How is it important for fiducial points.
Fiducials on the PCB are used as datum points that assist the machine head in pinpointing the precise location to place components. Correct locations and definitions of these markers during design have significant implications for assembly quality. Based on these markers, SMT machines carry out their precise actions, such as solder paste application, component placement, AOI, x-ray inspection and more. Since the machine’s visual field at the edge of the PCB is limited, locating fiducials on PCB edges may prevent the placement machine from identifying them. If the machine head cannot locate the fiducial points, component placement may be impossible. Therefore, it is important to meticulously define these important fiducials at a distance larger than 7mm from the edge of the PCB during design.
Additionally, release of solder mask must be allowed to prevent concealment of fiducials and enable rapid, clear and precise identification of the fiducials. In the same context, it is recommended to place three fiducial points asymmetrically on the PCB surface to permit the automated machine to identify the PCB has entered the conveyor belt in the correct direction. Placing four fiducial markers symmetrically on the PCB surface may cause a situation where a reversed insertion is perceived by the machine as a normal correct insertion and will trigger incorrect placement of the components. Likewise, for automatic stencil printers, place fiducial markers on the solder paste and solder mask files.
Moreover, to certify the fiducial is clear and easily identifiable during x-ray inspection, a component-free area (larger than 1mm) must be maintained beside the fiducials along the entire width of the PCB, that is, from the other side of the PCB as well. This is imperative since the x-ray beam penetrates the entire width of the PCB, and therefore images its other side as well. Placing the components next (<1mm) to the fiducials on the other side of the PCB will not permit identification of the fiducials by the x-ray machine, hence making inspection much harder.
