Fiducial marker

Marks are used on electrical circuit boards as optical reference points for the drilling of vias and the placement of electronic components by pick and place machines. They are optically detected by the cameras drilling or mounting head, and thus determines the position of the tool relative to the printed circuit board. The target positions are given relative to the fiducials. The precise positioning solder defects and electrical shorts between the solder pads ( pads ) should be avoided. Fiducials are only of production, have in operation of the circuit board no longer function.

Depending on requirements, one or more registration marks at suitable points are placed on the surface of the circuit board. For high accuracy requirements, usually at tolerances below 0.08 mm, registration marks are placed on the placed component in close proximity and positions within the insertion operation measured several times to tolerances in the placement machine to minimize. This is usual in integrated circuits with some 100 connector pins and housing types such as Quad Flat No leads package ( QFN ) and Ball Grid Array (BGA ).

Nature

For good visibility of a registration mark, which initially consists of a circle of copper, they should have the following properties:

• A diameter of about one millimeter;
• No coating with solder resist, because the colored solder mask complicates the optical detection of the fiducial;
• A paint-free copper ring around the circle of also at least one millimeter width to avoid glare from the paint surface;
• Not been covered by an inscription or logo ( silkscreen ) or coverage stocked components.

The diameter of one millimeter is common. Good image recognition also comes with smaller, then consume less space on the platinum surface. Older placement machines need something larger markers if necessary. When registration marks in a circular form at least two marks should be placed. Instead of circles and crossed lines are common. Longer lines without allowing a second mark, the exact orientation of the printed circuit board. At low accuracy requirements, other, regular solder pads can be used as a visual reference points instead of fiducial marks.

A printed circuit board equipped on both sides, which is accomplished in two separate steps, then registration marks are useful on both sides. The arrangement of the marks on the two sides can be carried out independently.