SMT (Surface Mount Technology) assembly has become a leading electronic manufacturing technology in terms of performance and efficiency of electronic products. Low cost, while ensuring high reliability, is undoubtedly two key factors that OEMs (Original Equipment Manufacturers) must consider.

　　The SMT assembly process involves many steps, each of which affects the quality of the final product. Furthermore, any modification to each manufacturing step can lead to significant cost fluctuations. Therefore, a comprehensive understanding of the SMT assembly process is highly beneficial, and it is a shortcut to reducing costs without sacrificing performance.

　　Generally speaking, the SMT assembly process mainly includes the following steps: solder paste printing, solder paste inspection (SPI), chip mounting, visual inspection, reflow soldering, AOI, visual inspection, ICT (in-circuit testing), functional testing, and board separation, etc. A thorough understanding of the entire process can help you reduce production costs.

　 　Step 1: Solder Paste Printing

　　SMT assembly begins with solder paste printing, the purpose of which is to apply the appropriate amount of solder paste to the pads where components will be soldered. The quality of solder paste printing mainly depends on three factors: solder paste condition, squeegee angle, and squeegee speed.

　　High-quality SMT assembly PCBs depend on the storage and application of solder paste. Solder paste must be stored at low temperatures in a refrigerator and must be restored to room temperature before use on the SMT production line. In addition, uncovered solder paste must be used within two hours. In addition to the condition of the solder paste, the parameters of the solder paste printer should also be properly set, especially the scraping angle and speed, as both are closely related to the specific amount of solder paste remaining on the pads.

　 　Step 2: Solder Paste Inspection (SPI)

　　Solder paste inspection itself is an optional method to reduce costs, as it is better to reduce solder defects now than to find them later. SPI is not a necessary step in the SMT assembly process, but applying it helps reduce manufacturing costs and improve product quality. After all, most defects in SMT assembly originate from solder paste printing, and if they can be found and addressed early, the threat of defects leading to later stages of manufacturing can be reduced or even eliminated. There are two types of SPI machines: 2D and 3D. PCBCart has a 3D SPI machine in its workshop to provide better inspection services for customers.

　 　Step 3: Chip Mounting

　　Chip placement is a core link in the SMT assembly process. Chip placement is completed by a chip placement machine. Chip placement machines mainly differ in speed and placement capacity. Some small components are generally mounted using high-speed placement machines. High-speed placement machines have fast mounting speeds, allowing components to be quickly attached to the solder paste on the pads.

　　However, large components such as BGAs, ICs, and connectors are usually mounted by multi-functional placement machines that run at relatively lower speeds. For these components, alignment is very important. Pre-placement alignment requires more time, which is why the speed of multi-functional placement machines is much lower than that of high-speed placement machines. In addition, due to size limitations, some components used in multi-functional placement machines do not rely on tape, while some rely on trays or tubes.

　 　Step 4: Visual Inspection + Manual Component Placement

　　After chip placement, a visual inspection is required to ensure, to the greatest extent, that there are no defects in the reflow soldering. The main problems found in this step include misalignment and missing parts. Once reflow soldering is complete, defects will be firmly fixed on the PCB, making them extremely difficult to handle. As a result, product reliability will decrease, and production costs will increase.

　　On the other hand, some components can be placed directly by hand in this step, including some large components, DIP components, or components that cannot be placed by the placement machine for some reason.

　 　Step 5: Reflow Soldering

　　During the reflow soldering process, the solder paste melts to form IMCs (intermetallic compounds), connecting the component leads to the circuit board. The temperature curve during the reflow soldering process includes preheating, heating, reflow, and cooling. Taking lead-free solder paste SAC305 as an example, its melting point is about 217℃, so unless the temperature of the reflow soldering furnace is higher than 217℃, the solder paste cannot be remelted. In addition, the higher temperature of the reflow soldering furnace should not exceed 250℃, otherwise many components will not be able to withstand such a high temperature and will not melt.

　　In fact, the setting of the temperature curve determines the quality of reflow soldering and helps reduce production costs. Therefore, it is important to find an experienced SMT assembler as a CM (Contract Manufacturer) who fully understands the factors affecting SMT soldering quality and improvement measures. All factors will lead to lower production costs.

　　Step 6: AOI (Automated Optical Inspection)

　　Up to this point, the components have been fixed to the PCB after reflow soldering, which means that the basic part of the tasks related to SMT assembly has been completed. However, assembled circuit boards can never be directly used in the final product unless they have been thoroughly tested and inspected. The performance of solder joints can be checked by applying AOI. AOI can expose some defects, such as tombstoning, bridging, missing components, misalignment, orientation, bridging, and open solder joints.

　 　Step 7: AXI (Automated X-ray Inspection)

　　X-ray inspection complements AOI because it can more clearly and directly indicate some defects. It is not a mandatory measure after reflow soldering. However, as long as the SMT assembler cares more about product quality and reliability, the X-ray inspection machine will definitely be applied to meet the strict requirements of some OEMs to achieve higher efficiency.

　 　Step 8: ICT or Functional Testing

　　The purpose of ICT is to test for open and short circuits in the circuit by measuring resistance, capacitance, and inductance, and to find defects in some components. Therefore, the components are tested to ensure their high performance after reflow soldering.

　　Functional testing complements ICT because ICT can only test for open and short circuits on bare boards, but cannot test the functionality of the assembled PCB. Therefore, functional testing must be used to test the functionality of the assembled PCB to maintain the high reliability of the final product.