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福英达
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​The Solder Paste Printing Process

2022-06-30

The Solder Paste Printing Process

 

1. Introduction

Solder paste printing is one of the most important and widespread processes in electronics manufacturing. As the name suggests, solder paste is printed onto the pads after using a specific medium. The most commonly used medium is a stencil. The aperture layouts of stencils vary according to different circuit board structures. However, the number of solder paste particles printed in a single aperture is limited. Typically, five solder paste particles can be accommodated by rectangular apertures. The solder paste should effectively pass through the apertures and cover specific locations on the circuit boards. The solder paste is supposed to stay evenly on the circuit board after demolding.

2. The Factors That Influence Printing Quality

Adhesion is a factor that affects the connection between the solder paste and the substrates. In addition, the printing solder paste should have an excellent viscosity to maintain outstanding release performance from the stencils. However, viscosity decreases with time and the number of prints. If it is a long printing time, the viscosity of the solder will decrease and affect the print quality.

The stencil needs to have good friction. The solder paste needs to effectively adhere to the stencil and become stable instead of flowing around. It is necessary to clean the stencil in time. If the stencil contains impurities, it will affect the friction of the stencil, and the effect of solder paste printing decreases.

The area ratio and aspect ratio of the apertures need to be above 0.66 and 1.5, respectively. Experiments show that if the aspect ratio is too small, the thickness of the aperture is larger than the width, which is likely to cause the solder paste to stick to the wall. Similarly, a large wall area will also affect the quality of solder paste printing.


Aspect Ratio Formula                Area Ratio Formula   

 

The squeegee should maintain proper friction and printing angle. The excellent friction force allows the solder paste smoothly separate from the squeegee after printing. The printing angle will affect the viscosity of the solder paste. It is recommended to keep the printing angle at 45-60°.

Printing speed and stencil separation speed affect the viscosity of solder paste. Since the shear speed is inversely proportional to the viscosity, a high printing speed will hinder the solder paste from passing through the apertures evenly. In addition, a high separation speed of the stencil will lead to too smaller viscosity, which will cause the solder paste to stick to the bottom of the stencil.

Other factors such as stencil geometry can affect solder paste release.

 

3. The Features and Components of Printing Solder Paste

Printing Solder Paste can be made of various alloy combinations with flux, such as tin-silver-copper, tin-bismuth/tin-bismuth-silver, tin-lead, etc. However, due to environmental protection, tin-lead solder has been gradually replaced. The use of flux can reduce the oxide on the surface of the circuit board and reduce the tension of the solder, and it can also adjust the viscosity of the solder paste for printing.


Type

Alloy Component

Melting Point

Viscosity (Adjustable)

Tin-Silver-Copper

Sn96.5Ag3Cu0.5

217℃

120±30Pas

Tin-Bismuth

Sn42Bi58

138℃

150±30Pas

Tin-Bismuth-Silver

Sn42Bi57.6Ag0.4

139℃

120±30Pas


Viscosity is affected by temperature, alloy particle content, and particle size. Viscosity can be adjusted as required. Printing solder paste is recommended to be used at 20-25℃.


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