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Naming Convention for Fiducial Footprints

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cgnd View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cgnd Quote  Post ReplyReply Direct Link To This Post Topic: Naming Convention for Fiducial Footprints
    Posted: 01 Oct 2024 at 11:44am
Is there a standard naming convention for board-level fiducial footprints? 

I know there is a pad stack naming convention for local fiducials embedded inside a land pattern, but I'm curious if IPC has a recommended naming convention for a footprint that is just a single global fiducial for use on a board/panel.

Also, if there is an IPC standard that defines standard fiducial requirements, can you point me to it?

Thanks!
Chris
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 01 Oct 2024 at 12:32pm
IPC does not have a footprint naming convention for Fiducials. 

There are 4 types of fiducials defined in IPC-7351B pages 33 & 34:
  • Local
  • Global 
  • Panel
  • Zonal
All 4 types fall into the same category as far as size and shape.

Local Fiducials: Local fiducial marks are used to locate the position of an individual component requiring more precise placement.

For these components, two local fiducial marks are required for correction of translational offsets (x and y position) and rotational offsets (theta position). This can be two marks located diagonally opposed within or outside the perimeter of the land pattern.

All fine pitch components should have two local fiducial systems designed into the component land pattern to insure that enough fiducials are available every time the component is placed, removed and/or replaced on the printed board. All fiducials should have a solder mask opening large enough to keep the optical target absolutely free of solder mask. If solder mask should get onto the optical target, some vision alignment systems may be compromised due to insufficient contrast at the target site.

If space is limited, one may be able to share a fiducial from an adjacent component within the location constraint. Also, if space is limited and the printed board is deemed by manufacturing to be sufficiently small, component fiducials may be omitted and global fiducials used in their place.



Panel and Global Fiducials: Global fiducial marks are used to locate the position of all circuit features on an individual printed board. When a multi-image circuit is processed in panel form, the global fiducials are referred to as panel fiducials.

A minimum of three fiducial marks is required for correction of offsets (x and y position), rotational offsets (theta position) and nonlinear distortions (scaling, stretch and twist). These should be located orthogonally as far apart as possible on the circuit or panel.

It is good design practice to locate global or panel fiducials in a three-point grid-based datum system. The first fiducial is located at the 0-0 location. The second and third fiducials are located in the X and Y directions from 0-0 in the positive quadrant. The global fiducials should be located on the top and bottom layers of all printed boards that contain surface mount as well as through-hole components since even through-hole assembly systems are beginning to utilize vision alignment systems.



Zonal Fiducials: To ensure the accurate placement of multiple surface mount components that are not near ‘component specific local fiducials’ or ‘global fiducials’, additional ‘zonal fiducial targets’ may be placed within a zone or an area of the board assembly to compensate for printed board dimensional stability.

Size and Shape of Fiducial: The optimum fiducial mark is a solid filled circle. The preferred diameter of the fiducial mark is 1.00 mm. The maximum diameter of the mark is 3.00 mm. Fiducial marks should not vary in size on the same printed board more than 25 um. A clear area devoid of any other circuit features or markings shall exist around the fiducial mark. The minimum size of the clear area shall be equal to twice the radius of the mark.



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Post Options Post Options   Thanks (0) Thanks(0)   Quote cgnd Quote  Post ReplyReply Direct Link To This Post Posted: 01 Oct 2024 at 1:32pm
Thanks Tom.

Couple follow up questions:

1. Footprint Expert has three pad sizes for fiducials depending on density level (1mm, 0.75mm, and 0.5mm). Are these sizes specified in an IPC standard or are they just the defaults in the Footprint Expert software? (IPC-7351 says "preferred diameter is 1mm", but does not seem to specify recommended sizes per density level, or a minimum fiducial size).



2. In the absence of an official IPC convention, I'm considering using the following convention for fiducial footprints in my library: FID + Pad size + Density Level

Examples: FID100M, FID75N, FID50L

I omitted the mask/keepout sizes from the naming scheme because IPC-7351B defines these as equal to 2R (Figure 3-13), but I'm not 100% sure this is always the case?

Alternatively, these dimensions could be specified explicitly using the modifiers from the padstack naming convention: FID + Pad size + M Mask size + K Keepout size + Density Level

Examples: FID100M200K200M, FID75M150K150N, FID50M100K100L

Curious if you have any feedback on either of these or would design this a fiducial naming convention differently than the options I show above.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cgnd Quote  Post ReplyReply Direct Link To This Post Posted: 01 Oct 2024 at 1:40pm
I just realized that the fiducials can be round or square, so maybe a better alternative might be: FID + [C|S] Pad size + M Mask size + K Keepout size + Density Level

Square fiducial, 100mm pad, 200mm mask, 200mm keepout: FIDS100M200K200M

Circular fiducial, 50mm pad, 100mm mask, 200mm keepout: FIDC50M100K100L
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 01 Oct 2024 at 1:46pm
1. the 3-Teir density level for fiducials in Footprint Expert are not an IPC guideline. They are there for users to define whatever they want. 

2. your recommendation for a footprint naming convention looks good. You're using the pad stack name c100m200k200 with a FID prefix. 

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