If you're having PCBs produced for the first time it can be quite daunting trying to making decisions about all the manufacturing options: substrate material, fire rating, soldermask, peelable masks, overlay inks, board thickness, copper thickness, through-hole plating, panelisation, and so on. One of the most important decisions though is definitely the selection of surface finish.
Unfortunately there's no one "best" PCB surface finish and all finishes have both good and bad points, so we can't just say "use finish X" and know that it's right for all situations. Selecting a surface finish is a matter of making a decision based on trade-offs that are most appropriate for the intended use.
ENIG
All of the Freetronics PCBs produced so far use a surface finish called "ENIG", which stands for "Electroless Nickel Immersion Gold". It consists of an underlying layer of nickel with a thin layer of gold over the top. The gold layer is very thin and not intended to provide the main structure of the track, it just acts as a protective coating for the nickel to prevent it tarnishing before it's soldered. Gold is extremely resistant to corrosion so ENIG has several good points: it can be touched with bare fingers without tarnishing, has a very long shelf life, and the pads / tracks are very flat and square-edged - something that can be important for fine-pitch surface-mount parts.
One one bad aspect to ENIG though is that it takes a bit more solder to complete a joint because the surface hasn't yet been pre-tinned, and because there isn't an existing layer of solder to melt against the iron and increase the initial contact area (hence increasing the rate of heat transfer) it can take an extra second or so to get the joint hot. You really need to make sure your iron has good contact with the joint to make sure enough heat is transferred to the track and the component lead.
HASL
The most common surface finish you see on PCBs is called "HASL", or "Hot Air Solder Levelling". HASL boards are dipped in molten solder and then the excess is cleaned off using hot-air knives to leave behind the thinnest possible layer of solder. The solder itself then protects the underlying track from corrosion while making it extremely easy to solder on parts, because the whole pad is pre-tinned. It's generally the cheapest finish available and a great choice for general-purpose boards. One downside to HASL is that even after the hot-air knife has cleaned off as much excess as possible, the meniscus of the solder will still cause the edges of pads to be slightly rounded. That makes surface-mount parts not sit down quite as flat as they would on an ENIG board.
So you'd expect that for a board like the Freetronics Prototyping Shield the obvious choice would be to select a HASL surface finish. But there's a catch. We're trying to stick to RoHS-compliance as much as possible, which means not using hazardous substances in our products. That would mean we couldn't use regular HASL: we would have to use lead-free HASL. Lead-free solder has a higher melting temperature than regular solder so if we went with lead-free HASL it'd be annoying for customers who don't have lead-free soldering equipment. We'd probably end up with lots of complaints from customers using regular solder and soldering irons who have trouble getting the lead-free solder hot enough.
Immersion Silver
Another possible PCB finish is "immersion silver", and it provides an amazingly good finish but has a terrible shelf life. For boards intended for machine assembly immediately after manufacture, silver is a great option. The problem is that it tarnishes quickly and is adversely affected by touch, so it's no good for boards that are intended for distribution to hobbyists for (potentially) long storage and hand assembly. If you held an immersion-silver PCB in your hands, put it on a shelf for a couple of months, and then pulled it out to use it, you would probably find the surface was terribly tarnished and you just couldn't get solder to stick to it.
The "Best" Finish?
In the end we settled on ENIG as the standard finish on our PCBs because of the benefits of long shelf life, tarnish resistance, RoHS-compliance, and easy soldering compared to other surface finishes. It's more expensive than HASL but overall the result is worth it.
