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Obviously the panel needs fiducials, and I also place three of them on each PCB. Some IC footprints however also show local fiducials. I've seen them for instance for certain TQFP footprints. When are they required?

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  • \$\begingroup\$ Do you mind explaining what a fiducial is? \$\endgroup\$ – Kellenjb May 27 '11 at 14:19
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    \$\begingroup\$ @Kellenjb - fiducials are reference points (as copper circles) for the pick and place machine. They should be clearly isolated from anything else on the board (including silkscreen). You typically add 3 or 4 of them near the corners of the PCB. \$\endgroup\$ – stevenvh May 27 '11 at 14:25
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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.

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  • \$\begingroup\$ I agree about the self-aligning of the BGAs, but doesn't that also go for the QFPs? All QFPs I've seen are perfectly centered on their pads. Would pitch be the criterion for using fiducials? Like required for pitch 0.5mm or less? \$\endgroup\$ – stevenvh May 27 '11 at 17:38
  • \$\begingroup\$ @stevenvh All parts will self-align to some extent. In the case of QFPs and TSSOPs it doesn't take much for a single pin to bridge the gap between two pads. If that happens then the part won't self-align. The worst case is when the part is slightly rotated. It's all because of the thin and long pads/pins on a fine pitch. BGAs are a bit more forgiving because of the geometry of the pads. \$\endgroup\$ – user3624 May 27 '11 at 17:54
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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.

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    \$\begingroup\$ A pick and place machine positions a part using absolute coordinates from fiducials (either global or local), not by trying to line up pins and pads. \$\endgroup\$ – Kevin Vermeer May 27 '11 at 15:57
  • \$\begingroup\$ Why the downvote? \$\endgroup\$ – Leon Heller May 27 '11 at 16:09
  • \$\begingroup\$ None of the boards I have with TQFP parts on them have fiducials, they were presumably assembled with pick and place machines. \$\endgroup\$ – Leon Heller May 27 '11 at 16:10
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    \$\begingroup\$ @Leon - The downvote is because "the answer is not useful": The pin/pad alignment idea is incorrect by my understanding, and I've seen fiducials on TQFPs (0.5mm pitch, rectangular profile used on some of the video ICs we work with) but not on BGAs. Also, as David pointed out, the lower pitch on TQFPs makes it more important. It's nothing personal! \$\endgroup\$ – Kevin Vermeer May 27 '11 at 16:30
  • \$\begingroup\$ They are required for manual placement of BGAs, according to assemblers I've used. \$\endgroup\$ – Leon Heller May 27 '11 at 16:33
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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.

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