For the fabrication of Surface Mount Technology stencils (SMT Stencils) is important to know the different manufacturing technologies available to create the stencils and select the one that improves the efficiency of the final product. This will depend on the requirement of the application and specially the design of the board. The stencil design ultimately determines the size, shape and positioning of the deposits, which are crucial to ensuring a high-yield assembly process.
Three stencil manufacturing technologies are the most important in the industry: Electroforming, Chemical Etching and Laser cutting.
ELECTROFORMING PROCESS
This is an additive process whereby the stencil foil is created by electroforming nickel. The process creates a nickel stencil with a unique gasketing feature that reduces solder bridging and minimizes the need for underside stencil cleaning. The stencil is created by imaging photoresist on a substrate (or mandrel) where the apertures are intended and then plating (atom by atom) the stencil around the resist to the desired thickness.
Figure 1. Electroforming manufacturing process
CHEMICAL ETCHING PROCESS
Chemical etching is a process used to remove material (metal) by way of masking an area with a protectant (photo resist) and then submerging the material into an etchant (acid) to remove the material that has not been protected by the photo resist. For this reason, it’s considered a subtractive process. It has a wide range of applications, offering tight tolerances and a long list of available materials and thicknesses. Chemical Etching is very useful to fabricate step stencils and hybrid stencils.
Figure 2. Chemical Etching manufacturing process
LASER CUTTING PROCESS
Laser Cutting is also a subtractive process since the removal of material from the stencil foil creates the aperture. The laser beam (Figure 5) 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. The smoothness of cut will depend on many parameters, including cut speed, beam spot size, laser power and beam focus. As with Chemically Etched stencils, the laser-cut aperture size must be adjusted to the post-processing treatment employed because aperture size change will occur during this process. Electro-polishing and nickel-plating are also used to further smooth surface walls and improve solder paste release.
Figure 3. Laser Cutting manufacturing process
Figure 4. Stencil design
Figure 5. Stencil Laser