Fiducial Marks

Revision 1, June 2001

 

A fiducial mark is a printed artwork feature (or features) that is created in the same process as the printed circuit board conductive pattern and that provides a common, measurable point for component mounting with respect to a land pattern or land patterns.

 

The above definition has been taken from SMEMA Fiducial Marks Standard 3.1 that defines fiducial mark properties on printed circuit boards. See http://www.smema.org/smema3.1.pdf

 

A fiducial mark is an etched feature in the copper pattern used by the pick-and-place equipment camera to ensure the most precise placing of a SMT component on the circuit board. It is important to have the highest possible contrast between the fiducial mark and the laminate surface; also, optimum is a fiducial mark with a 100 percent plane surface.

The fiducial mark surface is plated copper, and the shape is determined by several factors of which the most significant ones are the placing on the production panel and the pattern spread around the fiducial mark.

 

As a fiducial mark usually has the shape of a solid filled circle, typically with a diameter of 1-3 mm, it appears as a high-voltage area on which an increased copper electro-plating will take place.

 

Fiducial marks are often used to ensure an accurate placing of e.g. QFP components with many tie points, but they are also used as general optical targets for the positioning of less critical components, e.g. resistors and capacitors. With respect to the latter, the fiducial marks will be placed in the border material of the mounting panel where the pattern spread is less concentrated. Here, more concentrated copper plating may take place resulting in an uneven and larger surface than intended.

 

The contrast between the fiducial mark and the laminates becomes smaller. At the same time, the risk of scratches in the fiducial mark is increased as the higher placed solder mask over the nearest placed pattern parts do not give any protection.

Therefore, good design practice would be to place so-called copper thieves around the fiducial marks. Copper thieves are copper areas that will make the plating more even, see FIG. 1.

 

SMEMA Fiducial Marks Standard 3.1 specifies the following properties for fiducial marks:

 

3.3.1: Shape – The shape of the fiducial shall be a solid filled circle.

3.3.2: Size – The minimum diameter of the fiducial mark shall be 1,00 mm. The maximum diameter of the fiducial mark shall be 3,00 mm.

 

3.3.4: Clearances

3.3.4.1: Fiducial mark clearance – the size of the clearance area shall be at least twice that of the fiducial mark and shall be concentric with the centre of the fiducial mark, as shown in Fig 1.

3.3.4.2: Edge Clearance –The distance from a fiducial mark to the edge of a printed circuit board or fabrication panel shall not be less than the sum of 4.75 mm and the fiducial mark clearance.

3.3.5: Base material – The fiducial mark shall be bare or covered copper.

3.3.6: Coverings – The covering may be clear anti-oxidation coating, nickel plating, tin plating or a hot air leveled solder coating or (Chemitalic addition:) electroless nickel/gold.

3.3.6.2: Solder Resist Coatings – Solder resist coatings (masks) should not cover a fiducial mark and its clearance area.

 

FIG 1 shows the typical properties of a fiducial mark with the diameter 3 mm: