beamray , 05-01-2020, 02:51 PM
IEC 61188 can help a lot.(or IPC standards). Also, Yes Robert made grand video.
I only will add few things.
Fiducials are used for 2 purposes:they help to measure flatness (twist and bow) of a PCB and to make system of coordinate. More Feducials on a board is always better, because once optical control systems can make more systems of coordinates and also measure flatness more precisely. I always use my own rule of thumb: "if your pcb has dimension ratio not greater than 3 to 1, if larger side is not more than 100mm and feducials are placed in 3 corners than you do not need more. Is PCB is larger you may have additional fedutial triangle some where near centre. For ICs it works like that if it is second case, and ICs (QFN, LCC, BGA) has side not greater than 27 mm (goes for ARIA 5) you may have 2 fedutials because they are mostly for placing , in other case use 3 fedutials, because you need check flatness in the area one more time." That what I got from practice.
Also I will say, that you can do fedutials for group of ICs.
And moreover please always make clear for PWB manufacturer what twist and bow you need. IEC and IPC suggests 0,75% for smd, but when ICs getting big or your board is Class 3 board you really should do 0,5% twist and bow. and make sure that manufacturer didn't straighten it with pressure and/or heat. Because you may pay for their doing.