PCBgogo

 I'm looking at options for applying a serial number to each PCB which we assemble so we can track things like test results, operator, specific component reels, etc., to a specific board.  This is an internal manufacturing serial number, and won't be used/seen by customers.
The current thought is to apply 2-3 datamatrix barcodes to the board (unique number per board, all barcodes coded the same on a board).   At least one per side, maybe 2 on the top, 1 on the bottom.   These would be small - probably around 6mm square including the label.  Or, perhaps some other symbology (like a 5mm x 15mm code39 2d barcode, etc).   Regardless, I'd use a polyimide label which is rated to survive the reflow oven.   We're about done using the wave solder machine, so that probably isn't a concern.
Of course then you have the cost of the label and the cost to apply to the board.
The other thought  I've had was to laser or otherwise print these on the board. Or even better is if I could find a reasonably priced PCB house which will do the serialization for me in some way.
Curious what others are doing or have seen done?