QR codes/Datamatrix on PCB

[Alex Eisenhut]

I've been asked to implement QR codes on the blank PCBs we make, directly on the PCB, no sticker.

I don't even know what scanner they plan on using yet.

Is it better to have soldermask openings over exposed copper (enig finish), or over bare PCB material? I'd use silkscreen but I've noticed even when I ask for LPI it often comes back splotchy like an old old old school silkscreen. So I can't rely on that.

I have no clue what size I can get away with. The minimum of 15x15mm is already enormous for the kind of PCBs I make.

Datamatrix might be better?

Has anyone done this?

[jeremy]

I have dev boards from Nordic which do this. Nrf52840 usb dongle, about $10-15 iirc, the code is on the underside near the exposed USB pads. They use datamatrix on the soldermask, but the two boards I have from the same batch have different barcodes, so I assume that they add them later with a laser engraver. Based on the sharpness of the edges and fine detail, it’s not a CO2 but likely YAG, fiber or UV laser.

You need to be careful that the barcode is not inverted (dark where is should be white, etc) as many off the shelf scanners cannot read the inverted ones.

[jeremy]

Also found the same thing on a EFM8UB3 thunderboard dev kit

[Alex Eisenhut]

Yeah those are great but I forgot to mention that I'm tied to green soldermask, for now.
Maybe if my boss likes QR codes more than green mask, we'll change the soldermask color...

[jeremy]

I'm not sure green is a huge problem, you just need good lighting like any other machine vision project (maybe a colour filter to improve contrast), and the view needs to be off-axis to avoid the specular reflection from the light source on the shiny soldermask.

[Niklas]

We have a DMC about 6x6 mm that is laser marked just before the solder paste printing. We have products both with and without copper underneath the green solder mask. As already mentioned, avoid silk screen as the repeatability is poor for larger areas. Instead of the old mesh pattern you can get lines with LPI.
Make sure the DMC area + 1 mm frame is free from vias and try not to place any large solder pads close to it. Vias can be interpreted as dots and large pads can have an excess of flux that can distort the DMC.

[jeremy]

We have a DMC about 6x6 mm that is laser marked just before the solder paste printing. We have products both with and without copper underneath the green solder mask. As already mentioned, avoid silk screen as the repeatability is poor for larger areas. Instead of the old mesh pattern you can get lines with LPI.
Make sure the DMC area + 1 mm frame is free from vias and try not to place any large solder pads close to it. Vias can be interpreted as dots and large pads can have an excess of flux that can distort the DMC.

Out of interest, what kind of laser is it ? (What wavelength?)