When do I use fiducials for individual components on a PCB?
Fiducials are used by the pick and place machine to provide better accuracy when placing components on the PCB. There is a camera that recognizes the fiducials and uses it as a registration point to calibrate where the machine thinks it is on the PCB.
There are two types of fiducials: Global and Local.
Normally a PCB will have 3 global fiducials per side (top & bottom), and usually in the corners of the PCB. This is so it can recognize the boards overall orientation and position.
Local fiducials are located near some of the critical parts. Usually there are two fiducials for each part, in opposite corners. IF you have several critical parts that are close together then a fiducial can be shared by two or more parts-- reducing the number of fiducials required and the the PCB space taken up by them.
Where you need local fiducials really depends on the pick and place machine that will be used, and the placement accuracy required by the component. Chips with a finer pin pitch will need fiducials more.
It's interesting to note that TQFP's need fiducials more than most BGA's. Most TQFP's have a pin pitch of around 0.5mm, while most BGA's are 0.8 to 1.27mm. BGA's also have a cool ability to somewhat self-align due to the surface tension of the melted solder. But I need to stress that this is very component and machine dependent, so check with your assembly shop.
Also machine dependent is going to be the construction of the fiducial. Things like how big the pad is, and how much the soldermask is pulled back. Usually the fiducial is round, but sometimes square or bow-tie shaped.
Another thing is that some assembly shops will request fiducials to just feel good about things-- but don't really need them. My second to last PCB had had lots of fine pitch BGA's, QFN's, and TQFP's and had no fiducials on it, but there were no issues with parts placement. My current board is nowhere near as difficult but they are requesting fiducials. Go figure. I'll humor them and put the fiducials on it.
They are most often used with BGA devices, because the pads can't be seen when the chip is placed in position manually. Most assembly companies insist on them. I don't think I've seen them used with TQFP parts.
My experience is that if the pin pitch of the part is larger than .635mm, then the global fiducials are adequate. If the part has a pin pitch .635mm or smaller, then it should have it's own fiducials. I don't differentiate between qfp or bga parts. I use a 1mm smt dot inside a 3mm dia sm clearance for all my fiducials and place them at opposite corners where they will not interfere with pin 1 (A1) markings.