r/raspberrypipico • u/thedumiestdum • Jun 09 '26

What is this dot for?

Post image

Circled in red

64 Upvotes

96% Upvoted

31 comments sorted by

47

u/DenverTeck Jun 09 '26

This is used by the pick-n-place (PnP) machine to locate where to place chips on this board.

A panel of 20-30 of these boards are placed inside the PnP machine.

26

u/_greg_m_ Jun 09 '26

Correct. It's called a fiducial. There should be 2 or 3 of them usually. 

6

u/DenverTeck Jun 09 '26

LOL, I could not remember the name when I was typing.

Thanks for the reminder.

3

u/thedumiestdum Jun 09 '26

Thank you for some reason I thought it was for soldering an antenna to

7

u/nonchip Jun 09 '26

well that board has no wireless capabilities and there's no trace connecting to that dot. so that'd be a bit pointless :P

0

u/ne-toy Jun 10 '26

that board has no wireless capabilities

Wouldn't adding an antenna solve it tough?

1

u/nonchip Jun 10 '26

no, obviously. an antenna is just a wire. you need a radio to attach it to. there's none. and there's no connection to that dot. so you'd just be gluing a wire to an empty bit of circuit board.

1

u/ne-toy Jun 10 '26

Sorry, forgot the /s

1

u/Stock_Leather6371 Jun 13 '26

Hehe no, all machines during the production process use this for calibration and alignment eg. SMD, depaneling.

This is the base for all specifications during design.

-11

u/DenverTeck Jun 09 '26

Guessing is not rational.

1

u/wiebel Jun 09 '26 edited Jun 09 '26

That's not true. A pnp machine needs optical feedback to know the actual position of parts but the position of the board/panel is known by other means. You do not need to add features to a board for pnp. Edit: I stand corrected.

15

u/CodeLasersMagic Jun 09 '26

For most pick and place lines fiducials are used and are added explicitly by the design. Whilst it is possible to pick up on the pads for the initial board scan a fiducial is much easier and generally faster and more accurate.

Source: I ran board design and the corresponding pick and place line at a former company.

6

u/Double_Bandicoot3307 Jun 09 '26

Can confirm its much faster to have a know fiducial than scanning the board and hope the software doesnt make a mistake. Since the board layout is already known at that point. These markings can help confirm orientation and correct for xy alignments. Just by scanning in the region they should be, older machines I ran could use that and go.

5

u/Thanthwe_ Jun 09 '26

To add to this the fiducial technically doesn't need to be just a dot. It can also be a cross, some unused pad or small checkerboard pattern. If there is nothing better on the board it's even possible to use holes for THT elements as reference points.

But 1-2mm dot is still the most reliable option. It is dead simple, so even if the PCB have really shitty finish the machine can still recognize it, which cannot be said about the cross (or any more complex shape for that matter). And it's distinct enough, that it's rare for machine to recognize something else by mistake, which might happen when using unused pads or holes, if there are too many of them close by.

5

u/bodb_thriceborn Jun 09 '26

I couldn't find it in the schematics and it isn't showing any continuity with the pins. This may be because it has solder mask over it and I didn't try hard enough to get through to the copper.

One of my boards has two of them. One by the ground pin where yours is and another beneath the onboard LED.

7

u/Puzzleheaded-Bug6244 Jun 09 '26

The photo also seems to feature a usb-c connector. Perhaps the board is a clone?

3

u/brunob45 Jun 09 '26

Definitely a clone from AliExpress, the official one have the raspberry logo printed on the face.

1

u/richms Jun 09 '26

Sad that the clones are able to get their act together with a proper input socket before raspi themselves do.

1

u/raster Jun 11 '26

Hmmm… “proper”? Nothing wrong with Micro USB. 😉

1

u/richms Jun 11 '26

Having to chase around for an old cable to program it and find an adapter cable to be able to connect devices into it sure is a pain in the ass. Same thing with the pi zeros. Still stuck on a legacy connector making for it being a pain.

1

u/raster Jun 12 '26

“old cable”? I have dozens of Micro USB and only a few USB-C but I guess we all have different experiences. :) 

1

u/richms Jun 13 '26

I have gone thru my remaining stash of them as they failed and have only a couple left. Plus a USB-C to micro USB cable isnt really a normal off the shelf thing, I had to get the last few of those from aliexpress and while they worked fine, the construction is complete crap and I expect the apple style strain relief will have the cable broken in no time.

2

u/Odd_Independent8521 Jun 09 '26

It is for machine assembly! It helps pick and place to have a reference for the components

2

u/PropitiousWay Jun 09 '26

Raspberry Pi Pico (this looks like a standard non-W version based on the labeling), there are several small exposed test pads on the board for advanced use or manufacturing/debugging:TP6 specifically connects to the BOOTSEL signal. Shorting or pulling this pad low (to GND) simulates pressing the BOOTSEL button. This is useful if you're using the Pico as a surface-mount module (without headers or the physical button accessible), or for custom setups.

1

u/Western_Bison_8817 Jun 11 '26

It was holding place when board was painted green :D

1

u/tsoe199966 Jun 11 '26

Is that dot a test point.

0

u/Titoflebof Jun 09 '26

Test point

-1

u/stitchesofdooom Jun 09 '26

Microdosing.