For the past several decades, SMT production has required highly sophisticated techniques to produce challenging circuits with such components as ceramic BGAs, RF shields and increasingly small components. More and more, mass-produced technologies such as high-demand IoT devices are also requiring these levels of sophistication with an overriding emphasis on saving time and cost. Components requiring different paste volumes are often placed quite close to each other to avoid ESD or voltage surge. This kind of assembly requires methods beyond the adjustment of aperture dimensions. Connecting different size parts together cleanly and with maximum transfer efficiently means using stencils with varying degrees of foil thickness. These are called step-up and step-down stencils. Step-up and step-down stencils enable the adjustment of varying solder paste quantities on the same board, including clean application of paste around raised areas.
Step-up and step-down stencils (step stencils) have been in use for about ten years. They have proven an effective means for integrating mixed components with different paste requirements onto a single PCB. Step stencils streamline assembly time and may lower assembly costs, because the step areas are replaceable, allowing stencils to be reused. Step stencil technology has evolved into several different types for different uses. They can be manufactured by wet chemical etching, or by laser cutting/welding, milling, and electroforming.
Step-up areas are used when a greater volume and height of solder deposit is needed. Step-down areas reduce the volume and height of solder paste. “Stepless” steps use ramped areas to accommodate large components and parts that need increased paste deposits. Angled steps reduce damage from the squeegee. Step-relief cavities compensate for irregularities on the board such as bar codes or other raised areas.
Components will small footprints such as 0201 frequently need to vary their solder paste volumes to avoid short-circuit problems. Since the length and width are such a small fixed area, the solution is to vary the height (thickness) of the solder paste using step stencils. The need for precise thicknesses of solder paste typically is increased by circuits that employ components with both smaller and larger footprints.
Besides accommodating a component mix with both high and low paste requirements, some of the applications of step stencils include achieving coplanarity with components of different heights, and printing circuit boards with stickers or other elevations on the surface. Step height may vary, but a minimum distance of 0.5 mm from the component pad to the edge of the step is generally recommended for step-up/step-down areas. A number of factors can influence transfer efficiency when using step stencils, including the squeegee speed, angle, pressure, material, and direction, as well as the distance from the edge of the step the nearest aperture.
Stencil designs should optimize the solder paste deposit for each component. Too much paste can prevent perimeter joint formation, too little can inhibit thermal transfer and create voids in the paste application that negatively affect electrical performance. This makes a high-quality stencil an indispensable printing tool for many applications. The selection of materials and manufacture of stencils is also important. For example, stress relieved stainless steel enables a tighter aperture density, and electropolishing can smooth out rough edges that may cause irregularities during the printing process.
Printing circuit boards using step stencils presents challenges that must be met to produce accurately printed boards. Yet when all of the parameters have been carefully considered, step stencils offer beneficial design solutions for PCBs with increasingly smaller mixed components while providing faster turnaround. Surface-mount boards with different paste height requirements can be machine printed in one quick step using step-up and step-down stencils.
NPI Services, Inc. uses state-of-the-art automated equipment to maintain the highest quality standards and ensure that the products we manufacture exceed all specified IPC Class 2 and Class 3 requirements. Assembly certifications include IPC-A-610, Class 1, 2, and 3, as well as IPC/EIA J-STD-001 and IPC/EIA J-STD-001 with Space Addendum.
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