A number of years ago in a previous life, we discussed laser marking 2-d matrix codes. We had a number of boards for customers that just were too populated to get any sort of a label onto. 2-D laser units were avaialable to work in line with our automated equipment, as well.
Unfortunately, they turned out to not be cost effective for us at the time. We went with the good old fashion paint pens...and small s/n labels (heat resistant, of course) for boards that were too small...coupled with CofC's to track revision etc. A bit of a process hog when you up-rev a lot of boards, and need to keep track of it...but, still more effective for us than the marking machine.
If you have a adhesive application machine such as the Creative Automation machines we use on our SMT lines you could do as our company does.
We pick a corner or an empty area on the board and apply a series of dots specifying the version you are building.
We also use one other method of board marking that is foolproof, and that is if say you build 5 different versions of the same board. Add 5 sets of fake pads to the art work, and when your boardhouse builds your PCBs have them labeled Rev A, B, C, D and E.
Then in the chipshooters program ADD a placement of a component to the appropriate pad for that rev being built. That way when you run the correct program it AUTOMATICALLY labels it for you without human involvment which can lead to mistakes.
Hi Pete, we use and have been using Brady Lasertab markers, they hold up through oven and wash, we use a standard laser printer (I believe we use a special toner though) and a software package called labels unlimited (can find on ebay or amazon) which is a very cheap software package and will also print barcodes. Has worked very well for us.