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The Short Scoop: The Ins and Outs of Step Stencils

The demand for step stencils is increasing, but why? The answer: shrinking components. In the early days of SMT assembly, step stencils were used to reduce the stencil thickness for 25 mil pitch leaded device apertures. However, as SMT requirements became more complex and, consequently, more demanding, so did the requirements for complex step stencils. Today, fine-pitch components, which necessitate a thin stencil for acceptable paste release, are sandwiched beside a variety of devices and components that require higher paste height. Getting the solder paste volume needed for larger components, while still having an area ratio that allows for good paste release on small components, can be challenging. Step stencils offer the SMT process engineer significant flexibility in achieving the proper solder paste height and volume for the overall paste printing process.

Step stencils range from simple designs to complex multi-thickness steps. With them you can print different paste heights, with one stencil, in one print. Without them, a separate print run and stencil might be needed for each solder paste height or a manual application for additional paste deposit required. Using a step stencil is much faster, prints an actual solder brick, and allows for solder paste volume control. Dispensing and other printing methods are slower, many times more manual, yield less control on solder paste volume, and usually require a second step, after initial printing.

Step stencils are available in all stencil technologies: Electroformed, laser cut, and chemical etched. These steps are usually etched or electroformed depending on the application and the area ratio. Although traditionally etching was used for most stencil manufacturing and aperture formation, etching is now predominantly used for step stencils. The thickness of the step areas and the overall thickness of the stencil are determined by the area ratio of the component. Step stencils have a primary base thickness and step-up or step-down areas across the board pattern depending on the thickness and volume of solder paste a particular component requires. Read the full column here.Editor's Note: This column originally appeared in the August 2013 issue of SMT Magazine.