What is a mask on a PCB?

Question

I have an Arduino project that is mostly working on a solderless breadboard and I’m starting to plan on making it semi-permanent by soldering the components in place and mounting it into a project box.

I am thinking about using this Adafruit Perma-Proto Half-sized Breadboard PCB. Part of the description says (emphasis added):

The bottom has the 5-hole pad design that matches a classic breadboard, with 4 power bus lines on the sides, and no mask so you can easily cut traces when necessary.

Front: Back:

What does the part about no mask mean?

I think cutting traces means using a knife to scratch off the connection lines on the back so you can “split” one or more columns and gain more functionality. For example, assume I have a fictional micro-controller whose width would allow it’s pins to fit on this board’s rows E and F (straddling the middle) and its length would use pins 1-10 If I cut the connections horizontally from 1-10 between rows D and C and H and G, that would allow me to use additional components for rows A-C and H-J and columns 1-10 that I would not have been able to use without the cut. Is this correct?

ETA: Some of my questions here have been received poorly, so I took the time to make sure I added links and relevant pictures and quotes. Hope this helps.

Answer 1

Mask in this context means solder mask. An insulating paint that protects the traces from oxidation, dirt and shorts when soldering.

The bottom side shows you the raw material without the soldermask. The top side has white soldermask with black silkscreen (the text).

Answer 2

You are correct on the reason why cutting traces on these boards can be beneficial.

Now, regarding the mask: on regular PCBs, there is a layer of insulating "paint" that is silkscreened on the top of the boards after the copper is etched, mainly for the following reasons:

• prevent involuntary short-circuits if the board touches something conductive.
• protect the copper from dirt/contamination/oxidation.
• prevent solder bridge between adjacent pads during the reflow soldering process

It is usually green on most PCBs (this color comes from the mask, not the PCB core material). The mask goes all over the board surface, except where pads are, so that components can be soldered.

On this board, on the bottom side, they didn't put any mask at all (which is why it isn't green - or white, as the top side is), so that it doesn't annoy you when trying to cut traces. The mask could make it more difficult to reach the copper with an exacto knife.