Common quality defects in reflow soldering and improvement methods for tin bead improvement

Tin beads are similar to solder balls, but they are much smaller in size. Tin beads usually appear outside the solder pad due to various reasons before soldering, while they appear independently outside the solder pad and pins after soldering and fail to fuse with the solder paste, resulting in the formation of tin beads. Tin beads often appear on both sides of the component or between fine pitch pins, which can easily cause circuit board short circuits. The schematic diagram of tin bead quality defects is shown in the following figure.

 Schematic diagram of tin bead quality defects

The reasons for the formation of tin beads
① Improper temperature setting in the heating zone of the reflow welding machine. If the preheating time is short and insufficient, the activity of the flux is low, and the oxide film on the surface of the solder pad and solder particles cannot be removed, which is easy to produce tin beads; If the preheating temperature is raised too quickly, the water vapor and solvent in the solder paste can easily vaporize and expand, resulting in the formation of tin beads.
② The corrosion accuracy of the opening size of the steel mesh does not meet the requirements, which can easily cause burrs in the opening size of the steel mesh, poor solder paste formation after printing, burrs or gaps; If the steel mesh is too thick or the printing pressure is too high, it is easy to cause the solder paste to accumulate too thick, which affects the quality of solder paste printing. When reflow soldering is carried out, these burrs or excessive solder paste can easily condense into solder beads.
③ The printed circuit board is not cleaned thoroughly, and there is residual solder paste. After reflow soldering at high temperatures, the residual solder paste is distributed around the solder pads. The solder paste near the solder pads and component solder ends is pulled back to the welding position to form solder joints, while the solder paste away from the solder pads and component solder ends gradually contracts towards the middle of the component, producing solder beads around or on the side of the component.
Process improvement methods for tin beads
Based on the above reasons analysis, the following process improvement methods are formulated:
① Control the preheating temperature of the preheating zone at 120-150 ℃, with a temperature increase rate of 1-4 ℃/s and a duration of about 90 seconds. Secondly, optimize the reflow soldering curve reasonably. With the continuous increase of temperature, the wettability of the solder paste is obvious, reducing the generation of solder beads. However, if the reflow soldering temperature is too high, it is easy to damage components, circuit boards, and solder pads. Therefore, choose an appropriate soldering temperature, and control the reflow temperature between 220-245 ℃.
② Re optimize the shape and center distance of the solder pad pattern, select appropriate steel mesh materials and processes, adjust the printing parameters of the printing machine, set the printing pressure reasonably, thereby improving the quality of solder paste printing and effectively reducing the formation of solder beads.
③ The circuit board for secondary printing needs to be cleaned with alcohol and blown with an air gun to remove residual solder paste on the circuit board. At the same time, production must strictly follow the production process requirements and standardize the operating process.

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