The requirements for production of SMT stencils are always increasing. Numerous applications today use a variety of components on one circuit board, which requires the application of different amounts of solder paste in one printing process.
For mixed assembly, SMT stencils are therefore required that have partial thickness gradations. They are known as stepped, step up, step down or multi-level stencils.
Modern electronic assemblies usually consist of a variety of very small components, delicate rasters and some larger design elements. As a result, SMD stepped stencils must ensure that in smaller areas, there is a greater diameter for the increased paste volume.
These gradations are produced by additive or subtractive methods.
Stencils constructed using additives can only be produced galvanically, subtractively produced stencils are produced by etching or mechanical processing. Patchwork stencils are a special case, in which patches of variable material thickness are introduced into stamped sections.
All these options are expensive and time consuming, which has hitherto prevented widespread application.
The newly developed step-up stencil process makes possible for the first-time an additively constructed, affordable and quickly produced stepped stencils.
For the step-up stencil procedure, patches in various thicknesses are welded to a base sheet. The position of the patches is measured precisely using the LPKF laser machine and the patches are welded to the base sheet using spot welding. Nitrogen is used as the welding gas. This procedure allows us to create variable geometry patches and boost their durability.
After the patches have been welded on, the SMT layout is cut out using lasers. The inner walls of the pad apertures are cleanly cut as the stainless steel base melts whilst they are being cut. Another advantage of this procedure is that the distances between the steps and pad apertures can be kept very small.
Step stencils that are made via this process also have a longer service life than step stencils which are manufactured using subtractive processes. This is because the surface of the stencil is not damaged in this process, and its durability is actually increased as a result of the patches that are welded onto it. The step stencils that are produced using this process can be refined further, i.e. with electropolishing and nano-coating.
A stepped stencil should only be used if the standard variants of laser-cut SMD stencils are no longer sufficient. Above all that can be the case when there is a high component mix and thus different solder paste needs. A necessary second imprint would be a corresponding case of application.
During the design of stepped stencils there are particular guidelines to consider. Above all they refer to the arrangement and height of the steps and the appearance to necessary distances between the components and the step edges. The choice of a flexible squeegee, as well as squeegee direction/compression, are crucial factors in the use of stepped stencils.
Photocad offers two quick mounting systems for SMT stencils – Quattro-Flex and VectorGuard. Both systems are suitable for stepped templates.