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Stencil Clear Paste Printing Process In SMT

- Mar 11, 2018 -

As usual, note that some details can eliminate undesired conditions such as solder paste misprints and removal from the board to a cured solder paste. It is our goal to deposit the proper amount of solder paste in the desired location. Stained tools, dried solder paste, misalignment of the stencil with the board, may cause undesirable solder paste on the underside of the stencil or even on the assembly. During the printing process, the template is wiped according to certain rules during the printing cycle. Ensure that the template is located on the pad, not on the solder mask, to ensure a clean solder paste printing process. In-line, real-time solder paste inspection and inspection prior to reflow after component placement are all process steps that help reduce process defects before soldering occurs. For fine-pitch stencils, if there is damage between the pins due to bending of the thin stencil cross-section, it can cause the solder paste to deposit between the pins, creating print defects and/or short circuits. Low viscosity solder paste can also cause print defects. For example, a high operating temperature of the press or a high squeegee speed can reduce the stickiness of the solder paste in use, causing print defects and bridging due to the deposition of too much solder paste. In general, the lack of adequate control of materials, solder paste deposition methods and equipment is the main reason for defects in the reflow soldering process.

Removing the solder paste from the misprinted board with a small scraper blade may cause some problems. It is generally feasible to immerse the misprinted board in a compatible solvent, such as water with an additive, and then remove the small solder ball from the board with a soft brush. Rather than repeated soaking and scrubbing, rather than a violent dry brush or scraping. After solder paste is printed, the longer the operator waits for cleaning to be printed, the harder it is to remove the solder paste. The misprinted board should be put into the soaking solvent immediately after the problem is found, because the solder paste is easy to remove before drying.

Avoid wiping with a cloth strip to prevent solder paste and other contaminants from smearing on the surface of the board. After soaking, flushing with a gentle spray can often help remove the unwanted solder paste. Hot air drying is also recommended. If a horizontal stencil cleaner is used, the surface to be cleaned should face down to allow the solder paste to fall off the board.

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