I am looking at designing a board with a 256 ftBGA. I haven't used a BGA before in layout and I'm not sure what rules I should use in setting the dimensions of the pads and solder mask openings. A paper by Lattice recommends "PCB solder pads match the cor- responding package solder pad type and dimensions". But they don't give full details.
For this part they say the package is SMD (solder mask defined) and specify the pad diameter as 0.5 mm. Then they say the "PCB (SMD) Solder Mask Opening" should be 0.4 mm. They also specify "PCB (NSMD) Solder Land Diameter" at 0.35 mm.
Should I assume the package solder mask opening matches the PCB number provided, 0.4 mm? Why do they specify the NSMD pad size rather than the SMD pad size? It just seems odd they way they have worded this.
How do you design pads for BGAs?