PCB Fiducial Guidelines
This set of guidelines will help you design your PCB for manufacturing on an automated assembly line such as the one at Dena Technologies. Some of the specific details may vary with other service providers, but the general principles apply anywhere.
fiducial – a registration mark (target) which can be located precisely by a machine vision system.
IPC-7351 (Guidelines for SMT Design) section 3.4.4 explains the requirements for fiducials. IPC-2221 (Generic Standard on PCB Design) refers to the SMEMA recommended fiducial design, but does not contain specific information about fiducials.
Machine visions systems typically have black and white cameras set to relatively high contrast. Metal surfaces reflect light and appear white while non-reflective materials such as epoxy appear black. The fiducial will be a copper pad on the PCB with a clearance around it exposing the glass-filled epoxy FR4 substrate. Any metal features or components too close to the fiducial could confuse the machine vision system. Soldermask is often glossy and may appear irregularly reflective at the edge so it is recommended to have a soldermask expansion. In stainless steel stencils, the fiducial is an epoxy-filled aperture which appears black on a white background.
The ideal shape of a fiducial is a simple solid circle. The crucial piece of information that fiducials provide are the coordinates of their centers and a circle’s center is equidistant from the edge regardless of rotation. Complex shapes may confuse the vision system as it struggles to find edges.
The typical size of a fiducial is 40mil to 50mil (1.0mm to 1.27mm) diameter copper aperture with 80mil to 150mil (2.0mm to 3.81mm) diameter soldermask aperture. It may be possible to use even smaller fiducials, as theoretically a fiducial could be as small as the smallest terminal on the PCB, but very small pads are more prone to shape irregularity especially with lower cost finish. The soldermask aperture should be as large as possible, between 2 to 3 times the diameter of the copper. All of the fiducials on a board should be the same size.
Fiducials should be placed as components from a library, not as free pads, so that they appear as components with coordinates in the pick and place centroid data file.
There should be no traces or surface mount component bodies within the soldermask aperture which may confuse the vision system, but it may be okay to place through-hole components on top of fiducials in most cases.
It is suggested to place the two diagonal fiducials asymmetrically, at different distances from their nearest edges. That way if the board is mistakenly loaded rotated 180° the fiducials will not be in the expected positions. It is not recommended to use four symmetrical fiducials because it could enable the board to also be loaded upside down. The fiducials should be as far apart in both X and Y axes as possible. A third fiducial is not required, but can better correct for rotation especially if scaling is not accurate or if fiducials are fairly close together in one or both axes.
There are three tiers of fiducials in PCB assembly: component fiducials, board fiducials, and panel fiducials. Component fiducials are used around a specific component which must be placed especially precisely such as a BGA, but they are not commonly used as the board fiducials are usually close enough. Board fiducials are used for the placement of all SMT components on a single board. Panel fiducials are used mainly by the solder paste printer and are replicated on the stencil, but they are also used in multi-up panels when the individual board fiducials are used to indicate X-outs (boards which failed electrical testing and should not be populated by the machine).
After reviewing this guideline please familiarize yourself with the PCB Panelization Guideline.