Solder paste is applied to PCB metal pads by screen printing. The volume of the printed solder paste is determined by the stencil aperture and the stencil thickness. In most cases, the thickness of a stencil must be matched to the needs of all components on the PCB. Stencil apertures should be a circular shape. To ensure a uniform and high-solder paste transfer to the PCB, laser-cut stencil, made from mostly stainless steel, is typically used. Nickel Blank stencils, referring to stencils where the entire foil is laser-cut from a sheet of pure nickel material, can also be used. However, high-quality nano-coated stencils (laser cut from stainless steel) can perform as well as or better than Nickel Blanks.
Uniform Stencil Aperture Design
- For packages with 0.92 mm ball pitch, a uniform stencil aperture opening of 19.7 mils to 20.0 mils round is recommended, matching the PCB pad size.
- For packages less than 55 mm x 55 mm with 1 mm ball pitch, a uniform stencil aperture opening of 19.7 mils to 20.0 mils round is also recommended.
Bull’s Eye Stencil Recommendation
Another option is to use a Bull's Eye stencil aperture, where the board land-pad diameter increases from the center of the device outward, matching the variable aperture diameter of the stencil, at a rate that depends on the warpage as a function of thermal expansion and mechanical attachment. This can vary depending on the PCB. The bull’s eye offers a capture margin, because with an increased opening size with respect to the outer BGA balls, more solder paste is printed. A recommended stencil design for the bull’s eye design is shown in the following figure. Similar stencil designs are recommended for packages 55 mm x 55 mm and larger with 1 mm ball pitch. The final stencil design should be based on an evaluation of the board design. Designers should work with their CM to optimize the stencil design and assembly process.