SMT surface mount technology is a common mounting technique in the electronics industry. SMT technology allows for the mounting of more, smaller, and lighter components, enabling high-precision circuit boards and bringing certain conveniences to life. So, what are some common problems with SMT surface mounting? What aspects need attention during reprocessing?

　 　I. Common Defects in SMT Adhesive

　　1. Component offset is a common defect in high-speed placement machines. The main causes are: offset in the X-Y direction during high-speed PCB movement. This phenomenon is more likely to occur on components with small adhesive application areas, due to insufficient adhesion.

　　Inconsistent adhesive dispensing (e.g., two adhesive points under an IC, one large and one small). Uneven force during thermal curing leads to offset on the side with less adhesive.

　　2. Component loss during wave soldering has complex causes:

　　Insufficient adhesive strength. Impact before wave soldering. Excessive residue on some components. The adhesive is not resistant to high-temperature shock.

　　3. Mixing adhesives from different manufacturers results in significant chemical differences, leading to various defects:

　　Difficult curing; insufficient adhesion; severe component loss during wave soldering.

　　Solution: Thoroughly clean the stencil, scraper, and dispensing head to prevent mixing adhesives from different brands.

　　4. The required push strength for 0603 capacitors is 1.0 KG, and for resistors is 1.5 KG. For 0805 capacitors, it's 1.5 KG, and for resistors, it's 2.0 KG. Failure to meet these requirements indicates insufficient strength.

　　Reasons for insufficient push strength:

　　Insufficient adhesive, uncured adhesive, PCB or component contamination, brittle adhesive, lack of strength.

　　5. A 30ml syringe of adhesive needs to withstand tens of thousands of pneumatic impacts before being used up. Therefore, the adhesive must have excellent thixotropy; otherwise, unstable adhesive points and insufficient adhesive will lead to insufficient strength and component detachment during wave soldering.

　　Conversely, excessive adhesive, especially for small components, can stick to the pads, hindering electrical connection.

　　6. The printing stencil needs regular cleaning (every 8 hours with ethanol). Adhesive impurities. Unreasonable stencil apertures (too small) or low dispensing pressure resulting in insufficient dispensing. Air bubbles in the adhesive. Clogged dispensing head (clean immediately). Insufficient dispensing head preheating temperature (set to 38℃).

　　7. Stringing occurs when the adhesive does not break cleanly during dispensing, resulting in a string-like connection in the dispensing head's direction of movement. Excessive stringing, with adhesive covering the printed pads, can cause poor soldering, especially with larger dispensing nozzles.

　　Stringing is mainly affected by the resin's inherent stringiness and dispensing conditions. Solutions:

　　(1) Increase dispensing stroke and reduce speed, but this will reduce production efficiency.

　　(2) Lower viscosity and higher thixotropy materials are less prone to stringing. Therefore, choose such adhesives whenever possible.

　　(3) Slightly increase the temperature of the temperature controller to force the adhesive to become lower viscosity and higher thixotropy. Consider the adhesive's shelf life and dispensing head pressure.

　　8. Excessive adhesive fluidity can cause slumping. A common slumping problem is that the adhesive slumps after being left for too long. If the adhesive spreads to the PCB pads, it can cause poor soldering. Furthermore, slumped adhesive may not contact the main body of components with relatively high pins.

　　This can lead to insufficient adhesion, making it difficult to predict the slumping rate of easily slumping adhesives, making initial dispensing volume setting difficult.

　 　II. Precautions for SMT Surface Mount Processing

　　1. Precautions for using solder paste in SMT surface mount processing:

　　(1) Storage temperature: Recommended refrigerator storage temperature is 5℃-10℃. Do not go below 0℃.

　　(2) Inventory principle: Follow the FIFO (First-In, First-Out) principle. Avoid prolonged storage in the refrigerator.

　　(3) Thawing requirements: After removing the solder paste from the refrigerator, allow it to thaw naturally for at least 4 hours. Do not open the lid during thawing.

　　(4) Production environment: Recommended workshop temperature is 25±2℃, and relative humidity is 45%-65%RH.

　　(5) Used solder paste: Use opened solder paste within 12 hours. If storage is necessary, transfer it to a clean, empty container and reseal it before returning it to the refrigerator.

　　(6) Amount of paste on the stencil: The amount of solder paste on the stencil should not exceed 1/2 of the scraper height during printing. Observe frequently and add small amounts frequently.

　　2. Precautions during SMT surface mount printing operations:

　　(1) Squeegee: Steel squeegees are recommended for better solder paste shaping and film release on the pads.

　　Squeegee angle: 45-60 degrees for manual printing; 60 degrees for machine printing.

　　Printing speed: 30-45 mm/min for manual printing; 40-80 mm/min for machine printing.

　　Printing environment: Temperature 23±3℃, relative humidity 45%-65%RH.

　　(2) Stencil: Choose stencil thickness and aperture shape/ratio according to product requirements.

　　QFP\CHIP: Chips with a center-to-center spacing of less than 0.5mm and 0402 chips require laser drilling.

　　Stencil Inspection: Weekly tension tests are required for the stencil, with a tension value above 35N/cm.

　　Stencil Cleaning: Clean the stencil with a dust-free stencil wipe after printing 5-10 PCBs continuously. Do not use rags.

　　(3) Cleaning Agent: IPA Solvent: When cleaning the stencil, use IPA and alcohol solvents. Do not use solvents containing chlorine, as this will damage the solder paste components and affect overall quality.