Could someone point me to the documentation that describes the gold dot and surrounding gold circle on the front of my arduino mega v2 board. It is right above the ICSP header and reset button, and directly underneath the letter 'U' in the silk screened word 'communication' I've not been able to find any reference to it anywhere. Is is a jumper, a test pad, RF pad???


That is most likely a Fiducial Mark for the vision system of a pick-and-place assembly robot to key off of in order to more precisely place the components on their pads.

If you look carefully, you will see a second one located below the DC barrel jack.

Boards like this are manufactured in a gridded panel having a number of copies in each direction, and only cut apart after they are largely complete. With such a large workpiece, any linearity error in the manufacturing could result in a high-density component being misplaced far enough to cause problems. Having a pair of fiducials on each individual board in the panel can help calibrate out errors not only which might otherwise accumulate over the large dimensions of the entire panel, but also rotational skew of the panel in its holder.

Picture of the Arduino 2560 from arduino.cc site

You can see both on this picture of the Arduino Mega 2560 from the arduino.cc site


To add to Chris's answer: fiducials appear at three levels. First there are fiducials at the panel level, which contains one or more PCBs, depending on their size. Next there are fiducials for the PCB, acting as a position reference for component placement. Sometimes you can see component fiducials close to some fine pitch components, to make sure they are placed with high precision. You'll sometimes see these latter fiducials on opposite corners of QFP (Quad Flat Package) parts. (The AVR controller on the Arduino Mega 2560 is in QFP.)

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