. This step relies on precise mechanical positioning and visual recognition technology to ensure that the components are aligned with the pads. The placement accuracy and efficiency mainly depend on the performance of the pick and place machine.
The pick and place machine is divided into high-speed mounting machines and multifunctional mounting machines. High-speed machines are used for mounting components with large pin spacing. High-speed assembly machines have high efficiency and speed, and they can quickly complete the placement of a huge number of components. SMT high-speed mounting machines are widely used to produce electronic products, especially in production lines that require a big number of SMD components, such as the production of mobile phones and computers. Multifunctional mounting machines are slow and generally suitable for small-batch production. It can mount high-precision large and special-shaped components, and generally can also mount small sheet-shaped structural components, which can almost cover all the component ranges. Its structure is flexible and has high positioning accuracy. In addition, multifunctional mounting machines do not have strict requirements on material packaging methods and can accept a variety of packaging design methods. When mounting difficult special-shaped components, multifunctional mounting machines have more advantages due to their higher mounting accuracy. In summary, when batch and efficient mounting is required, high-speed mounting machines are suitable; when the scale is small and needs to adapt to different types of component parts, multifunctional mounting machines are more suitable.
#Step 5. Reflow Soldering
After finishing the above steps, now there are solder paste and components on the PCB pads, and they need to be connected. Now we need to do reflow soldering which is the process of melting the solder paste on the PCB pads to achieve mechanical and electrical connection between the components’ pins and the circuit board pads. The reflow soldering process generally includes four stages: preheating, melting, reflow, and cooling. The temperature profile is an important parameter for reflow soldering. The temperature profile setting needs to meet the requirements of the PCB material, the components specifications, and the solder paste properties. So the circuit boards can have the best soldering effect. A skilled reflow soldering engineer will precisely control the temperature and time in the soldering process, so that make soldering quality between electronic components and printed circuit boards stable.
#Step 6. AOI (Automatic Optical Inspection)
After finishing reflow soldering, now the components are soldered to the board. The next step is the AOI (automatic optical inspection). It uses high-resolution cameras and image processing software to automatically check the placement status of components and soldering quality in the SMT PCB assembly production. AOI can detect many problems such as missing components, polarity errors, position offsets, soldering defects, and foreign matter contamination. It is a very important step to ensure PCBA quality in the SMT assembly production line. Through AOI inspection, PCB assembly manufacturers can promptly discover and correct PCBA board defects, reduce rework and scrap rates, and improve overall production efficiency and product qualification rates.
#Step 7. X-ray testing
AOI can detect the soldering quality of daily ICs (such as QFP/SOP) because the pins of such ICs are exposed to the outside. However, if the pins are at the bottom or inside of the IC (such as BGA/QFN), AOI cannot detect the soldering quality. Therefore, X-RAY testing is needed. X-RAY testing uses the light spots formed by X-ray penetration to check whether the solder balls are soldered well and whether there are any PCBA failures like cracks, open circuits, short circuits, voids, delamination, and other defects.
After completing the above PCB SMT assembly process, if the board is found to have a defect, it needs to be repaired. If it passes the AOI and X-RAY test, it can be separated by a depanelized machine and then cleaned before entering the subsequent process. If you have any questions or inquiries about the circuit board SMT Assembly Process, please feel free to contact us at service@pcbbee.com, and we will reply within 2 hours.
