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 This 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.
RECOMMENDED: AMD recommends using a 4 mil to 5 mil thick stencil.
RECOMMENDED: AMD recommends using a uniform stencil aperture opening of 19.7 mils to 20 mils round, matching the PCB pad size for packages less than 60 mm x 60 mm with 1.0 mm ball pitch.
RECOMMENDED: AMD recommends using a non-uniform ( Bull's Eye ) stencil aperture opening for packages larger than 60 mm x 60 mm and smaller than 77.5 mm x 77.5 mm with 1.0 mm ball pitch, including 2892 and 3824 pin packages.
