Introduction to the causes of the grapevine effect in the SMT process

Grape ball effect refers to the SMT reflow process, solder paste can not be completely melted and wet pad, and the formation of a small ball similar to grapes, affecting the reliability and appearance of the solder joints. The causes of the grape ball effect is mainly related to the quality of the solder paste, printing process, reflow profile and other factors. In this paper, we will focus on analyzing the effective life of the solder paste after printing on the impact of the grape ball effect.

 

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Effective Life after Solder Paste Printing

Post-printing effective life of solder paste refers to the time that the solder paste can maintain good printing shape and performance at room temperature after it is printed on the PCB. Reasonable management of the effective life is critical to the control of the grapevine effect, as solder paste may oxidize, dry out, and collapse after exceeding the effective life, resulting in lower print quality and increased soldering risk.

 

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Factors Affecting the Effective Life of Solder Paste After Printing

The effective life of solder paste after printing is affected by a number of factors, including the brand, type, and formulation of the paste, as well as storage conditions. Solder pastes from different vendors may have different effective life, usually between 2 and 8 hours.

 

Stencil opening size and board in-line time also affect solder paste life. The printed PCB is called a solder paste board, and the effective exposure time of the board in air shrinks as the stencil aperture size decreases. The smaller the stencil aperture size, the thinner the stencil, the smaller the amount of printed solder paste, the faster the drying of the paste, the mechanism and the evaporation of water droplets on the glass is consistent: the larger the droplets, the slower the disappearance, and vice versa, the faster the disappearance.

 

In order to avoid problems caused by the expiration of the effective life of solder paste after printing, the following are some suggested measures:

 

Use solder paste in strict accordance with the instructions and specifications provided by the solder paste manufacturer, and observe the expiration date and storage conditions after opening.

Reasonably arrange the printing quantity and time according to the production plan to avoid the solder paste being exposed to air for a long time.

Regularly check the status of the solder paste on the printing plate, and replace or replenish the fresh solder paste in time if any abnormality is found.

Use a special viscosity tester or other methods to test the viscosity of solder paste to ensure that it is within the appropriate range.

Stir the solder paste with a stirring tool to make it uniform and consistent and avoid layering or settling.

Feed the printed board into the reflow oven as soon as possible before reflow to shorten the dwell time and reduce oxidation and volatilization.

Select the appropriate reflow profile to ensure that the solder paste can completely melt and wet the pads to form a complete solder joint.

 

Fitech Solder Paste

 

Printing solder paste produced by Fuyingda in Shenzhen has excellent printing performance, lower tin release performance and long time in-line performance up to 8 hours, its slumping performance and thixotropic performance are also outstanding. For more information, please feel free to contact us.