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福英达
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What are the Factors Influencing Solder Paste Printing-Part2

2021-12-24

Factor two: Solder paste

Solder paste is a stable mixture consisting of a solder alloy powder, viscous flux, and some additives, with a certain viscosity and good thixotropy. It has a certain viscosity at room temperature, which can mount the components initially.

When heated to a certain temperature, the flux and other substances are volatilized, and the alloy powder melts into a liquid form. By the effects of surface tension and wettability, the solder paste fills pads to connect the pads and components, forming firm solder joints.

 

The solder paste is printed onto the PCBs through the stencil apertures by a squeegee utilizing the thixotropy property of the solder paste. When the solder paste is under a certain temperature and shear force, the viscosity drops rapidly, allowing the solder paste to pass through the apertures. After the shear force is removed, the solder paste viscosity returns to normal. Solder paste is a shear-thinning material. When shear stress is present, the viscosity will decrease.

 

Solder paste should be stored at low temperatures and used within the validity period. The solder paste is used in a cycle at room temperature. After the solder paste reaches room temperature, the solder paste container can be opened to prevent water vapor from condensing, and then the solder paste is stirred to meet the printing requirements. After conducting a number of experiments, the solder paste was taken out of the refrigerator and placed at a room temperature of 25 °C. The corresponding room temperature could be reached after 4 hours. At the same time, the solder paste was rewarmed and the viscosity was measured. When the stirring time is 60-130 seconds, the viscosity of the solder paste meets the requirements. There is a positive correlation between ambient temperature and solder paste mixing time and rewarm temperature. The general trend is that the lower the ambient temperature, the longer the stirring time is required, and vice versa. Through relevant experiments, a questionnaire for mixing time and ambient temperature was finally determined, helping people to identify whether the viscosity meets the printing requirements. Generally speaking, the viscosity of solder paste can also be determined by visual inspection if a dedicated solder paste mixer is not available. The manufacturer can stir the solder paste with a tool for 30 seconds, then stir up a part of the solder paste and let it drip spontaneously. If the solder paste does not drop, it is too sticky. If it falls continuously, it is too thin.

 

Fitech is a semiconductor solder R&D and production company with 20 years of experience. Fitech has a complete set of solder paste development and inspection equipment, including the equipment to measure the solder paste viscosity and thixotropy, which ensures the product quality can meet user needs.


Water-Soluble SAC305 Solder Paste



Factor three: PCB printed circuit boards

The straightness of the PCB board is one of the important factors affecting the quality of solder paste printing, especially the large PC board, which needs to be supported by a special stencil to avoid the warping of PCBs and the changes in the amount of solder paste printing. The warping of the PCBs affects the solder paste release ratio.

 

Therefore, it is necessary to reasonably design the printing stencil of the PCBs to ensure its straightness and solder paste printability.

 

Factors four: Adjusting the printing parameters

The adjustment of printing process parameters is important, including printing speed, squeegee angle, pressure, mesh and PCB separation speed, etc. The faster squeegee speed cause faster solder paste rolls and lower viscosity, which is beneficial to the filling of the solder paste. However, the high squeegee speed leads to short solder filling time. Therefore, the pressure of the squeegee should be appropriate. If it is too large, the squeegee and the stencil will be damaged, and the surface of the stencil will be stuck with the solder. If it is too small, an insufficient volume of solder paste will be deposited onto the pads. During the sorting process of the stencil and the PCBs, due to the friction of the sidewall of the stencil and the bonding effect of the solder paste, the flow velocity at the sidewall of the stencil is lower than the separation speed near the center of the apertures, resulting in residual solder paste on the sidewall and decrease in solder release ratio. The smaller the squeegee angle, the greater the downward pressure on the solder, and it is not easy to scrape the solder paste on the mesh surface. If the angle is too large, the solder paste cannot be rolled, which negatively affects the release of the solder paste. Generally, the automatic printing machine is about 60°.

 

Solder Paste Inspection

After the solder paste printing is completed, it is necessary to judge the printing quality. The inspection is generally carried out through special equipment, and it is usually required to detect whether the solder paste height, volume, size, deviation, and three-dimensional shape meet the requirements. The height, area, and volume in IPC7527 require the amount of solder paste to account for 75% to 125% of the calculated amount of the stencil. However, solder paste printing for fine-pitch devices requires strict control in actual production.

 

Conclusion

There are many factors affecting solder paste printing. This article mainly discusses the factors that affect solder paste printing, such as stencils, solder paste, PCBs, printing parameter adjustment, etc. Various defects in the process are analyzed and solved, which provides a reference for improving the quality of the SMT production line.

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