Different Types of SMT Stencils and Their Uses

By Bob Wettermann

There are a variety of different configurations of SMT stencils depending on the intended end usage. SMT stencils are used from prototyping a few boards to mass assembly of hundreds of thousands. Their usage and configurations are varied for a variety of reasons.

Typical Framed SMT Stencil

Prototype SMT stencils are used to build the first few assembled prototypes of the PCB or they may be used to prove out a concept of only a single function of a PCB. Typically the prototyping quantities are from a few pieces up to 50 pieces for mass volume assemblers. These SMT stencils tend to be “frameless” stencils in that they are held tautly to non-permanent frames. “Foil only” or “frameless” SMT prototype stencils are foils with the appropriate aperture patterns lining up to the locations on the PCB requiring solder paste. These foil only SMT stencils can be manufactured from either plastic (Kapton™) or nickel-based stainless steel alloys. Plastic stencils have limitations on the size of the apertures and the distances between apertures because the limited rigidity of the plastic is not as high of that of stainless steel.

Kapton™ SMT Stencil

In another derivative of this stencil “flaps” can be added to the sides of the metal SMT prototype stencils which will help limited solder paste flow to other areas of the PCB.

Prototype SMT Stencil with Flaps

Another form of SMT stencils are frameless stencils. These are used when there are many board “spins” requiring lots of different stencils or when the PCB assembly area is space constrained. A single frame system has air pressure applied to the frame system and therefore creates the tension on the face of the stencil in order to create a quality print. These stencils are more cost effective than framed versions as the frame is eliminated and the shipping costs are reduced.

Finally, there are a variety of framed SMT stencil options which are fit to a given machine configuration. The frames provide the rigidity and the tensioning on the stencil. They come in a variety of different sizes and materials. Older stencil printing machines have cast steel frames while more modern stencil printing machines accommodate aluminum frames. Sometimes the SMT stencil frames are color-coded for a variety of reasons including identifying the alloy or solder paste versus adhesive versions.