Laser stencil opening design
A general question about stencils that the laser stencil design guide will address is how aperture design affects printing performance. Three main performance issues in solder paste printing. The opening size [width (W) and length (L)] and the thickness of the stencil metal foil (T) determine the volume of solder paste printed on the PCB. During the printing cycle, as the squeegee passes over the stencil, the solder paste fills the openings of the stencil. Then, during separation of the board from the stencil, the solder paste is released onto the pads of the board. Ideally, all solder paste filling the opening is released from the walls of the opening and adheres to the pads of the board, forming a complete solder brick. The ability of solder paste to release from the inner hole wall is mainly determined by three factors: the area ratio/aspect ratio of the stencil design, the geometry of the side wall of the opening, and the smoothness of the hole wall.
Generally accepted design guidelines for acceptable solder paste release are aspect ratios greater than 1.5 and area ratios greater than 0.66. The aspect ratio is a one-dimensional simplification of the area ratio. When the length is much greater than the width, the area ratio is the same as the aspect ratio. When the stencil is separated from the board, solder paste release encounters a competing process: will the solder paste transfer to the pad or stick to the side hole walls of the SMT die stencil? When the pad area is greater than 2/3 of the inner hole wall area, 85% or better solder paste release capability can be achieved.
Stencil technology also plays a major role in the percentage of solder paste released. The geometry of the sidewall of the opening and the finish of the hole wall are directly related to the stencil technology. Electropolished laser-cut stencils have smoother inner cell walls than non-electropolished laser-cut stencils. For a given area ratio, the former releases a higher percentage of solder paste than the latter. For aspect ratios approaching 1.5 and area ratios approaching 0.66, some stencil technologies achieve higher percentages of solder paste release better than others.
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