Stencil Aperture Creation


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There are several accepted methods for stencil manufacture, such as etch, laser-cut, and electroform.

Additive Processes

Electroformed stencils are made by an additive process as opposed to the subtractive (etching) process used for chem-etch and laser-cut. A nickel bath containing nickel ions and a nickel-hardening additive is used to electroplate onto a substrate called a mandrel. Photo-resist on the mandrel is exposed and developed, forming photo-resist pillars that delineate stencil apertures. Nickel is electroplated one ion at a time until the desired foil thickness is achieved. Then, nickel foil is removed from the mandrel, creating the completed stencil.

The Laser-cut Process3

Laser cutting stencils is a subtractive process. Gerber data is translated into a CNC-type language that the laser understands. The aperture is cut out by moving the laser head only, moving the table holding the stencil only, or a combination of each. The laser beam enters inside the aperture boundary and traverses to the perimeter, where it completely cuts the aperture out of the metal, one aperture at a time. Cut smoothness depends on many parameters, including cut speed, beam spot size, laser power, and beam focus. Typical beam spot size is about 1.25 mils.4 Laser-cut stencils can have aperture widths as small as 0.004″ with an accuracy of 0.0005″, suitable for ultra-fine-pitch component printing. Laser-cut stencils also produce ragged edges because the vaporized metal is transformed into metal slag during the cutting process. This can cause paste clogging. Smoother walls can be produced by micro etching.

Etch

In a chemical etching process, the rosin is photo-developed based on positive Gerber data onto a metal foil; the metal foil is then submerged in a acid bath to wet the exposed areas and etch the steel, often leaving a rough aperture wall. This type of tool manufacturing method is one of the least expensive; however, due to the resulting aperture geometry and walls, it is not suitable for fine-pitch printing requirements.

Etching manufacturing methods tend to leave a rough surface on the aperture walls of the stencil. A post-processing step is often suggested to reduce aperture wall surface tension and improve paste release. Results include finer material removal from aperture walls, and post-process starting-aperture sizes must be adjusted to the post-processing treatment employed, as their finished size will change.

This excerpt is taken from Laser-cut Electropolish and Laser-cut Nanocoat Stencils: A Comparison of Finish Performance for Complex Designs, by Miguel A. Lara, Celestica. Read it in the SMT Printing center.

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