Solder mask stencils are metal or plastic perforated sheets used to apply solder paste to land locations where the paste will be re-flowed to attached electronic components. Wafer bump stencils are used for screen-printing solder paste or conductive adhesives onto wafers to form bumps used in Chip-on-Board connections. Stencils are made of stainless steel, Nickel or Polyimide (Kapton®). Since a large number of small apertures must be drilled in the thin metal or plastic sheet, precision, quality and speed are paramount.
The apertures can be produced by etching or standard laser cutting. The last has the advantage of high flexibility, speed, and is able to cut out very small apertures. However, the problem with classical laser cutting is cut quality. In fact, standard laser cutting needs a post-treatment to clean the stencil and remove burrs on its backside. Using a short pulse laser guided by a water jet, a burr-free, clean cut can be reached. In comparison with the conventional laser the difference is, at the very least, quite remarkable. Additionally, the water jet prevents thermal damage and oxidation. No post-treatment is therefore necessary.