How to Apply Solder Paste

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For SMT mass production, solder paste is applied by stencil printing: the stencil covers the PCB, the squeegee sweeps across it at a steady speed, and the paste prints precisely through the apertures onto the PCB pads. For small batches and repair, you can dot paste with a solder-paste pen, or manually print with a small squeegee and mini stencil. Apply paste in a uniform thickness, just covering the pad—not too much, to avoid bridging, and not too little, to avoid cold joints. Place components as soon as possible after printing, to prevent the paste surface from skinning over. For hand application, control the pressure so the paste stays uniform.

The Solder Paste application process must be strictly carried out in accordance with the equipment manual, while flexibly adjusted based on the actual conditions of the production site. For example, when using a fully automatic printing machine, attention should be paid to the matching relationship between squeegee pressure and speed. Excessive pressure will cause the solder paste to be squeezed out of the stencil openings, while insufficient pressure may result in inadequate solder paste filling.

During the solder paste coating process, if burrs or solder paste accumulation are found at the edges of the pads, it indicates that the squeegee angle or pressure settings are unreasonable and require recalibration. At the same time, check whether the stencil is worn or blocked by foreign matter, as these factors will affect the final printing quality.

For PCBs with high-density BGA or QFN packages, more attention must be paid to printing accuracy during solder paste coating. Laser-cut stencils are usually used to ensure the accuracy of opening dimensions, while controlling the viscosity and slump properties of the solder paste to ensure it maintains a good shape after printing, avoiding bridging or cold soldering during Reflow Soldering.

In actual production, the storage and use of solder paste must also strictly follow specifications. For example, unopened solder paste should be stored in a refrigerated environment below 4°C, and opened solder paste must be used up within 24 hours; otherwise, performance degradation caused by oxidation or solvent evaporation will affect soldering quality.

Operators must wear anti-static wrist straps before performing solder paste coating to prevent electrostatic discharge from damaging sensitive components. At the same time, regularly clean the squeegee and stencil of the printing machine to ensure no residual solder paste affects the next batch of products.

After solder paste coating is completed, AOI inspection or manual visual inspection is also required to confirm the printing quality. If abnormalities such as solder paste offset, insufficient material, or excessive material are found, production should be stopped immediately and the cause investigated to prevent defective products from flowing out in batches.

Solder paste coating is not a simple repetitive action, but a systematic task involving equipment debugging, material selection, environmental control, and process monitoring. Only by fully mastering all details can high-quality SMT production be truly achieved.