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, and/or the package coplanarity of ≤2 mils, and/or the substrate warpage at peak reflow temperature is ≤25µ. Under these conditions, the nonuniform stencil aperture opening is recommended, and the stencil thickness is 5 mil.
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.