No matter how far SMT (Surface Mount Technology) has developed today, fundamentally improving the quality and performance of assembled PCBs (Printed Circuit Boards) remains the core of SMT technology, and the assembly of PCBs directly affects the reliability of electronic products. For SMT assemblers, the quality of assembled products not only represents the factory's production level but also reflects the strength and potential of electronics manufacturers. To obtain high-quality assembled products, promoting the rationalization, standardization, and standardization of the production process, it is necessary to combine the actual production situation and formulate a reasonable and effective production quality process control plan. Therefore, SMT assembly process control is the fundamental work in SMT assembly process optimization, and effective process control helps to timely discover potential faults in assembly production, reduce the product defect rate, and avoid economic losses caused by defective products.

　　Although SMT assembly involves complex PCBA processes, its process control mainly occurs in the main steps of the entire process, namely printing, mounting, and reflow soldering. Therefore, this article will discuss some methods regarding these steps of SMT assembly process control. All methods are based on the actual manufacturing experience of PCBCart factory.

　 　Solder Paste Printing PCB IQC

　　Although the PCBs are produced in the same factory, each batch of PCBs must undergo IQC (Incoming Quality Control).

　　Aspects of the PCB that need to be checked before SMT printing include:

　　a. Confirm whether the circuit board is deformed;

　　b. Confirm whether the circuit board pads are oxidized;

　　c. Check the board surface for scratches, broken lines, exposed copper, etc.;

　　d. Ensure that the PCB surface is flat, smooth, and uniform.

　　During the solder paste printing process, the following aspects must be considered to properly handle the PCB:

　　a. Gloves must be worn when picking up the circuit board;

　　b. Visual inspection, the distance between the eyes and the board surface is 30cm to 45cm, and the angle is 30° to 45°;

　　c. When picking up and placing the board, be careful to avoid collisions and drops. Do not stack or place it vertically to avoid broken lines;

　　d. The reference marks on the board should be checked to ensure that they perfectly match the positioning holes on the stencil.

　　 Solder Paste Usage and Management

　　During the solder paste application process, the following regulations should be followed:

　　a. The workshop environment temperature is controlled at around 25℃, and the relative humidity is controlled within the range of 35%~75%;

　　b. Solder paste that is not in use should be kept away from the production line to avoid misuse;

　　c. If newly opened solder paste is to be used with "old" solder paste, it should be mixed at a ratio of 3:1.

　　Maintaining uniform solder paste is very important. The following aspects should be noted:

　　a. The effectiveness of the solder paste should be strictly monitored, and expired solder paste should not be used;

　　b. When not in use, the solder paste should be stored in the refrigerator.

　 　Process Control Measures During Printing

　　To ensure the quality of solder paste printing, the following process control measures should be taken:

　　a. The printed part must be complete. If it is incomplete, the parameters of the circuit board, stencil, and printing knife should be modified;

　　b. Bridging should not occur during printing;

　　c. The printing thickness should be uniform. If it is not uniform, the squeegee force should be adjusted in time;

　　d. It is necessary to check whether there are any downward-turned edges on the pads. If so, check whether the stencil holes are blocked;

　　e. Check whether there is any deviation in the printing effect. If there is any deviation, the stencil position should be modified in time.

　　In addition, the stencil needs to be cleaned to prevent the flux from drying on the stencil and blocking the stencil holes. For electronic products that need to withstand strong vibrations in practical applications, the solder paste thickness also needs to be adjusted to ensure the solderability and reliability of the products.

　　 Chip Mounting

　　As a key equipment in the SMT assembly production process, the pick-and-place machine mainly mounts SMDs (Surface Mount Devices) onto the corresponding pads on the PCB. Therefore, the pick-and-place machine has high requirements for mounting accuracy, especially in terms of material supply, programming, testing, and assembly.

　　 Process Control Measures During Chip Mounting

　　Measure 1: All SMDs must be completely correct and compatible with the design documents.

　　Measure 2: As a control instruction signal, the writing process of the program must be accurate, and the corresponding data must comply with the pick-and-place machine program manual.

　　Measure 3: The assembly between SMDs and component suppliers should be as accurate as possible to prevent errors from recurring.

　　Step 4: Accurate debugging should be carried out on the pick-and-place machine before assembly manufacturing, and defects should be handled promptly and appropriately during the SMT assembly process.

　 　Process Control Measures After Chip Mounting

　　The pick-and-place machine has a complex structure, including a transmission mechanism, servo system, identification system, and sensors. Various defects are prone to occur in the SMT pick-and-place machine during production. The following are solutions.

　　Solution 1: It is necessary to analyze the working order and logical relationship between the transmission components of the pick-and-place machine.

　　Solution 2: The location, link, and degree of defects during the operation of SMT assembly should be understood, and different defects should be classified and identified through strange sounds.

　　Solution 3: The operation process should be clarified before defects occur.

　　Solution 4: The location where the defect occurred should be clarified.

　　As a high-precision electronic manufacturing equipment, the pick-and-place machine undertakes a large number of SMT pick-and-place tasks. In order to keep the equipment in good condition and operate better, a regular maintenance plan must be formulated.

　　 Reflow Soldering

　　Reflow soldering aims to attach SMDs to a PCB board through thermal convection. As the temperature rises, the solder paste connecting the components and pads begins to melt, and as the temperature falls, the solder paste will solidify, permanently adhering the components to the board.

　　Process control requirements for reflow soldering in SMT assembly include:

　　1. An appropriate reflow soldering temperature curve should be set, and real-time testing should be conducted regularly;

　　2. During soldering, the soldering direction specified in the PCB design document should be strictly followed;

　　3. Vibration should be avoided during the soldering process.

　　To inspect the quality of reflow soldering, the following aspects can be used as references:

　　1. Ensure the component soldering parts are complete;

　　2. The surface of the solder joint should be smooth;

　　3. The solder joint should be crescent-shaped;

　　4. There should be no solder balls or residue on the surface of the circuit board;

　　5. Bridging and cold solder joints should not occur.