Registration mark

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Two registration marks on a circuit board

Registration marks are used on electrical circuit boards as optical reference points for automated manufacturing processes. These are among others the drilling of vias , the placement of electronic components by placement machine , the automatic optical inspection , the benefits milling or painting with protective paint.

The registration marks are optically recorded by cameras and thus the position of the tool relative to the circuit board is determined. The target positions are specified relative to the registration marks. Precise positioning prevents soldering errors and electrical short circuits between the soldering surfaces ( pads ). Registration marks are only used for production, they no longer have any function when the circuit board is in operation. Sometimes registration marks are used as test points at the same time.

Depending on the requirements, one or more registration marks are placed at suitable locations on the surface of the circuit board. With high accuracy requirements, usually with tolerances below 0.08 mm, registration marks are placed in the immediate vicinity of the assembled components and positions are measured several times during the assembly process in order to minimize tolerances in the automatic assembly machine. This is common for integrated circuits with a few 100 connection pins and housing designs such as Quad Flat No Leads Package (QFN) or Ball Grid Array (BGA).

nature

A round registration mark with gold plating

For easy recognition, a registration mark, which initially consists of a disc made of copper, should have the following additional properties:

  • About a millimeter in diameter .
  • No coating with solder mask , because the colored solder mask makes the optical detection of the registration mark difficult.
  • A paint-free ring around the copper washer, also at least one millimeter wide, to avoid reflections from the paint surface.
  • No covering with lettering or a logo ( assembly print ) or covering with assembled components.

The diameter of one millimeter is common. Good image recognition is also possible with smaller ones, which then take up less board area. Older pick and place machines may need slightly larger brands. At least two marks should be placed for circular registration marks. Instead of circles, crossed lines are also common. Longer lines allow precise orientation of the circuit board even without a second registration mark. In the case of low accuracy requirements, other, regular soldering surfaces can be used as optical reference points instead of registration marks.

If a board is populated on both sides, which is done in two separate steps, registration marks on both sides are helpful. The marks on both sides can be arranged independently of one another.

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