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福英达
福英达

What are the Factors Influencing Solder Paste Printing-Part 1

2021-12-09

Abstract: The printing of solder paste is an important step in surface mount technology (SMT). The quality of solder paste printing will directly affect the effect of reflow soldering.

 

Keywords: solder paste printing, influencing factors, reflow soldering, SMT, stencil printing

 

Influencing factors of solder paste printing

 

Printing equipment, stencil quality, squeegee, solder paste, PCB substrate, printing process parameters, operating environment, and other factors will affect the quality of solder paste printing. In the actual operation of the SMT production line, since the parameters of the purchased printing machine have been determined, the material, type, shape, hardness, etc. of the squeegee have been confirmed, and the temperature, humidity, and cleanliness have been set. Therefore, the factors affecting solder paste printing are mainly analyzed from the aspects of stencils, solder pastes, PCBs, and printing parameters.

 

Influencing factor 1: Design, manufacture, and use of stencils

 

The stencil affects the printing release rate of the solder paste. The release rate of solder paste is the ratio of the capacity of solder paste released on the pad to the size of the aperture of the stencil.

 

The solder paste printing release rate is equal to the solder paste release amount divided by the aperture volume.

 

Stencil printing


Figure 1


The most important control parameters in stencil design are aperture size and stencil thickness. The release of solder paste is directly affected by the size of the opening and the thickness of the stencil. Other factors can also affect the solder paste release rate, such as the geometry of the stencil wall, the smoothness of the stencil aperture, the separation speed of the stencil and the PCB, the gap between the stencil and the PCB, and the size accuracy of the stencil aperture, etc.

 

The stencil area ratio is the ratio of the vertical panel area to the sidewall area. The aspect ratio is the ratio of the aperture width to the aperture thickness. Figure 2 shows a view of the aperture on the stencil.


Stencil aperture

Figure 2

Ratio=opening area/opening wall area=(LXW)/[2X(L+W)XT]

 

Aperture ratio=opening width/stencil thickness=W/T.

 

With the improvement of the integration of electronic devices, the pin pitch becomes smaller, the corresponding pad size becomes smaller, and the corresponding stencil aperture becomes smaller. The miniaturization of the aperture leads to high requirements for the performance of the stencil. Therefore, when designing the stencil, in order to ensure that the solder paste release rate reaches 75%, the ratio of the width to the thickness of the stencil must be greater than 1.5, and the area ratio must be greater than or equal to 0.66. The IPC7525B standard specifies the aperture size for various components. However, in actual production, the specific component pin spacing required for PCB assembly should be comprehensively considered, and a reasonable optimization should be made to meet the required release rate.

 

The manufacture of stencils has a significant influence on the smoothness and accuracy of the sidewall of the stencil. The manufacturing methods of stencils include chemical etching, laser cutting, and electroforming. The first two belong to the subtraction process, while the last belongs to the addition process. At present, in actual production, laser cutting is mostly welcomed when considering cost and cycle factors, especially for the production of stencils with a spacing of less than 0.5mm. The advantages are that no image conversion is required, the positioning accuracy is high, and the error rate is low. The aperture wall of the laser-cut stencil can form a taper angle of about 2°, forming a tapered hole with a bottom opening slightly larger than the squeegee surface to facilitate the release of solder paste.


-To Be Continued-

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