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
Uniform stencil aperture can be implemented for the package size of up to 60 mm x 60 mm for 1 mm pitch and 0.92 mm pitch packages. Under these conditions, the uniform stencil aperture opening of 19.7 mils to 20.0 mils round is recommended, matching the PCB pad size and the stencil thickness is 5 mil.
Non-Uniform (Bull’s Eye) Stencil Aperture Design
Another option is to use a non-uniform stencil aperture, where the board land-pad diameter increases from the center of the device outward with a variable stencil opening 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.
For 1 mm pitch packages, the non-uniform stencil aperture design can be implemented for a package size up to 77.5 mm x 77.5 mm. Under these conditions, the non-uniform stencil aperture opening is recommended, and the stencil thickness is 5 mil.
For 0.92 mm pitch packages, the non-uniform stencil aperture design can be implemented for the package size up to 70 mm x 70 mm. Under these conditions, the non-uniform stencil aperture opening is recommended, and the stencil thickness is 4 mil.
Non-Uniform (Bull’s Eye) Stencil Aperture Design with Spacer
Non-uniform stencil aperture with spacer design can be implemented for the package size of greater than 70 mm x 70 mm or less than 80 mm x 80 mm for a 0.92 mm pitch package. Under these conditions, the non-uniform stencil aperture opening with spacer is recommended, and the stencil thickness is 4 mil. See the following table for recommended spacer dimensions. Spacer pads should be 35 mils (0.89 mm) in diameter (NSMD) with a solder mask opening of 39 mils (0.99 mm) and located on each corner with the center 0.96 mm from each edge, as show in the following figure. The stencil aperture for each spacer should be 30 mils (0.76 mm) in diameter.
The final stencil design should be based on an evaluation of the board design. Designers should work with their contract manufacturer to optimize the stencil design and assembly process.