A Few Details of Note about Solder Mask
I'm going to oversimplify this, so my apologies to our colleagues over at Sunstone Circuits, but a PC board basically has four parts: metal, substrate, solder mask and silk screen. I might simplify it even more to say that it only has one important part: the metal. But that would even require an apology to myself. Basically, all four of those systems (see what I did there?) play a crucial role in the making of a reliable PCBA (printed circuit board assembly).
Out on our production floor, we regularly see the benefits of paying careful attention to the design and construction of PCBs, and we also see the reverse. With that said, here are a few important tidbits:
Note 1: Solder mask far more important than just for aesthetic purposes.
Solder mask has become something of an adornment over the last few years. There was a time when you could get your boards in green or with no solder mask and that was about it. Today though, a wide variety of colors are available, and people take advantage of that for brand and style. However, solder mask is still a mission critical component. Solder mask plays an important role in controlling where solder will flow and where it will not. (Read about one aspect of that here.)
Note 2: Specifications such as mask thickness are also important.
The image below illustrates two thicknesses of solder mask. When you’re dealing with very small parts—such as 0402 or smaller passive components—thick solder mask may hold the component up high enough so that the metal ends don’t make good contact with the solder paste. If one side makes good contact and the other doesn’t, the surface tension of the melted solder on the side with contact may tug on the part, pulling it up vertical, which is called tombstoning. Mask thickness isn't a super common issue, but we do sometimes see it with very tiny parts on board from board houses that are pushing a bit beyond their actual capability.
Note 3: Solder mask does affect the solder joint.
That might not have been true in ancient times when we were all hand soldering our thru-hole boards, but it's true today with surface mount. Component manufacturers calculate land patterns based largely on the amount of solder required to make a reliable electrical and mechanical connection. The solder paste stencil cut-outs are sized to match the land and deposit just the right amount of solder on the pad.
During reflow, the solder will spread out and fill the metal area. When your land pad has traces connected to it, the solder mask is what keeps the solder from spreading out down the trace, which could result in an insufficient solder joint.
The board above left is roughly equivalent in functionality to the two above right, but uses small SMT parts. The two boards on the right were less expensive to buy, and are fine for this application (hand soldered by me), but won't cut it for surface mount.
The bottom line is that solder mask is important. Pick a cool color if you want, but use mask and don't take it for granted. Head over to Screaming Circuits and get a quote on your boards regardless of what color mask you use.
The solder mask of Zorro is not the same thing - but it would be pretty cool.