When you look at the board, there is a weird part of the PCB right of the top left mounting hole. There is also another one above the top right mounting hole.

****                            ↓ Here's one                                            ** Arduino Uno

This image is directly copied from https://arduino.cc/en/Main/ArduinoBoardUno.

What are these? Is this just created when this is manufactured (to grip to to move the board or to grip during solder mask creation), or does it serve a purpose (something to hook a multimeter to)?

  • \$\begingroup\$ You should point at them or circle them. \$\endgroup\$
    – taco
    Apr 19, 2013 at 4:46

1 Answer 1


The points highlighted in this image (produced by @AnindoGhosh):

Arduino Uno - what are these

Are called Fiducial markers.

A fiducial marker or fiducial is an object placed in the field of view of an imaging system which appears in the image produced, for use as a point of reference or a measure. It may be either something placed into or on the imaging subject, or a mark or set of marks in the reticle of an optical instrument.

Basically, most modern electronics are assembled by a robotic system. To place the electronic components in exactly the correct place on a circuit board, the robot needs to know exactly where the circuit-board is with reference to it's robotic arm.

These markers are designed so that they're easy to measure using a computer-vision system. Basically, the pick-and-place robot has a camera on it's arm, and it takes a picture of each fiducial, performs a centroid operation on the little circle in the middle, and uses the centroid's results to establish the precise position of each dot. Since there are two dots, it can derive a precise position (both X-Y and R (rotational)) in a 2-dimensional plane.

The reason the soldermask (the blue covering over most of the PCB) is held back from covering the fiducial is that the components need to be correctly aligned to the copper layer (it's composes the actual pads), not the solder mask. Generally, there is some mis-match between the soldermask, the copper, and the silkscreen(the white printing) called "registration error".

centroid operation

Article on fiducials from LadyAda.

  • \$\begingroup\$ Given the fiducial is a well-defined circle, and that the robotic system is on a parallel plane to the PCB, why isn't just one fiducial sufficient to establish the X-Y match? \$\endgroup\$
    – boardbite
    Jun 15, 2013 at 3:58
  • 5
    \$\begingroup\$ @boardbite - Because the board can rotate on that X-Y plane. You need to know X, Y and Theta (rotation) to place parts accurately. \$\endgroup\$ Jun 15, 2013 at 5:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.