Are these PCB fiducials? - Page 1
Forum section: electronics / beginners

author: salvagedcircuitry
author_rank: Regular Contributor
timestamp: January 11, 2018, 11:08:14 pm
content: I was wondering if these "+" marks are fiducials or indications of a positive voltage plane / trace. The PCB fiducials I am used to seeing are a square pulled back soldermask area with an exposed copper circle in the middle. I know they are used for alignment purposes, so I was surprised when I saw these "+" signs implemented in a similar way. As you can see from the photo, the soldermask is pulled back in the background of the "+" sign, the + sign is in copper and not a silkscreen character.This particular PCB is the input power board on an ARRI cinema camera. There is an 8 pin lemo connector for input power and the camera is rated for 10.5-34v DC.This is clearly just a simple input board with a few caps and inductors.Thanks!

author: martys
author_rank: Regular Contributor
timestamp: January 11, 2018, 11:55:58 pm
content: Not very likely that + signs are anything more than a fiduciary mark, positive ground circuits are rare and the bottom + sign seems to be on a ground plane. Probably locating marks that were used during alignment used during PCB or solder-mask work or alternatively, places to drill mounting holes.

author: salvagedcircuitry
author_rank: Regular Contributor
timestamp: January 12, 2018, 12:02:59 am
content: Thanks! I had a feeling they were just fiducials. I traced it out and there are + rails and ground rails on both sides of the board and vias to connect each side.Thanks.

author: rx8pilot
author_rank: Super Contributor
timestamp: January 12, 2018, 12:07:25 am
content: My P&P can use + as a fiducial, but I have never tried it.I am sure that ARRI had assigned a small group of engineers to analyze the pros/cons of the chosen fids so that those 3 parts land in the correct spot.

author: salvagedcircuitry
author_rank: Regular Contributor
timestamp: January 12, 2018, 01:03:43 am
content: Huh. That would make sense. I thought you only needed one fiducial per side of the board. I guess having 2 or 3 can only aid in accuracy? Do you use multiple fiducials? Thanks.

author: salvagedcircuitry
author_rank: Regular Contributor
timestamp: January 12, 2018, 01:39:04 am
content: Having multiple fiducials does aid in automation with pick and place machines. I thought only one was needed. This web page has a nice writeup about it. Thanks for everyone's inputhttps://www.worthingtonassembly.com/blog/2014/12/29/what-are-fiducials-and-why-are-they-useful

author: rx8pilot
author_rank: Super Contributor
timestamp: January 12, 2018, 02:04:16 am
content: I use a minimum of 3 fids per board but also have panel fids and local fids.The 'local' fids are usually a single mark close very fine parts like BGA's.You need enough data to determine position, rotation, skew, stretch, etc.Short and misplld from my mobile......

author: dmills
author_rank: Super Contributor
timestamp: January 12, 2018, 09:36:54 am
content: '+' was once commonplace as a FID, but modern stuff seems to favour the circle on copper with a pulled back mask and I second the notion of three widely spaced across the board, it also helps catch a board loaded the wrong way around (Seen it!).Local fids are a good thing is you are going smaller then about 0.5mm pitch, or if the board is very large.Fid type tends to be a case of asking the board house what they like to see, I have had answers varying from "Don't need them" to "both circles and pluses".Regards, Dan.