Challenges for Step Stencil Printing



Challenges for Step Stencil Printing
This paper focuses on the printing performance of step-up/step-down stencils and the paper ends up with a short outlook on 3D cavity printing.
Production Floor

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Authored By:


Carmina Lantzsch, Georg Kleemann
LaserJob GmbH, Furstenfeldbruck, Germany

Transcript


The stencil printing process is one of the most critical processes in electronic production. Due to the requirement for faster and smaller, it is necessary to place components with different paste volume close together without regard to solder paste printing. It is no longer possible to control the solder paste volume only by adjustment of the aperture dimensions. The requirements of solder paste volumes for specific components are realized using a so called step stencil. The step-down stencil is required when it is desirable to print fine-pitch devices using a thinner stencil foil, but print other devices using a thicker stencil foil. The paper presents the innovative technology of step-up and step-down stencils in a laser cutting and laser welding process. The step-up/step-down stencil is a special development for the adjustment of solder paste quantity, fulfilling the needs of placement and soldering. The focus of this paper is on the printing performance of step-up/step-down stencils and the paper ends up with a short outlook on 3D cavity printing.

Summary


The stencil printing process is one of the most critical processes in the electronic production. Due to the requirement: "faster and smaller" it is necessary to place components with different paste volume close together without regard to solder paste printing. In our days it is no longer possible to control the solder paste volume only by adjustment of the aperture dimensions.

The requirements of solder paste volumes for specific components are realized by different thicknesses of metal sheets in one stencil with so called step stencils. The step-down stencil is required when it is desirable to print fine-pitch devices using a thinner stencil foil, but print other devices using a thicker stencil foil.

The paper presents the innovative technology of step-up and step-down stencils in a laser cutting and laser welding process. The step-up/step-down stencil is a special development for the adjustment of solder paste quantity, fulfilling the needs of placement and soldering. This includes the laser cutting and laser welding process as well as the resulting stencil characteristics and the potential of the printing process.

Influencing factors on the printing process for step stencils like squeegee speed, squeegee angle, squeegee pressure, squeegee material, printing direction and distance from the step-edge to the nearest aperture are shown in this paper. A test layout was developed with different step heights and different distances from the nearest apertures to the step-edge to give proposals and guidelines for future designs. The transferred solder paste volume was measured with highly sophisticated systems and the results which are gained in this study allow new design guidelines.

The focus of this paper is on the printing performance of step-up/step-down stencils and the paper ends up with a short outlook on 3D cavity printing.

Conclusions


The step stencil technology has been a well-established technology for years and offers a multiplicity of solutions and applications for a mixed component range. The flexibility of the laser cut and laser welding process is enormous and allows the application of 3D cavity as well. Even used stencils step areas are replaceable and thus offers a cost saving factor.

Through the laser welding process the edges are automatically rounded off and the squeegee can easily adjust to step heights or depths without damaging the squeegee blade. The settings of the printing process influence the transferred solder paste volume distinctly, especially in the depression of the very close area to the step edge with increasing step heights.

The remaining paste residues in the depression affect the paste release and in consequence the transferred paste volume. The parameter settings like squeegee speed, squeegee angle, squeegee material and squeegee pressure show a significant influence on the wipe off behavior of the solder paste in the depression of the step stencil and therefore on the transfer efficiency. Especially with uncritical aperture geometries the access of solder paste in the depression has to be considered.

The aperture geometry plays a significant role in the transfer efficiency of solder paste in step down stencils especially by increasing step heights.

Initially Published in the IPC Proceedings

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