Hi Mahmood,

Both solder mask and legend are photoimagible 2-part epoxies from LPKF. There are others out there. I've heard of a solder mask that can be machined, but I've never actually seen it. Pretty much everything I've seen, whether film or liquid, is photoimagible. You'll pretty much need to set up a UV source and you'll need to do the chemical developer. I like LPKF's because the only disposal issue I have is the developer, and it comes with a neutralizing conditioner that makes it compliant for sewer disposal.

I can and will tell you this. If you are going to do fine pitch SMT, a solder mask is necessary in the most practical sense. It's not just about making the board look good. With the larger formats, the ones where you can use a pneumatic dispenser to apply solder paste to the pads (e.g. 0805, or even the 0.050" pitch soic parts if your hand is steady enough), solder mask looks good but isn't really necessary. On the other hand, once you get down to the 0.025" pitch parts, solder mask starts to become important for it's functionality, keeping the solder in its place during reflow. And you simply aren't going to put down a CP2102 without solder mask.

Legends, I must admit, are handy but not abosolutely necessary. It's nice to be able to just look at the board and know which header pin is 5V, which is GND and which is data, but I could just refer to a printout if I had to. In truth, I only include the legend because I'm already set up to do the process (for solder mask), it doesn't really add much to the cost of the board, and it looks better when I'm finished.

Good luck,

Joe