Array/Rails/Fiducials

This Gold Phoenix PCB Knowledge Center article explains the key elements of PCB assembly panel design: arrays, rails, fiducial marks and tooling holes.

Array

An array is combining a single PCB multiple times to make a larger array of connected boards.

This process is referred to as “stepping out” the individual boards. It is often referred to as “step and repeat.” Other terms used to describe an array are: panelized, stepped out, palletized and rout and retain.

Why Would Someone Want an Array?

The reason a manufacturer would want their board in array/panelized form is to help with the manufacturing process. It gives them the ability to load the parts at a much faster rate because they can do the whole array at once instead of loading one board at a time. The boards are loaded using automated equipment referred to as “pick and place” machines. This equipment simply picks up parts (normally surface mount devices) and places them down on the board.

Arraying/panelizing the boards will typically raise the cost of the individual boards. This happens because you can get fewer boards on a production panel when they are in an array versus when they are run as individual boards. Even though the individual board cost will go up, the cost of assembling the boards is less because of the array configuration.

If you do not have any special requirements for array panelization, Gold Phoenix PCB can panelize by its routine. If you have requirements, you can send Gold Phoenix PCB your panel drawing (panel size, panel ways, etc.), and Gold Phoenix PCB will follow your requirement to do it.

Rails

Rails, also called breakaway or coupon, are extra PCB materials that are added to the sides of the array to allow for easier handling during the assembly process. The rails could be on all four sides of the array or only on two sides. Normally 0.25 inch rails are added around all four sides.

Fiducial Marks

Normally 3 or 4 fiducial marks are added on the side rails around the PCB. Fiducial marks need to be added to the side that surface mount parts exist on. If both sides need solder parts, add fiducial marks on both sides.

It is acceptable to add fiducial marks only on the top side if surface mount parts exist on the top side only. 0.050" × 0.050" round fiducial marks are acceptable.

The fiducial is a copper pad circle with the same finish as the rest of the board. This provides alignment targets for the automated assembly equipment to get things lined up. Some manufacturers may require fiducials on each individual board as well. However, for the most part, there are pads or other features on the individual boards that can be used to provide additional alignment targets.

Tooling Holes

Tooling holes in the four corners of the array (in the rails) are fairly common as well. Again, these are used to aid in alignment and orientation when assembling the board. The typical size is 0.125 inch and they are non-plated.