Ball grid array (BGA) packages having 0.4mm ball pitch require careful attention to printed circuit board (PCB) design parameters to successfully yield reliable and robust assemblies; the standard rules of thumb don't apply anymore. In fact, the design guidelines for 0.4mm and 0.5mm differ primarily due to issues surrounding shorts or opens between balls under the processor.
In addition to the design rules, fine-pitch board design is a team effort. Close coordination and communication between the device supplier, the PCB designer, the board fabricator, and the assembly shop is mandatory.
The following factors have a major effect on the quality and reliability of PCB assembly: pad design, via-in-pad (VIP) guidelines, via finishing, stencil design, solder paste requirements, solder paste deposition and reflow profile. This application report provides a starting point for understanding the current set of guidelines. It is strongly recommended that you perform actual studies in conjunction with your assembly house and board supplier to optimize the process.