Fiducial marks design for PCB assembly

In PCB assembly process, in order to align the entire PCB accurately, it is usually necessary to place fiducial marks at the four corners of the PCB. Sometimes, only three can be placed. There are three types of fiducial mark: panel fiducial mark, unit fiducial mark, and local fiducial mark.

PCB designers must follow some restrictions for drawing fiducial mark in layouting, to ensure they can be recognized by a machine's optical system. Shenzhen PCB maker SYS Tech introduces as below, these restrictions include:

1. The surface is same with PCB surface treatment. The flatness should be paid attention to. The edge is smooth and neat. The color is obviously different from the surrounding background color. The soldermask opening of fiducial mark is concentric with the mark. soldermask opening diameter for PCB panel or single PCB is 3mm, for the local mark, diameter is 1mm.

2. In order to increase the contrast between the fiducial mark and the substrate, a copper foil can be placed at inner layers under the mark point. The fiducial mark on the same board should have the same inner layer background, that is, the copper foil should be consistent under the mark point.

3. In order to ensure the recognition effect of solder paste printing machine and pick/place machine, there should be no pads, vias, test points, traces and silk screen marks in the fiducial mark area. Fiducial mark cannot be cut by the V-cut machine.

4. Fiducial mark for single PCB and PCB panel should be designed as components, and local fiducial mark should be designed as part of the component package. It is convenient to give accurate coordinate values for positioning.

5. Fiducial mark on the PCB, the mark center to component is not less than 5mm; fiducial mark on the break away tab, the mark center to component is not less than 3mm.